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Construction Joints in Structures

By
Ubani Obinna
-

Concrete joints are essential components of various structures and are implemented on construction sites based on specific construction requirements and design considerations. Construction joints in concrete structures are almost unavoidable, given that structures cannot be built in a single casting step. Therefore, with the presence of joints, building the structure piece by piece is possible until it becomes a complete whole.

There are different types of joints in concrete such as movement joints, contraction joints, expansion joints, sliding joints, construction joints, temporary joints, and isolation joints. Some of these joints can exist in structures of different configurations, addressing specific technical needs based on the design and nature of the structure.

Construction Joints

Construction joints are deliberate separations or discontinuities created within concrete structures. In other words, construction joints allow for the placement of concrete in stages. This is because it is not practical to pour concrete for an entire structure in a continuous manner, except for small construction works. 

For example, depending on the technical capacity of the contractors, it may not be feasible to pour the concrete of a floor and the continuation columns of the floor on the same day. Moreover, the maximum quantity of concrete that may be mixed and placed on the construction site is also determined by the batching and mixing capacity, the number of the workforce, and the allocated time duration.

In effect, accurately positioned and effectively implemented construction joints establish boundaries for consecutive concrete placements, without causing any negative impact on the structure. A properly constructed joint in monolithic concrete ensures a robust and watertight surface, allowing the joint to maintain its flexural and shear continuity. In the absence of this continuity, the region becomes weaker, potentially functioning as a joint for either contraction or expansion.

Continuity of reinforcement is critical in construction joints
Continuity of reinforcement is critical in construction joints

Preparation of Construction Joint Surfaces

To establish a strong and impermeable joint, several prerequisites must be met before the application of the newly poured concrete. It is necessary for the concrete to be thoroughly cleaned and free of any laitance residue.

When there is a short period of time between consecutive placements, it is essential to visually inspect and ensure that any loose particles, dirt, and laitance have been thoroughly removed. To ensure proper bonding between the new concrete and the hardened concrete, the new concrete should be extensively vibrated throughout the entire area.

For more mature joints, additional surface preparation is required. The concrete can be cleaned using an air-water jet or wire brooming while it is in a state where the laitance can be removed but the aggregate remains securely in place. To prepare hardened concrete, it is recommended to use either a wet sandblast or an ultra-high-pressure water jet, as stated by the ACI Committee 311 (2019).

According to ACI 318, it is necessary to thoroughly dampen the existing concrete before placing fresh concrete. Green concrete does not require any more water, but concrete that has dried up may require saturation for a duration of one day or longer. During concrete placement, there should be no stagnant water on the wet surface.

Location of the joint

When designing construction joints, it is important to consider the precise placement or location of the joint in the structure. If construction capacity allows, construction joints should be positioned in a way that minimizes the impact on the structural strength of the structure while still being in harmony with the building’s aesthetics.

The positioning of joints varies depending on the nature of the element being constructed. To provide clarity, beams and slabs will be discussed as distinct entities from columns and walls.

construction joint for different phases of pour

Beams and slabs

When it comes to the structural integrity of beam and slab floor systems, it is recommended to position joints perpendicular to the main reinforcement at sites where shear is minimal or at points of contraflexure. At the point of contraflexure, the bending moment is expected to be very minimal. Therefore, it is going to be the least stressed section of the structure.

Typically, joints are placed at the midpoint or within the central third of the span, but their exact placement should be confirmed by the structural engineer. According to ACI 318, in beam and girder construction, when a beam crosses a girder at the place where the shear is minimal, the construction joint in the girder should be positioned at a distance that is twice the width of the incident beam.

It is often discouraged to use horizontal construction joints in beams and girders. A conventional approach involves integrating beams and girders seamlessly with the slab. To achieve flexural continuity, it is necessary to extend the reinforcement through the joint with a length that is sufficient to provide an appropriate splice length for the reinforcement. Shear transmission occurs through either shear friction between the existing and new concrete, or dowel action in the reinforcement across the joint.

In Nigeria, it is usually typical to cast the beams of a suspended floor first, when it is not feasible to cast the entire floor in a single pour. At the subsequent casting, the slab deck is poured to sit on the hardened beam.

Columns and Walls

When it comes to concrete placement, it is common practice to limit the height of concrete to one story. Construction joints in columns and bearing walls should be positioned at the lower surfaces of floor slabs and beams, and at the upper surface of floor slabs for columns that extend to the subsequent floor. The column capitals, haunches, drop panels, and brackets must be integrated seamlessly with the slab.

The positioning of newly poured concrete on a level surface can impact the connection point. Traditionally, it is customary to apply a layer of mortar, with the same proportions as the concrete, before pouring new concrete over the joint. The ACI Manual of Concrete Inspection (ACI Committee 311, 2019) recommends using a bedding layer of concrete that contains a slightly higher proportion of cement, sand, and water than the design mix specified for the project.

Material measuring less than 3/4 inch can remain in the bedding layer, whereas any material above 3/4 inch should be removed. The recommended depth for placing this mixture is between 4 to 6 inches, and it should be vibrated vigorously along with the standard mixture placed on top.

Conclusion

Construction joints play a crucial role in the performance and buildability of reinforced concrete structures. They serve as specially selected interfaces between different sections or phases of a structure, during the concrete pour. Construction joints are typically formed by placing a layer of concrete, then allowing it to set before placing the next layer. The joint is then formed by removing any loose material and cleaning the surface of the concrete. 

It is important to ensure that there is continuity of reinforcements at construction joints to ensure maximal shear transfer between the joints. Furthermore, in flexural members, joints should be located where stresses are minimal, while in compression members such as columns and walls, they should be preferably located at the top and bottom of the members.

Construction joints are crucial elements in ensuring the structural integrity, constructability, and aesthetics of concrete structures. Understanding their types, design considerations, and benefits helps optimize their role in creating safe, durable, and visually appealing buildings.

Geostatic (In-situ) Stresses and At-Rest (K0) Condition in Soils

By
Ubani Obinna
-

The stress in the soil mass is affected by the previous loading history. When a new foundation is constructed, the new load can either increase or decrease the existing stresses in the soil. The response of the soil mass to the new load depends on the previous stress history. Therefore, it is important to understand the stress imprint of the soil mass before designing a foundation.

The term “stress imprint” refers to the state of stresses that are locked into the soil structure. These stresses are locked in because the soil particles have rearranged and formed bonds. The bonds prevent the stresses from being released, even when the load is removed. The stress imprint can be either positive or negative.

image 20
Figure 1: Stresses in a soil mass

A positive stress imprint is created when the soil is loaded to a higher stress than it is currently experiencing (normally consolidated soil). A negative stress imprint is created when the soil is unloaded to a lower stress than it is currently experiencing (overconsolidated soil).

The stress imprint can affect the behaviour of the soil in a number of ways. For example, a positive stress imprint can make the soil more resistant to shearing, while a negative stress imprint can make the soil more susceptible to settlement.

The stress imprint can be difficult to measure, but there are a number of techniques that can be used. One common technique is to use a pressuremeter. A pressuremeter is a device that is inserted into the soil and used to measure the stresses in the soil.

In-Situ Stresses in Soil

In situ, the vertical stresses act on a horizontal plane at some depth z. These can be computed in any general case as the sum of contributions from n strata of unit weight γi and thickness zi as;

po = ∑γizi —— (1)

In the context of the formation of soil deposits, the area of the land where soil accumulates is typically extensive, and the depth of the deposit keeps increasing until either the accumulation process or the internal weathering process halts. This transition leads to a gradual downward compression of the soil at any specific depth.

Likewise, the vertical stress also increases due to this compression, and in almost all instances, the unit weight of the soil is a function of depth. Given the substantial lateral extent of the deposit, there is limited justification for notable lateral compression to take place.

Consequently, it is reasonable to anticipate that the vertical locked-in effective stresses (p’o) would surpass the effective lateral stresses (σ’h) at the same location. This relationship between horizontal and vertical stresses can be defined as the ratio of the two.

K = σh/po ——– (2)

which is valid for any depth z at any time.

K0 Conditions

Over geological time the stresses in a soil mass at a particular level stabilize into a steady state and strains become zero. When this occurs the vertical and lateral stresses become principal stresses acting on principal planes. This effective stress state is termed the at-rest or K0 condition with K0 defined as;

K0 = σ’h/p’o ——– (3)

Therefore, Ko conditions refer to the state of stresses in a soil mass when there is no lateral strain. This means that the soil is not allowed to deform horizontally. Ko conditions are typically found in undisturbed soil, where the soil has not been subjected to any significant lateral stresses.

