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Engineers React to Large Span Cantilever Slab Construction

By
Ivy Ugorji
-

A structural engineer posted a picture of a large cantilever slab construction on LinkedIn, and so many reactions have followed the picture due to its size and dimensions. Cantilever structures are generally structures that are fixed (or continuous) at one end, and free (unsupported at the other end). As a matter of fact, they are one of the most challenging structures to design given that the bending moment increases with the square of the length, while the deflection increases to the fourth power of the length. As a result, controlling deflection and dealing with stability of large span cantilever structural systems is always a challenge in design.

Read Also…
Design of large span cantilever structures
Structural design of cantilever slabs

We have posted the comments from various professionals on the post to enable us review, scrutinize, and assess the possibility of designing and constructing such type of structure. However the exact span, design data, calculation sheet, and drawings of the structure were not made available, but a lot of people still managed to lend their voices to the construction.

Let us look at their responses below;

Cantilever slab

(1) A senior engineer from New Delhi area of India doubted the stability of the structure citing inadequate support conditions. He suggested that cables could have been used to support the slab further. According to him,

Doesn’t look like it is safe. The slab is too thick whereas the supporting structures are too thin. Also, normally such thick slabs are designed as flat slabs but I don’t see the flat slab connection type being followed at the support junctions. It looks like to take care of the cantilever load and moments coming on the slab the thickness has been increased. Hopefully the designer has checked all the loads and moments properly otherwise it is going to fail in the long run. The best option would have been to provide some sort of pulling arrangements at the top corner of the slab via cables or pin jointed stereo frames at the outmost corner where there is no support from the bottom of the slab and that way the designer would have succeeded in decreasing the slab thickness as well as given an aesthetic look to it.

(2) Another structural engineer from India attributed it to the power of post tensioned concrete design as he wrote;

Power of PT Design

(3) A registered Professional Engineer (P.E) from the USA commented that the loading on the slab may not be high enough to cause serious problems. According to him,

There are some issues, but if the math and science can be proved it can be done. Usually in situations like this, the loading is majorly under developed so when you compare the loading to the strength, it will be sufficient. But in reality the loading is not high enough. Expect 250 psf live load to 500 psf live load and use a heavy concrete weight with 2000 pci strength or 1500 pci strength. They tend to screw up the mix design on these.

(4) A retired engineer from India wrote;

This is possible. The slab must have been designed as two adjecent edges supported slab. I did come across such a design in early stages of my career as structural engineer

(5) A registered civil engineer from Nigeria expressed doubts over the safety of the slab and said,

The span of this cantilever slab seems to be much, and may be unsafe.

(6) A senior structural engineer (M.Tech) from India also considered the safety of the structure to be doubtful due to the lack of a back span and said,

That seems to be a dangerous cantilever. I don’t see a continuous backing portion either. I hope it must have been designed and checked well.

(7) The idea of lack of back-span was also raised by another engineer who said that it does not matter whether the slab was post tensioned or not. According to him;

No back span, seems to be very dangerous. Whether it is PT or NonPT the cantilever behaviour is same. Looks very huge self weight due to higher thickness

(8) A civil engineer from Nigeria pointed out that a blend of lightweight materials on a steel framing can be used to achieve the structural system. According to him,

It depends on the material used to achieve the cantilever. POP or any lightweight material can be used to achieve this cantilever then the frame can be metal. The rest will be a good aesthetic work to produce a good look. No load should be on the artificial slab.

However, materials like POP may not be adequate or durable given the exposure of the structure. However, the use of polystyrene can work, after which it will be screeded with mortar to give a finished appearance of concrete. Polystyrene has be extensively used for doing concrete fascia and other building elements.

(9) Another Engineer pointed out that the image could be edited, because the support system provided for the structure (walls and columns from the picture) looked too small to support the massive weight of the cantilever structure. According to him,

Looks to me like a edited image of the structure but if it’s true I don’t know how this structure is safe. Supports looks weaker than the heavy cantilever slab resting on it.


