For a building to be useful, the structural integrity and building services such as plumbing, electrical fittings, HVAC, etc must be functional and efficient. Unfortunately, it is common to see MEP services and pipes clash with structural members such as beams and columns in buildings. However, this can be prevented or handled in the design stage through the use of Building Information Modelling (BIM). One of the commonest solutions to avoid the clash of columns and water closet (WC) waste pipes is through the use of vanity walls behind the WC.
Traditionally, vanities are fittings that are provided in rooms or water closets to aid storage, provide counter top and improve aesthetics. They can be wall mounted or supported on the floor, but they are typically placed next to the wall of a building, hence the name vanity walls.
Vanity around a sink in a bathroom
In our own context, the aim of the vanity is to conceal the waste pipe of the WC, and to also avoid clashing of the waste pipe with the columns of the building. This is usually dependent on how the architect positioned the WC. Some contractors observe the very bad practice of chiseling structural members to allow pipes and services pass through. This is completely unacceptable and should be discouraged.
Some modern designs of WC require vanity walls by default (to accommodate the water cistern), but in some cases vanities can be constructed using properly finished sandcrete walls to provide an alternative route for waste pipes, especially when the position of the WC is not very favourable. To be favourable to plumbers, WC’s are usually positioned to back an external wall. However, when a WC is backing an internal wall, the waste pipe may clash with some sensitive structural members, and vanities may be needed. The height of the vanity can be properly raised to accommodate the flush cistern, and the top can also provide some storage as shown in the image below.
Vanity accommodating WC waste pipe, cistern tank, and providing counter top storage
We are going to use an example to show how vanities can be used to avoid waste pipe conflict with columns.
Original drawing showing route of waste pipes
In the floor plan shown above, a WC is provided adjacent to an internal wall, and the location of the duct is as shown in the plan layout. A little observation of the preliminary drawing shows that the route of the discharge pipe (in green lines) is conflicting with the column close to the duct (all columns are highlighted in red). To solve this problem, a vanity wall can be provided adjacent to the main wall as shown below. The new route of the waste pipes as they pass through non-structural members directly to the duct can also be observed in the new layout.
Introduction of vanity walls as an alternative route for waste pipe
The only disadvantage of this method is that the size of the toilet will be reduced by size of the vanity wall introduced, but it is usually a viable solution.
Toilet with vanity walls
Building construction requires good technical coordination among all the consultants at the design stage. By so doing, all potential conflicts and difficulties that can be encountered during construction will be minimised.
The civil engineering industry is wide and unique. There are different fields in civil engineering such as structures, highway and transportation, water resources, environmental, and geotechnical engineering. Each of these fields appear unique in their own way, but one common thing among them is that there will always be design, and then execution (construction). Therefore, I will describe the two aspects of civil engineering as design and construction. Having experienced both faces of the industry, I can confidently talk about the characteristics of each aspect.
A design engineer deals with lines, shapes, figures, numbers, calculations, regulations, and codes of practice. This is usually done on a paper or on a computer. The final output of a design engineer is a construction drawing or model which is taken to site for execution. Therefore, a design engineer must be technically skilled with good analytical capacity. He should be computer literate, and good with numbers. A knowledge of the requirements of the building code of practice is a must, and he should have the basic ones on his fingertips without consulting any literature. His knowledge in his area of expertise should be vast.
For instance, a structural engineering consultant should be able to talk about the stresses he is expecting in any part of a structure, and the acceptable disposition of reinforcements. A geotechnical engineer on looking at soil report and diameter of a pile should be able to say something about the expected range of safe working load of the pile. A water resources and environmental engineer should be able to stand on his feet and talk about effluent water treatment design of an industry and so many more.
These people are the consultants, and they do not do the dirty job in civil engineering. Since they are mainly office guys, they should always dress smart, and be able to communicate eloquently. Whenever they visit a construction site, it is usually for inspection, supervision, sign-off, or site meetings. Anybody who is not intellectually smart has no business being in consultancy. This is because they are the people that the man on site will call once there is any challenge, and he should be able to say something meaningful while standing on his feet. Construction projects are usually time-bound, and there should be no room for delay from any stakeholder. If the challenge on site is a major one, he should be able to issue the right instruction and then provide the solution in the shortest possible time. This is because he made the design.
The expected qualities and characteristics of a civil engineering consultant are as follows;
Good communication skills (written and oral)
Strong analytical skills
Sound theoretical knowledge of civil engineering
Good knowledge of latest industry codes and standards
Good computer skills
Ability to make good engineering judgement
Ability to provide technical leadership
Confidence and integrity
Civil engineers who will end up in the design office as consultants usually have a minimum CGPA of 3.5 on a 5.0 scale. This does not mean that people will lower class of degree cannot be consultants if they develop themselves. They are usually mathematically sound, and find topics in structural engineering interesting. They can be software enthusiast, always looking out for the latest engineering software that will make their work easier. Typical consultants may have a hard time adjusting to full time site conditions, especially if they have not had a prior site experience. Furthermore, they can be distracted by designs and other unrelated activities when given a site to manage.
A contractor takes over a site after receiving the approved drawings and contractual terms to go ahead and build what the consultant has designed. A site manager deals with a lot of issues that are wider than what a design engineer faces. He is interested in understanding ALL the technicalities in the drawings handed over to him. Building construction drawings comprise of architectural drawings, structural drawings, mechanical drawings, and electrical drawings. He is also expected to have construction methodology document and standard safety policy documents. As a matter of fact, all the ‘madness’ experienced by people in different professions is experienced by the man on site.
A site manager is always dealing with workers, sub-contractors, logistics, quality control, safety, time, technical accuracy, and general management. While technical ability is the key characteristic of a design engineer, managing people is the key characteristic of a site manager. This is because his success or failure depends on people.