Figure 2 shows the range of K0 and the relationship between po and σh in any homogeneous soil. The figure also shows the qualitative curves for preconsolidation in the upper zone of some soil from shrinkage/chemical effects. The figure clearly illustrates the anisotropic (σv ≠ σh) stress state in a soil mass.

image 10
Figure 2: Qualitative vertical and lateral pressures in a soil.

In the figure above, although the linear vertical (also called geostatic) pressure profile is commonly used, the p’0 effective pressure profile is more realistic of real soils since γ usually increases with depth. The lateral pressure profile range is for the geostatic pressure profile and would be curved similarly to the p’o curve for real soils.

K₀ conditions are critical for:

  • Retaining wall design.
  • Excavation support.
  • Earthquake-induced lateral pressures.
  • Soil-structure interaction.

Because of the sampling limitations, it is an extremely difficult task to measure K0 either in the laboratory or in situ. Some field methods are available, but note that they are very costly for the slight improvement—in most cases—over using one of the simple estimates following. In these equations use the effective angle of internal friction ‘ and not the total stress value.

Jaky (1948) presented a derived equation for K0 that is applicable to both soil and agricultural grains (such as corn, wheat, oats, etc.) as;

K0 = [(1 – sinφ’)/(1 + sinφ’)] × ( 1 + 2/3sinφ’) ——– (4)

which has been simplified—and erroneously called “Jaky’s equation”— to the following:

K0 = 1 – sinφ’ ——— (5)

This equation is very widely used and has proved reasonably reliable in comparing initial to back-computed K0 values in a number of cases and for normally consolidated materials. Kezdi (1972) suggests that for sloping ground Jaky’s equation can be used as follows:

K0 = (1 – sinφ’)/(1 + sinβ) ——– (6)

where β is the angle with the horizontal (with sign) so that K0 is either increased or reduced as site conditions dictate. This reference also gives a partial derivation of the Jaky equation for any interested user.

Brooker and Ireland (1965) (for normally consolidated clay) suggest;
K0 = 0.95 – sinφ’ ——– (7)

Alpan (1967) (for normally consolidated clay) suggests;
K0 = 0.19 + 0.233 log10 Ip ——– (8)

An equation similar to Eq. (8) is given by Holtz and Kovacs (1981) as;
K0 = 0.44 + 0.0042Ip ——– (9)

where Ip is the plasticity index of the soil in percent.

We can readily derive a value for K0 in terms of Poisson’s ratio based on the definition of K0 being an effective stress state at zero strain. From Hooke’s law, the lateral strain in terms of the effective horizontal (x, z) and vertical (y) stresses is;

εx = 0 = 1/Es(σx – μσy – μσz) = εz ——– (10)

For a cohesionless, soil μ is often assumed as 0.3 to 0.4, which gives K0 = 0.43 to 0.67, with a value of 0.5 often used. It is extremely difficult to obtain a reliable estimate of K0 in a normally consolidated soil, and even more so in overconsolidated soils (OCR > 1). A number of empirical equations based on various correlations have been given in the literature. Several of the more promising ones are:

Alpan (1967) and others have suggested that the overconsolidated consolidation ratio K0,OCR is related to the normally consolidated value K0,nc in the following form;

K0,OCR = K0,nc × OCRn (2-23) ——– (11)

where n = f(test, soil, locale) with a value range from about 0.25 to 1.25. For overconsolidated sand, n can be estimated from Figure 3.

image 19
Figure 3: Exponent n for sands.

For cohesive soil, Wroth and Houlsby (1985) suggest n as follows:

n = 0.42 (low plasticity — IP < 40%)
n = 0.32 (high plasticity — IP > 40%)

However, n ≈ 0.95 to 0.98 was obtained from in situ tests on several clays in eastern Canada. Mayne and Kulhawy (1982) suggest that a mean value of n = 0.5 is applicable for both sands and clays and that n = sinφ’ is also a good representation for sand. Their suggestions are based on a semi-statistical analysis of a very large number of soils reported in the
literature.

The exponent n for clays was also given by Alpan (1967) in graph format and uses the plasticity index IP (in percent). The author modified the equation shown on that graph to obtain;

n = 0.54 × 10(-IP/281) ——– (12)

And, as previously suggested (for sands), we can use;

n = sinφ’ ——— (13)

Conclusion

Geostatic conditions refer to the stress state within a soil mass due to the weight of the overlying soil. These stresses are primarily vertical and compressive, but they also induce horizontal stresses due to the frictional resistance of the soil particles. The magnitude of these stresses depends on the depth of the soil layer, the unit weight of the soil, and the geometry of the soil mass.

The K0 condition, also known as the at-rest earth pressure, represents a specific stress state within a soil mass where there is no lateral strain. This condition occurs when the soil is horizontally constrained, such as by a retaining wall or a layer of bedrock. In K0 conditions, the horizontal effective stress (σ’h) is typically about 0.5 to 0.7 times the vertical effective stress (σ’v). The factors affecting K0 values are the overconsolidation ratio, type of soil, confining pressure, stress history, and effective stress state.

Design of Fillet Weld Connections

By
Ubani Obinna
-

In modern construction, the commonest methods of connecting steel members are bolting and welding. Fillet welds are generally used for connecting parts where the fusion faces form an angle of between 60° and 120°. Although there are other special cases and generalities stated in the code regarding fillet weld (kindly refer).

EN 1993-1-8 (Eurocode 3 Part 8) identifies different types of welding such as;

  • fillet welding
  • butt welding
  • plug welding,
  • and flare groove welds.

The effective weld thickness, denoted as ‘a‘, is determined by measuring the height of the largest inscribed triangle inside the fusion faces and the weld surface. This measurement is taken perpendicular to the outer side of the triangle, regardless of whether the triangle has equal or unequal legs.

image 18
Figure 1: Fillet weld thickness for different penetration

Types of Fillet Welds

There are three main types of fillet welds:

  • Equal leg fillet welds: These welds have legs of equal length.
  • Unequal leg fillet welds: These welds have legs of different lengths.
  • Lap fillet welds: These welds are used to join two pieces of metal that are overlapping.
fillet weld connection

Fillet Weld Design

The design of a fillet weld depends on the thickness of the metal being welded, the strength of the weld required, and the type of weld being used. The following factors should be considered when designing a fillet weld:

  • Weld size: The weld size is the length of the weld leg. The weld size should be large enough to provide the required strength but not so large that it causes distortion of the metal.
  • Weld throat: The weld throat is the thickness of the weld. The weld throat is typically equal to 0.7 times the weld size.
  • Weld length: The weld length is the length of the weld along the joint. The weld length should be long enough to provide the required strength but not so long that it is wasteful of material.

According to clause 4.5.3.1(1) of EN 1993-1-8, the design resistance of a fillet weld should be determined using either the directional method or the simplified method.

Directional Method

The directional method is based on the comparison between the design tensile strength and the applied stress in the most severely loaded part of the weld throat. When using the directional method for analyzing fillet welds, the force acting on the weld is divided into components that run parallel and perpendicular to the weld throat plane. This results in evenly distributed normal and shear stresses.

The applied stress, being determined from a Von Mises formulation, accounts for the influence on the weld strength of the inclination of the resultant force per unit length to the weld axis and plane.

In the directional method, a uniform distribution of stress is assumed on the section of the weld, leading to the normal stresses and shear stresses shown in Figure 2, as follows:

σ⊥ normal stress perpendicular to weld axis
σ|| normal stress parallel to weld axis
τ⊥ shear stress (in plane of fillet weld surface) perpendicular to weld axis
τ|| shear stress (in plane of fillet weld surface) parallel to weld axis

directional method fillet weld
Figure 2: Stresses in a weld

Simplified Method

The simplified method is based on the design shear strength of the weld which is compared directly to an applied weld throat shear stress obtained by dividing the resultant force per unit of length of the weld throat size. This method does not necessitate determining the individual stress components within the weld.

Instead, the overall stress is calculated as the vector sum of all force components, and the weld’s strength is assumed to be equal to its shear strength, regardless of the force’s direction. Since fillet welds are weakest under pure shear, this method consistently produces more conservative results than the directional method.

Design Example

For the welded connection (K-Joint) in a steel truss given as shown below, establish the minimum throat thickness of the fillet weld for the gusset plate. The basic assumption is that the gusset plate transfers axial forces acting in its plane and in the direction of the member axes.

DESIGN%2BOF%2BWELDED%2BCONNECTION%2BFOR%2BROOF%2BTRUSS


Analysis Results
NA,Ed = 250 kN (compression)
NB,Ed = 250 kN (compression)
NC,Ed = 17.5 kN (compression)
NB,Ed = 17.5 kN (compression)

Geometric Data
Gusset plate thickness      tg = 10mm
Web thickness                  tw = 8mm
Angle between gusset and web      αa = 90°
Number of fillet welds      na = 1
Length of weld                  Lw = 400mm
Effective throat thickness a = ?

