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Design of Reinforced Concrete (R.C.) Beams

By
Ubani Obinna
-

Beams are horizontal structural elements designed to carry lateral loads. When they are inclined or slanted, they are referred to as raker beams. Floor beams in a reinforced concrete building are normally designed to resist load from the floor slab, their own self-weight, the weight of the partitions/cladding, the weight of finishes, and other actions as may be applied. The design of a reinforced concrete (R.C.) beam involves the selection of the proper beam size and area of reinforcement to carry the applied load without failing or deflecting excessively.

Under the actions listed above, a horizontal reinforced concrete beam will majorly experience bending moment and shear force. Depending on the loading and orientation, the beam may experience torsion (twisting), as found in curved beams or beams supporting canopy roofs. For raker beams, the presence of axial force can be quite significant in the design.

Longitudinal reinforcement is used to resist bending moment, and also enhance the shear force capacity of a beam. Stirrups (links) are used for resisting any excess shear force and torsion (where applicable). In deep beams or beams subjected to torsion, sidebars can be used to enhance the torsion capacity and also prevent cracking. The depth (d), width (b), and disposition of reinforcements define the load-carrying capacity of a beam and forms the essence of their design.

Design of reinforced concrete beams
Figure 1: Section of a reinforced concrete beam

In typical reinforced concrete buildings, floor beams can be categorised into;

  • T – beams,
  • L – beams, or
  • rectangular beams
types of beam in a building
Figure 2: Types of beams in a framed concrete building

T-beams and L-beams are generally referred to as flanged beams. T- beams are usually internal beams, while external beams (perimeter beams) are usually L – beams. Beams that are not carrying any slab load are more often rectangular beams. Beams in a reinforced concrete building can also be described in terms of their support condition such as simply supported, cantilever beams, or continuous beams.

The steps in the design of a reinforced concrete beam are as follows;

(a) Preliminary sizing of members
(b) Estimation of design load and actions
(c) Structural analysis of the beam
(d) Selection of concrete cover
(e) Flexural design (bending moment resistance)
(f) Curtailment and anchorage
(g) Shear design
(h) Check for deflection
(i) Check for cracking
(j) Provide detailing sketches


Preliminary sizing of beams

Preliminary sizing of a beam can be influenced in a lot of ways. However, deflection criteria can be used as a starting point in the analysis, even though experience is the best. Long span beams will require deeper sections, and at the same time, the magnitude of loading will also have an effect on the sizing.

Sometimes, architectural constraints can limit your options in selecting the depth and width of reinforced concrete beams. If the headroom of the building is low, you cannot afford very deep sections unless the beams are directly aligned with the partitions. The width of the block walls can also constraint the width of the beam. This is however different if a suspended ceiling will be used to conceal the beams and give the soffit of the floor a uniform look.

A general guide to the size of a beam may be obtained from the basic span-to-effective depth ratio from Table 7.4N of the BS EN 1992-1-1. The following values are can be used as a guide;

Overall depth of beam ≈ span/15.
Width of beam ≈ (0.4 to 0.6) × depth.

The width may have to be very much greater in some cases, especially when a larger width is needed to reduce the shear stress in the beam. As said earlier, the size is generally chosen from experience. Many design guides are available which assist in design.

Efective flange width of beams

Clause 5.3.2.1 of EN 1992-1-1:2004 covers the effective flange width of beams for all limit states. For T-beams, the effective flange width, over which uniform conditions of stress can be assumed, depends on the web and flange dimensions, the type of loading, the span, the support conditions, and the transverse reinforcement. The effective width of the flange should be based on the distance lo between points of zero moment, which is shown in Figure 3.

lo%2Bfor%2Beffective%2Bflange%2Bwidth
Figure 3: Assumed points of zero moment for beams (Figure 5.2 of EN 1992-1-1:2004 (E))

The flange width for T-beams and L-beams can be derived as shown below. The notations are shown in figure 4.

beff =Σbeff,i + bw ≤ b

where;
beff,i = 0.2bi + 0.1lo ≤ 0.2lo
and
beff,i  ≤ bi

effective%2Bflange%2Bwidth%2Bparameters
Figure 4: Effective flange width parameters ((Figure 5.3 of EN 1992-1-1:2004 (E))

Estimation of design loads

In the manual analysis of floor beams, loads are transferred from slab to beams based on the yield line assumption. However, finite element analysis will use a numerical approach to transfer loads from the slab to the beam. The magnitude of load transferred depends on if the slab is spanning in one-way or two-way. Typically, for a two-way slab, the loads are either triangular (for the beam parallel to the short span direction of the slab) or trapezoidal (for the beam parallel to the long span direction of the slab) as shown in Figure 5.

Load transfer from slab to beam 1
Figure 5: Load transfer from slab to beam

Typical load distribution from slab to beam is shown in the figure above. The analysis of trapezoidal and triangular loads on beams can be tedious especially for continuous beams with variable loading and unequal spans.

However, for beams of equal span and uniform loading, coefficients for bending moment and shear can be obtained from Chapter 12 of Reynolds and Steedman (2005). For the sake of convenience, the load transferred from the slab to the beam can be approximated as a uniformly distributed load, and the formulas for the transfer of such loads are given in Chapter 13 of Reynolds and Steedman (2005). They are presented in Table 1.

Table 1: Equivalent UDL load transferred from slab to beam

Type of slabUDL transferred to beam on the long Span (kN/m)UDL transferred to beam on the short Span (kN/m)
One-way slabnlx/2nlx/5
Two-way slabnlx/2(1 – 0.333k2)nlx/3

Where;
n = ultimate pressure load on the slab = 1.35gk + 1.5qk
lx = length of short span
ly = length of long span
k = ly/lx

Beams in a building can also be subjected to other loads and the typical values are;

  • Self-weight – Depends on the dimensions of the beam = (unit weight of concrete × width × depth)
  • Demountable lightweight partitions = 1 kN/m2
  • Block work with plaster = 3.5 kN/m2

Structural Analysis of Reinforced Concrete Beams

The primary purpose of structural analysis in building structures is to establish the distribution of internal forces and moments over the whole or part of a structure and to identify the critical design conditions at all sections. The geometry is commonly idealised by considering the structure to be made up of linear elements and plane two-dimensional elements.

Linear elastic analysis (with or without redistribution) or plastic can be carried depending on the one suitable for the problem. However for most buildings, linear elastic analysis is very adequate.

For the ultimate limit state only, the moments derived from elastic analysis may be redistributed (up to a maximum of 30%) provided that the resulting distribution of moments remains in equilibrium with the applied loads and subject to certain limits and design criteria (e.g. limitations of depth to neutral axis).

Different methods can be used for the elastic analysis of statically indeterminate beams such as;

  • Moment distribution method
  • Clapeyron’s theorem of three moments
  • Force method
  • Slope-deflection method
  • Stiffness method

Some coefficients are also provided in the code of practice for the evaluation of the bending moment and shear force in continuous beams.

Concrete Cover

An adequate concrete cover should be provided in reinforced concrete beams for the following reasons;

■ safe transmission of bond forces
■ durability
■ fire resistance

The minimum cover to ensure adequate bond should not be less than the bar diameter, or equivalent bar diameter for bundled bars, unless the aggregate size is over 32 mm. Under normal conditions, concrete cover of 35mm to 40 mm is usually adequate for beams.