As a result of this, people from other related professions such as architecture, building, and quantity surveying can function satisfactorily as site managers provided they follow instructions. The fundamental ability needed by a site manager is the capacity to read and understand construction drawings properly. Once he can do that satisfactorily, the rest has to do with his leadership and managerial skills. A site engineer is expected to meet his targets, and must always have a working plan of achieving them.
The expected qualities of a site manager are as follows;
Integrity and resolve to always do the right thing and not cut corners
Good technical ability
Resources management
Good communications skills (written and oral)
Being proactive
Team player
Ability to think ahead
Good organisation and planning skills
Positive vibes and energy
Possession of relevant construction skills and use of common site equipment/tools
The highest technical skill required of a site manager is the ability to produce as accurately as possible, what the consultant has designed. With experience, he can however make suggestions on how the construction can be made better. This however is a function of training. A site manager with background in building or architecture will be more natural with the shell of a structure such as masonry works and finishes, while someone with structural engineering background will be more natural with the core of the structure such as reinforcements, concrete mixes, and formwork. What an architect will call the structural engineering consultant for might not bother someone with civil engineering background.
A site manager must always have an open communication line, and makes a lot of phone calls in a typical day at work. He is meant to coordinate the activities of sub-contractors such as electricians, plumbers, HVAC contractors, carpenters, interior decorators, etc. He is also meant to coordinate logistics and delivery of materials to site. During site meetings and inspections, he is supposed to lead the team around the site, and offer report on the progress of work, his targets, and the challenges they are facing. Typically, a good site manager should see challenges ahead of time and work against them from the onset. However, with the dynamics of a typical construction site, whenever an unexpected challenge comes up during construction, he should rise to the challenge and solve it immediately.
With all these, it is therefore obvious that site managers do not work alone. It is basically about people, and he is meant to offer leadership and direction at all levels. A site manager may not necessarily be a high flying academic guru. The key thing in hiring a site manager should be the person’s passion for construction, and his basic training.
A typical site manager doesn’t like to do designs, and will prefer to work with someone else’s drawing. Making designs during an ongoing project is a distraction for a site manager. A site engineer should be focused on the project a hand, and typically thinks about the project and makes plans for the following day before sleeping. His first activity in the morning could be calling a sub-contractor or supplier to know if he is already on his way to site.
Therefore, recruiters should watch out for the passion of the person they are hiring. Being passionate for either design or construction does not make you less an engineer. Do not fall for the stereotype that you should excel in both; it is not necessarily the true. Design and construction are two special but closely related aspects of engineering. From experience however, people with site experience tend to make better and more efficient design than people without site experience.
Find your passion, monetize it properly, work hard, and live your dream life. God bless civil engineers all over the world.
A lot of engineers and builders from all over the world have reacted to the weird partial collapse of a concrete slab in a residential building. The failure was shown in a picture that has been circulating across several civil engineering social media platforms since this week.
In the picture, some section of a concrete slab of an apparently live bedroom spalled off and crashed to the floor of the room in what could have been a life threatening situation. The failure showed no reinforcements, and evidence from the rubble suggested that no reinforcement was provided in the zone that gave way. While half of the section collapsed, the remaining section stayed in place, leaving engineers to wonder what could have led to that pattern of failure.
On reacting to the failure, some people said that it is due to poor concrete mixture.
Poor concrete mixture
Segregation caused by poor materials and Inadequate proper mixture of concrete
Some others attributed the failure to lack of reinforcement.
I can’t see any reinforcement
Is there bottom reinforcement there?
Another person suggested that the ceiling fan could have been part of the cause of the failure. According to him;
If we critically look at the image, I will say the failure is as a result of weak thick sand-mortar which must have cracked due to vibration force generated from the ceiling fan. I am seeing mortar of up to 100 mm thick without mesh reinforcement. I will recommend that the ceiling fan should be removed.
Someone else attributed it to excess concrete cover as he said;
This is critical the reinforcement cover seems to be 5 inches thick
While there may not be enough data to make proper conclusion, what is your opinion on the cause of the failure?
The use of arches as a structural form can be dated back to antiquity. Arch structures are unique structural forms which resists forces majorly by converting them to compressive forces, in a process popularly referred to as arch action. By transferring the compressive forces through the arch rib or barrels, they are transferred to the base of the arch as outward thrusts, which implies that the final support of the arch must be stiff and stable. According to [1], the economic viability of any arch bridge depends on the suitability of the site’s geology and soil condition. This article aims to explore the analysis and design of a concrete arch bridges subjected to Load Model 1 of Eurocode, using Staad Pro software.
Historically, most arch bridges were associated with stone masonry, which later gave way to the use of bricks in the nineteenth century. These structures were designed to minimise the development of tensile stresses in the members, and hence often gave rise to very massive structures. However, with advances in materials such as concrete and steel, more slender and aesthetically pleasing structural forms can be achieved.
Masonry Arch Bridge
Arch bridges can be classified according to the following;
Materials of construction
Structural scheme, and
Shape of arch
As far as materials is concerned, arch bridges can be constructed from timber, stone masonry, bricks, concrete, or steel. However, in recent times, timber bridges are usually restricted to small spans. An example of a timber arch bridge is the Mur River Bridge in Austria (also called the Holzeuropabrücke wood bridge) which is made of three-hinged parabolic timber. It is considered to be the largest cantilevered timber bridge in Europe. Also, the Eagle River Timber Bridge in Michigan is another example of three-hinged arch timber bridge with a span of about 23-24 metres. The Tynset Bridge in Norway built in the year 2001 is considered the longest timber bridge in the world designed for full highway loading with a span of 70 m. The structural form consists of tied timber truss arches supporting the bridge deck with the use of suspension cables.