Material Data
Steel grade = S275
Yield strength fy = 275 N/mm2
Ultimate tensile strength fu = 430 N/mm2
Partial factor (resistance of weld) γM2 = 1.25 (recommended)

Design forces in the gusset plate at the chord web face
The effects of the small eccentricity from the chord axis will be neglected.
The gusset plate section is verified for the following forces:

Ng,Ed is the axial force at an eccentricity of ez = 0  to the centreline of the gusset plate
Vg,Ed is the shear force

nhyy

αi = 35°
Ng,Ed =  2(250 cos 35) = 409.576 kN
Vg,Ed =  2(250 sin 35) = 286.788 kN

Assuming a uniform distribution of load in the section, the normal stress is given by;

Normal%2Bstress

Ag is the cross-sectional area and it is given by Ag = tg. Lw = 10mm × 400mm = 4000 mm2

σg,max = (409.576 × 1000) / (4000) = -102.394 N/mm2

The mean shear stress is given by;
τg,max = (286.788 × 1000) / (4000) = 71.697 N/mm2

By directional Method;
Assuming a uniform distribution in the throat section of the weld;

σ⊥ = the normal stress to the throat plane
τ⊥ = the shear stress (in the plane of throat) perpendicular to the axis of the weld
τ// = the shear stress (in the plane of throat) parallel to the axis of the weld

On the throat section of the weld, the force per unit length are:

dfgh

Where eg is the average eccentricity. In our own case, we will be taking the eccentricity as unity.

a σ⊥ = [(102.394 × 1.0) / 1.0] / (sin 45°) = 72.403 N/mm2
a τ⊥ = [(102.394 × 1.0) / 1.0] / (cos 45°) = 72.403 N/mm2

awee

a τ// = [(71.697 × 1.0) / 1.0] = 71.697 N/mm2

The design resistance of the fillet weld will be sufficient if the following conditions are both fulfilled:

drty

σw = [72.4032 + 3(72.4032 + 71.6972]0.5 = 190.762 N/mm2
βw = 0.85 (Table 4.1 EN 1993-1-8:2005)

Therefore;
fu / βw.γM2 = 410 / (0.85 × 1.25) = 385.882 N/mm2

190.762 N/mm2 < 385.882 N/mm2    OK

Also verify;
0.9fu / γM2 = 0.9 ×  410 / (1.25) = 295.2 N/mm2
72.403 N/mm2 < 295.2 N/mm2    OK

From these conditions, a minimum value for the effective throat thickness can be derived;

Throat%2Bthickness%2Bformular%2Bfor%2Bwelding

a1,min = 190.762 / 385.882 = 0.494 mm
a2,min = 72.403 / 295.2 = 0.245 mm
amin = max[a1,min , a2,min ] = 0.494 mm

However according to clause 4.5.2(2), the effective throat thickness of a fillet weld should not be less than 3mm, therefore, adopt 4mm effective throat thickness.

By Simplified Method

The design resistance of a fillet weld may be assumed to be adequate if, at every point along its length, the resultant of all the forces per unit length transmitted by the weld satisfies the following criterion (see Eurocode 3, Part 1-8):

Fw,Ed ≤ Fw,Rd

Where Fw,Ed is the design value of the weld force per unit length and Fw,Rd is the design weld resistance per unit length. The design strength resistance per unit length (Fw,Rd) is given by;

Fw,Rd = Fvw,d a

Where Fvw,d is the design shear strength of the weld and a is the throat thickness of the weld.

fwdv

Fw,Ed = τg,max = (286.788 × 1000) / (400) = 716.97 N/mm (shear force per unit length of weld)
fvw,d = (430 / √3) / (0.8 ×  1.25) = 233.657 N/mm2

Assuming a weld throat thickness of 4 mm
Fw,Rd = Fvw,d a = 233.657 × 4 = 934.628 N/mm
Fw,Ed  (716.97 N/mm) <  Fw,Rd  (934.628 N/mm) Ok

Note: 
According to clause 4.5.1(2) of EN 1993-1-8:2005, a fillet weld with an effective length less than 30 mm or less than 6 times its throat thickness, whichever is larger, should not be designed to carry load.

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Strengthening Flat Slabs against Punching Shear

By
Ubani Obinna
-
strengthening flat slab against punching shear

Punching shear is one of the most critical failure modes in flat slabs, characterized by its catastrophic and brittle nature. In existing buildings, there may come a time when there is a need for strengthening flat slabs against punching shear failure.

A flat slab is a type of suspended floor without beams, allowing for direct transfer of loads from the slab to the columns. They are frequently employed in building construction due to their ease of construction (especially in formwork and reinforcement placement), cost efficiency, and ability to accommodate lower floor heights. A flat slab construction, however, experiences significant transverse loads concentrated around the column, which can result in non-ductile, sudden, and brittle punching failure.

Most design codes worldwide have well-established design guidelines to prevent punching shear failure during the design process. Incorporating steel reinforcement, such as stirrups, shear heads, or shear studs, in the high shear zone enhances ductility, a crucial requirement in the design process.

punching shear failure
Figure 1: Typical punching shear failure

However, there are several factors that can lead to a decrease in the punching shear strength of flat slabs. These include casting concrete with a compressive strength lower than the intended value, incorrectly placing the punching shear reinforcement, and neglecting tension forces caused by shrinkage restraint or temperature changes, which have been shown to reduce the shear strength of the concrete.

Additionally, insufficient punching shear capacity of flat slab structures can be attributed to factors such as changes in usage and loading, new installations, or design and construction errors. Sometimes, this necessitates either upgrading existing structures or replacing them entirely. However, replacing deficient structures is often not a feasible option due to the substantial costs involved. Therefore, repairing and/or strengthening inadequate structures represents a viable solution, offering cost and time savings.

Strengthening is necessary to regain or improve the punching shear strength of the slab in this situation. To enhance the structural integrity, one can employ several methods such as incorporating drop beams, increasing the thickness of the slab near the columns with steel or concrete caps, or introducing punching shear reinforcement.

Techniques for Strengthening Flat Slabs against Punching Shear

To enhance the punching shear capacity of a flat slab-column connection, one can employ methods such as using a thicker slab, a larger column, concrete with higher compressive strength, more flexural reinforcement, or more shear reinforcement. These options can be generally categorised into;

In any project, some techniques for increasing the punching shear capacity of flat slabs are better than others, especially when considering architectural features and cost. Some exclusive methods of strengthening flat slabs against punching shear are discussed below;

Slab Strengthening by Columns Caps

Steel or concrete caps can be used at the top of columns to shift the critical section of the slab in punching shear away from the column face, thereby increasing the overall punching shear capacity of the slab. A typical steel cap used for slab strengthening is shown in Figure 2.

Punching shear strengthening using steel caps
Figure 2: Punching shear strengthening using steel caps (Abdelrahman, 2023)

The steel cap is attached to the column and the slab soffit using steel dowels. Steel stiffeners are used to increase the flexural stiffness of the steel plates attached to the slab. Grout is used to fill the gap between the steel cap and the bottom slab soffit to ensure good contact between the cap and the slab soffit. Strengthening efficiency will be greatly reduced if cracks or small gaps exist between the slab and the cap, as no vertical deformations of the slab are expected in this zone under the effect of gravity loads.

Punching Shear Reinforcement

To enhance the overall punching shear strength of the slab, it may be necessary to add steel reinforcement. The main types of shear reinforcement used for enhancing punching shear capacity are;

  • anchored bolts with nut, washer and plate
  • headed bolts
  • bonded bolts
  • grids or fans (only for composite materials)
  • stirrups (only for composite materials)

This process usually involves drilling through the slab near the column, where the flexural steel reinforcement is often densely packed. To prevent the need for cutting steel rebars during slab drilling, one can utilize steel detecting equipment to accurately locate the reinforcement.

image 12
Figure 3: (a) Anchored bolts with nut, washer and plate (b) headed bolts

After identifying the rebars, holes with a diameter bigger than that of the stirrups are drilled. The steel shear bars are then put through these holes and secured to the top and bottom concrete soffits using steel nuts, as depicted in Figure 4. To safeguard the steel bars and prevent corrosion, it is imperative to pump epoxy into the holes.

image 13
Figure 4: Strengthening flat slabs using shear reinforcements (Abdelrahman, 2023)

The quantity of shear bars is established according to the design specifications and the aspect ratio of the columns. The design standard dictates that the entire area of the steel bars at the critical section must be determined.

