Column Axial Shortening in Tall Buildings

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October 22, 2020

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.

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

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October 21, 2020

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 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.

Structural Analysis and Design of Portal Frames

By

Ubani Obinna

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July 13, 2024

Table of Contents

Buildings with large spans can be economically constructed using steel portal frames. Such structures are very common in business parks, industrial complexes, warehouses, places of worship, leisure houses, and sports complexes. The functionality of these types of buildings is usually influenced by factors such as the usage of the building, property development laws, availability of space, development plan, and the desired quality of the finished building.

Portal frames are characterized by their unique structural configuration and rigid connections. They comprise two vertical or inclined columns, rigidly connected at their eaves by a horizontal or inclined beam called the rafters. This arrangement forms a rigid frame that is either rectangular or trapezoidal in profile, with inherent in-plane stability.

The design of portal frames involves the selection of adequate column and rafter steel sections that will satisfy critical performance criteria such as bending, shear, axial compression, flexural buckling, lateral torsional buckling, and deflection, under the anticipated loading conditions.

In portal frames, the beam or rafters act as a portal, effectively transferring gravity and wind loads to the columns. Typically, a rigid bolted end-plate connection facilitates this load transfer and plays a critical role in the overall stability of the frame. The columns can be supported as fixed or pinned members connected to the base plates. When the columns are supported as fixed connections, moment-resisting base plate design will need to be carried out.

Steel as a construction material offers numerous possibilities to achieve both pleasant and flexible functional use economically.

Portal frames with hinged column bases are the most common type of industrial building and are used all over Nigeria and the rest of the world. Portal frames possess adequate stability in-plane and majorly require bracing for out-of-plane stability. Other structural forms that can also be used in industrial frames are lattice trusses, cable-stayed structures, etc.

industrial portal frame with mezzanine — Industrial portal frame with mezzanine

As a structure designed to accommodate complex human activities, it is very important to pay adequate attention to the details that will make the building functional, aesthetically pleasing, safe, and efficient. Expertise in the area of human factor engineering should be employed to make every aspect of the building efficient. The general aspects that should be considered before the detailed design of an industrial building are;

Space optimization.
Speed of construction.
Access and security.
Flexibility of use.
Environmental performance.
Standardization of components.
Infrastructure of supply.
Service integration.
Landscaping.
Aesthetics and visual impact.
Thermal performance and air-tightness.
Acoustic insulation.
Weather-tightness.
Fire safety.
Design life.
Sustainability considerations.
End of life and re-use

Loadings on Portal Frames

Steel portal frames designed and constructed in Europe typically follow the guidance set out in the Eurocodes (EC). These are a series of harmonized standards that define the requirements for the safety and serviceability of structures. Here’s a breakdown of the anticipated loadings considered according to the Eurocodes for steel portal frames:

Permanent Actions

Dead loads: This includes the self-weight of the steel frame itself, cladding materials (roofing, wall panels), any permanent fixtures or suspended ceilings.
Superimposed dead loads: These are permanent loads that are not part of the structure itself but add weight. Examples include partitions, fixed building services (HVAC units, piping), and permanent building equipment.

Variable Actions

Snow loads: The weight of snow accumulation on the roof, determined based on the geographical location and specific snow load zone.
Wind loads: Wind pressure acting on the entire structure, considering the building’s shape, size, and location. Both positive (wind suction) and negative (wind pressure) wind loads are evaluated.
Imposed loads: These are live loads acting on the structure due to occupancy or use. The specific value depends on the building function (residential, office, storage) and any specific use cases.
Seismic loads: In seismic zones, earthquake loads are considered to ensure the structure can withstand potential earthquake forces. The specific design approach depends on the seismic hazard level of the location.

Combination of Actions

The Eurocodes also require considering various combinations of these actions for design. This accounts for the possibility of multiple loads acting simultaneously. Specific factors are applied to each action type depending on whether it’s a permanent, variable, or accidental load. This ensures the design considers realistic scenarios the structure might encounter.

Methods of Portal Frame Analysis

Two primary approaches can be adopted for analyzing portal frame structures. They are:

elastic analysis, and
plastic analysis.

Elastic Analysis

The elastic analysis approach is based on the assumption that the frame exhibits purely elastic behaviour, meaning it does not experience any permanent deformations beyond its elastic limit when subjected to loading.