Eagle River Timber Bridge, Michigan
Mur River Bridge (Holzeuropabrücke wood bridge) in Austria
Reinforced concrete and steel arches are altogether much lighter structures than masonry arch bridges. The structure consists basically of the arch, the deck and usually some supports from the arch to the deck – in that order of importance. The basic parts of an arch bridge are;
The deck
The crown
The spandrel
The arch rib or barrel
The springings
The extrados or back
The intrados or soffit
The skewback or abutment
The rise, and
The span
These components are shown in the figure below;
Components of an arch bridge [1]
Concrete arches can be made of full width curved arch, or series of ribs. Steel is usually made of series of ribs.
Full width curved arch bridge under construction
Concrete arch bridge with two ribs
Steel arch bridges can be lower arch bridge or through trussed arch bridge.
Lower steel trussed arch bridge
Through-trussed steel arch bridge
The structural scheme adopted in any arch bridge can be influenced by a lot of factors such as the type of deck, environmental conditions, cost, and feasibility. However in terms of structural form, arches can be broadly classified as hinged or fixed. A hinged arch can be two-hinged arch or three-hinged arch. While the former is statically indeterminate, the later is statically determinate. Statically determinate arch structures are free from secondary stresses from indirect actions such as differential settlement and temperature difference. An example of a three-hinged arch bridge is the Rossgraben Bridge in Switzerland.
In terms of shape, an arch bridge can be segmental (circular), parabolic, or elliptical. However, the parabolic arch is the most popular shape for arch bridges. According to [1], the ratio of span to rise should generally be in the range of 2:1 to 10:1. The flatter the arch the greater the horizontal thrust and this may affect the structural form selected, i.e. whether or not a tie should be introduced, or the stiffness of the deck relative to the arch.
Analysis of an arch bridge on Staad Pro
It is possible to model and analyse arch bridges using Staad Pro software. We are going to demonstrate this using a parabolic arch bridge. The general structural form of the bridge is shown below. The structural form can be said to be a bit similar to that of Krka River Bridge in Croatia.
Schematic of the arch bridge
The equation of the parabolic arch bridge is given by;
y = 4x/5 – x2/50 ———– (1)
Using equation (1), the nodes for the vertical coordinates of the arch were established at 1m interval along the horizontal axis, and connected using linear line elements.
The arch is made of two ribs connected to each other with rigid reinforced concrete members along the axis of the arch and at the deck level. The arch ribs are made of concrete members 1500 mm deep, and 750 mm wide. The spandrels are made of concrete columns of dimensions 600 mm x 600 mm transferring the load of the bridge deck to the arch. The rendered structural form of the bridge deck is shown below;
Structural scheme of the arch bridge
The longitudinal girders of the bridge deck are 5 in number with dimensions of 1000 mm x 400 mm, spaced at 2 m centre to centre. They are supported by transverse girders of the same dimension, which transfer the deck load to the columns. The columns (spandrels) ultimately transfer the deck load to the arch ribs. The deck slab is 200 mm thick and has a total width of 10.4 m. The vehicle carriage way is 8.0 m wide, with 1.2m cantilever on either side with raised kerbs for pedestrian walkway. The overall dimensions of the arch bridge is shown below.
Dimensions of the arch bridge
The bridge deck has been subjected to Load Model 1 on 2 notional lanes, and a remaining area of 2 m. The tandem load system on the bridge was modelled as a moving load on Staad Pro. Hence, the loads considered on the arch bridge are the self weight, UDL traffic action, and wheel load traffic action.
The analysis results are as follows;
(1) Self weight
Support reactions under self weight
Bending moment under self weight
Maximum sagging moment in arch rib = 259 kNm Maximum hogging moment in arch rib = 901 kNm Maximum Bending moment in column (spandrel) = 138 kNm (4th column from the left)
Shear force under self weight
Maximum shear force in arch rib = 359 kN Maximum shear force in column (spandrel) = 101 kN (4th column from the left)
Axial force under self weight
Maximum axial force in arch rib = 3276 kN (compression) Maximum axial force in column (spandrel) = 501 kN (1st column from the left)
(2) Traffic UDL
Bending moment under UDL traffic action
Maximum sagging moment in arch rib = 82 kNm Maximum hogging moment in arch rib = 222 kNm Maximum Bending moment in column (spandrel) = 39.7 kNm (4th column from the left)
Shear force under UDL traffic action
Maximum shear force in arch rib = 93 kN Maximum shear force in column (spandrel) = 28.8 kN (4th column from the left)
Axial force diagram under UDL traffic action
Maximum axial force in arch rib = 855 kN (compression) Maximum axial force in column (spandrel) = 160 kN (1st column from the left)
(3) Traffic Wheel Load
The variation of bending moment as wheel load travels through the bridge deck is shown below;
Variation of bending moment as wheel load travels through bridge deckVariation of bending moment as wheel load travels through bridge deckVariation of bending moment as wheel load travels through bridge deck
For moving traffic action; Maximum hogging moment = 1593.285 kNm Maximum sagging moment = 900.343 kNm Maximum shear force = 646.233 kN Maximum axial compression = 1414 kN Maximum axial tension = 804.699 kN
There are other forces such as torsion that should be checked in the analysis result. For design purposes, the self weight and traffic actions can be combined using 1.35Gk + 1.5Qk. Other actions on bridges should also be considered.
Thank you for visiting Structville today, and God bless you. Remember to contact us for your structural designs, detailing, and project management and training. You can send an e-mail to ubani@structville.com or a whatsapp message to +2347053638996.