(10) Another engineer who is rounding up his PhD in Spain pointed out that the structure must be post tensioned, however, it seems unsafe especially in the presence of any seismic event. According to him;

Post-tension! But even if it’s not behaving similarly in cantilever spans. The thickness! Huge Self weight! with moderate seismic load or any lateral load, it will make severe damages! Please, treat Infrastructure in high responsibilities.

(11) An independent Structural Engineer with over 39 years of experience from India suggested that the most important thing is to balance the weight of the structure and the moment generated at the supports. According to him,

Main thing is to see the balancing weight and Moment at the support and the section of slab should be with max. thickness at supports and minimum thickness at free end , we have already designed and constructed a stadium at Dehradun in 2001 & standing safe till date.

However, in as much as the exact details of the structure are not available, what do you think?

Understanding Human-Structure Interaction

By
Ubani Obinna
-

Recent researches have shown that human beings interact with structural systems undergoing vibration. Initially it was thought that human body is an inert mass during vibration of structures, but studies have proved otherwise. Human-structure interaction is a study which describes the independent human system and structural system working together as a whole. It also studies the vibration of structural systems where people are involved, and how human body responds to structural vibration.

Vibration serviceability issues are becoming increasingly important in structures subjected to crowd action such as foot bridges, galleries, stadiums, places of assembly etc. This is important given the increasing use of slender and aesthetically pleasing elements in structures. It is understood that the excess lateral vibration of the Millennium bridge which caused the bridge to ‘wobble’ on its opening day gave the impetus for researches on how human beings interact with structural vibration.

According to B.R. Ellis [1] who researched on human-structure interaction in the year 1997;

It has occasionally been assumed that the effect of the people is simply that of an added mass on the system, however, both
site and laboratory tests demonstrate that human bodies do not act solely as mass on the structure and show that the problem is somewhat more complex

In an experiment reported by Ji and Ellis [2], the frequency of a beam was tested when it was empty, and compared with when someone was standing on it. The frequency of the bare beam was observed to be 18.68 Hz, while the frequency of the occupied beam was observed to be 20.02 Hz. If the human body acted as an inert mass, the frequency of the human occupied beam would be smaller rather than bigger. It was therefore observed that the human body did not act as an inert mass but as a mass-spring-damper system. This particular discovery formed the basis for the new topic called human-structure interaction.

Modelling human-structure interaction

Two approaches are usually adopted when studying the dynamic property of a human body based on the bio-mechanical method. One approach is to develop the part-body model and the other approach is to develop the whole-body model [3]. In the body part model, the attention is on the local dynamic properties of body parts such as the hands, heads, or the vibration of the soft tissues, under the assumption that the forces acting on the model are already known. However, the whole-body model is concerned with the holistic dynamic property of the body (for example a human standing on a beam subjected to external excitation). In this case, the force acting on the body is unknown in advance and the interaction between the body and the structure occurs [3].

Instead of the single degree of freedom (S-DOF models that have been extensively investigated, it is recommended that human-structure interaction be modelled with two degrees of freedom (2-DOF), where one degree of freedom is for the structure, and the other for the human being. To develop the two degree of freedom system, one approach is to model two separate single degree of freedom systems (one for the structure and one for the human being), and join the two models together to form a 2-DOF. This method is called separative modelling, and has the advantage of being easily reproducible in the laboratory for experiment [3]. On the other hand, when the system is modelled inseparably as a whole, it called integrative modelling.

human structure interaction
Fig. 1. The separative modelling of human-structure interaction. (a) Structure model, (b) Body model, (c) Coupled model [3]
Integrative modelling of human body
Fig. 2. The integrative modelling of human-structure interaction. (a) Body on SDOF structure, (b) Coupled model [3]

In a parametric research work that was carried out to compare the two models, the natural frequency, damped natural frequency and decrement coefficient of the first mode from the integrative model were found to be smaller than those from the separative model. The authors further concluded that integrative model is more reasonable than the conventional separative model, stating that the separative model can introduce up to 20% error in the calculation of the natural frequency [3].