Flexural Design of Reinforced Concrete beams

The stress block of a singly reinforced beam section (Eurocode 2) is shown in Figure 6;

stress block of a singly reinforced beam section
Figure 6: Rectangular stress block Eurocode 2

From EC2 singly reinforced concrete stress block, the moment resistance capacity of the beam MRd is given by;

MRd = Fcz —— (1)
fcd = design strength of concrete = (αccfck)/γc = (0.85 × fck)/1.5 = 0.5667fck

Compressive force in concrete = Design stress (fcd) x Area of compression block
Fc = 0.5667fck × 0.8 x b = 0.4533bfck

From the stress block distribution;

z = d – 0.4x —— (2)

Clause 5.6.3 of EC2 limits the depth of the neutral axis to 0.45d for concrete class less than or equal to C50/60. Therefore for an under reinforced section (ductile failure);

x = 0.45d —— (3)

Combining equation (1), (2) and (3), we obtain the ultimate moment of resistance (MRd)

MRd = 0.167fckbd2 ———- (4)

Also from the reinforced concrete stress block, we can obtain the tensile design moment as;

MEd = Fsz ——- (5)

Where;
Fs = fyk/1.15As1 ——-(6)
As1 = Area of tensile reinforcement required

Substituting equation (6) into (5) and making As1 the subject of the formula;

As1 = MEd/(0.87fykz) ——(7)

The lever arm z in EC2 is given from equation (2), z = d – 0.4x
Therefore, x = 2.5(d – z)
M = 0.453 × fck × b × 2.5(d – z)z

Let k = M/(fck bd2)
k can be considered as the normalised bending resistance

Hence;
M/(fck bd2) = 1.1333 [(fck bdz)/(fckbd2) – (fck bz2)/(fck bd2)]

Therefore;
0 = 1.1333[(z/d)2 – (z/d)] + k
0 = (z/d)2 – (z/d) + 0.88235k

Solving the quadratic equation;
z/d = [1 + (1 – 3.529k)0.5]/2

Rearranging;
z = d[0.5 + √(0.25 – k/1.134)] —— (8)
z = d[0.5 + √(0.25 – 0.882k)]

where ;
k = MEd/(fckbd2) —– (9)

Anchorage of Reinforced Concrete Beams

Reinforcing bars should be well anchored so that the bond forces are safely transmitted to the concrete avoiding longitudinal cracking or spalling. Transverse reinforcement shall be provided if necessary. Types of anchorage are shown in Figure 7 below (Figure 8.1 EC2).

anchorage of reinforcement
Figure 7: Different methods of anchorage (Fig 8.1 EN 1992-1-1:2004)

For bent bars, the basic tension anchorage length is measured along the centreline of the bar from the section in question to the end of the bar, where:

lbd = α1α2α3α4α5lb,req ≥ lb,min

where;
lb,min is the minimum anchorage length taken as follows:
In tension, the greatest of 0.3lb,rqd or 10ϕ or 100mm
In compression, the greatest of 0.6lb,rqd or 10ϕ or 100mm

lb,rqd is the basic anchorage length given by, lb,rqd = (ϕ/4)σsd/fbd

Where;
σsd = The design strength in the bar (take 0.87fyk)
fbd = The design ultimate bond stress (for ribbed bars = 2.25η1η2fctd)
fctd = Design concrete tensile strength = 0.21fck2/3 for fck ≤ 50 N/mm2

η1 is a coefficient related to the quality of the bond condition and the position of the bar during concreting
η1 = 1.0 when ‘good’ conditions are obtained and
η1 = 0.7 for all other cases and for bars in structural elements built with slip-forms, unless it can be shown that ‘good’ bond conditions exist

η2 is related to the bar diameter:
η2 = 1.0 for φ ≤ 32 mm
η2 = (132 – φ)/100 for φ > 32 mm

α1 is for the effect of the form of the bars assuming adequate cover
α2 is for the effect of concrete minimum cover
α3 is for the effect of confinement by transverse reinforcement
α4 is for the influence of one or more welded transverse bars ( φt > 0.6φ) along the design anchorage length lbd
α5 is for the effect of the pressure transverse to the plane of splitting along the design anchorage length

Curtailment of Reinforced Concrete Beams

Sufficient reinforcement should be provided at all sections to resist the envelope of the acting tensile force, including the effect of inclined cracks in webs and flanges.

curtailment of beams eurocode
Figure 8: Illustration of the curtailment of longitudinal reinforcement, taking into account the effect of inclined cracks and the resistance of reinforcement within anchorage lengths (Fig 8.7 EN 1992-1-1:2004)

For members with shear reinforcement the additional tensile force, ΔFtd, should be calculated according to clause 6.2.3 (7). For members without shear reinforcement ΔFtd may be estimated by shifting the moment curve adistance al = d according to clause 6.2.2 (5). This “shift rule” may also be used as an alternative for members with shear reinforcement, where:

al = z (cotθ – cotα)/2 = 0.5z cotθ for vertical shear links
z = lever arm, θ = angle of compression strut
al = 1.125d when cotθ = 2.5 and 0.45d when cot θ = 1

Shear Design of Reinforced Concrete Beams

shear model of a reinforced concrete beam
Figure 9: Shear force model EC2

In EC2, the concrete resistance shear stress without shear reinforcement is given by;

VRd,c = [CRd,c k(100ρ1 fck )1/3 + k1.σcp]bw.d ≥ (Vmin + k1.σcp) ———- (10)

Where;
CRd,c = 0.18/γc
k = 1 + √(200/d) < 0.02 (d in mm);
ρ1 = As1/bd < 0.02 (In which As1 is the area of tensile reinforcement which extends ≥ (lbd + d) beyond the section considered)
Vmin = 0.035k(3/2)fck0.5
K1 = 0.15; σcp = NEd/Ac < 0.2fcd
(Where NEd is the axial force at the section, Ac = cross sectional area of the concrete), fcd = design compressive strength of the concrete).

The compression capacity of the compression strut (VRd,max) assuming θ = 21.8° (cot θ = 2.5)
VRd,max = (bw.z.v1.fcd )/(cot⁡θ + tanθ) ———- (11)

v1 = 0.6(1 – fck/250)
Let z = 0.9d

If VRd,c < VEd < VRd,max
Asw/S = VEd/(0.87fykz cotθ)

If VRd,max > VEd, calculate, the compression strut angle.
θ = 0.5sin-1 [(VRd,max/bwd))/(0.153fck (1 – fck/250)] ———- (12)

If θ is greater than 45°, select another section.

Minimum shear reinforcement
Asw/S = ρw,min × bw × sinα (α = 90° for vertical links)
ρw,min = (0.08 × √fck)/fyk ———- (13)

Deflection of Reinforced Concrete Beams

A beam should not deflect excessively under service load. Excessive deflection of beams can cause damages and cracking to partitions and finishes. It can also impair the appearance of the building and cause great concern to the occupants of the building.

The selection of limits to deflection which will ensure that the structure will be able to fulfill its required function is a complex process and it is not possible for a code to specify simple limits which will meet all requirements and still be economical. Limits are suggested in the code but these are for general guidance only; it remains the responsibility of the designer to check whether these are appropriate for the particular case considered or whether some other limits should be used.

For deemed to satisfy basic span/effective depth (limiting to depth/250);

Actual L/d must be ≤ Limiting L/d × βs

The limiting basic span/ effective depth ratio is given by;

L/d = K [11 + 1.5√(fck)ρ0/ρ + 3.2√(fck) (ρ0/ρ – 1)1.5] if ρ ≤ ρ0 ——- (14)

L/d = K [11 + 1.5√(fck) ρ0/(ρ – ρ’) + 1/12 √(fck) (ρ0/ρ)0.5 ] if ρ > ρ0 —— (15)

Where;
L/d is the limiting span/depth ratio
K = Factor to take into account different structural systems
ρ0 = reference reinforcement ratio = 10-3 √(fck)
ρ = Tension reinforcement ratio to resist moment due to design load
ρ’ = Compression reinforcement ratio

The value of K depends on the structural configuration of the member and relates the basic span/depth ratio of reinforced concrete members. This is given in the table 2;

Table 2: Basic span/effective depth ratio of different structural systems

Structural SystemKHighly stressed ρ = 1.5%Lightly stressed ρ = 0.5%
Simply supported beams1.01420
End span of interior beams1.31826
Interior span of continuous beams1.52030
Cantilever beams0.468

βs = (500As,prov)/(fykAs,req) —–(16)

For flanged sections with b/bw ≥ 3, the basic ratios for rectangular sections should be multiplied by 0.8. For values of b/bw < 3, the basic ratios for rectangular sections should be multiplied by (11 – b/bw)/10. For beams with spans exceeding 7 m, which support partitions liable to be damaged by excessive deflections, the basic ratio should be multiplied by 7/span.