When a portal frame undergoes gravity loading, the bending moment reaches its peak values at the eaves (the horizontal line where the roof meets the wall) and at the apex (the highest point of the frame). At the eaves, the bending moment exhibits a hogging behaviour (convexity upwards), while at the apex, it displays a sagging behaviour (convexity downwards).

Elastic analysis tends to yield higher maximum bending moments at both the eaves and apex compared to plastic analysis. Consequently, design based solely on elastic analysis often results in less economical frames, as it necessitates the use of larger and potentially more expensive members to accommodate these higher moments.

Plastic Analysis

In contrast to elastic analysis, plastic analysis of portal frames acknowledges the potential for inelastic deformations within the structure. This allows for a more significant redistribution of bending moments throughout the structure. This redistribution is facilitated by the formation of plastic hinges at specific locations within the frame.

These plastic hinges typically develop at sections where the bending moment reaches the material’s plastic moment resistance, signifying the point at which the material yields. As a result of this redistribution, plastic analysis often leads to the design of lighter and more economical portal frames compared to those solely based on elastic analysis.

Second Order Effects in Portal Frames

While plastic analysis offers a more realistic representation of portal frame behaviour compared to elastic analysis, it is essential to acknowledge that neither method explicitly incorporates the influence of the frame’s stability under load, also known as the second-order effect. This is particularly relevant for portal frames, which are typically slender and lightweight structures susceptible to experiencing significant deformations under load. As a consequence, they are inherently more prone to these second-order effects.

To address this critical aspect, BS EN 1993-1-1 Clause 5.2.2 provides a comprehensive framework encompassing various methodologies for accounting for second-order effects during the design and analysis of steel structures. Furthermore, Clause 5.2 of the same standard establishes well-defined criteria to assist engineers in determining the significance of second-order effects in specific steel structures.

In the context of portal frame structures, a critical parameter known as α_cr is employed to assess the frame’s sensitivity to second-order effects. This factor represents the ratio between the structure’s elastic critical buckling load (F_cr) for global instability and the applied design load (F_Ed). When α_cr meets or exceeds a value of 10 for elastic analysis, second-order effects are generally considered negligible.

α_cr = F_cr/F_Ed ≥ 10

However. BS EN 1993-1-1 has a simple approximate method to evaluate α_cr when the roof slope is less than 26° and the axial force in the rafter is not significant. The axial force in the rafter is significant if N_Ed ≥ 0.09N_cr

The buckling amplification factor α_cr can be calculated using the relationship below;
α_cr = h/(200 × δ_NHF)

Where;
h is the height to the eaves
δ_NHF is the horizontal deflection at the eaves under a notional horizontal force applied at each eaves node, equal to 1/200 of the factored vertical base reaction.

Member Stability Analysis

Portal frames are typically constructed from open steel sections and are susceptible to a specific buckling mode known as lateral-torsional buckling. This phenomenon occurs when the member experiences combined bending and twisting deformations. To mitigate this risk, it is often necessary to incorporate various restraint mechanisms within the frame.

Both the rafters (horizontal beams) and columns (vertical supports) in a portal frame require careful evaluation to ensure they possess adequate stability against buckling. To address this challenge, three primary categories of restraints can be employed in portal frames:

Lateral Restraint: As the name suggests, lateral restraints primarily focus on preventing lateral movement of the compression flange. This is often achieved through:
- Purlins or Side Rails: These horizontal or slightly inclined members, when securely connected to the top flange of the rafter, act as a barrier against lateral movement, particularly when the compression flange is on the top side of the rafter.
Torsional Restraint: This type of restraint aims to prevent the entire member (rafter or column) from twisting about its longitudinal axis. It typically involves a combination of:
- Purlins or Side Rails: Similar to lateral restraints, purlins or side rails can contribute to torsional restraint when used in conjunction with:
  - Rafter or Column Stays: These are additional steel members that connect the flange experiencing compression (typically the top flange of a rafter) to a more stable structural element, like a column or another rafter. The stay effectively restricts the twisting motion.

Intermediate Restraints

An additional concept to consider is the use of intermediate restraints. These can also be purlins or side rails, but strategically placed between the primary torsional restraints. Their primary function is to provide lateral support to the tension flange (typically the bottom flange of a rafter) when it’s experiencing tension. This allows for increased spacing between the more robust torsional restraint systems, potentially offering a more economical design.