Applications of human-structure interaction

(1) Prediction of human body response to structural vibration
(2) Evaluate structural vibration where people are involved
(3) Identification of some dynamic characteristics of the human body
(4) Reduction of human induced structural vibrations
(5) Improvement of human comfort in a working environment
(6) Improvement of animal welfare and meat quality

References
[1] Ellis B. R. (1997): Human-structure interactions in vertical vibrations. Proceedings to Institution of Civil Engineers, Structures and Buildings (1997)122 Panel Paper 11023
[2] T. Ji, Ellia B.R: Human-Structure Interaction
[3] Zhou D., Han H., T. Ji, Xu X. (2016): Comparison of two models for human-structure interaction. Applied Mathematical Modelling 40(2016): 3738 – 3748 http://dx.doi.org/10.1016/j.apm.2015.10.049

How to Plaster a Block Wall

By
Ubani Obinna
-

Plastering is the process of covering a wall surface with a thin layer of mortar for protection and decorative purposes. It is usually the first stage in the finishing works of a building, and it is categorised under ‘wet works’. The mortar for plaster works comprises of a mixture of fine sand, cement and water. The requirements for mortar and rendering is given in BS EN 998-1:2003.

Download…
A guide to BS EN 998-1 and BS EN 998-2
How to determine the quantity of mortar for laying block

Plastering of a wall is a skilled labour, and requires the services of an experienced mason. Poorly done plastering work can ruin the aesthetics of a building, and lead to high repair/maintenance costs. Plaster surfaces can be made relatively smooth or rough, depending on the next stage finishes to be applied. The typical thickness of plasters ranges from 12 mm to 15 mm. If the thickness of plaster exceeds 15 mm, it will be important to plaster in two coats. Some common plaster defects are:

(1) Cracks
(2) Spalling/falling off of plasters from the wall
(3) Undulating surfaces
(4) Efflorescence
(5) Popping etc

In order to carry out a good plastering job, here are some important steps to follow especially where accuracy is important.

(1) Check for straightness of the wall

Before the commencement of any plastering work, it is important to be very familiar with the wall to be plastered by carrying out simple checks. It is important to check the edges of the wall for squareness, and the length and height for perfect horizontality and verticality. If the walls are in conformity with the construction drawings, then there is no problem. If there are deviations, then know that the thickness of the plastering work will be affected, and that will affect the cost and duration of the work. A decision will have to be taken before the next stage of the plastering work can commence.

check for straightness

(2) Preparation of the surface

It is a good practice to lay blockwork with recessed joints to receive plaster, but this is not always the case. Either way, the walls shall be brushed clean of all dust, thoroughly wetted and surface dried before plaster is applied. If there are bonding agents specified by consultants or manufacturers, then it shall be applied according to the instructions before the plastering can commence. All put-log-holes (i.e. holes left for scaffolding) shall be properly filled in advance of the plastering.

(3) Installation of gauges

To ensure an even thickness and a true surface, gauges of plaster 15 mm x 15 mm, or broken clay tiles set in mortar shall be first established on the entire surface at about 2 metre intervals both vertically and horizontally. The thickness of the plaster specified excludes the key i.e. the grooves or open joints in the block work (if any). The minimum thickness of the plaster over any portion of the surface shall not vary from the specified thickness by more than 3 mm. However, if the wall is not straight, there will be very large variations in the thickness of the plaster. That is why step one is very important.

(4) Material

The specified mortar material in the drawing should be followed to the later. The mix ratio often recommended is 1:3, which means that one bag of 50 kg cement to 6 head pans of plaster sand should be used.