Check for Cracking in Reinforced Concrete Beams

According to clause 7.3.1 of EN 1992-1-1:2004, cracking is normal in reinforced concrete structures subject to bending, shear, torsion or tension resulting from either direct loading or restraint or imposed deformations. However, cracking shall be limited to an extent that will not impair the proper functioning or durability of the structure or cause its appearance to be unacceptable.

Crack width in beams may be calculated according to clause 7.3.4 of EN 1992-1-1:2004. A simplified alternative is to limit the bar size or spacing according to clause 7.3.3. If crack control is required, a minimum amount of bonded reinforcement is required to control cracking in areas where tension is expected. The amount may be estimated from equilibrium between the tensile force in section just before cracking and the tensile force in reinforcement at yielding or at a lower stress if necessary to limit the crack width.

Detailing of Reinforced Concrete Beams

The maximum and minimum areas of steel required in reinforced concrete beams are given in the Table 3.

Table 3: Reinforcement detailing of reinforced concrete beams

Specific member issueValue
Minimum area of steel required (As,min)0.26(fctm/fyk)btd ≥ 0.0013bd
Maximum areas of steel required (As,max)0.04bd
Minimum spacing of main bars max{dg + 5mm, ϕ, 20mm}
Minimum area of shear links (Asw, min)(0.08bwS√fck)/fyk
Maximum spacing of links0.75d

In the most simplified manner, the detailing guide for beams is as shown below. However, in the most practical terms for simple residential buildings with moderate spans, reinforcements are rarely curtailed (except at the supports hogging areas) for ease in construction, especially where manual iron bending is employed. Note that to satisfy anchorage requirements, take the bob length for beams as 15  (15 x diameter of reinforcement).

simplified detailing of a reinforced concrete beam
Figure 10: Simplified detailing of a reinforced concrete beam

Design Example of a Reinforced Concrete Beam

A continuous beam in a residential building is loaded as shown below. The beam is an L-beam with effective flange width of 895 mm. The depth and width is 450 x 230 mm. Concrete cover = 35mm, fck = 25 MPa, fyk = 460 MPa

loaded beam

The bending moment and shear force diagram due to the applied load is shown below;

bmd and shear

Effective depth (d) = 450 – 35 – 16/2 – 8 = 399mm
Beff = 895mm
k = MEd/(fckbeff d2) = (36.66 × 106)/(25 × 895 × 3992) = 0.01029

Since k < 0.167 No compression reinforcement required

z = d[0.5 + √(0.25 – 0.882K)]
z = d[0.5 + √(0.25 – 0.882(0.01029))] = 0.95d

As1 = MEd/(0.87fyk z) = (36.66 × 106)/(0.87 × 460 × 0.95 × 399) = 241.667 mm2

Provide 2Y16 mm BOT (As,prov = 402 mm2)

The minimum area of steel required;
fctm = 0.3 × fck2⁄3 = 0.3 × 252⁄3 = 2.5649 N/mm2 (Table 3.1 EC2)

As,min = 0.26 × fctm/fyk × b × d = 0.26 × (2.5649/460) × 230 × 399 = 133.04 mm2
Check if As,min < 0.0013 × b × d (119.301 mm2)
Since, As,min = 168.587 mm2, the provided reinforcement is adequate.

Check for deflection at the span
K = 1.3 for simply supported at one end and continuous at the other end
ρ = As/bd = 402/(895 × 399) = 0.0011257 < 10-3√25

Since ρ < ρ0
L/d = K [11 + 1.5√(fck) ρ0/ρ + 3.2√(fck) (ρ0/ρ – 1)3⁄2]
L/d = 1.3 [11 + 1.5√25 × (0.005/0.0011257) + 3.2√25 (0.005/0.0011257 – 1)3⁄2] = 1.3(11 + 33.313 + 102.158) = 190.4123

Modification factor βs = 310/σs
σs = (310fykAs,req)/(500As,prov) = (310 × 460 × 241.667)/(500 × 402) = 171.451 N/mm2
βs = 310/171.451 = 1.808

Since the beam is flanged, check the ratio of b/bw = 895/230 = 3.89
Since b/bw is greater than 3, multiply the allowable L/d by 0.8

The allowable span/depth ratio = 0.8 × βs × 190.4123 = 0.8 × 1.808 × 146.471 = 275.412

Taking the distance between supports as the effective span, L = 3825 mm
Actual deflection L/d = 3825/399 = 9.5864
Since 275.412 > 9.5864, deflection is deemed to satisfy
Use 2Y16mm for the entire bottom span.

Top reinforcements (Hogging moment)

Support 3
MEd = 36.296 KNm

Since flange is in tension, we use the beam width to calculate the value of k (this applies to all support hogging moments)

k = MEd/(fckbw d2) = (36.296 × 106)/(25 × 230 × 3992) = 0.0396
Since k < 0.167 No compression reinforcement required
z = 0.95d

As1 = MEd/(0.87fyk z) = (36.296 × 106)/(0.87 × 460 × 0.95 × 399) = 240 mm2

Provide 2Y16mm TOP (As,prov = 402 mm2)

Shear Design
Using the maximum shear force for all the spans
Support A; VEd = 65.19 KN
VRd,c = [CRd,c.k. (100ρ1 fck)1/3 + k1.σcp]bw.d ≥ (Vmin + k1.σcp)bw.d

CRd,c = 0.18/γc = 0.18/1.5 = 0.12

k = 1 + √(200/d) = 1 + √(200/399) = 1.708 > 2.0, therefore, k = 1.708

Vmin = 0.035k3/2fck1/2
Vmin = 0.035 × 1.7083/2 × 251/2 = 0.390 N/mm2

ρ1 = As/bd = 402/(230 × 399) = 0.00438 < 0.02;

σcp = NEd/Ac < 0.2fcd (Where NEd is the axial force at the section, Ac = cross sectional area of the concrete), fcd = design compressive strength of the concrete.) Take NEd = 0

VRd,c = [0.12 × 1.708(100 × 0.00438 × 25 )1/3] 230 × 399 = 41767.763 N = 41.767 KN
Since VRd,c (41.767) < VEd (65.19 KN), shear reinforcement is required.

The compression capacity of the compression strut (VRd,max) assuming θ = 21.8° (cot θ = 2.5)
VRd,max = (bw.z.v1.fcd)/(cotθ + tanθ)
V1 = 0.6(1 – fck/250) = 0.6(1 – 25/250) = 0.54
fcd = (αcc fck)/γc = (0.85 × 25)/1.5 = 14.167 N/mm2
Let z = 0.9d

VRd,max = [(230 × 0.9 × 399 × 0.54 × 14.167)/(2.5 + 0.4)]× 10-3 = 217.879 KN

Since VRd,c < VEd < VRd,max
Hence Asw/S = VEd/(0.87fyk zcot θ) = 65190/(0.87 × 460 × 0.9 × 399 × 2.5) = 0.18144

Minimum shear reinforcement; Asw/S = ρw,min × bw × sinα (α = 90° for vertical links)
ρw,min = (0.08 × √fck)/fyk = (0.08 × √25)/460 = 0.00086956
Asw/S (min) = 0.00086956 × 230 × 1 = 0.2000

Since 0.200 > 0.18144, adopt 0.200 as the minimum shear reinforcement
Maximum spacing of shear links = 0.75d = 0.75 × 399= 299.25
Provide Y8mm @ 250mm c/c as shear links (Asw/S = 0.4021) Ok

Beam detailing sketches

Reinforced Concrete Structures

By
Ubani Obinna
-

Concrete is arguably the most widely used construction material in the world. It is produced from a mixture of cement, sand, gravel, and water through a process known as hydration reaction. In its fresh state, concrete can be poured into different moulds and forms to achieve the desired shape. This is one of the reasons why it is an attractive construction material.