Use of FRP Sheets

Glueing fibre-reinforced polymer (FRP) strips in both perpendicular directions of the slab around the column head can enhance the slab’s flexural capacity. Employing bonded FRP strips in both orthogonal directions as a strengthening technique solely impacts the load-rotation curve, whereas the failure criterion remains relatively unchanged. The load-rotation curve post-strengthening exhibits increased stiffness due to the augmented longitudinal reinforcement.

frp sheets strengthening
Figure 5: Use of FRP sheets in slab strengthening

Generally, this method of flat slab strengthening enhances the punching shear resistance but diminishes the ductility of the connection. The failure becomes more brittle as the maximum rotation after reinforcement is lower than that of the current slab. The efficacy of this strengthening technique is closely tied to the quantity of flexural reinforcement in the existing slab.

Bonded Reinforced Concrete Overlay

A bonded reinforced concrete overlay (BRCO) may be used if the fibre-reinforced polymer (FRP) strengthening is insufficient to reach the desired punching capacity. The application of a BRCO (Figure 6) may be included in the flexural strengthening techniques, but it actually affects both the failure criteria and the load rotation curve. This technique increases the punching strength by improving the failure criteria curve.

image 14
Figure 6: Bonded reinforced concrete overlay (BRCO)

In this method, a reinforced concrete overlay of a specified thickness is placed around the column head of the flat slab. Mechanical shear connectors are used to bond the old and new concrete for maximum load transfer. The disadvantage of this method is its potential to disrupt the architectural features of the building.

Enlargement of the support

To enhance the punching shear capacity of flat slabs, one might consider enlarging the column, adding a concrete capital, or inserting a steel capital after the initial installation (Figure 7). If the failure does not affect the capital, the latter two solutions can be considered similar to the first one. Moreover, the capital must possess enough rigidity to provide enough support to the slab; otherwise, the effectiveness of the strengthening technique may be compromised.

image 15
Figure 7: Strengthening of flat‐slab by enlargement of the support: (a) column widening; (b) casting new concrete capital; (c) post‐installing steel capital

The expansion of the support impacts both the failure criterion curve and the load rotation curve. Nevertheless, the alteration of the load-rotation curve is solely observed in the vicinity of the horizontal plateau, where the maximum flexural capacity is attained. Due to this characteristic, this particular strengthening approach is less susceptible to the initial rotation compared to other techniques.

Post-tensioning

Post-tensioning systems can be used to enhance the strength of existing reinforced concrete flat slabs. These strengthening techniques can be divided into two categories: flexural strengthening and shear strengthening. Flexural strengthening typically involves the installation and prestressing of FRP strips on the top surface of the slab. The second method involves the use of inclined steel or FRP straps that are fastened through steel plates or by bonding.

image 16
Figure 8: Prestressing system for flexural strengthening: FRP strips and anchor plate
image 17
Figure 9: Prestressing system for shear strengthening – CFRP straps anchored with steel plates 

The impacts of prestressing systems can generally be categorized into three main aspects:

  • In-plane compressive forces
  • deviation forces caused by the inclination of tendons
  • bending moments caused by the eccentricity of tendons;

The load rotation curve and the failure criterion curve are both influenced by in-plane forces. The compression field enhances the flexural strength and increases the rigidity of the slab. Moreover, compressive stresses within the same plane increase the strength of interlocking that is supported by the critical shear crack.

Conclusion

In this article, some of the methods of improving the punching shear capacity of reinforced concrete flat slabs were reviewed. The major takeaways are as follows;

Post-installed shear reinforcement has proven to be an effective and reliable method for structural strengthening of flat slabs against punching shear. Numerous studies have successfully determined the optimal quantity and type of shear reinforcement for most structures, preventing punching failure and enabling the slab to achieve its maximum flexural capacity.

However, in some instances, designing shear reinforcement to avert punching failure has not been feasible due to premature concrete strut crushing. The introduction of shear steel bolts can enhance the slab’s deformation capacity by 47.4% and its load-bearing capacity by 15.4%.

Reinforcing a slab with FRP strips for flexural strength indirectly improves its punching resistance by increasing the slab’s overall stiffness. Consequently, the effectiveness of this technique is directly linked to the existing slab’s longitudinal reinforcement ratio (ρ). As ρ increases, the effectiveness of flexural strengthening with FRP strips diminishes.

In a certain study, the use of CFRP sheet as externally bonded flexural reinforcement, in addition to steel reinforcing bars on the tension face of the slab, increased the punching shear capacity and ultimate deflection by about 50 and 30% respectively.

Using a BRCO (bonded reinforced concrete overlay) offers superior outcomes compared to FRP strips. The increased slab depth due to the BRCO improves the failure criterion, enabling the achievement of higher punching strength. However, to prevent premature debonding of the reinforced concrete overlay, the utilization of mechanical connectors is advisable.

Enlarging the support can be accomplished by casting a new concrete capital or installing a steel capital. This strengthening method influences both the failure criterion and the load-rotation curve, but its effectiveness is unaffected by the shear load at the time of strengthening.

With a new support size twice that of the existing support, punching strength increases by 30-50%. The primary factor influencing punching strength after strengthening is the ratio between the critical perimeter after and before strengthening.

Flexural and shear post-tensioning systems are also viable options for strengthening against punching shear. However, flexural post-tensioning systems have encountered issues due to anchorage failures. Conversely, shear prestressing systems demonstrate greater reliability and enable substantial increases in punching capacity. The effectiveness of this technique is time-independent, and results obtained on both unloaded and loaded slabs are expected to be similar. Moreover, shear post-tensioning can reduce slab deflection and crack width under service loads.

Combining various techniques is also a feasible approach. For instance, FRP strips on the slab’s top surface can be coupled with post-installed shear reinforcement, achieving both flexural and shear strengthening. Shear post-tensioning systems can be applied prior to flexural strengthening techniques like FRP or BRCO to minimize initial slab rotation and enhance the efficacy of the strengthening measures.

References

Abdelrahman A. (2023): Strengthening of Concrete Structures – Unified Design Approach, Numerical Examples and Case Studies. Springer Nature

Analysis of Slope Stability

By
Ubani Obinna
-

An earth slope can be either a naturally occurring incline or one that is artificially constructed through excavation or engineered fill to produce an embankment. A slope failure refers to the downward displacement of a section of the slope mass in relation to the mass below the sliding surface. Therefore, slope stability assessment is an important aspect of earthworks and geotechnical designs in civil engineering works.

The magnitude of a slope failure ranges from a few meters in height to the displacement of a substantial portion of a high land or mountain. An instance of this is the 1974 Rio Mantaro landslide in Peru, which consisted of a sliding mass of 6 kilometres in length, 2 kilometres in height, and a volume of 1.5 billion cubic meters.

Figure 1 shows a significant slope failure that took place in Oso, Washington, USA, on March 22, 2014. The failure surface displays a rotational sliding surface.

oso slope failure
Figure 1: Slope failure that took place in Oso, Washington, USA,

This article focuses on the stability analyses of unreinforced soil slopes under static loading.

Causes of Slope Failure

A slope failure occurs when the external shear stress (or sliding moment) exceeds the internal shear strength (or resisting moment) of the slope.

Shear stress > Shear strength or:
Rotational moment > Resisting moment

The following factors may increase the shear stress or sliding moment:

  • Additional surcharge at the top of a slope.
  • Application of lateral force that may be caused by seepage, earthquake, or pile driving.

The following factors may decrease the shear strength or resisting moment of a slope:

  • Weathering of a rock slope.
  • Discontinuities such as weak seams and faults that are developed in the slope.
  • Saturation of the slope.
  • Removal of lateral support of the slope, for example, the cut toe of a slope.

Types of Slope Failure

Slope failures can be classified into different types based on the shape of the failure surface and the nature of the slope movement.

  • Surficial (or translational) slope failure: The sliding surface is parallel to the slope surface. This type of failure can occur when there is a weak layer of soil or rock beneath the surface, or when a loose topsoil layer rests on a hard subsoil layer.
  • Rotational slope failure: A large mass of the slope rotates along a curved failure surface. This type of failure is often caused by heavy rainfall or by excavation at the toe of the slope.
  • Landslide: A landslide is a large-scale slope failure that can involve multiple types of slope movements. Landslides can be caused by a variety of factors, including heavy rainfall, earthquakes, and volcanic eruptions.
  • Lateral spread: Lateral spread is the lateral movement of a fractured soil mass. It is typically caused by earthquakes.
  • Debris flow: A debris flow, or mudslide, is a rapid movement of soil that is entrained by flowing water or wind. Debris flows are often triggered by heavy rainfall.
  • Creep: Creep is a slow and almost imperceptible movement of the failure portion of a slope. It can be caused by a variety of factors, including gravity, groundwater flow, and frost heave.
  • Rock falls: Rock falls occur when rocks on a slope are mobilized by wind, runoff, or gravity. Rock falls can be very dangerous, as they can travel at high speeds and cover long distances.
types of slope failure
Figure 2: Types of slope failures. (a) Surficial slope failure, (b) rotational slope failure, (c) landslide.