The choice of the most suitable restraint system depends on several factors, including:

Span length: Longer spans generally necessitate more robust restraints, like closely spaced purlins or combined torsional restraint systems.
Loading conditions: Heavier loads require stronger restraints to maintain stability.
Cost and complexity: Bracing systems with frequent purlins or closely spaced torsional restraints might be more expensive and labour-intensive compared to using intermediate restraints with wider spacing between the primary torsional restraints.
Architectural considerations: The visual impact of different restraint systems should be considered. While closely spaced purlins might be visually busy, strategically placed intermediate restraints can offer a more streamlined appearance.

New Textbook Publication on Portal Frame Design

In our commitment to spreading civil engineering knowledge, a simple textbook has been written (part of Structville webinar proceedings) to present a brief but important aspect of portal frame design. The publication contains explanations on different types of portal frames, considerations in the design of portal frames, functional components of portal frames, actions on portal frames, and a design example of portal frames using elastic analysis. The design code adopted in the publication is BS EN 1993-1-1:2005 (Eurocode 3).

In addition, anyone who purchases the publication (price is ₦2,050) receives a video tutorial on modelling of portal frames in Staad Pro for free.

The cost of the publication is ₦2,050 only. To purchase, click HERE

To purchase the full webinar materials (including videos of discussions and models) for ₦4,100 only, click HERE

Which Engineering Judgement Informed this Decision?

By

Ivy Ugorji

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October 18, 2020

Looking at the image above, it can be seen that the contractor lapped and cranked the column reinforcement to an entirely new position. In your own opinion, which engineering judgement or principle informed this decision taken by the contractor, or is it plain lack of structural engineering knowledge?

Comment your answer below with reasons and stand a chance of winning our new publication on ‘Structural Analysis and Design of Industrial Portal Frames‘. Winners will be chosen randomly. Kindly show appreciation by notifying us in the comment section when you receive yours.

To purchase this publication for ₦2,050 only, click HERE.

Application of Wind Load to Shear Walls – A Manual Approach

By

Ubani Obinna

-

February 8, 2023

The higher a building goes, the higher the effects of lateral forces such as the wind on the structure become. Shear walls are often used for providing lateral stability against destabilising actions of wind in reinforced concrete structures. Shear walls can be independently relied on to provide lateral stability in a building, or sometimes other elements such as beams, columns, and staircases can interact with shear walls to provide the required lateral stiffness to a building.

Shear-wall frame interaction for resisting lateral forces is usually complex and may involve tedious manual calculations. However, this can be made simple by using finite element analysis software. It is however easy to transfer wind load manually to shear walls without recourse to the columns. This is the concept that this article explores.

To determine the wind load transferred to shear walls in a building, the load is divided among the stabilising elements based on their stiffness (second moment of area). The stiffer elements attract a greater portion of the load. If the shear walls and other stabilising elements are symmetrically arranged (which is recommended), the centre of gravity and the shear centre (centre of stiffness) coincides and eliminate any potential torsion (twisting) in the building due to the lateral load. However, if the arrangement is unsymmetrical, there will be a twisting moment in the structure that must be properly accounted for in the design.

Let us use an example to show how this is done. Consider the general arrangement of a 7-storey building shown below;

The characteristic wind pressure coming from the y-direction to the building is 1.25 kN/m². The thickness of the shear walls and the lift core is 225 mm. Let us distribute the wind load to the stabilising elements of the building (shear walls and lift core).

The relative stiffness of the elements are as follows;

Moment of inertia
Wall 1 = Wall 2 = I_w1,_x = I_w2,_x =bh³/12 = (0.225 x 5³)/12 = 2.34 m⁴
Lift core = I_L,x = (2 x 2³)/12 – (1.55 x 1.55³)/12 – (1.0 x 0.225³)/12 = 0.851 m⁴

∑I_x = 2I_w,x + I_L,x = 2(2.34) + 0.851 = 5.531 m⁴

The ratio of wind force transferred to each wall (wall 1 and wall 2) = I_wi,x/∑I_x = 2.34/5.531 = 0.423

Therefore each wall will carry 42.3% of the wind force. This implies that about 85% of the wind force is resisted by the shear wall, while the remaining 15% is resisted by the lift core.