In its hardened state, concrete is very good in compression, but weak in tension. In order to augment this inherent weakness of concrete in tension, steel reinforcement is usually introduced to take up the tensile stresses. Any structure made up of steel reinforcement embedded in concrete to form a load resisting composite is known as a reinforced concrete structure. The process of specifying the member sizes of concrete and the area of steel required to ensure good performance of a structure under load is known as reinforced concrete design.

The key to the good performance of reinforced concrete structures lies in the complementary action of concrete and steel. This composite but complementary action is highlighted in the Table below;

PropertyConcreteSteel
Tensile strengthPoorGood
Compressive strengthGoodGood (but slender members will buckle)
Shear strengthFairGood
DurabilityGoodFair (will corrode if unprotected)
Fire resistanceGoodPoor (will lose strength at elevated temperature)

By looking at the table above, you can see that all the desirable properties listed will be achieved if the two materials are combined. The structural design of reinforced concrete structures aims at taking advantage of the different but complementary characteristics of concrete and steel. Some of the basic theoretical assumptions that are made in design are as follows;

  • Concrete’s resistance to tension is zero (not practically true, the tensile strength of concrete is about 10% of its compressive strength, but this strength is usually ignored in ultimate limit state design)
  • The bond between steel and concrete is perfect

Based on these assumptions, all tensile stresses in a structure are transferred to the reinforcements during design. These tensile stresses are transferred by the bond between the concrete and the reinforcement. Perfect bond assumption demands that the strain in the reinforcement is identical to the strain in the adjacent concrete (compatibility of strains). Furthermore, the coefficients of thermal expansion for steel and for concrete are of the order of 10 x 10-6 per ℃ and 7-12 x 10-6 per ℃ respectively. These values are sufficiently close that problems with bond seldom arise from differential expansion between the two materials over normal temperature ranges.

Practically, if the bond between reinforcement and steel is not adequate. the reinforcing bars will slip within the concrete and there will be no composite action. Adequate bonding is ensured by detailing the structure such that the reinforcement is properly anchored in the concrete. Reinforcement bars are also ribbed in order to facilitate bonding with concrete.

reinforcement bars
Reinforcement bars are ribbed to enhance bonding with concrete

It is normal for cracking to occur in concrete when it subjected to tensile or flexural stress. This cracking, however, does not mean that the structure is not safe provided it is adequately reinforced that the crack width is kept to a minimum. If the crack width is excessive, there may be serviceability and/or durability issues (corrosion of reinforcement) in the structure.

Furthermore, when the compressive or shearing forces exceed the strength of the concrete, then steel reinforcement must again be provided to supplement the load-carrying capacity of the concrete. For example, compression reinforcement is generally required in a column, where it takes the form of vertical bars spaced near the perimeter. To prevent these bars buckling, steel binders are used to assist the restraint provided by the surrounding concrete.

Reinforced concrete has a lot of applications in construction, and has been applied in a lot of structures worldwide – bridges, industries, residential buildings, highrise buildings, swimming pools, retaining walls, highways (rigid pavement) etc. The design of any reinforced concrete structure should start with the understanding and behaviour of the structure to be designed under load. The designer will need to specify the load path (how the load will be transferred from the superstructure to the foundation).

For instance, to design a building, the structure can be broken down into the following elements. This is what is called the general arrangement of the building.

  • Beams: horizontal members carrying lateral loads
  • Slab: horizontal plate elements carrying lateral load
  • Columns: vertical members carrying primarily axial load but generally subjected to axial load and moment
  • Walls: vertical plate elements resisting vertical, lateral or in-plane loads
  • Bases and foundations: pads or strips supported directly on the ground that spread the loads from columns or walls so that they can be supported by the ground without excessive settlement. Alternatively, the bases may be supported on piles.

Knowledge of reinforced concrete design starts from knowing how to design the separate elements listed above. However, it is important to recognize the function of the element in the complete structure and that the complete structure or part of it needs to be analysed in order to obtain actions for design.

Designers are expected to follow a generally accepted code of practice in their design and detailing. This is to enable a quick check and understanding of the design by other engineers. Some codes of practice used in the design of concrete structures across the world are;

EN 1992-1-1:2004 – Eurocode 2: Design of concrete structures – Part 1-1: General rules and rules for buildings (European Union)
BS 8110-1:1997 – Design of reinforced concrete structures – Rules and general rules for buildings
ACI 318-19: Building Code Requirements for Structural Concrete and Commentary
IS 456-2000: Plain and Reinforced Concrete – Code of Practice (Indian Standards)
CSA A23.3:2014 – Design of concrete structures (Canadian Standards Association)
AS 3600:2018 – Concrete structures (Standards Australia)

Aerodynamics of High-Rise Buildings

By
Ubani Obinna
-

To improve the safety and serviceability of super-tall buildings in strong winds, aerodynamic optimization of building shapes is considered to be the most efficient approach. Aerodynamic optimization is aimed at solving the problem from the source in contrast to structural optimization which is aimed at increasing the structural resistance against winds [1]. Aerodynamics is concerned with the study of air in motion and how it interacts with solid objects around it.

According to Tanaka et al. [2], the free form architecture being expressed in the modern construction of tall buildings today do not only have the advantage of reflecting the architect’s idea, but also have the advantage of reducing the effects of wind on the structure due to aerodynamic effects. Some aerodynamic modifications in architectural design are one of the effective design approaches which can significantly reduce the effect of the lateral wind force and thus, the building motion [3, 4].

The modifications usually employed to improve aerodynamic response of tall buildings are [5];

  • tapered cross-section
  • setback
  • sculptured top
  • modifications to corner geometry, and
  • addition of openings through building
areodynamic optimzation of tall buildings
Some aerodynamic optimization of tall buildings [4]

By changing the flow pattern around the building due to aerodynamic modifications of the building shape, (i.e. an appropriate choice of building form), wind response can be moderated when compared to the original building shape. As far as wind loading and resulting motions are concerned for tall and slender buildings, the shape is critical and a governing factor in the architectural design. Wind tunnel test is the most popular method of evaluating the aerodynamic behaviour of a high-rise building.

distribution of wind pressure
Distribution of mean vertical velocity around a square and helical building form [2]

Improving the aerodynamic performance of a tall building can be achieved by local and global shape mitigations. Local shape mitigations, such as corner mitigations, have a considerable effect on structural and architectural design, while global shape mitigations have a minor effect on structural and architectural design [6]. Essentially, the precise selection of the outer shape details of a building can result in a significant reduction in forces and motions caused by wind.

Aerodynamic forms in general reduce the along-wind response as well as across-wind vibration of the buildings caused by vortex-shedding by “confusing” the wind (i.e., by interrupting vortex-shedding and the boundary layer around the façade and causing mild turbulence there).

Absolute MAD 1020 by iwan baan
Absolute towers, MISSISSAUGA, CANADA

While irregular forms pose challenges to structural engineers for developing the structural framework, they can be advantageous in reducing wind load effects and building responses [7]. Examples employed in contemporary tall buildings are chamfered or rounded corners, streamlined forms, tapered forms, openings through a building, and notches.

The Shanghai World Financial Center and the Kingdom Center in Riyadh (see below) employ a large through-building opening at the top combined with a tapered form. The proposed Guangzhou Pearl River Tower’s funnel form facades catch natural wind not only to reduce the building motion but also to generate energy using wind. Due to the nature of the strategy which manipulates building masses and forms, this approach blends fittingly with architectural aesthetics.