Slope Stability Analysis

Slope stability analysis methods typically use the limit equilibrium approach, which means that the forces or moments that cause a slope to fail (slide) are balanced by the forces or moments that resist the slope from sliding. This is known as the critical condition. A factor of safety (FS) is used to measure slope stability and is based on the force or moment equilibrium.

FS = 𝜏f/𝜏
or:
FS = Mresist/Mslide

where;
𝜏f = the maximum shear stress at failure, which is equal to the shear strength,
𝜏 = shear stress that causes the sliding of a failure portion, which is caused by external loads such as gravity, foundation loading, seismic force, etc.,
Mresist = total resisting moments that resist a rotational sliding,
Mslide = total sliding moments that cause a rotational sliding.

In general, a value of 1.25–1.5 is used as an acceptable factor of safety. Alternatively, within the context of limit state design, it should be mentioned that:

Ed ≤ Rd

where;
Ed = design effect of the actions (e.g., sliding forces)
Rd = design resistance (dependent on soil strength)

The shear strength is expressed by the Mohr–Coulomb failure criterion:

𝜏f = c + 𝜎 tan 𝜙

where;
𝜎 = total normal stress,
c = soil’s cohesion based on the total stress,
𝜙 = soil’s internal friction angle based on the total stress.

The Mohr–Coulomb failure criterion can also be expressed using the effective stress:

𝜏f = c′ + 𝜎′ tan 𝜙′

There are two main approaches for analyzing slope stability: the total stress method and the effective stress method. The total stress method uses undrained shear strength parameters (c and ϕ), while the effective stress method uses drained shear strength parameters (c′ and ϕ′)

In slope stability analyses, two “artificial” factors of safety are defined on the basis of c and 𝜙, respectively:

FSc = c/cm
FS𝜙 = tan 𝜙/tan 𝜙m

where;
FSc = factor of safety on the basis of c,
FS𝜙 = factor of safety on the basis of 𝜙,
cm = mobilized cohesion that is actually developed along a slip surface, also denoted as cd,
𝜙m = mobilized internal friction angle that is actually developed along a slip surface, also denoted as 𝜙d.

In slope stability analysis, partial factors of safety can be used to account for uncertainties in the soil properties and the loading conditions. These factors are similar to those used in other engineering design codes. In a stable slope, not all of the soil’s strength is needed to resist the forces that cause it to slide.

The mobilized shear strength, which is the amount of shear strength that is actually used to resist sliding, is always less than or equal to the peak shear strength. This means that there is always a factor of safety built into the design of a stable slope. The developed or mobilized shear strength that is needed for slope stability is represented by cm and 𝜙m. Therefore, cm ≤ c and 𝜙m ≤ 𝜙.

At equilibrium, the developed or mobilized shear stress can be expressed as:

𝜏m = cm + 𝜎 tan 𝜙m

At the critical condition, the entire shear strength is needed for equilibrium. So, cm = c, and 𝜙m = 𝜙. Therefore, the minimum value for both FSc and FS𝜙 is 1.0.

If: FSc = FS𝜙 = a (constant)

Then: c = a⋅cm, and tan 𝜙 = a⋅tan𝜙m

FS =𝜏f/𝜏m = (c + 𝜎 tan 𝜙)/(cm + 𝜎 tan 𝜙m) = a
So ∶ FS = FSc = FS𝜙

Methods of Analysis of Slope Stability

Different methods can be used in the analysis of stability of slopes. The methods can be generalised as;

  • Mass methods
  • Methods of slices
  • Finite element methods

In the mass methods, the sliding soil mass is analyzed as one entity. The mass methods are applicable only to homogeneous slopes and can be employed in the analysis of finite and infinite slopes. In the analysis of finite slope, Culmann’s method is normally used for planar failure surfaces, while Taylor’s chart and Michalowski’s chart are used for curved failure surfaces.

In the methods of slices, the sliding soil mass is divided into numerous slices and the stability of each slice is analyzed. Then all the slices are combined to derive the factor of safety of the slope for the assumed failure surface. To obtain the true factor of safety of the slope, numerous trial surfaces are analyzed that provide the minimum factor of safety. The methods are applicable to homogeneous or heterogeneous slopes.

The popular methods in this category are the ordinary method of slices (Fellenius method of slices), Bishop-simplified method of slices, Bishop and Morgensten method, Spencer method with consideration of pore water pressure, Morgenstern charts for rapid drawdown, etc.

The finite element method involves modelling the soil using finite elements and simulating the failure of the slope. Computer packages such as Plaxis are capable of such models.

Method of Slices

The method of slices is a numerical procedure that has been developed to handle stability analysis of slopes where conditions are nonhomogeneous within the soil mass making it impossible to deduce closed-form solutions. Some of the non-homogeneous conditions commonly encountered are irregularity of failure planes, variable soil properties, significant variation in the distribution of pore pressure along the failure plane, irregular slope geometry, etc.

The analysis requires the selection of a trial failure plane and discretization of the resulting failure wedge into a convenient number of slices as shown in Figure 3.

image 8
Figure 3: Ordinary method of slices. (a) Slices and failure circle, (b) forces on the ith slice.

The analyst is required to devise the slicing in a manner that can incorporate any nonhomogeneity within the slope so that each resulting slice would be a homogeneous entity. Then, the stability of each slice can be analyzed separately using the limit equilibrium method and principles of statics. The static analysis of the slices can be obtained in most geotechnical engineering textbooks.

Assumptions of the Method of Slices

The method of slices makes several simplifying assumptions to facilitate analysis:

  1. The slope is divided into vertical slices of uniform or variable thickness.
  2. The failure surface is assumed to be planar or circular for simplicity.
  3. The soil within each slice is assumed to be homogeneous and isotropic.
  4. The normal and shear stresses at the base of each slice are assumed to be uniformly distributed.
  5. Interslice normal and shear forces are neglected or assumed to be constant.

The factor of safety of the slope on any assumed circular slip surface using the method of slices is:

image 9

The ordinary method of slices assumes that the interslice forces on both sides of each slice cancel one another. Therefore, the factor of safety derived from this method is overconservative and is lower than other methods of slices.

Worked Example

Calculated the factor of safety of the earth slope shown in Figure 4 using the ordinary method of slices. Assume the failure circle shown in Figure 5.

SLOPE STABILITY PROBLEM
Figure 4: Earth slope stability problem

The selected failure surface and the number of slices are shown in Figure 5.

Earth slope stability failure circle
Figure 5: Earth slope stability failure circle

The solution can be presented in the tabular form shown below;

Slice No [1]Area (m2) [2]Slice Weight (kN/m) [3]α (degrees) [4]b (m) [5]cos α [6]sin α [7]b/cos α (m) [8]Wsin α (kN/m) [9]Wcos α  (kN/m) [10]
113.121236.178613.6450.4850.8747.515464206.4196114.5463
229.829536.9224940.6560.7546.097561404.8392352.2208
332.108577.9443940.7770.6295.148005363.5268449.0625
430.006540.1083040.8660.54.618938270.054467.7335
523.794428.2922240.92710.3754.314529160.6095397.0695
616.095289.711440.970.2424.12371170.10982281.0187
75.717102.906740.99250.1214.03022712.45163102.1342
Total35.848431488.012163.786

The factor of safety can then be calculated thus;

FS = (c∑col[8] + ∑col[10]tan𝜙)/∑col[9]
FS = (21 × 35.848 + 2163.786 tan 25)/1488.01 = 1761.794/1488.01 = 1.18

Conclusion

Slope stability analysis is a crucial aspect of geotechnical engineering, ensuring the stability of natural slopes, embankments, and excavations. It evaluates the likelihood of a slope failure, which can have catastrophic consequences for infrastructure, property, and human safety.

The most common approach to slope stability analysis is the limit equilibrium method, which assumes that the slope is either in a state of equilibrium or on the verge of failure. It involves calculating the driving forces that cause the slope to slide (e.g., gravitational forces) and the resisting forces that prevent sliding (e.g., shear strength of the soil).

The method of slices is a widely used limit equilibrium technique for analyzing slope stability. It involves dividing the slope mass into a series of vertical slices and analyzing the forces acting on each slice. The forces acting on each slice include its weight, the normal and shear forces at its base and sides, and any external forces such as water pressure.