To calculate the shear centre of the structure, the following procedure can be followed;

We will need to determine the centre of gravity of the lift core. We can easily form a table for that;

	Area A (m²)	Lever arm (x) m	Ax (m³)
2.0 x 2.0	4	1.0	4
-1.55 x 1.55	-2.4	1.0	-2.4
-1.0 x 0.225	-0.225	1.0	-0.225
	∑A = 1.375 m²		∑Ax = 1.375 m²

Therefore the centre of gravity in the x-direction is ∑Ax/∑A = 1.0 m

Taking moment about the centreline of wall 2;

(I_w1,_x × 24) + (I_L,x × 12) = d_x∑I_x
(2.34 × 24) + (0.851 × 12) = (d_x × 5.531)

Therefore, d_x = 12 m = L/2 = 12 m

This shows that in the direction considered, the shear centre coincides with the centroid of the building, hence no torsion. Had there been significant torsion this would have been resolved into +/– forces in a couple based on the shear walls.

The force on each shear wall is therefore as follows;

w_k1 = w_k2 = 0.423 x 1.25 x 24 = 12.69 kN/m

Normally Consolidated and Overconsolidated Soils

By

Ubani Obinna

-

July 8, 2022

When a saturated clay is subjected to external pressure, the pressure is initially taken up by the water in the pores thereby leading to excess pore water pressure. If drainage is permitted in the system, a hydraulic gradient is developed and the excess water begins to flow out of the soil mass.

As water dissipates from the system, the pressure gradually gets transferred to the soil skeleton (soil grain) and they begin to rearrange and compress until the water is completely dissipated from the soil. This process continues until the entire pressure is transferred to the soil grains. This process is known as consolidation and occurs in cohesive soils only. Pore water dissipation in granular soil is immediate.

Consolidation can occur in a soil mass for a lot of reasons such as;

Application of external static load from structures
Lowering of the groundwater table
Desiccation
Self-weight of the soil (an example is recently placed fill)

Clay soil is said to be normally consolidated if the effective overburden pressure that it is currently experiencing is the maximum it has ever experienced in its history. On the other hand, it is said to be overconsolidated if the present overburden pressure is less than the effective overburden pressure it has experienced in the past.

The ratio of the maximum overburden pressure it has experienced in the past and the current overburden pressure is known as the ‘overconsolidation ratio’ (OCR). The overconsolidation ratio of a normally consolidated clay is unity, while the overconsolidation ratio of an overconsolidated soil is greater than unity.

Overconsolidation can occur in clays due to reasons such as;

Self-weight of the soil which has eroded
Weight of continental ice sheet which has melted
Desiccation of layers close to the surface

The nature of consolidation of a clay soil affects its behaviour in the field and when tested in the laboratory. For instance, the natural moisture content of a normally consolidated clay is usually close to the liquid limit, while the natural moisture content of an overconsolidated clay is usually close to the plastic limit.

Clay soils in their natural state have memory of the magnitude of the highest pressure they have experienced in the past. This memory is locked in the soil structure and can only be broken when the clay is remoulded or reconstituted at a moisture content that is equal to or greater than the liquid limit.

The stress-strain curve of an overconsolidated clay is likely to exhibit more elastic behaviour when compared with the stress-strain curve of a normally consolidated clay. Since a normally consolidated clay is experiencing the maximum pressure of its history, it is more likely to get compressed without recovery (plastic recovery) when subjected to additional external pressure in their natural state. This is not the same for overconsolidated soils which will exhibit elastic behaviour whenever an external pressure that is less than what they have experienced in the past is placed and removed.

Preconsolidation pressure can be determined from the field or in the laboratory from the e-log P curve plot (void ratio against log of pressure) after carrying out a consolidation test in the laboratory. The most widely used method was proposed by Casagrande in the year 1936 (see Figure below).

The method involves locating the maximum point of curvature in the curve and drawing a tangent and horizontal line at that point. The angle between these two lines is then bisected. The abscissa of the point of intersection of this bisector with the upward extension of the inclined straight part corresponds to the pre-consolidation pressure p_c.

Pre-consolidation pressure is the yield point that indicates the beginning of the plastic deformation during the compressive loading in a soil. The ratio of the preconsolidation pressure and the applied effective pressure is the overconsolidation ratio.