(5) Application of plaster

Plastering of walls shall commence after completion of ceiling plastering if any. The plastering shall be started from the top and worked down towards the floor. Mortar shall be applied between the gauges to slightly more than the require thickness. Furthermore, the plaster shall be well pressed into the joints, levelled and brought to a true surface by working on a straight edge (range) reaching across gauges, with small upward and sideways movement. Finally the surface shall be finished true with a wood float or trowel according to the type of finish required.

If a sandy granular texture is needed (for example plastering to receive wall tiles), the surface shall be wood floated. If a smooth finish is needed, trowelling shall be done to the extent required to produce the desired smoothness.

(6) Finishes

The plaster shall be finished to a true and plumb surface and to the degree of smoothness required. The work shall be tested frequently as the work proceeds with a true straight edge not less than 2.5m long and with plumb bobs. The gap between the straight edge and any point on the plastered surface shall not exceed 3 mm. All horizontal lines and surface shall be tested with a level and all jambs and corners with a plumb bob as the work proceeds. At corners, plastic edge trims (corner angle beads) could be used to avoid wavy edges.

corner bead

(7) Stopping work at the end of the day

In stopping work at the end of the day, the plaster shall be left cut clean to line both horizontally and vertically. When recommencing the plastering, the edge of the old work shall be scraped, cleaned and wetted before plaster is applied to the adjacent areas, to enable the two to be properly jointed together. Plastering work should be closed at the end of the day on the body of the wall not nearer than 150 mm to any corners or arrises. Horizontal joints in plaster work shall not be formed on parapet tops and copings, as these invariably lead to leakages.

(8) Curing

Curing shall be started 24 hours after finishing the plaster. The plaster shall be kept wet for a period of seven days. During this period it shall be suitably protected for all damages, at the contractor’s expense.

Top Civil Engineering Consultancy Firms in Nigeria

By
Ubani Obinna
-

The construction industry in Nigeria remains a very promising area for the present and for the future. Infrastructure and housing deficit in Nigeria has inspired government support, and public-private partnership (PPP) investments in construction, towards improving the standard of living for the masses. Civil engineering consultants are at the forefront of providing designs, consultancy, and supervision for most civil engineering projects in Nigeria. Consultancy firms are legally required to be registered with COREN in order to practice Engineering in Nigeria. Companies can also belong to the Association of Consulting Engineers in Nigeria (ACEN). This post will give the list of some of the top civil engineering consulting firms in Nigeria.

Read Also…
Problems of civil engineering consultancy in Nigeria

While there are many civil engineering consulting firms in Nigeria, here are some of the top consulting firms.

(1) Ove Arup and Partners

Ove Arup started civil engineering practice in London in the 1946, and today, is one of the leading civil engineering consultancy firms in the world with offices in over 34 countries including Nigeria. Arup has been operating in Nigeria for over 60 years and has delivered numerous projects across the country.

Address:  26, McCarthy Street, Onikan, PO Box 2088, Lagos
Telephone: +234 (0) 1 462 2580-4
E-mail: lagos@arup.com

(2) Sanni Ojo and Partners

Sanni Ojo and Partners is one of the leading consultancy and project management firms in Nigeria who started operation in the year 1991. They offer consultancy in civil engineering, buildings, an oil and gas services. Sanni Ojo and partners have delivered numerous projects in Nigeria, and they are members of Council for Tall Building and Urban Habitats (CTBUH).

Address: 1 George Alade Lane, Off Fola Agoro Street, Abule Ijesha, Shomolu Lagos
Telephone: 01-7928849
Website/URL Address: http://www.sop-consulting.com

(3) Etteh Aro and Partners

Etteh Aro and partners was established in the year 1970 by two founding partners, and have delivered over 1000 projects across Nigeria. The firm has recorded success in the design of major highways, bridges, fluid retaining structures, stadia, multi-storey buildings, environmental engineering fields, offshore infrastructures and so on.