Kingdom centre 1
Kingdom Centre, Riyadh

References

[1] Xie J. (2014): Aerodynamic Optimization of Supertall buildings and its effectiveness assessment. Journal of Wind Engineering and Industrial Aerodynamics (130):88-98
[2] Tanaka H., Tamuro Y., Ohtake K., Nakai M., Kim Y.C., and Bandi E.K. (2013): Aerodynamic and flow characteristics of tall buildings with various unconventional configurations. International Journal of High-Rise Buildings 2(3):213-228
[3] Ilgin E.H. and Gunel M.H. (2007): The role of aerodynamic modifications in the form of tall buildings against wind excitation. METU JFA 24(2):17-25
[4] Elshaer A., and Bitsuamlak G. (2018): Multi-objective aerodynamic optimization of tall buildings openings for wind-induced excitation reduction. Journal of Structural Engineering 144(10):27-38
[5] Kareem, A., Kijewski, T. and Tamura, Y. (1999): Mitigation of Motion of Tall Buildings with Recent Applications. Wind and Structures, 2(3): 201-251.
[6] Elshaer A., Bitsuamlak G., and El Damatty A (2016): Aerodynamic shape optimization of tall buildings using twisting and corner modifications. 8th International Colloqium on Bluff Body Dynamics and Applications, Northeastern University, Boston Massachussets, USA
[7] Ali M.M., and Moon K.S. (2007): Structural developments in tall buildings: Current trends and future prospects. Architectural Science Review 50(3):205-223

Feature Image belongs to [2].

Negative Skin Friction on Pile Foundation

By
Ivy Ugorji
-

Negative skin friction is a downward drag force exerted on a pile by the soil surrounding it. This is the reverse of the normal skin friction or shaft resistance needed to support piles. If the downward drag force is excessive, it can cause the failure of the pile foundation.

Where applicable, negative skin friction must be allowed when considering the factor of safety on the ultimate load-carrying capacity of a pile. The factor of safety (FOS) where negative skin friction is likely to occur is given by;

FOS = Ultimate load carrying capacity of pile/(Working load + Negative skin friction load)

Negative skin friction may occur in pile foundations due to the following circumstances;

  1. If a fill of clay soil is placed over a granular soil or completely consolidated soil into which a pile is driven, the consolidation process of the recently placed fill will exert a downward drag of the pile during the process of consolidation.
  2. If a granular fill material is placed over a layer of compressible clay, it will induce the process of consolidation in the clay layer and exert a downward drag on the pile.
  3. Lowering of the water table will increase the vertical effective stress on the soil at any depth which will induce consolidation settlement of the clay. If a pile is located in the clay layer, it will be subjected to a downward drag force.
negative skin friction

According to section 7.3.2.2 of EN 1997-1:2004 (Eurocode 7), if ultimate limit state design calculations are carried out with the downdrag load as an action, its value shall be the maximum, which could be generated by the downward movement of the ground relative to the pile.

Furthermore, the calculation of maximum downdrag loads should take account of the shear resistance at the interface between the soil and the pile shaft and downward movement of the ground due to self-weight compression and any surface load around the pile. An upper bound to the downdrag load on a group of piles may be calculated from the weight of the surcharge causing the movement and taking into account any changes in groundwater pressure due to ground-water lowering, consolidation or pile driving.

Computation of negative skin friction on a single pile

The magnitude of negative skin friction (Fn) for a single pile in filled up soil may be taken as;

(a) Cohesionless soil
Fn = 0.5K’γf‘Hf2tanδ’

Where;
K’ = coefficient of earth pressure = Ko = 1 – sinφ’
γf‘ = Effective unit weight of material causing down drag
Hf = Depth of compressible layer causing down drag
δ’ = soil-pile angle of friction ≈ 0.5φ’ – 0.6φ’

(b) Cohesive soil
Fn = PHfS

Where;
P = Perimeter of pile
S = Shear strength of soil

Negative skin friction on pile groups
When a group of piles passes through a compressible fill, the negative skin friction may be obtained using any of the following methods;

(a) Fng = nFn
(b) Fng = sHfPg + γf‘HfAg

Where;
n = number of piles in the group
γf‘ = Effective unit weight of material causing down drag
Pg = Perimeter of the pile group
Ag = Cross-sectional area of the pile group within the perimeter Pg
S = Shear strength of the soil along the perimeter of the group


Minimum Area of Reinforcement Required for Reinforced Concrete Beams

By
Ubani Obinna
-

In reinforced concrete beams, the minimum area of longitudinal reinforcement required is given by;

As,min = 0.26 (fctm / fyk) bt d   ≥ 0.0013 bt d —- (1)

where:
fctm is the mean value of axial tensile strength of concrete at 28 days, see Table 3.1 of EN 1992-1-1:2004
fyk is the characteristic yield stress of reinforcement steel
bt is the mean width of the tension zone; for a T-beam with the flange in compression, only the width of the web is taken into account in calculating the value of bt
d is the effective depth of concrete cross-section.

Any section containing less than As,min as given in equation (1) should be treated as an unreinforced section.

The minimum area of reinforcement for different classes of concrete strengths assuming fyk = 500 MPa is given in the Table below;

fck (MPa)fctm (MPa)0.26fctm/fyk
252.60.13%
282.80.14%
302.90.15%
323.00.16%
353.20.17%
403.50.18%
453.80.20%
504.20.21%

For a T-beam with the flange in tension (such as upstand beams or ground beams), the minimum area of reinforcement is given by equation (2);

As,min = 0.26 (fctm / fyk) Ac,t —– (2)

Where Ac,t is the area of the tension zone above the neutral axis of the reinforcement.

BS 8110-1:1997 made express provisions for flanged beams for different stress states of the web or the flange. The requirements are given in the table below;

Flanged beam in flexure (tension reinforcement)Definition of PercentageMinimum Percentage (fy = 500 MPa)
(a) Web in tension
bw/b < 0.4100As/bwh0.18%
bw/b ≥ 0.4 100As/bwh0.13%
(b) Flange in tension
T-beam100As/bwh0.26%
L-beam100As/bwh0.20%

Maximum area of longitudinal reinforcement
The maximum area of tension and compression reinforcement in reinforced concrete beams is;

As,max = 0.04Ac —- (3)

Where;
Ac is the area of the cross-section

Shear Reinforcements (links and stirrups)
The minimum area of shear reinforcement in beams should be calculated from;

Asw/sbw ≥ ρw,min —– (4)

Where;
ρw,min = (0.08√fck)/fyk
Asw = Area of shear reinforcement
s = spacing of shear reinforcement
bw = width of the web

Solved Example
For the concrete section shown below, find the minimum area of longitudinal reinforcement required for;
(a) When the web is in tension
(b) When the flange is in tension
fck = 25 MPa, fyk = 500 MPa

Beam section

(a) When the web is in tension
Using Eurocode;
As,min = 0.26 (fctm / fyk) bt d 
As,min = 0.26 x (2.6/500) x 250 x 695 = 235 mm2 > 0.0013btd (0.0013 x 250 x 695) = 226 mm2

Using Table 3.25, BS 8110-1:1997;
bw/b = 250/1500 = 0.167 < 0.4
As,min = 0.18bwh/100 = (0.18 x 250 x 750)/100 = 338 mm2

(b) When the flange is in tension
Using Eurocode;
As,min = 0.26 (fctm / fyk) Ac,t
Ac,t = (550 x 250) + (1500 x 145) = 355000 mm2 
As,min = 0.26 x (2.6/500) x 355000 = 480 mm2

Using Table 3.25, BS 8110-1:1997;
As,min = 0.26bwh/100 = (0.26 x 250 x 750)/100 = 488 mm2

Construction Commences on the World’s Longest Underwater Tunnel

By
Muyiwa Ajumobi
-

The construction of the world’s longest immersed tunnel has officially begun. The Fehmarnbelt Tunnel that will connect Denmark and Germany, is scheduled to be officially opened by 2029. It’s one of Europe’s largest ongoing infrastructure projects, with a budget of more than US$8 million.

The tunnel will have an 18 kilometers extension and will be built across the Fehmarn Belt, a strait between the German island of Fehmarn and the Danish island of Lolland. It will be an alternative to the current ferry service, which takes 45 minutes. Traveling through the tunnel will take seven minutes by train and ten minutes by car.