Design of Glass Swimming Pools

By

Ubani Obinna

-

December 5, 2020

The use of swimming pools is increasingly becoming an important part of the urban lifestyle. Apart from the public swimming pools in parks and other recreation areas, a private swimming pool in a home offers a lot of benefits to the homeowner, and also increases the value of the property.

Swimming pools can be constructed of different materials such as reinforced concrete, shotcrete, plastics, composites, glass, etc. In all cases, the material to be used in the construction of a swimming pool must be able to resist the water pressure, some degree of impact, and must be watertight.

Glass swimming pools have gained popularity over the years especially due to their aesthetic appeal. The use of glass is more popular in suspended or surface swimming pools for obvious reasons. For suspended pools, glass can be used as the side panels and/or the base, while for surface pools it can be used for the side panels only. Advances in glass technology have improved the engineering properties of glass to make them less brittle, with improved toughness and durability. The basic design requirements for glass swimming pools are;

Maximum transparency
Robustness
Water tightness, and
Strength

Structural glass for swimming pools is usually made of heat strengthened laminated glass, which offers shatterproof behaviour when subjected to an impact force. Multiple glass sheets laminated together are usually used in glass engineering for additional functionality in case one of the glass sheet fails.

Furthermore, different materials such as PVB (Polyvinyl butryl) or ionoplast can be used as the interlayer between different panes of glass to form laminated glass. Ionoplast interlayer, however, offers the best behaviour for glass subjected to adverse loading conditions. The safety of the people inside the pool and the people in the building should be given top priority in the design.

Structural Design of glass swimming pools

In the structural design of glass swimming pools, the thickness of the glass is determined from the anticipated hydrostatic pressure. The glass can be modelled as plate elements with simply supported boundary conditions. Glass possesses some elastic properties which allows it to regain its shape after unloading, but has no plastic properties.

This implies that fracture occurs in the material before any permanent deformation can take place. As a result, linear elastic analysis using Kirchoff’s thin plate theory is sufficient for the analysis of glass swimming pools, and the material can be idealised as a homogenous and isotropic. Failure in a glass is usually in tension because glass has good compressive strength.

Laminated glass has a modulus of elasticity several thousand times larger than the modulus of elasticity of the weakest interlayer material, PVB or ionoplast. This great mismatch makes the behaviour complex as well as analysis and modelling of laminated glass. To account for this during modelling, full relaxation of the interlayer (no cooperation between the sheets) can be assumed. This is deemed a conservative approach. Therefore, the thickness of the glass to be used in finite element modelling can be taken as;

t_m = t_pl √n

Where t_m is the thickness to be used in modelling, t_pl is the nominal thickness of each individual sheet, and n is the number of glass layers.

It is a good practice to investigate different types of support stiffness depending on the actual support conditions of the glass. Furthermore, the pool can be analysed for other conditions such as when one or more sheets are broken, and the effect evaluated.

For bottom glass panels, the self-weight of the glass and the water pressure can be applied as UDL on the surface of the glass, while for the walls, the water pressure should be applied as a triangular hydrostatic load. For suspended swimming pools, wind pressure and other anticipated forces should also be checked. After the analysis, the deflection, tensile stresses, and support reactions of the glass panes should be checked. The occurring tensile stress from the applied loading should not exceed the tensile strength capacity of the glass.

Detailing
Since every joint in a water retaining structure is a potential source of leakage, the number of joints should be minimised. The openings in the concrete floor or wall supporting the glass should be closed with a glass made of one piece. The joint between the concrete and the glass should be filled with a selected adhesive which should not react with the interlayer material. The concrete notch for the glass should be treated with primer to bind properly with the adhesive.

Application of Effective Stress Concept in the Analysis of Cohesive Soils

By

Ubani Obinna

-

October 17, 2020

The application of load on a saturated soil increases the pore water pressure. This sets up a hydraulic gradient which causes the water to flow out of the soil. As water dissipates from the soil mass, the pore water pressures gradually move toward their long-term equilibrium value. This is referred to as consolidation and it is time-dependent. The time taken for consolidation increases with decreasing stiffness and decreasing permeability. In sands and gravel, it is almost instantaneous.

Effective stress condition is used to represent the state when all the excess pore water pressure within the soil mass have dissipated. i.e. the drained state. The reverse is usually referred to as ‘total stress state’. Granular (cohesionless) soils are free-draining, therefore excess pore water pressures created during construction will dissipate so quickly that “effective stress” conditions exist in both the short and long term. Hence the effective angle of internal friction (ø′) is used for any stress analysis involving granular materials.