References Melbourne C. (2008): Design of Arch Bridges. In ICE Manual of Bridge Design, Institution of Civil Engineers, UK
Stays can be used to support orthotropic bridge decks which consists of continuous girders. The stays which are inclined cables passing over or pinned to the piers are attached to the girders and forms part of the supporting system of the bridge deck. Wide application of cable stayed bridges have been achieved recently due to the development of high strength steel, improved bridge deck systems, and analytical tools/software. In this article, we intend to explore the applicability of Staad Pro software (v8i) in the non-linear analysis of cable-stayed bridges under moving traffic load. This will be evaluated using a simple bridge deck model.
In cable-stayed bridges, the tension members (inclined cables) should be able to develop high degree of stiffness due to prestress under the dead load of the brdige deck, with additional capacity to take loads from the live load. In the example adopted in this article, the radial or converging cable stayed system was adopted, which is deemed the most efficient arrangement due to its ability to carry the maximum component of the dead and live load, while keeping the axial load in the bridge deck to a minimum. An example of this arrangement is the Ikoyi-Lekki link bridge in Lagos, which is the first cable-stayed bridge in West Africa.
Ikoy-Lekki Link Bridge, Lagos, Nigeria
In Staad Pro, the simplified arrangement adopted is shown below.
Model of a cable stayed bridge on Staad Pro
The properties of the sections used in the analysis are as follows; Pylons – Concrete (1000 x 500)mm Main girders – Steel (UB 762 x 267 x 147) Cross girders – Steel (UB 457 x 152 x 60) Stays – 50 mm diameter steel cables
The pylons were placed on a fixed support, while the two ends of the bridge deck were placed on pinned and roller support respectively. The geometry and dimensions of the bridge components are shown below;
Dimensions of the bridge
The load cases that were considered in the model are the self weight of the bridge and moving traffic wheel load. Since the deck slab was not modelled, it was not accounted for in the self weight of the bridge. Also, the UDL components of traffic action and full wheel load recommendations according to EN 1991-2 were not considered. Therefore, the results from this analysis are only representative of the assumptions made, and may not fully reflect the behaviour of cable-stayed bridges.
A tandem load of 4 wheels (300 kN each) with a width of 3 m and longitudinal spacing of 1.2 m was used in the model, and applied at the centre of the bridge.
Definition of vehicle wheel load on Staad Pro
Linear Analysis Results
When analysed on Staad Pro using linear elastic analysis, the bending moment, shear forces, axial force, and displacements were obtained due to the self weight and the moving load.
The variation of the bending moment as the wheel load travels on the bridge deck is shown in the figures below;
The deflection and bending moment of the structure under self weight is shown below. Note that this linear analysis was carried out under zero initial tension in the cable.
Deflection of the bridge under self weight – Linear Analysis
Bending moment of the bridge under self weight – Linear Analysis
The tension in the cables are; Cable 1 (closest to the piers) = 19 kN Cable 2 (intermediate) = 22.3 kN Cable 3 (farthest from the pier) = 11.2 kN
Non-Linear Cable Analysis
A preliminary attempt to run a non-linear cable analysis for moving load using Staad Pro was not successful. However, a non-linear analysis result was obtained for the self weight of the bridge members. The non-linear cable analysis command used is shown below. You can click HERE to see the definition of the terms. An initial tension force of 0.5 kN was applied to the cables.
Non-linear cable analysis command on Staad Pro
Using the parameters above, the self weight load case converged 100% with zero errors and zero warnings. The non-linear displacement of the structure under self-weight is shown below.
Deflection of the bridge under self weight – Non-Linear Analysis
The value of deflection was observed to be higher than the deflection obtained from linear analysis. The tension in the cables are shown below, but were observed to be lesser than the result obtained from static linear analysis.
Cable 1 (closest to the piers) = 17.3 kN Cable 2 (intermediate) = 16.5 kN Cable 3 (farthest from the pier) = 7.7 kN
The maximum moment under the self weight for non-linear analysis is shown below;
Bending moment of the bridge under self weight – Non-Linear Analysis
Further analysis showed that the result of non-linear cable analysis is heavily influenced by ‘Sag Minimum’. The closer ‘Sag Minimum’ is to 1.0, the closer the non-linear result is to linear analysis and vice versa. ‘Sag minimum’ is a factor used to account for sagging in the cable when the tension is low. This is achieved is Staad Pro by modification of the modulus of elasticity of the cable. However, if the value is too low, the analysis will not converge properly.
Since the non-linear cable analysis of the structure was not successful for moving load, I recommend you run linear analysis of the moving load and obtain the critical load locations. Later, you can apply the loads statically at the critical location, and run the non-linear cable analysis under a single load case. A example of this process is shown below.
Application of wheel load statically for non-linear cable analysis
The wheel load shown in the figure above has been applied statically, and the load case was combined with the self weight of the structure. Using the same non-linear parameters used above, the analysis results are as follows;
Deflection of the bridge under self weight and wheel load – Non-Linear Analysis
Bending moment of the bridge under self weight and wheel load – Non-Linear Analysis
The tension in the cables at the left hand side of the piers are; Cable 1 (closest to the piers) = 259 kN Cable 2 (intermediate) = 185 kN Cable 3 (farthest from the pier) = 0
The tension in the cables at the right hand side of the piers are; Cable 1 (closest to the piers) = 14.8 kN Cable 2 (intermediate) = 76 kN Cable 3 (farthest from the pier) = 195 kN
A better option for handling this can as well be recommended.