Address: 35, Oshuntokun Avenue, Bodija Ibadan, Ibadan
Telephone: 09-5231845
Website/URL Address: http://www.etteharo.com/

(4) Nexant Consulting

Nexant Consulting Limited is an ISO 9001:2015 certified consulting firm with extensive expertise and experience in railway engineering works (tracks, signalling, telecoms, power & electrification), major civil engineering projects (stations, depots, highways, bridges, buildings, etc.), structural engineering works, as well as asset management. They have over 30 years experience across the globe.

Address: CITY HALL, 2nd Floor, West Wing, Catholic Mission Street, Lagos Island, Lagos State
Telephone: 01-2956505
E-mail: info@nexantconsult.com
Website: https://nexantconsult.com/

(5) PinConsult Associates Limited

PinConsults was established in the year 1985 to provide highly competent professional consultancy services in the fields of civil engineering, structural engineering and building management. They have delivered numerous projects across Nigeria.

Address: 27/29 King George V Way Onikan, Lagos
Telephone: 01-271-6259
Website: http://pinconsultnigeria.com

(6) Aurecon Group

Aurecon is an engineering, design and advisory company with a global presence. The Aurecon brand was created in 2009, through a merger of Connell Wagner with two African based businesses.  In October 2019, faced with changing conditions in Africa, where global models are no longer considered an advantage, Aurecon decided to demerge its African business, which is in the process of developing its own brand and identity. This process is expected to be completed in 2020.

Address: 5, Admiralty Street, Waterside, Off Admiralty way, Lekki Phase 1 Lagos State
Telephone: +234 7069 527 330
E-mail: nigeria@aurecongroup.com
Website: https://www.aurecongroup.com/

(7) Integrated Advanced Analysts Associates Limited

IAA Associates Limited is an independent organization providing consultancy and advisory services in the fields of structural and civil engineering. IAA Associates has well qualified engineers and technicians who have considerable expertise in both traditional and high-tech engineering. They delivered the popular Nestoil towers building.

Address: LAPLACE HOUSE Block III, Plot 3 Oniru Estate Off Ligali Ayorinde Street Victoria Island Annex, Lagos
Telephone: (+234) 1-7731029
E-mail: info@iaaassociates.com
Website: http://www.iaaassociates.com/

(8) Advanced Engineering Consultants

AEC provides complete consulting services from the initial investigation stages, through feasibility studies, inventory, condition survey, outline planning, production of detailed designs, preparation of contract documents, evaluation of tenders to construction supervision with successful completion of design and supervision for various types of projects. The firm has worked for Government, State and International organisations. They are the consultants for the proposed 4th Mainland Bridge project in Lagos.

Address: 18, Town Planning Way, Ilupeju, P.O. Box 6925, Shomolu, Lagos, Nigeria.
Telephone: +234 909 388 7975
E-mail: info@aec.org.ng
Website: https://aec-group.org/

(9) Consultants Collaborative Partnership

With over 25 years of experience in the Nigerian and African environment, CCP offers professional Architectural Design, Engineering, Project Management and quantity Surveying Consultancy Services. They have executed projects of excellent value and professional acclaim in most major cities of Nigeria – from private residences to shopping malls; from multi-storey residential blocks to 500 unit housing estates – cutting across the Residential, Commercial, Institutional, Hospitality and Industrial sectors.

Address: CCP Place, Plot 17, Block 25, Chief Abiodun Yesufu Way Oniru, Victoria Island, Lagos, Nigeria
Telephone: +234- 9038001564
E-mail: info@consultantscollaborative.com
Website: https://www.consultantscollaborative.com/

(10) UF-A Consultants

UF-A is a contemporary and innovative engineering firm with vast experience in civil/structural engineering within and outside Nigeria. UF-A have undertaken numerous projects of all sizes and levels of complexity including commercial, leisure, educational and worship center projects.