Structural Analysis Example: Arch Bridges

By
Ubani Obinna
-

For the given arch bridge loaded, as shown above, determine the following;

(a) The support reactions
(b) The bending moment diagram of the girder
(c) The shear force diagram of the girder
(d) The axial force diagram of the girder

Support Reactions

∑MB = 0
30Ay – (4 × 15) × (15/2 + 15) = 0
30Ay – 1350 = 0
Therefore; Ay = 1350/30 = 45 kN

∑MGL= 0
15Ay – (4 × 152)/2 + 5H = 0
15(45) + 5H = 0
Therefore; H = -225/5 = -45 kN

∑MA = 0
30By – (4 × 152)/2 = 0
30By – 450 = 0
Therefore; By = 450/30 = 15 kN

∑MGR= 0
15By + 5H = 0
15(15) + 5H = 0
Therefore; H = -225/5 = -45 kN

Analysis of the Joints

Joint 2

image 1

φ = tan-1(1/6) = 9.46°

∑FX = 0
-F2-1cosφ + H = 0
-F2-1cos9.46 – 45 = 0
-F2-1 = 45/cos 9.46 = 45.62 kN
F2-1 = -45.62 kN

∑FY = 0
-F2-1sinφ – F2-6 = 0
-F2-6 = F2-1sinφ
-F2-6 = -45.62 sin9.46 = 7.498 kN
F2-6 = 7.498 kN

Joint 1

image 2

α = tan-1(4/6) = 33.69°

∑FX = 0
-F1-Acosα + F1-2sinφ = 0
-F1-Acosα – 45.62sin(9.46) = 0
-F1-A = 45.62sin(9.46)/cos(33.69) = 54.082 kN
F1-A = -54.082 kN

∑Fy = 0
-F1-Asinα + F1-2sinφ – F1-5 = 0
54.082sin(33.69) – 45.62sin9.46 – F1-5 = 0
22.501 – F1-5 = 0
F1-5 = 22.501 kN

Therefore, the loading on the girder can be summarised as follows;

image 3

When the inclined forces are resolved into their vertical and horizontal components;

Fy = 54.082sinα = 54.082sin(33.69) = 30 kN
Fx = 54.082cosα = 54.082cos(33.69) = 45 kN

image 4

Check;
∑Fy ↑ = 45 + 15 + 2(22.501)+ 2(7.498) = 120 kN
∑Fy ↓ = (4 × 15) + 2(30) = 120 kN

Bending Moment

(Coming from the left)
MA = 0
M5 = (45 × 6) – (30 × 6) – (4 × 62)/2 = 18 kNm
M6 = (45 × 12) – (30 × 12) – (4 × 122)/2 + (22.501 × 6) = 27 kNm
MGL = (45 × 15) – (30 × 15) – (4 × 152)/2 + (22.501 × 9) + (7.498 × 3) = 0
(Coming from the right)
MB = 0
M8 = (15 × 6) – (30 × 6) = -90 kNm
M7 = (15 × 12) – (30 × 12) + (22.501 × 6) = -45 kNm
MGR = (15 × 15) – (30 × 15) + (22.501 × 9) + (7.498 × 3) = 0

image 5

Shear Force

VA = 45 – 30 = 15 kN
V5L = 45 – 30 – (4 × 6) = -9 kN
V5R = 45 – 30 – (4 × 6) + 22.501 = 13.501 kN
V6L = 45 – 30 – (4 × 12) + 22.501 = -10.499 kN
V6R = 45 – 30 – (4 × 12) + 22.501 + 7.498 = -3 kN
VG = 45 – 30 – (4 × 15) + 22.501 + 7.498 = -15 kN
V7L = -15 kN
V7R = -15 + 7.498 = -7.502 kN
V8L = -7.502 kN
V8R = -7.502 + 22.510 = 15 kN
VB = 15 kN

image 6

Axial Force

NA-B = H = Ax = 45 kN (Tension)

image 7


Number and Spacing of Piles in a Group

By
Ubani Obinna
-

Typical structures are rarely supported only on a single pile. Therefore, the number and spacing of piles required to support a given structure is an important aspect of the design. The number of piles required to support a given load is dependent on the magnitude of the load and the load-carrying capacity of the piles, while the spacing of piles in a group is dependent on the type of pile, soil properties, method of installation, and the size of the pile.

However, in some cases, lightly loaded columns in a building or other structures can be supported on a single pile, provided the piles and pile caps are eventually chained together with continuous ground/plinth beams. In such cases, the load-carrying capacity analysis of the pile can be based on the case of a single pile.

In other cases, the group of piles are typically positioned in close spacing beneath the structures they support, thereby necessitating the consideration of the collective behaviour of the entire group of piles. This is especially important when friction piles are used. The bearing capacity of a pile group does not solely depend on the capacity of each individual pile multiplied by the number of piles in the group. This difference in capacity is attributed to the phenomena known as ‘Group action of piles’.

Grouping of Piles

It is typical to avoid the exclusive usage of driven piles beneath a column or wall due to the tendency of the pile to deviate horizontally during installation, leading to uncertainty in aligning the pile accurately beneath the foundation. Failure can occur either at the interface between the pile and column or within the pile itself, when unexpected eccentricities arise.

Therefore, it is usual practice to construct piles for walls in a staggered configuration on both sides of the wall’s centre line. When a single pile configuration is not feasible, a higher number of piles are utilized in a specific configuration for supporting a column. If the needed capacity exceeds three piles, the piles will be arranged symmetrically around the point or area where the load is applied. Figure 1 illustrates the representative configurations of pile groups for column loads.

Typical pile spacing and configuration for column loads
Figure 1: Typical pile spacing and configuration for column loads

Typically, column and wall loads are transmitted to the pile group by means of a pile cap. This pile cap is a thick reinforced concrete slab that is connected to the pile heads, facilitating the collective behaviour of the group (see Figure 2).

The requirement for the collective arrangement of driven piles does not extend to bored piles. Drilled shafts can be placed with a high degree of precision. Residential buildings can utilize a single, large-diameter drilled shaft pile to provide support for its columns. This can be utilized when the arrangement of three piles results in an excessive amount of additional load capacity, particularly in the scenario of driven piles.

In a typical scenario, the number of piles required to support any load is calculated using the serviceability limit state loads. This is then compared with the stated safe load-carrying capacity of a given pile with known diameter and embedment depth. For instance, if the calculated service axial load is 2450 kN, and the load carrying capacity of a 600 mm diameter pile at an embedment depth of 15 m is 724 kN, the number of piles required is given by;

Number of piles = Service Axial Load/Allowable pile load capacity = 2450/724 = 3.38 (Adopt 4 piles)

Spacing of Piles

The spacing of piles in a group is determined by several criteria, including the overlapping of stresses between adjacent piles, the cost of the foundation, and the intended efficiency of the pile group. The stress isobars of a single pile supporting a focused load will resemble the illustration in Figure 2(a).

stress isobars of piles and pile groups
Figure 2: Stress isobars of single piles and groups of piles

When piles are driven in close proximity, there is a potential for the stress isobars of neighbouring piles to overlap, as illustrated in Figure 2(b). To prevent potential failure due to either shear or excessive settlement, it is advisable to increase the space between the overlapping elements, as seen in Figure 2(c). Wide spacing is disadvantageous as it necessitates a larger pile cap, hence increasing the overall cost of the foundation.

Driven piles result in increased stress overlap owing to soil displacement. When piles are driven into loose sands, compaction occurs, resulting in smaller spacing. However, in the case of piles being driven into saturated silt or clay, compaction does not occur, but the piles may undergo uplift. To mitigate this issue, one can increase the pile spacing.

Reduced spacings can be employed for cast-in-situ piles due to a decreased tendency for disturbance. End-bearing piles can be positioned at a closer spacing compared to friction piles. Different building codes typically stipulate the minimum spacing between piles. The spacing of straight uniform cylindrical piles may range from 2 times the diameter (2d) to 6 times the diameter (6d). The required minimum distance for friction piles is three times the diameter of the pile (3d).

When point-bearing piles pass through a compressible stratum, the minimum spacing required is 2.5 times the diameter of the piles if they are resting on compact sand or gravel. However, if the piles are resting on stiff clay, the minimum spacing should be 3.5 times the diameter of the piles. The minimum distance required for compaction piles may be 2 times the diameter (2d).

In general, piles should be driven starting from the centre and moving outward, unless the soil is soft clay or very soft. In such cases, the pile driving should start from the edges of the foundation and go towards the centre to prevent the sideways movement of dirt during the driving process.

Take Away

The primary considerations for determining the number of piles include the load-bearing capacity of the soil, the expected loads on the structure, and the desired safety factors. Pile capacity can be estimated through various methods such as static load tests, dynamic load tests, or geotechnical analysis.

Engineers must also consider the distribution of loads across the piles and assess how they interact to distribute the structural loads effectively. Additionally, the structural design of the piles themselves, including their type (e.g., driven piles, drilled shafts, or micropiles), size, and material, may also affect the number of piles needed.

The spacing of piles is equally important. It influences the overall performance of the foundation system, affecting factors like settlement, cost, and load-carrying capacity. When piles are spaced so closely such that the stress isobars overlap, the pile group will undergo higher settlement than previously envisaged. From experience, you rarely go wrong by placing your spacing between 2.5d to 3d. Proper spacing design involves a balance between load distribution and economic considerations.