Clays are naturally complicated materials, and most clay deposits in the world are usually overconsolidated. The analysis of clay soils are usually idealised using;

Total stress approach (purely cohesive behaviour), or,
Effective stress approach (purely frictional behaviour)

Frictional behaviour corresponds to conditions where pore water pressures can be defined or assumed and thus allow the strength of the soil to be characterised in terms of effective stress. Cohesive behaviour refers only to the immediate short term when pore water pressures cannot be conveniently defined or assumed. Then, the strength of the soil is represented by the original undrained shear strength, c_u.

In cohesive soils, the change from total stress (undrained conditions) to effective stress (drained conditions) generally occurs over a much longer period of time. The exception being the presence/addition of fine silts/granular material which can greatly reduce the time in which effective stress conditions are reached. During this period, the strength parameters of the cohesive soil may change significantly due to pore water pressures changes induced following the construction of a retaining structure. The change in strength is caused by equalisation of negative pore water pressure in the soil and results in reduced values of cohesion c′ but increased values of angle of internal friction (ø′).

Mohr CoulombFailureCriterionintermsofeffectivestresses

Whilst all cohesive soils are subject to these changes, the effective stress condition is not usually critical when fine silts and naturally consolidated and slightly overconsolidated clays (those with cohesion values of less than about 40kN/m²), are involved, since the change from effective parameters gives an overall increase in soil strength. However, the reverse is true for over-consolidated clays, (those with undrained values in excess of about 40kN/m²). The overall strength will, in most cases, be reduced as the stress condition changes from total to effective because the loss of substantial cohesive strength is not compensated adequately by the increasing angle of internal friction. Hence it is advised that for cohesive soils both short and long-term stress analyses be carried out to determine the more onerous design case.

Typically, long-term calculations are carried out using effective stress parameters – drained analysis. Short-term calculations use total stress parameters – undrained analysis. Traditionally, many generalist engineers have used the cohesion model and total stress procedures. However, any tendency for the pore water to drain will lead to changes in voids ratio and hence the value of undrained cohesion. Voids in soil are not fixed in place; water can move and voids can collapse or expand. Therefore undrained cohesion values are difficult to predict with certainty. In the long term, clays behave as granular soils exhibiting friction and dilation. For this reason, calculations to predict long-term values of pressures are undertaken using the effective stress method.

When using the effective stress method the friction and cohesion can be characterised by the two parameters angle of shearing resistance, φ’ and cohesion intercept in terms of effective stress, c’ (rather than peak angle of shearing resistance, φ’_max). In the absence of test data, BS 8002 recommends values of φ’ that can be used with c’ = 0, and these are given in the Table below.

Plasticity index (%)	15	30	50	80
φ’ (degrees)	30	25	20	15

Engineered Wood Products and their Applications in Structural Engineering

By

Ubani Obinna

-

October 17, 2020

A wide range of derived wood products are obtained by binding and fixing of wood particles, boards, veneers, fibres, or strands with adhesives or other mechanical means to form composites. The reconstituted wood products derived using such means are generally referred to as engineered wood products (EWPs). There are different types of engineered wood products such as plywood, particleboard, laminated timber, finger joints, cross-laminated timber, etc. In the field of civil engineering, EWPs have been used in construction for over 40 years. Glued laminated timber (glulam), connected plated trusses, finger joints, plywood, mechanically and adhesive bonded web beams, etc have all found useful applications in structural engineering for a long period of time.

finger jointed timber — Timber Finger Joint *(Source: http://www.structuraltimber.co.uk/)*

Engineered wood products are increasingly becoming important in the field of structural engineering. Over the last decade, high rise timber structures have been constructed in different parts of the world as a result of improved engineered wood products. Moreover, timber has been identified as the most environmentally friendly construction material when compared with steel and concrete. According to Structural Timber Engineering Bulletin, there have been significant developments in the range of EWPs for structural applications with materials such as laminated veneer lumber (LVL), parallel strand lumber (PSL), laminated strand lumber (LSL), prefabricated I-beams, metal web joists and ‘massive’ or cross-laminated timber (CLT) becoming more widely available.