1280px Fehmarn bridge.svg

It will be the longest combined road and rail tunnel in the world, with two double-lane motorways, separated by a service passageway, and two electrified rail tracks. Besides the benefits to passenger trains and cars, it will have a positive impact on the flow of freight trucks and trains. 

image

According to Femern website, the contract for the construction of the 18km tunnel was signed on 30 May 2016. The contract which is worth almost 4 billion Euro was signed between Femern A/S  (the Danish state-owned company tasked with designing and planning the link) and international contractors responsible for the establishment of the 18 km Fehmarnbelt tunnel between Rødbyhavn and Puttgarden. The contract with the Dutch consortium, Fehmarn Belt Contractors (FBC) came into force in November 2019. This covers dredging and reclamation.

In May 2020, Femern A/S initiated conditional contracts for the tunnel as well as the portals and ramps, which were signed with the consortium, Femern Link Contractors (FLC). The contracts will be activated with effect from 1 January 2021.

Construction work started in the summer on the Danish side. Work will carry on for a few years in Denmark before moving into German territory. Workers are now building a new harbor in Lolland and in 2021 they will start construction of a factory, both meant to support work on the tunnel. Located behind the port, the factory will have six production lines to assemble the 89 massive concrete sections that will make up the tunnel.

The immersed tunnel and the tunnel factory as well as the portals and ramps

Consortium: Femern Link Contractors (FLC)
Contractors:
VINCI Construction Grands Projets S.A.S. (France)
Per Aarsleff Holding A/S (Denmark)
Wayss & Freytag Ingenieurbau AG (Germany)
Max Bögl Stiftung & Co. KG (Germany)
CFE SA (Belgium)
Solétanche-Bachy International S.A.S. (France)
BAM Infra B.V. (Holland)
BAM International B.V. (Holland)
Sub-contractors:
Dredging International N.V. (Belgium)
Consutants:
COWI A/S (Denmark)

Lateral Load Resistance of High-Rise Buildings

By
Ubani Obinna
-

The lateral load resistance and stability of buildings get increasingly important as the height of the building increases. Gravity loads on buildings can be said to vary linearly with height. In a fairly regular building, the increment in axial load in columns can be said to increase linearly as you move from the roof to the ground floor. On the other hand, lateral loads are quite variable and increase rapidly with height [1].

For example, under a uniform wind load, the overturning moment at the base varies in proportion to the square of the height of the building, while the lateral deflection varies as the fourth power.  However, wind load distribution actually increases with height, and this gives rise to a greater base bending moment [2].

In actual sense, the pressure exerted by wind on a building are not steady, but dynamic and highly fluctuating [3]. This fluctuating pressure could bring severe damage to the building other than the wind force itself, especially due to fatigue [4].

winds
Wind action simulation on a tall building [5]

For structural designs, one of the ways to safely assume wind pressures is through the quasi-steady assumption in which the building is subjected to a steady lateral wind force. This is the approach reported in many codes of practice for the loading of buildings. It is the duty of the structural engineer to select the structural system that will resist the gravity and lateral actions, and at the same time satisfy serviceability requirements of the structure, especially in terms of occupancy comfort due to vibration and sway.

One of the main tasks when designing high-rise buildings is to give the structure the ability to absorb the horizontal forces, and to transmit the resulting moment into the foundation [2, 4]. The loads acting on a tall building can be simply divided into vertical and horizontal actions.

The vertical loads are the weight of the building, imposed load and snow loads (where applicable). The horizontal loads are wind loads, seismic response, unintended inclinations/tilt, impact forces, blasts etc.

The vertical loads are taken up by the bearing walls, columns or towers and are led to the foundations. The loads occurring from the wind are first taken by the façades and are then further distributed to the slabs [6]. The floor slabs act as diaphragms and are often considered to be stiff in their plane and deformations in its plane are usually disregarded. The slabs are connected to the stabilising units, such as shear walls, cores, or stabilising columns.

wind load on a tall building
Wind load distribution to floor slabs [6]

If the façade which takes the wind load is supported by the floor slabs, then the floor slabs will be subjected to a distributed load. However, when the facades have columns attached directly to them, the loads are first transferred to the columns resulting in concentrated loads on the floor slabs. The stress distributions have to be dealt with through careful planning of how the slabs and the facade are connected.

Floor slabs are often considered to be stiff, and the horizontal load distribution through the building is due to the stiffness of the different stabilising components. If the floor slab is not stiff enough, or slip occurs in joints between slab elements in the same plane, then the displacement of the floor slab will not be the same along the loaded side of the floor slab. Stress distribution in floors depends on both loads and supports [6].

The floor of a tall building is supported by the stabilising units through a shear force distributed along the width of the wall. The walls are subjected to both bending and shear deformations but in low robust walls the bending contribution is negligible. If slender units are used for stabilising then bending mainly occurs and shear deformation is negligible.

bending and shear deformation of a floor slab
Bending and shear deformation of a floor slab [6]

Based on holistic considerations, shear walls become more slender in taller structures, even though shear walls are usually considered as low and robust on each floor. It is, therefore, necessary to consider both bending and shear deformations when designing tall buildings. The deformation from bending is curved in the opposite direction to the shear deformation. The deformation from shear is due to the shear forces applied through the floor slabs in each storey. As the loads accumulate and increase through the building the largest singular deformation occurs at the first floor for the shear contribution.

bending and shear deformation of a shear wall

Bending and shear deformation of a shear wall [6]

References

[1] Bungale S. T. (2010): Reinforced Concrete Design of Tall Buildings. CRC Press, Taylor and Francis Group
[2] Hallebrand E., and Jakobsson W. (2016): Structural design of high-rise buildings. M.Sc thesis presented to the Department of Construction Sciences (Division of structural mechanics), Lund University, Sweden
[3] Nizamani J., Thang B.C., Hiader B., and Sharrif M. (2018): Wind load effects on high rise buildings Peninsular Malaysia. IOP Conference Series: Earth and Environmental Science 140(2018)01215
[4] Sandelin C. and Bujadev E. (2013): The stabilization of high-rise buildings: An evaluation of the tubed mega frame concept. M.Sc Dissertation submitted to the Department of Engineering Science, Applied Mechanics, Civil Engineering Uppsala University.
[5] https://www.theb1m.com/video/how-tall-buildings-tame-the-wind Assessed on the 23rd of October, 2020
[6] Gustaffson D., and Hehir J. (2005): Stability of tall buildings. M.Sc thesis submitted to the Department of Civil and Environmental Engineering, Chalmers University of Technology, Sweden

Bentley Systems Announces Winners of ‘Year in Infrastructure 2020’ Awards

By
Ivy Ugorji
-

Bentley Systems Incorporated has announced the winners of the Year in Infrastructure 2020 Awards. The annual awards program honors the extraordinary work of Bentley users advancing design, construction, and operations of infrastructure throughout the world. 

Bentley Systems is an infrastructure engineering software company. They provide innovative software to advance the world’s infrastructure – sustaining both the global economy and environment. The software solutions from Bentley Systems are used by professionals, and organizations of every size, for the design, construction, and operations of roads and bridges, rail and transit, water and wastewater, public works and utilities, buildings and campuses, and industrial facilities. 

The Year in Infrastructure annual awards program honors the extraordinary work of Bentley users advancing design, construction, and operations of infrastructure throughout the world. Sixteen independent jury panels selected the 57 finalists from over 400 nominations submitted by more than 330 organizations from more than 60 countries.  