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The International Association of Bridge and Structural Engineering (IABSE) has announced that the IABSE Congress Ghent 2021 will be held from 22 to 24 September 2021. The International Association of Bridge and Structural Engineering (IABSE) is a scientific / technical association with members including renowned and top-level engineers in more than 100 countries. The upcoming Congress will be held at International Convention Centre (ICC) Ghent, and is organised by Belgian and Dutch Groups of IABSE in co-operation with Ghent University. The theme of the Congress is ‘Structural Engineering for Future Societal Needs‘.
Download the preliminary invitation card and call for abstracts HERE
According to the information on IABSE website, the Congress will be an excellent forum to discuss the latest innovations on structural and bridge engineering, especially regarding the future needs of society. Scientists, students, designers, contractors, owners and experts from international organisations around the world are given the opportunity to share their latest experiences. In addition to the congress, plenty of technical and social events are organised in order to expand your knowledge and network. All of this is organised with the historical city centre of Ghent as the background.
Based on the theme, the scientific committee of the congress stated that future societal needs comprise building and maintaining safe and reliable buildings and infrastructures while coping with the effects of climate change in a world with scarcer resources and satisfying the ambition to reduce mankind’s CO2 footprint. Anticipated sub-themes are therefore amongst others ‘Structural safety and reliability with respect to climate change’ and ‘Circularity, re-use and sustainability of structures’.
Important Dates for the congress are as follows;
February 28, 2020 September 15, 2020 November 30, 2020 January 21, 2021 March 1, 2021 April 30, 2021
June 15, 2021 June 15, 2021 September 20-21, 2021 September 22-24, 2021
Call for abstracts Deadline for abstract submission Notification of acceptance abstract Submission deadline for full papers Final Invitation and registration opens Notification of acceptance of full papers and announcement of presentation types Registration deadline for all presenting authors Deadline for early-bird registration IABSE Annual Meetings IABSE Congress Ghent 2021
Anticipated sub-themes amongst others are:
• Structural safety and reliability with respect to climate change • Circularity, re-use and sustainability of structures • Emission free building of structures
Special sub-themes are:
Enhancing resilience of civil infrastructure to hurricane and thunderstorm hazards under changing climate
Structural bearings and anti-seismic devices: innovation, standards and testing requirements
Towards extending the service life of existing concrete infrastructure through advanced assessment methods
However, also more traditional sub-themes may be addressed, if the abstract and paper refer to the main conference theme, e.g.: • All types of bridges • Large span structures • Light-weight structures • High-rise buildings • All structural materials • Structural health monitoring • Design for earthquakes • Case studies • Failures and forensic engineering • Strengthening and retrofitting • Dynamics of structures • Innovative structures • Fatigue and fracture • Structural analysis and optimisation • Parametric design • Structural behaviour under fire conditions • Soil-structure Interaction • Exceptional loads on structures • Safety, reliability and risk • Architecture of structures • Additive manufacturing
According to the scientific committee, all written papers submitted will be peer-reviewed. The large majority of the presentations will be delivered orally in normal presentation sessions but also poster sessions, poster ‘elevator pitch’ sessions, discussion sessions, a Pecha Kucha session and special sessions will take place. In order to accommodate all sessions and speakers, parallel sessions run simultaneously in 6 to 8 breakout rooms on the three main congress days. Keynote speakers will introduce relevant topics to the congress theme and/or give a state-of-the-art overview on these topics. The technical programme will attract all those involved and interested in the state of the art and the future of bridge and structural engineering. You are hereby invited to contribute to this IABSE Congress and join IABSE community in Ghent in 2021!
Disclaimer: www.structville.com is neither an agent, staff, nor representative of IABSE. This information obtained from IABSE website has been shared here as news in our commitment to spread qualitative information that will beneficial to the civil engineering community for their general development and career enhancement. Therefore all further inquiries regarding this congress should be directed to IABSE official website.
Deflection is one of the most important serviceability limit state criteria in the design of reinforced concrete structures. Excessive deflection in a structure can lead to cracking of finishes, impaired functionality, and unsightly appearance. Different codes of practice have provisions for controlling deflection in concrete structures. In BS 8110-1:1997 and EN 1992-1-1 (Eurocode 2), deflection can be assessed using the simplified span/depth ratio (deemed-to-satisfy rules) or by following the rigorous method (direct calculation of deflection). However, rigorous method is lengthy and has numerous uncertainties involved in the calculation, hence the span/depth ratio approach is widely used. In both codes of practice, compression reinforcement has been shown to be beneficial to the deflection response of reinforced concrete structures.
Concrete is good in compression but weak in tension. As a result, steel reinforcement is usually provided in the tension zone of concrete structures to resist tensile stresses. Sometimes, steel is provided in the compression zone to assist concrete in resisting compression, and in this case, is normally referred to as doubly reinforced section. The major reasons for providing compression reinforcement in concrete structures are;
to reduce long-term deflection due to sustained loading,
to increase ductility,
to changing failure mode from compression-initiated to tension-initiated, and for
ease in construction (for example using hanger bars to provide support for stirrups).
However, it has been recognised that deflection in a concrete beam and slab increases with time for up to nine years[1]. The long-term deflection of a reinforced concrete member under sustained load is mainly due to creep and shrinkage, and it is usually larger than the immediate deflection[2]. It is important to realise that creep and shrinkage are time-dependent properties of concrete, which are influenced by environmental conditions and the concrete mix design[3]. While creep depends on the sustained load, shrinkage is independent of the load. Other important concrete properties used in assessing level of deflection are the modulus of elasticity, modulus of rupture, and tensile strength properties.