open web joists — Open Web Joist *(Source: http://www.structuraltimber.co.uk/)*

According to Tupenaite et al (2019), the most popular engineered timber products used in high-rise timber buildings are produced based on laminating and gluing. The examples of such products are;

Glued laminated timber (glulam)
Cross laminated timber (CLT)
Laminated veneer lumber (LVL)

Glued Laminated Timber (glulam)
Glued laminated timber (glulam) is a structural member made by gluing together a number of graded timber laminations with their grain parallel to the longitudinal axis of the section. Glulam is the oldest glued structural product (over 100 years). It is generally composed of lumber layers (2×3 to 2×12), planed and pre-finger-jointed, and then bonded together with moisture- resistant structural adhesives longitudinally. Members can be straight or curved, horizontally or vertically laminated and can be used to create a variety of structural forms (see figure below).

Laminations are typically 25mm or 45mm thick but smaller laminations may be necessary where tightly curved or vertically laminated sections are required. The requirements for the manufacture of glulam are contained in product standard BS EN 14080: 2013.

Glulam is used in large structural elements such as portal frames, bridges, beams, columns, trusses etc.

glulam in construction — Glulam in Construction

Cross Laminated Timber (CLT)
Cross laminated timber (CLT) is a structural timber product with a minimum of three cross-bonded layers of timber, of thickness 6mm to 45mm, strength graded to BS EN 14081-1:2005 and glued together in a press which applies pressure over the entire surface area of the panel. CLT was developed around 15 years ago in Central Europe and is made up of a solid engineered wood panel, made up of cross angled timber boards which are glued together.

CLT panels typically have an odd number of layers (3,5,7,9) which may be of differing thicknesses but which are arranged symmetrically around the middle layer with adjacent layers having their grain direction at right angles to one another .

The structural benefits of CLT over conventional softwood wall framing and joisted floor constructions, include:

• large axial and flexural load-bearing capacity when used as a wall or slab
• high in-plane shear strength when used as a shear wall
• fire resistance characteristics for exposed applications
• superior acoustic properties

CLT can be used in floor slabs, roofs, beams, columns, load bearing walls, and shear walls. Length up to 20m can be pproduced, with thickness ranging from 50 – 300 mm. Width of up to 4800 mm can be achieved with CLT.

clt wall in construction — CLT walls in building construction

clt beams and columns 1 — CLT beams and columns in a building

Laminated veneer lumber (LVL)
Laminated veneer lumber (LVL) is a structural member manufactured by bonding together thin vertical softwood veneers with their grain parallel to the longitudinal axis of the section, under heat and pressure. In some cases cross grain veneers are incorporated to improve dimensional stability. LVL is a type of structural composite lumber. Due to its composite nature, it is much less likely than conventional lumber to warp, twist, bow, or shrink, and has higher allowable stress compared to glulam.

LVL is often used for high load applications to resist either flexural or axial loads or a combination of both. It can provide both panels and beam/column elements. It can be used for beams, walls, other structures and forming of edges.

lvl frame — LVL framing for a building in New Zealand

The requirements for LVL are contained in product standard BS EN 14374:2004

High Rise Timber Building to be Constructed in Tokyo

By

Ubani Obinna

-

October 17, 2020

Mitsui Fudosan and Takenaka Corporation has commenced plans to develop a 70 metres high timber building in Tokyo’s Nihonbashi district. The 17 storey building when completed will be among the tallest timber buildings in Japan.

The interest in multi-storey timber buildings has increased around the world. Timber building materials cause considerably lower climate change impact compared to materials like steel and concrete. Moreover, modern engineered timber products provides opportunities to build high. In the last decade, 6 storeys timber buildings and higher have been constructed around the world, and engineers have begun to look at the possibility of building much taller with timber.

Construction of the timber high rise building in Tokyo is tentatively scheduled to start in the year 2023 with a possible completion date of 2025. The building’s floor area would be 26,000 square meters and will be constructed from 1,000 cubic meters of domestic lumber. The main structure would be a hybrid that incorporates Takenaka’s fire-resistant laminated wood with materials sourced from Mitsui-owned forestry in Hokkaido. Carbon dioxide emissions would be 20 percent less than those when construction a standard steel-frame office building of the same size and scale.

Mitsui Fudosan Group owns approximately 5,000 hectares of forestry in Hokkaido, all of which has been certified by the Sustainable Green Ecosystem Council.

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