Address: S242 Ganiyu Crescent, Gbagada Phase II, Lagos
Telephone: 0801 7122 6944
Website: http://www.uf-a.com/

(11) Hancock Ogundiya and Partners

Hancock Ogundiya & Partners is a partnership of Civil, Structural Engineers and Project Managers engaged in engineering practice throughout Nigeria. This organization was first established in 1972 as Hancock & Partners, and the office was expanded in 1977 with the formation of Hancock Ogundiya & Partners. The consulting services of the organisation provides a complete design facility from feasibility studies to design and preparation of working drawings and tender documents and the subsequent project site supervision/management.

Address: 33 Glover Street, Banilux Compond Ebute-metta, Yaba, Lagos State
Telephone: +234-1-295-6522
E-mail: info@hancockogundiya.com
Website: https://hancockogundiya.com

(12) Royal HaskoningDHV

Royal HaskoningDHV is an independent international engineering and project management consultancy firm that has been around since the year 1881. They have been working with clients to successfully deliver projects which contribute to improving living circumstances around the world for more than 137 years. The firm has over 6,000 colleagues, spread over 140 countries in the world.

Address: 10 Ondo Street Osborne Estate Phase 1 Ikoyi Lagos
Telephone: +234 818 0817 555
E-mail: info.ng@rhdhv.com
Website: https://www.royalhaskoningdhv.com/en-gb

(13) Morgan Omonitan and Abe Limited

Founded in 1972, MO&A has over 40 years of experience and expertise in providing integrated engineering & development consulting services such as civil & structural engineering, project management and independent expert services to the private & public sectors. MO&A has built a legacy of several successfully executed projects and partnerships with clients and stakeholders. They were behind the Alpha One towers, Eko Atlantic Lagos, Dangote head office building Ikoyi, Access bank head office, and many other developments.

Address: 241, Igbosere Road, Lagos Island, Lagos
Telephone: +234(0)808-913-7683
E-mail: info@moanigeria.com
Website: https://moanigeria.com/

There are hosts of other consulting firms in Nigeria, but we will stop here for now.

Webinar 1

Structville Webinar on Structural Design (May, 2020)

By
Ubani Obinna
-

Structville Integrated Services Limited in their commitment to qualitative knowledge of civil engineering designs will be organising monthly webinars on selected topics in civil engineering. In this month of May, we will host the following webinars outlined below;

(1) Structural Analysis and Design of Raft and Pile Foundation
Date: Saturday, 9th of May, 2020
Time: 11:00 am – 1:00 pm (WAT)
Facilitator: Engr. Ubani Obinna (MNSE)

Features:
(1) Theories and philosophies in the design of shallow and deep foundations
(2) Practical design of pile foundations and pile caps using real life data
(3) Rigid and flexible approach to design of raft foundation
(4) Structural design of raft foundation
(5) Full design material (mini textbook) with detailed drawings covering the above topics

Webinar 2 1

(2) Structural Design of Industrial Framed Structures – Steel Portal frames
Date: Sunday, 10th of May, 2020
Time: 1:00pm to 3:00 pm (WAT)
Facilitator: Engr. Ubani Obinna (MNSE)

Features:
(1) Special features of industrial structures
(2) Considerations in the design of industrial structures
(3) How to apply wind load on portal frames
(4) Structural analysis and design of portal frames
(5) Full design material (mini textbook) with drawings covering the above topics

Discussions on real life projects followed with question and answer sessions will commence after every discussion. Participants will receive webinar materials and instructions on how to participate in their e-mail prior to the webinar date, therefore it is important that you register on time with a correct e-mail address.

Price: NGN 3,000:00 only for both events
Platform: Zoom/Telegram
To book your space for the month of May, click HERE

For further inquiries contact:
Phone calls: +2348060307054
WhatsApp: +2347053638996
E-mail: ubani@structville.com


Bentley System Software Solutions for Building Design

By
Ivy Ugorji
-

Bentley Systems is a software development company that supports the professional needs of those responsible for creating and managing the world’s infrastructure, including roadways, bridges, airports, skyscrapers, industrial and power plants as well as utility networks. Bentley delivers solutions for the entire lifecycle of the infrastructure asset, tailored to the needs of the various professions – the engineers, architects, geospatial professionals, planners, contractors, fabricators, IT managers, operators and maintenance engineers – who will work on and work with that asset over its lifetime.