In normal strength concrete, compression reinforcement helps in reducing creep under sustained load in the compression zone, and this helps in reducing the final deflection of the structure. However, research has suggested that the effect of compression reinforcement may be minimal in reducing deflection in high strength concrete structures due to low creep[4]. Studies have shown that the effect of compression reinforcement in reducing long term deflection is dependent on the compressive strength of the concrete[4]. However, compression reinforcement is rarely used for the purpose of resisting deflection in concrete structures.
When the props and formwork are removed from the soffit of a slab, it undergoes immediate elastic deflection if the load is low. If the load is high, there will be cracks in the tension zone (say mid-span), and the deflection may not be entirely elastic[1]. However in the long term, the initial deflection under sustained load will increase because of shrinkage, creep, and increased cracking (note that cracking is deemed normal in concrete members).
Concrete drying shrinkage will cause warping of the slab in zones where there are unequal areas of top and bottom reinforcement. This is typical in the mid-span of a reinforced concrete slab, hence the slab will warp downwards and adds to the long-term deflection. The magnitude of warping is found to be directly proportional to the drying shrinkage of the concrete. It continues at a decreasing rate with increasing time for several years after pouring, and is independent of load. Therefore, provision of top reinforcements can curtail the effect of warping, thereby reducing the final deflection.
Furthermore, it is important to note that the actual long-term deflection of a normally reinforced slab or beam in the typical floor of a multi-level building depends on some other factors which may not be duly paid attention to during design. Some of these factors are materials supply, construction techniques, loading history, weather and time. Loading history and method of construction is known to affect the deflection of slabs. The construction load coming from the floor above have been shown to significantly influence the deflection behaviour of slabs in a multi-story building construction[5].
Assessment of deflection in BS 8110
BS 8110-2:1985 says that the sag in a concrete member will become noticeable if the deflection exceeds span/250. According to clause 3.4.6.1 of BS 8110, deflections in a structure may be calculated and then compared with the serviceability requirements given in BS 8110-2:1985. However, in all normal cases the deflections of a beam will not be excessive if the ratio of its span to its effective depth is not greater than the appropriate ratio. The basic span to effective depth ratio is given in Table 3.9 of BS 8110-1:1997 with values of 7 (cantilever), 20 (simply supported), and 26 (continuous members). The basic span/effective depth ratio should be multiplied by modification factors given in in Tables 3.10 and 3.11 of the code. BS 8110-1:1997 acknowledges that compression reinforcement influences deflection.
The modification factor for tension reinforcement of reinforced concrete is given in Table 3.10 of BS 8110-1;
Where fsis the service stress in the member and it is given by;
fs = 2fyAsreq/3Asprov
Where; Asprov = Area of steel provided Asreq = Areas of steel required fy = yield strength of reinforcement M = Applied bending moment at ultimate limit state
The modification factor for compression reinforcement is given in Table 3.11 of BS 8110-1 as;
Where As’prov is the area of compression steel provided.
In BS 8110-1, the area of compression reinforcement used for the calculation may include all bars in the compression zone, even those not effectively tied with links.
Clause 7.4.1(4) of EC2 says that the utility and appearance of a structure may be impaired when the deflection exceeds span/250. This limit accounts for both long term and short term deflection. Eurocode 2 follows a slightly different approach in assessing the deflection of a reinforced concrete member using the span/depth ratio approach.
EC 2 allows span/effective depth limits (L/d)to be calculated from equations 7.16a, 7.16b and 7.17 of the code:
L/d = K[11+(1.5√fck x ρ0/ρ) + 3.2√fck(ρ0/ρ – 1)1.5] if ρ0 ≤ ρ (7.16a)
L/d = K(11+(1.5√fck x ρ0/(ρ – ρ’)) + √fck √(ρ0‘/ρ))/12) if ρ0 > ρ (7.16b)
where K is a factor which depends on the static system of the structure (structural form). It has a value of 1.0 for simply supported structures, 1.5 for continuous structures, 1.3 for continuous – end span structures, 1.2 for flat slab, and 0.4 for cantilevers.
ρ0 = reference reinforcement ratio = 10-3√fck ρ = required tension reinforcement ratio ρ’ = required compression reinforcement ratio fck = concrete cylinder strength (N/mm²)
Equations 7.16 above have been derived on the assumption that the steel stress at SLS at a cracked section at the midspan of a beam or slab or at the support of a cantilever is 310 MPa (corresponding roughly to fyk = 500 MPa). Where other stress levels are used, the values obtained using Expression (7.16) should be multiplied by 310/σs. It will normally be conservative to assume that:
310/σs = 500/(fykAsreq/Asprov) (7.17)
Where;
σs = tensile stress at mid-span (support for cantilevers) under the design load at SLS Asprov = area of steel provided at the section Asreq = area of steel required at the section for ultimate limit state.
A little observation of equation 7.16 will show that the direct value of compression reinforcement provided cannot be used in EC2 unless the reference reinforcement ratio of 10-3√fck is greater than the actual reinforcement ratio. This goes on to say that the value to be used should be based on the area of compression steel required and not the area of compression steel provided (say H12 mm hanger bars). This is where BS 8110 greatly differs from EC2.
In BS 8110, the area of steel provided can be increased to order to reduce the service stress in the reinforcement. This usually increases the modification factor and the allowable L/d value. However, some questions have been asked on the area of reinforcement to be used in Eurocode assessment of deflection such as that from Beal[6];
It is not clear how Eq. 7.16(a) and (b) are intended to be applied when the reinforcement service stress varies from 310N/mm² as it is not clear how the reinforcement ratio ρ in eq. 7.16 is to be calculated. Is it the actual amount of reinforcement present, or is it the reinforcement which would have been required for a design stress of 310N/mm²?As currently drafted, it is not clear which of these interpretations is correct.