In conclusion, determining the number and spacing of piles for a foundation system demands careful engineering consideration of soil characteristics, structural loads, safety margins, and economic implications. The chosen arrangement must guarantee the structure’s stability and performance while keeping construction expenses within budget. Selecting the right number and spacing of piles is an important aspect of deep foundation design and has a substantial impact on the overall success of a construction project.

Selecting the Most Appropriate Roof Hatch for Your Building 

By
Ubani Obinna
-

When replacing or installing a new roof hatch, making the right selection is important for safety and functionality. A wide range of hatches exist for different weight ratings and use cases. This blog post lets you explore the key factors, like material, latching systems, and dimensions, to help you select the ideal roof hatch solution for your specific house or commercial establishment project’s needs. 

7 Essential Factors to Check When Purchasing Roof Hatch 

Picking a roof hatch that meets your structure project’s requirements and demands takes some planning. Below are seven crucial factors to check when buying the most suitable roof hatch for your home or commercial building’s access necessities. 

  • Size and Dimensions 

Measuring the roof area size is crucial. You must make sure the hatch will fit in that space properly. Consider what needs to go through the opening, like tools, supplies, and people. You want to choose a hatch big enough that nothing gets stuck when moving it in or out of the roof. 

roof hatch 2
  • Materials 

Manufacturers commonly make roof hatches out of three materials—aluminum, steel, and fiberglass. Aluminum-built hatches, like BA-ALRF-LA aluminum roof hatch, are light but easily dent if bumped. Steel is tough but will rust over time in some weather conditions. Fiberglass is very sturdy and can handle rain and sun without issues.  

It would be best if you chose based on what climate you live in. If it rains a lot where you are, fiberglass or steel would be better since they don’t rust. Aluminum would work well in dry areas. Remember to also think about how much bumping and banging the hatch may get, as steel could take more abuse than aluminum before getting damaged. 

  • Door Style 

Hatches come with doors of multiple panels or a single solid piece. Panel doors are lighter to open and close since they have separate sections that move. However, wind and cold air can blow through the cracks between panels.  

Solid doors, on the other hand, grant better insulation since they are one solid piece that seals tightly. But they are very heavy to lift open and shut. You should also consider whether drafts will be an issue for your use and whether you can effortlessly handle the weight of a solid door, given that ensuring safety is vital. Pick the door style best suited to your work needs and ability. 

  • Insulation 

For temperature control, seek out insulated hatch options. Insulation helps maintain a stable temperature in the roof area, allowing building occupants and employees to work throughout all seasons.  

In very hot or cold places, insulation makes a big difference in keeping enclosed spaces livable year-round. It prevents outside heat or chill from easily transferring through the hatch material. Hence, you must highly consider this, especially if the condition sees major temperature swings from season to season. 

  • Lifting System 

Hatches have presented various alternatives for helping lift the door open and closed. Common systems include gas shocks, springs, and pulley setups. Gas shocks and springs provide power to assist with lifting.  

Meanwhile, pulleys make raising or lowering the door easier while keeping balance. Consider your strength and pick a lifting system you can work with safely using only one hand. That way, your other hand will be free for holding tools, supplies, or other items when going in and out of the hatch opening. 

  • Safety Railings 

Railings play an important safety role. They help guard against accidental falls through the open hatch area. Be sure any railings surround the perimeter of the hatch opening.  

It protects you while in or around the space. These railings must match the hatch size so protection extends across the access area. Also, double-check its compliance with building codes for fall safety measures to ensure security from injuries and tumbles off the roof. 

access hatch
  • Hardware 

The hinges, latching mechanisms, and other small parts on a hatch are important to guarantee the component’s proper functioning over the long run. Hatches need hardware strong enough to stand firm against heavy usage over time. Among hardware materials, stainless steel weathers environmental elements like extreme heat, cold, rain, and snow better than alternatives.  

Using stainless steel avoids common issues, such as rust that can weaken other metal parts quicker than expected. So, when looking for hatches, consider the hardware’s quality, too. Adequate sturdiness guarantees the hatch continues operating seamlessly for all seasons as required for roof access. 

Pick What’s Best for Your Project 

Choosing the right hatch requires considering your building’s unique access needs and environment. Taking the time to measure and assess materials and systems compatibility will save costs. A hatch is an important safety investment that also enhances work efficiency. With so many options, the essential factors listed in this blog aim to help you purchase a hatch solution tailored for reliable, durable performance on your job sites for years to come. 

Design of Cylindrical Steel Tanks | Tank Farms

By
Ubani Obinna
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Cylindrical steel tanks are important structures used for the storage of different kinds of fluids. They are more popular in the petrochemical industry where it used for storing petroleum products, water, chemicals etc. Storage tanks are containers, either above or below the ground, that are used to store chemicals, petroleum, and other liquid products. This article is focused on the design of above above-ground cylindrical steel storage tank.

Cylindrical steel tank farms are usually constructed using a series of steel plates which are welded together on site. The thickness of the plates usually varies, usually being maximum at the bottom and minimum at the top. When the diameter of the tank is very large, internal steel columns are introduced to support the roof rafters and shell cover.

tank farm design

A cylindrical tank or shell may be described as thin or thick depending upon the thickness of the plate in relation to the internal diameter of the cylinder. Above-ground cylindrical tanks are typically thin-walled structures that are prone to buckling, or losing their stability, especially when they are empty or have a low liquid level. This is because external loads, such as wind and earthquakes, can more easily cause these tanks to collapse.

Traditionally, the ratio of t/d = 0.05 has been considered a suitable line of demarcation between thick and thin cylinders. If the thickness of the shell is equal to or greater than the internal diameter/20, the shell is regarded as a thick shell. In thin cylinders, the stress is usually assumed to be uniformly distributed over the wall thickness. When these storage cylinders are subjected to internal static pressure due to the stored fluid, the following types of stresses are developed.

(1) Hoop or circumferential stresses. These stresses act in a tangential direction to the circumference of the cylinder.
(2) Longitudinal stresses. These act parallel to the longitudinal axis of the shell
(3) Radial stresses. They act radially to the circumference of the shell.

Cylindrical Steel Tanks design

The design of cylindrical steel tanks such as tank farms usually involves the determination of the proper thickness of the tank shell that will resist the bursting force from the stored fluid, and also remain stable under the action of other external forces.

If the bursting force (pressure) = Resisting strength

Therefore,
pdl = 2ltσc

σc = pd/2t

Where;
d = internal diameter of the cylinder
t = thickness of the cylinder
p = internal pressure in the cylinder
σc = circumferential or hoop stress in the cylinder

Similarly for longitudinal stress for tanks with both ends covered with two end plates;

σl = pd/4t

Where σl = longitudinal stress

The maximum shear stress in a cylindrical shell is given by;

τmax = (σc – σl)/2 = pd/8t

Therefore, if it is required to determine the wall thickness of a thin cylindrical shell so that it can withstand a given internal pressure p, we have to determine that the maximum stress developed in the shell does not exceed the permissible tensile stress of the shell material (fy). Therefore,

t ≥ pd/2fy

Design of Cylindrical Steel Tanks

The document EN 14015:2005 provides the specifications for the structural design and verification of flat-bottomed, above-ground, welded steel tanks for the storage of liquid at ambient temperature. The EN14015 design standard uses a simplified approach to determine the thickness of tank shell courses.

This approach is based on the tank’s geometry, the liquid level, the material used, the liquid’s density, and a corrosion allowance. It aims to limit the tensile stresses in the shell due to hydrostatic pressure. However, it does not explicitly consider tank shell buckling.

Fluid Action

When a tank is in operation, the pressure load from the liquid inside it must be considered, from the maximum liquid level to when the tank is empty. The internal pressure load must also be considered, including the load due to the specified test pressure and test internal negative pressure.

For a full tank, the design hydrostatic pressure is the highest liquid pressure that acts at the bottom of the tank shell course. This is calculated as the product of the liquid’s specific weight and the maximum height of the liquid, measured from the top of the tank shell. This fluid pressure is resisted by making the bottom shell courses thicker.

In other words, the design hydrostatic pressure is the maximum pressure that the tank shell course must be able to withstand when the tank is full. This pressure is calculated based on the weight of the liquid and the height of the liquid column. The bottom shell courses are made thicker to resist this pressure.

The computation of the thickness of the tank shells is influenced by factors like the geometry of the tank, material to be contained, material strength, corrosion allowance, design pressure and height of shell courses from the top of the tank shell. Furthermore, the design of the shell for external wind loads and internal negative pressure, which is the buckling resistance of the tank shell, is prevented by providing ring stiffeners and determining the minimum spacing of secondary rings using an equivalent minimum shell thickness.