The Year in Infrastructure 2020 Special Recognition awardees are: 

Category 1: Advancing Project and Asset Longevity 

hdr basnight bridge large


Firm/Organisation/Institution: HDR 
Project: Marc Basnight Bridge 
Location: Dare County, North Carolina, United States  

Category 2: Advancing Bridge Asset Performance Modeling 
Firm/Organisation/Institution: Ulsan National Institute of Science and Technology (UNIST) 
Project: A Smartwatch on the Bridge 
Location: Ulsan, Ulju-gun, South Korea 

Category 3: Advancing Industrial Asset Performance Modeling 
Firm/Organisation/Institution: The Institute of Engineering and Ocean Technology/Oil and Natural Gas Corporation Limited 
Project: Challenges in Addressing Life Extension of Ageing Platforms in Western Offshore of India 
Location: Mumbai, India  

Category 4: Comprehensiveness in Industrial Digital Twins 

Volgogradnefteproekt LLC

Firm/Organisation/Institution: Volgogradnefteproekt, LLC 
Project: Ethane-Containing Gas Processing Complex Construction Support 
Location: Ust-Luga, St. Petersburg, Russia  

Category 5: Comprehensiveness in Transportation Digital Twins 

PT Waskita Manggarai realitymodel2

Firm/Organisation/Institution: PT. WASKITA Karya (Persero) Tbk 
Project: Railway Facility for Manggarai to Jatinegara: Package A – Phase II ( Main Line II) 
Location: South Jakarta, Jakarta, Indonesia  

Category 6: Comprehensiveness in Urban Digital Twins 
Firm/Organisation/Institution: JSTI Group Co., Ltd. 
Project: Hengjiang Avenue Rapid Transformation Project  
Location: Nanjing, Jiangsu, China 

Category 7: Comprehensiveness in a Connected Data Environment 
Firm/Organisation/Institution: Roads & Transport Authority (RTA) 
Project: Collaborative Information System Implementation – Whole Lifecycle Common Data Environment 
Location: Dubai, United Arab Emirates  

Category 8: Advancing Virtualization through Digital Twins 
Firm/Organisation/Institution: Network Rail 
Project: Overcoming Challenges Under COVID-19 Lockdown 
Location: Wales and Western Region, United Kingdom 

Category 9: Advancing Model-based Delivery through Digital Twins 
Firm/Organisation/Institution: NYS Department of Transportation 
Project: Model Based Contracting – NYS RT 28 over the Esopus 
Location: Mount Tremper, New York, United States 

Category 10: Advancing Mixed-Reality Workflows 
Firm/Organisation/Institution: Liaoning Water Conservancy and Hydropower Survey and Design Research Institute Co., Ltd. 
Project: Chaoyang Underground Pumping Station Project of the LXB Water Supply Project Phase II 
Location: Chaoyang, Liaoning, China 

Category 11: Advancing Sustainability Digital Twins 
Firm/Organisation/Institution: Shanghai Institute of Mechanical and Electrical Engineering Co., Ltd. 
Project: Shanghai Electric Environmental Protection Group Technology Renovation and Expansion Project for Nantong Thermoelectric Waste Incineration  
Location: Nantong, Jiangsu, China  

Category 12: Advancing Sustainable Architecture 

swatch headquarters

Firm/Organisation/Institution: Swatch Ltd., Shigeru Ban, Itten+Brechbühl AG 
Project: Swatch Headquarters 
Location: Biel, Bern, Switzerland  

Category 13: Advancing Sustainable Energy  
Firm/Organisation/Institution: Guangdong Hydropower Planning & Design Institute 
Project: Guangdong Yangjiang Pumped Storage Power Station  
Location: Yangjiang, Guangdong, China 

Category 14: Advancing Sustainable Water 
Firm/Organisation/Institution: Jacobs 
Project: San Jose Headworks 
Location: San Jose, California, United States 

The winners of the Year in Infrastructure 2020 Awards for going digital advancements in infrastructure are: 

4D Digital Construction 
Firm/Organisation/Institution: DPR Construction 
Project: 2019 LSM DS Tech Upgrade 
Location: Durham, North Carolina, United States 

Bridges 
Firm/Organisation/Institution:
Chongqing Communications Planning, Survey & Design Institute Co., Ltd., 
Guizhou Communications Construction Group Co., Ltd., 
Guizhou Bridge Construction Group Co., Ltd. 
Project: Digital Design and Construction of Taihong Yangtze River Bridge 
Location: Chongqing, China 

Buildings and Campuses 
Firm/Organisation/Institution: Voyants Solutions Private Limited 
Project: Bangladesh Regional Waterway Transport Project 1 – Shasanghat (New Dhaka) IWT Terminal 
Location: Dhaka-Shasanghat, Narayanganj, Chandpur, and Barisal; Bangladesh 

Digital Cities 
Firm/Organisation/Institution: City of Helsinki 
Project: Digital City of Synergy 
Location: Helsinki, Finland 

Geotechnical Engineering 
Firm/Organisation/Institution: Golder Associates Hong Kong Ltd. 
Project: Tuen Mun-Chek Lap Kok Link Tunnel, Southern Landfall 
Location: Hong Kong 

Land and Site Development 
Firm/Organisation/Institution: AAEngineering Group 
Project: Dzhamgyr Mine – Project Implementation in Extreme Conditions 
Location: Talas Region, Kyrgyzstan 

Manufacturing 
Firm/Organisation/Institution: MCC Capital Engineering & Research Incorporation Ltd. 
Project: BIM Technology-Based Construction of Digital Plant for Iron & Steel Base in Lingang, Laoting of HBIS Group Co., Ltd. 
Location: Tangshan, Hebei, China 

Mining and Offshore Engineering 
Firm/Organisation/Institution: AAEngineering Group 
Project: Digital Twin of AKSU Plant: From Concept to Startup 
Location: Aksu, Akmola Region, Kazakhstan 

Power Generation 
Firm/Organisation/Institution: Shanghai Institute of Mechanical and Electrical Engineering Co., Ltd. 
Project: Shanghai Electric Environmental Protection Group Technology Renovation and Expansion Project for Nantong Thermoelectric Waste Incineration 
Location: Nantong, Jiangsu, China 

Project Delivery 
Firm/Organisation/Institution: Sweco 
Project: Sweco | Digitalisation with BIM 
Location: United Kingdom 

Rail and Transit 
Firm/Organisation/Institution: POWERCHINA Huadong Engineering Corporation Limited 
Project: Innovative Application of Digital Engineering Technology in Shaoxing Rail and Transit Construction 
Location: Shaoxing, Zhejiang, China 

Reality Modeling 
Firm/Organisation/Institution: Khatib & Alami 
Project: Geo-enabling Reality Model Tips and Tricks 
Location: Muscat, Oman 

Road and Rail Asset Performance 
Firm/Organisation/Institution: Roads and Transport Authority (RTA) 
Project: Collaborative Information System Implementation – Whole Lifecycle Common Data Environment 
Location: Dubai, United Arab Emirates 

Roads and Highways 
Firm/Organisation/Institution: Sichuan Road & Bridge (Group) Co., Ltd. 
Project: BIM Technology Application on Chengdu-Yibin Expressway 
Location: Chengdu, Sichuan, China 

Structural Engineering 
Firm/Organisation/Institution: WSP 
Project: WSP Overcomes Complex Challenges with Bentley’s Technology to Deliver Principal Tower 
Location: London, England, United Kingdom 

Utilities and Communications 
Firm/Organisation/Institution: Sterlite Power Transmission Limited 
Project: Sterlite BIM 
Location: Tripura, India 

Utilities and Industrial Asset Performance
Firm/Organisation/Institution: Shell’s QGC business 
Project: Evolution of Engineering Data, Documents and Information Management 
Location: Brisbane, Queensland, Australia 

Water and Wastewater Treatment Plants 
Firm/Organisation/Institution: Hatch 
Project: Ashbridges Bay Treatment Plant Outfall 
Location: Toronto, Ontario, Canada 

Water, Wastewater, and Stormwater Networks 
Firm/Organisation/Institution: DTK Hydronet Solutions 
Project: Digital Water Network Engineering & Asset Management of Dibrugarh Water Supply Project 
Location: Dibrugarh, Assam, India 

Detailed descriptions of all nominated projects are in the print and digital versions of Bentley’s 2020 Infrastructure Yearbook, which will be published in early 2021. 

Column Axial Shortening in Tall Buildings

By
Ubani Obinna
-

When a vertical compressive load is applied to a column, it shortens. Axial shortening takes place in all structures but when reaching great heights its effect has significant importance [1]. As the columns shortening are added together the overall shortening of a high-rise building becomes big enough to have real consequences. Shortening has to be taken into account by the structural engineer when the building is being erected since it will vary as more stories are added [2, 3].