In a rapidly evolving world where design and construction is becoming increasingly digitized, here are some products of Bentley systems that are very useful for design of buildings and infrastructures. These software support the building design and documentation process throughout all phases of the project – from conceptual design and documentation to coordination and construction.

(1) OpenBuildings Designer

Open buildings

This software can design, analyze, document, and visualize buildings of any size, form, and complexity. It can also simulate real-world performance and evaluate building system performances so you can quickly discover the best design choices. OpenBuildings Designer can inform decision on energy cosumption, carbon emission, and fuel cost of a building at the design stage.

By its multidisciplinary integration, collaboration between architects, electrical engineers, mechanical engineers, and structural engineers is enhanced irrespective of geographical barrier. OpenBuildings Designer also provides building information modeling (BIM) advancements that can enable you deliver buildings faster and with greater confidence in design, workflow, capabilities, and deliverables. 

Ultimately, the software can design buildings and facilities, MEP systems, building structures, refine design alternatives, improve collaboration between professionals, and generate building documentation and reports.

(2) OpenBuilding Station Designer

OpenBuildings Station Designer 1

OpenBuilding Station Designer is a multidisciplinary rail station and pedestrian simulation software. It can design, analyze, visualize, and simulate rail and metro stations of any size, form, and complexity. It improves design quality by optimizing the functional space layout of the station building and the path of travel for the pedestrian.

As a design and simulation software, it can carry out design of station building structures, stations and facilities, MEP, functional space layout, furniture fixture and equipment, pedestrian simulation, geographic coordination, etc.

(3) Legion Simulator

Legion

This is a simulation software which predicts and explores how pedestrians and crowds interact with infrastructure. It performs virtual experiments on the design and operation of a site and assess the impact of different levels of pedestrian demand. With sophisticated modeling, analysis, and presentation capabilities for projects ranging from airports to train stations to sports venues, LEGION Simulator helps enhance pedestrian flow and improve safety by allowing the users to test evacuation strategies at any point of the simulations.

Using the software, you can set up and run user-defined analyzes and generate rich outputs based on a variety of metrics. The software can mimic all aspects of an individual’s movement including personal preferences, surrounding awareness, spatial restrictions, and perception of behaviors. Play back and re-run of simulation capability is also available in the software.

(4) ProStructures

ProStructures is a concrete and steel design software which lets you create design drawings, fabrication details, and schedules that automatically update whenever you change the 3D model. The software enables you to accurately and efficiently model 3D structures for structural steel, metal work, and reinforced concrete structures.

The software models parametric structures in concrete and steel, produce structural details, quantities, structural design documentation, construction documentation, steel fabrication drawings, share structural models, etc.

Disclaimer: All videos and pictures belong to the copyright owners

TMT Reinforcements Can Change Design Specifications in Nigeria

By
Ubani Obinna
-

Nigeria’s reinforcement market is usually categorised into ‘local’ and ‘foreign’ reinforcements which are priced differently in the market. Local reinforcements are manufactured in the country, while foreign reinforcements are usually imported into the country from Germany, Ukraine, and Russia. Different researchers and quality control agencies have often reported low quality of reinforcements manufactured in the country, which usually fail to meet the standards required for reinforcing steel. Due to the problems of quality assurance of reinforcements in Nigeria, most structural engineers in Nigeria use yield strength of 410 MPa in reinforced concrete design (using BS 8110-1:1997).

However, in the middle of local and foreign reinforcements in Nigeria are the Thermo Mechanically Treated (TMT) reinforcements. TMT reinforcements are currently produced worldwide on a large scale for high strength steel. They are usually manufactured using recycled metal scraps.