According to EN 14015:2004, the maximum allowable design stress in shell plates shall be two-thirds of the yield strength of the material with a maximum design stress of 260 MPa (when the temperature is equal to less than 100 degrees celsius). The specified thickness of the shell plates shall not be less than the specified nominal thicknesses specified in Table 16 of the code.

Tank Diameter D (m)Minimum nominal shell thickness (mm) for carbon and carbon manganese steels
D < 45
4 ≤ D < 105
10 ≤ D < 155
15 ≤ D < 306
30 ≤ D < 458
45 ≤ D < 608
60 ≤ D < 9010
90 ≤ D 12
Table 1: Minimum nominal shell thickness

The code specifies that in no case should the specified thickness of the shell or the reinforcing plate be greater than 40mm (clause 9.1.6). Also the minimum circumferential dimensions of a shell plate shall be 1m.

The required minimum design thickness of the shell plates is derived from equation (1) below or the secified values provided in Table 1, whichever is the greatest.

ec = D/20S[98W(Hc – 0.3) + p] + c —— (1)

where:
c is corrosion allowance, for the shell assumed fully painted (c = 0),
D is tank diameter in [m],
Hc is the distance from the bottom of the considered course to the design liquid height [m],
W is the maximum design density of the contained liquid under storage conditions, in kg/l
p is the design pressure [mbar],
S is the allowable design strength [MPa].

According to EN14015, 9.1.1a, the allowable design stress, S is a minimum of 2/3 of the yield strength of the shell material or 260 MPa, P is the design pressure [MPa], and W is the density of the liquid to be contained.

Hp1 = K(emin5/D3)0.5 —— (2)

where:
Hp is the minimum spacing between the stiffeners,
emin is the minimum shell thickness, and
K is a factor determined by an empirical formula in equation (3).

K = 95000/(3.563VW2 + 580PV) —— (3)

where:
VW is a 3-second gust wind speed and
PV is the design internal negative pressure.

For the specified nominal thickness of the bottom plates shall not be less than 6mm for lap welded bottoms or 5mm for butt welded bottoms (Carbon and Manganese Carbon steels) excluding the corrosion allowance. In addition, the bottom plate thicknesses in the corroded condition shall be sufficient to resist uplift due to the design internal negative pressure.

Bottom of tanks greater than 12.5m diameter shall have a ring of annular plates, having a minimum nominal thickness ea, excluding corrosion allowance either;

(a) not less than that given by the following equations;
ea = 3.0 + e1/3
where e1 is the thickness of the first course excluding corrosion allowance in mm.

But must not be greater than 6mm. Bottom of tanks up to and including 12.5m diameter may be constructed without a ring of annular plates.

storage design

Conclusion

Cylindrical steel storage tanks are one of the most common types of tanks used for storing a wide variety of liquids, including petroleum products, chemicals, and water. These tanks are typically designed to be above ground, although below-ground tanks are also used in some applications.

The design of a cylindrical steel storage tank is based on a number of factors, including the type of liquid to be stored, the tank’s capacity, and the operating conditions. The tank must be strong enough to withstand the hydrostatic pressure of the liquid, as well as any external loads, such as wind, earthquake, and snow.

The tank shell is typically made of steel plates that are welded together. The thickness of the shell plates is determined based on the design pressure and the allowable stress of the steel. The bottom of the tank is typically thicker than the sides and top to withstand the higher hydrostatic pressure at the bottom of the tank.

The tank roof can be either fixed or floating. Fixed roofs are typically used for tanks storing volatile liquids, such as gasoline, to prevent the release of vapours. Floating roofs are typically used for tanks storing non-volatile liquids, such as water, to reduce evaporation.

The construction of a cylindrical steel storage tank typically involves the following steps:

  1. Site preparation: The site is prepared to provide a level and stable foundation for the tank.
  2. Tank assembly: The tank shell is assembled by welding together the steel plates.
  3. Roof assembly: The tank roof is assembled and installed on the tank shell.
  4. Internal and external coatings: Internal and external coatings are applied to the tank to protect it from corrosion.
  5. Piping and instrumentation: Piping and instrumentation are installed to allow the tank to be filled and emptied, and to monitor the liquid level and pressure.

After the tank is constructed, it must be tested to ensure that it meets the design specifications. This typically involves a hydrostatic test, in which the tank is filled with water and the pressure is gradually increased to the design pressure.

The Benefits of Removable Drywall Access Doors in Commercial Building Design 

By
Ubani Obinna
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We often focus on aesthetics, functionality, and safety when designing commercial buildings. However, we also often overlook one crucial aspect: the access doors. These hidden doors provide convenient entry to concealed plumbing and electrical systems, offering surprising benefits for commercial buildings.  

This article will explore why removable drywall access doors are becoming increasingly popular and how they can enhance efficiency, aesthetics, security, and compliance in commercial building design.

Benefits of Removable Drywall Access Doors 

  1. Convenient Accessibility for Repairs and Maintenance 

Removable drywall access doors serve a simple but vital purpose: they provide access to hidden plumbing and electrical systems in commercial buildings. These systems can be quite complex and spread out into commercial structures. In the past, the usual way to access them was by cutting through the drywall, which took time and cost a lot in labor and materials. 

Drywall Access Doors

But with removable drywall access doors, you don’t need to go through all that trouble. You can open and close these doors easily, allowing maintenance workers to access the systems without causing much damage to the nearby walls. This makes it much cheaper and faster for businesses in the building, saving them money and reducing downtime. 

  1. Seamlessly Blending with Aesthetics 

People often have one concern when thinking about access doors: how they’ll look in a space. But removable drywall access doors are designed to blend with the walls around them. Removable drywall access doors can be customized to perfectly match the surrounding drywall’s texture, color, and design. This makes them virtually invisible and ensures they blend seamlessly with the wall when not being used. 

This ability to fit in with the aesthetics means these access doors will maintain the overall look of the space. In a business setting, where first impressions matter a lot, this careful attention to detail can make the building look better while still being useful. 

  1. Enhanced Safety Measures 

Security is a major worry in commercial building design, and access doors are crucial in addressing this concern. Removable drywall access doors can have strong locks, stopping unauthorized people from entering. This extra security layer is essential for protecting valuable items and keeping people in the building safe. 

Additionally, these doors can meet strict fire safety rules, ensuring they don’t undermine the fire-resistant quality of walls. This dual function makes them vital for maintaining safety and security in commercial properties. 

  1. Versatile Design Solutions 

Commercial buildings come in various shapes, materials, and sizes—each with unique design needs. Also, there are numerous technical systems running behind the scenes, such as cabling for internet connectivity, HVAC systems, and electrical wiring for various devices. Maintenance and upgrades are inevitable, and this is where the clever integration of Babcock-Davis BRGBR removable drywall access door becomes essential. 

Additionally, removable drywall access doors are incredibly versatile, allowing architects and designers to incorporate them into different layouts and setups easily. Whether it’s a tiny office, a bustling retail shop, or a vast industrial facility, these access doors can be adapted to fit the space’s specific needs precisely.  

This flexibility also applies to the size of the access doors, making it possible to tailor them for different entry points. This means no area becomes off-limits when maintenance or repairs are needed, ensuring that every part of the building remains accessible as necessary. 

  1. Long-Term Financial Benefits 

While installing removable drywall access doors might seem like an extra cost initially, it’s a smart long-term investment. These doors simplify repairs and maintenance, cutting down on labor costs. Moreover, they’re designed to be long-lasting, reducing the need for frequent replacements. 

Additionally, considering the increasing emphasis on energy efficiency in commercial buildings, you can insulate these access doors. This insulation prevents heat from escaping or entering them, lowering energy consumption and utility bills.

 

dry wall access door 2
  1. Compliance with Building Regulations 

Compliance with regulations is a non-negotiable requirement in commercial construction. Removable drywall access doors adhere to these codes, guaranteeing that the building remains safe and in line with standards.  

This compliance not only prevents legal issues but also enhances the overall safety and functionality of the space. It ensures that the building is aesthetically pleasing, structurally sound, and secure, meeting all legal requirements. 

Final Thoughts 

The benefits of removable drywall access doors in commercial building design extend far beyond their unassuming appearance. They offer easy access for repairs and maintenance, seamlessly integrate with the aesthetics of a space, enhance security, provide design versatility, and contribute to long-term cost savings. Moreover, they ensure compliance with building codes and minimize disruption during installation.  

These surprising advantages make removable drywall access doors essential in creating efficient, functional, and aesthetically pleasing commercial spaces. Architects, designers, and building owners should consider their inclusion as a fundamental aspect of any commercial building project, ultimately leading to more efficient and sustainable buildings. 

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