References
[1] Taylor J. P. (2009): The deflection of reinforced concrete. Taylor Lauder Bersten Pty Ltd [2] Zhon W., Kokai T. (2010): Deflection calculation and control for reinforced concrete flexural members. Canadian Journal of Civil Engineering 37(1):131-134 https://doi.org/10.1139/L09-121 [3] Elaghoury Z. (2019): Long-term deflection of reinforced concrete beams. M.Sc thesis Submitted to the Department of Civil and Environmental Engineering, The University of Western Ontario [4] Muhaisin M.H., Jawdhari A.R., Ammash H.K. (2019): Revised formula for predicting the long-term deflection ultiplier of normal and high strength concrete. Rev. IBRACON Estruct. Mater. 12(6):1345-1352 http://dx.doi.org/10.1590/s1983-41952019000600007 [5] Vollum R. L. (2009): Comparison of deflection calculations and span-to-depth ratios in BS 8110 and Eurocode 2. Magazine of Concrete Research 61(6):465-476 https://doi.org/10.1680/macr.2009.61.6.465 [6] Beal A. N. (2009): Eurocode 2: Span/depth ratios for RC slabs and beams. The Structural Engineer, Vol. 87 No. 20, 20th October 2009
Piles are the most preferred type of foundation for supporting the piers of overhead bridges. They offer greater resistance to vertical and horizontal actions from the bridge deck than shallow foundations. Furthermore, they extend the foundation of piers beyond scouring zones so that there will be no loss of bearing capacity. In this article, we are going to show how to design the foundation of a bridge pier using piles.
We have previously talked about the design the piers and pier cap of an overhead bridge. If you missed the post, read it below.
Let us go ahead and design the foundation of the bridge pier using the soil investigation report shown below.
Step 1: Determine the load from the superstructure
The loading on the pier cap at ultimate limit state and serviceability limit state are shown below;
Actions on the pier cap
When analysed on Staad Pro, the following support reactions were obtained;
ULS Vertical support reaction on each pier = 6001.483 kN Horizontal reaction = 368.443 kN Bending Moment = 702.032 kNm
SLS Vertical support reaction on each pier = 4259.247 kN Horizontal reaction = 260.901 kN Bending Moment = 497.125 kNm
Step 2: Determination of the number of piles required and the layout
We normally use the service loads to determine the number of piles required for the structure, and use ultimate loads (factored loads) for the structural design.
Using 900 diameter piles, Number of piles required per pier = 4259.247/1210 = 3.52 Therefore, provide 4 Piles per pier leg. The layout of the pile cap is shown below. However, attempts were made to combine the two piers under one pile cap, but no economical solution was reached due to the wide spacing of the piers (6 m).
Adopted layout of the pile cap
Step 3: Provide longitudinal reinforcement for the piles
The reinforcements required for the piles is usually based on the minimum required for the selected pile diameter
Relationship between minimum area of reinforcement and pile diameter
For a pile diameter of 900 mm, the minimum area of steel required is 2500 mm2 Therefore provide 9H20 mm (Asprov = 2826 mm2) Provide helical links of H10@250 pitch
The required thickness of the pile cap is usually estimated from the diameter of the piles.
h = 2 x diameter of piles + 100 = 2(900) + 100 = 1900 mm deep pile cap
Different approaches can be used for analysing the pile cap such as bending theory, strut and tie method, or finite element analysis. Let us obtain the internal forces in the pile cap from finite element analysis using Staad Pro.
3D model of the pile cap on Staad Pro
When analysed at ULS, the maximum moment in the pile cap is shown below;
Step 5: Check for punching shear around the pier perimeter
VEd = 6001.483 kN
Column perimeter u = πd = π x 1200 = 3769.9 mm
v = V/ud v = (6001.483 × 1000)/(3769.9 × 1800) = 0.8844 N/mm2 VRd,max = 0.2(1 – fck/250)fcd VRd,max = 0.2 (1 – 40/250) x 40/1.5 = 4.48 N/mm2
0.8844 N/mm2 > 4.48 N/mm2 This is okay
This shows that the punching shear around column perimeter is ok.
Critical shear checks for pile caps
Step 6:Check for critical punching shear
Consider the critical section for shear to be located at 20% of the pile diameter inside the pile cap.
Distance of this section from the column face; av = 0.5(Spacing between piles – width of column) – 0.3(pile diameter) av = 0.5(2700 – 1200) – 0.3(900) = 480 mm
Length of corresponding perimeter for punching shear u = 4(2160) = 8640 mm
Perimeter of pile cap = 4(3900) = 15600 mm
The contribution of the column load to the shear force may be reduced by applying a factor β = av/2d, since av < 2d where 0.5d ≤ av ≤ 2d
But a little consideration will show that av(480 mm) < 0.5d(900 mm), therefore, take av as 0.5d (900) Therefore β = 900/2(1800) = 0.25
v = βV/bd V = 6001.483/4 = 1500.37 kN
v = (0.25 x 1500.37 x 103)/(1000 x 1800) = 0.208 N/mm2
Since VRd,c (0.318 N/mm2) > VEd (0.208 N/mm2), No shear reinforcement is required Okay.
Step 7: Check for Anchorage Length
Let us calculate the design tension anchorage length of H32mm bar (fyk = 500 N/mm2, concrete cover = 100 mm, Concrete cylinder strength fck = 40 N/mm2) for a bent bar assuming good bond conditions.
where; fctk 0.05 = characteristic tensile strength of concrete at 28 days = 2.5 N/mm2 (Table 3.1 EC2) γc = partial (safety) factor for concrete = 1.5 αct = coefficient taking account of long-term effects on the tensile strength, this is an NDP with a recommended value of 1.