Floor slabs can start to tilt because of differential column shortening which in turn affects the cladding, partitions, mechanical equipment, and more, a possible result is shown in Figure 1. Axial shortening of columns can also affect stress distribution in horizontal structural elements.

Differential column axial shortening
Figure 1: Differential Column Shortening in a Building [4]

Depending on the material used for the construction, the magnitude of column shortening varies. Steel columns have more tendency to be affected by column axial shortening than reinforced concrete [5]. However, reinforced concrete does have length changes from creep and shrinkage making the two materials almost equal in total length change [4]. While shortening in steel columns have been attributed to elastic deformation only, it has been attributed to the summation of elastic strain caused by load application, shrinkage strain caused by drying, and creep strain induced by sustained stress over a long-term period in reinforced concrete.

differential shortening
Figure 2: Shortening in Columns [11]

Concrete columns and walls can potentially shorten at different rates within the same floor resulting in differential shortening [3, 6]. However, according to The Concrete Society (UK), column shortening is not significant in reinforced concrete buildings less than 10 – 15 storeys. At each storey height, a maximum shortening of 4 – 5 mm corresponding to deformation of about 1.4 mm/m is possible. Report suggests that it is difficult to reduce this shortening significantly. A better strategy is to minimise differential shortening by designing all columns to the same criteria.

In a research carried out by Ali [7] on steel buildings using SAP 2000 finite element analysis package, it was observed that the maximum axial shortening occurs in the top storey of the structure and almost zero magnitude at the bottom level. The increasing rate of axial deformation of steel column is quite high from ground level to the mid-floor levels and finally it becomes moderate from the mid floor level to the top floor. Interior columns experience higher axial shortening compared to side and corner columns which is responsible for the development of differential shortening in steel columns. This is because interior columns are more heavily loaded than exterior columns.

By designing the vertical structural members’ connection to deform without stressing the affected elements (cladding, partitions, etc.) column shortening can be contained. The problem of differential shortening between adjacent vertical elements still remains however and must be taken into consideration.

To compensate for columns shortening, a few different methods can be applied;

  • If the stresses are made equal between the vertical elements the length change will be more equal, especially if the material is the same.
  • Designing all columns to the same criteria
  • Keeping long clear spans between different structural types such as cores and columns
  • Steel columns can be compensated by making them longer in fabrication.
  • Concrete columns can be adjusted by the formwork.
  • Counteraction can be made by tilting the floor slabs the opposite way than that because of column shortening during construction
  • Use of shims [2]

Calculation of exact values of axial shortening in reinforced concrete structures is not a straight forward task. It depends on a number of parameters such as the type of concrete, reinforcement ratio, and the rate and sequence of construction [8]. This information may not be available to the structural engineer at the design stage.

Samarakkody et al [9] developed a technique to evaluate the differential axial shortening in a high-rise building with composite steel and concrete columns. This technique incorporated the effects of construction sequence and concrete levelling, stress relaxation of concrete due to the presence of the steel tube, time-dependent material properties such as creep, and effects of belt and outrigger systems.

Correia and Lobo [10] developed a simplified method of assessing axial shortening in tall buildings using the construction sequence approach and obtained favourable results that are comparable with outputs from finite element packages.

References
[1] Cargnino A., Debernardi P.G., Guila M., Taliano M. (2012): Axial shortening compensation strategies in tall buildings. A case study: The new Piedmont Government Office Tower. Structural Engineering International 22(1):121-129
[2] Sandelin C. and Bujadev E. (2013): The stabilization of high-rise buildings: An evaluation of the tubed mega frame concept. M.sc Dissertation submitted to the Department of Engineering Science, Applied Mechanics, Civil Engineering Uppsala University
[3] Matar S.S. and Faschan W.J. (2017): A structural engineer’s approach to differential shortening in tall buildings. International Journal of High-Rise Buildings 6(1):73-82
[4] Fintel M., Ghosh S. and Iyengar H. (1987): Column shortening in tall structures – Prediction and compensation. Portland Cement Association
[5] Patil D. and Bajad M.N. (2016): Predicting axial shortening of vertical elements in high rise buildings by using PCA method. International Journal of Innovative Research in Science, Engineering and Technology 5(7):12512-12519
[6] Fragomeni S., Whaikawa H., Boonlualoah S. and Loo Y.C. (2014): Axial shortening in an 80-storey concrete building, in ST Smith (ed.), 23rd Australasian Conference on the Mechanics of Structures and Materials (ACMSM23), vol. II, Byron Bay, NSW, 9-12 December, Southern Cross University, Lismore, NSW, pp. 1231-1236. ISBN: 9780994152008
[7] Ali S. (2014): Analysis of Effects of Axial Shortening of Steel Columns in Frame Structure. Proceedings of the World Congress on Engineering and Computer Science 2014 Vol II. WCECS 2014, 22-24 October, 2014, San Francisco, USA
[8] Jayasinghe M. T. R., Jayasena W. M. V. P. K. (2004): Effects of Axial Shortening of Columns on Design and Construction of Tall Reinforced Concrete Buildings. ASCE Practice Periodical on Structural Design and Construction 9(2) https://doi.org/10.1061/(ASCE)1084-0680(2004)9:2(70)
[9] Samakkody D.A., Thambiratnman D.P., Chan T.H.T. and Moragaspitiya P.H.N. (2017): Differential axial shortening and its effects on high rise buildings with composite concrete filled tube columns. Elsevier – Construction and Building Materials (143):659-672 https://doi.org/10.1016/j.conbuildmat.2016.11.091
[10] Correia R. and Lobo P.S. (2017): Simplified assessment of the effects of column shortening on the response of tall concrete buildings. Procedia Structural Integrity 5(2017):179-186
[11] Lee Y., Kim J., Seol H., Yang J., Kim K. (2017):3D numerical analysis of column shortening and shore safety under construction of high-rise building. Elsevier – Engineering Structures (150): 242-255. https://doi.org/10.1016/j.engstruct.2017.07.049




Autodesk Launches BuildingConnected in Australia and NewZealand

By
Ubani Obinna
-

BuildingConnected, a construction tender program owned by Autodesk had been launched in Australia and NewZealand. With this expansion, the BuildingConnected solution is now available in Australia, New Zealand, the United Kingdom and Ireland. More than one million construction professionals use the platform in North America alone, with more than 2000 general contractors and owners actively bidding out projects, totalling USD56 billion in project values each month.

According to Autodesk’s official website, BuildingConnected is a preconstruction solution that combines the largest real-time, construction network with an easy-to-use platform that streamlines the bid and risk management process. BuildingConnected can enhance preconstruction operations by helping contractors find the best subcontractor for the right project, qualify subcontractors and manage project risks, centralise bids and manage them in real time.

buildingconnected

BuildingConnected allows preconstruction teams to:

  • Quickly solicit bids with customisable templates and accurately compare those bids in a side-by-side “apples-to-apples” fashion
  • Track against internal budgets with real-time cost updates
  • Easily collaborate with other estimators on the team, and follow communications and bid versions
  • Export bids and summary sheets for transparent collaboration with owners
  • Gain valuable insight into historical bid data and reports to optimise for future projects

According to Autodesk Construction Solutions Managing Director, Asia Pacific Operations Tomy Praveen, “BuildingConnected will enable construction businesses in Australia and New Zealand operating in single or multiple states and locations to discover and engage with leading subcontractors. With this connected network, they will be able to benchmark pricing and better deliver projects on time and to budget,”.

Join webinars to learn more about preconstruction and BuildingConnected

To learn more about connected construction and tackling procurement challenges in preconstruction, Autodesk will host a webinar that explores recently conducted research.

It will host a panel of building procurement experts who will focus their experiences on procurement technology, plus share findings from our recent research examining how owners, head contractors and subcontractors in Australia and New Zealand approach procurement today.

You can find the registration link to attend here.

BuildingConnected is now available for customers in Australia, New Zealand, United Kingdom and Ireland. For more details and key features, you can visit here.

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