Bridge pier caps are horizontal structural members used for transferring bridge deck loads to the piers, before they are transferred to the foundation. They can be made of different shapes or forms, depending on the required aesthetics of the bridge. However, pier caps (or heads) must be designed to resist the actions transferred from traffic, self weight of the bridge deck, wind actions, or accidental actions due to vehicle impact. By the nature of their configuration, the piers and the pier cap/head of bridges should be designed as monolithic rigid frames.
However, some of the different configurations that are obtainable in bridge piers are;
Solid wall pier
Hammer head bridge pier
Rigid frame bridge pier
Solid wall Pier
Hammer head Pier Cap
Rigid Frame Pier Cap
In this article, we are going to show how to design reinforced concrete rigid frame bridge pier and pier cap using Staad Pro software. Different methods can be used for obtaining the reactive forces from the bridge deck such as grillage analysis, finite element analysis, or classical methods such as Courbon’s theory. We will pick the reactive forces from finite element analysis, and apply them to the bridge pier cap.
The configuration of the bridge deck is shown below;
Finite element model of the bridge deck
The length of the bridge deck is 15 m, and the loading and configuration is shown below;
Structural configuration of the bridge deck
Traffic action on the bridge deck (LM 1)
The maximum reactive forces from finite element analysis are as follows;
These reactions are transmitted through the elastomeric pad bearing to the pier cap. Note that there are accompanying horizontal actions due to braking, acceleration forces, skidding of vehicles etc. Design value of braking or acceleration forces depend on the vertical loads applied on notional lane 1. The values of horizontal forces should be combined with appropriate value for vertical traffic load corresponding to its frequent value. However, when the leading traffic loads are vertical ones, the accompanying value of the braking and acceleration forces are to be defined in the National Annex and can be set to zero. But for simplicity, horizontal actions have not been considered in this article.
Therefore the ultimate load that will be transferred to the pier cap is given by;
Since this is assumed to be an intermediate pier, the pier cap will be subjected to double of the load above. Therefore, the maximum ULS load from each girder will be 1146.39 x 2 = 2239 kN. Note that the factored self weight of the pier and pier cap should be included in the model.
Fro proper analysis and design of the foundation, it is important to separate the service and ultimate loads properly. At SLS, the maximum reactive force from each girder is 791.889 kN. The preliminary sizing of the rigid frame pier cap and the design data of the pier system is shown below;
Diameter of piers = 1200 mm Dimensions of pier cap = 950 x 1200 mm Support of frame = Fixed
Bridge Deck Load on the Pier Cap
3D Model of bridge pier on Staad Pro
When analysed on Staad Pro, the internal forces in the rigid frame pier cap are as follows;
Bending moment diagram
Shear force diagram
Axial force diagram
The summary of the internal forces at ULS are as follows;
Pier Cap Sagging moment = 3298.257 kNm Hogging moment = 2911.468 kNm Maximum shear force = 3468.85 kN Maximum axial force = 368 kN (compression)
Piers Maximum moment = 1508.62 kNm Maximum shear force = 368.443 kN Maximum axial force = 6001.483 kN
Structural design of the Pier Cap
Let cover to reinforcements = 50 mm Yield strength of reinforcement fyk = 500 MPa Compressive strength of concrete fck= 40 MPa
Span – Sagging
MEd = 3298.257 KN.m
Effective depth (d) = h – Cnom – ϕ/2 – ϕlinks Assuming ϕ25 mm bars will be employed for the main bars, and ϕ12 mm bars for the stirrups (links) d = 1200 – 50 – 12 – 12 = 1126 mm
Maximum spacing of shear links = 0.75d = 0.75 × 1126 = 844.5 mm Provide 6H12mm @ 200mm c/c (Asw/S = 3.39) Ok
Note that this link must be properly closed with adequate anchorage length, because it will assist in resisting torsion.
Side Bars Provide H16@200 c/c
The typical detailing of the pier cap is shown below. Note that bar marks and layer information were not included.
Typical detailing sketch of the pier cap
Design of the piers
You are expected to go through the process of designing columns according to Eurocode 2. You will need to check if the column is short or slender. If the column is slender, you will need to obtain the additional moments due to second order effects.
However for simplicity in this post, let us add the effect of imperfection to the column moment. You can go through the rigourous process of ensuring that you are designing for the worst effect.
Let us assume that effective length of the pier is 0.85L = (0.85 x 6000) = 5100 mm (note that Eurocode requires a more rigourous approach for calculating the effective length of columns)
The geometric imperfection of the column ei = [(1/200) x (5100/2)] = 12.75 mm
eiNEd = 6001.483 x 0.01275 = 76.5189 kNm
Let us therefore take the column design moment MEd to be = 1508.62 + 76.5189 = 1585.13 kNm
Hence, NEd = 6001.483 kN MEd = 1585.13 kNm d = 1200 – 50 – 50 = 1100 d/h = 0.9 (say)
Let fcd = (0.85 x 40)/1.5 = 22.667 N/mm2
NEd/h2fcd = (6001.483 x 1000)/(12002 x 22.667) = 0.183 (Conservatively say 0.2) MEd/h3fcd = (1585.13 x 106)/(12003 x 22.667) = 0.04
Therefore, Asfyd/h2fcd = 0.1 As.req = (0.1 x 12002 x 22.667)/(0.87 x 500) = 7504 mm2
The minimum area steel required is 0.1NEd/fyd = 1379 mm2 or 0.002Ac = 2262 mm2 whichever is greater.
Therefore provide 20H25 (Asprov = 8380 mm2)
To determine the shear reinforcement, shear design should be carried out. The reinforcement provided should be used to check the pier against vehicle collision. Note that this accidental load case should be unfactored and considered to be acting alone.