TMT reinforcements are characterised by their softer inner core, and hardened outer core, and are manufactured by a process called Thermo Mechanical Treatment. This process combines plastic deformation processes such as forging, rolling, etc with thermal processes like heat treatment, water quenching, heating, and cooling at various rates into a single process. During the cooling process of TMT reinforcements, the inner core remains red hot, while the cooled outer surface gets auto tempered due to heat flow from the core to the surface, and turns the outer surface into a hardened martensitic layer.

rebars

A recent research carried out in Nigeria have shown that TMT reinforcements can change the face of design in the country, given the gradual switch of design code from BS 8110-1 to Eurocode 2. Researchers from Nnamdi Azikiwe University, Awka, tested the mechanical properties of 70 samples of TMT reinforcements produced by four different companies in the country. The reinforcements tested were;

The diameter of reinforcement tested ranged from 10 mm diameter to 25 mm diameter. The research was published in the Journal of Science and Technology Research.

Out of the 70 samples tested, 91.5% met the required characteristic strength of 500 MPa (Eurocode 2, UK), and the percentage elongation at fracture satisfied all the requirements of BS 4449:2005. The mean yield strength of the samples was found to be 532.8 MPa with a standard deviation of 24.926 MPa, and coefficient of variation of 4.678%. The probability of the samples tested falling below the yield strength of 500 MPa was found to be 9.4% with a reliability index of 1.316. However, the ultimate tensile strength to yield strength ratio (Rm/Re) of the samples were found to be averagely high (with a mean of 1.356 and a standard deviation of 0.095).

According to the authors,

Design engineers are free to decide on the characteristic value of yield strength to use for design, since Eurocodes permits the use of yield strength ranging from 400 – 600 MPa. Manufacturers should however follow the recommendations in clause 8.2.2 of BS 4449 for assessment of long-term quality level of their characteristic strength… Future work should involve extensive testing of the chemical properties of TMT reinforcements produced in Nigeria, to see how they impart on the mechanical properties. Subsequently, reinforced concrete designers in Nigeria can confidently use fyk = 500 MPa, and a material factor of safety of 1.15 at ultimate limit state (design strength = 0.87fyk = 435 MPa) provided TMT reinforcements have been specified.

The mechanical properties of TMT reinforcements as reported in the study will therefore likely change the way designers specify reinforcements during designs. It is obvious that any design done using fyk = 410 N/mm2 will be more expensive than design done using reinforcement of yield strength, 500 N/mm2. Reinforcement dealers in Lagos are complaining that the demand for foreign reinforcement has reduced as more attention is being paid to TMT reinforcement by top construction companies. It will be interesting to see how TMT reinforcements will influence the construction market and design standards in Nigeria in the nearest future.

Reference:
Ubani O.U., Okonkwo V.O., Osayanmon O. (2020): Variability of Mechanical Properties and Reliability of Thermo Mechanically Treated Reinforcements in Nigeria. Journal of Science and Technology Research 2(1):1-12

To download the full research publication and findings, click HERE

How to Offset Beams in Staad Pro (with video tutorials)

By
Ubani Obinna
-

In some cases, structural members are not perfectly connected to each other along their centroidal axis. This can be as a result of structural arrangement, construction specifications, or feasibility of execution. There are provisions on Staad Pro software to offset beam members in order to reflect as closely as possible the real structural arrangement. In this article, we are going to show how you can offset beams on Staad Pro.

Step 1: Do a little calculation
You should know the value through which you wish to offset on structural member from the other. A little but simple calculation is required in order to know the value to input into staad Pro. Let us assume that a primary beam of 450 mm depth is supporting a secondary beam of 300 mm depth. By default, Staad will join the beams along their centrelines as shown in Figure 1.

Connection of two beams
Fig 1: Schematic representation of default connection of two beams on staad Pro

Let us assume that you want the top fibre of the primary and secondary beams to flush, you will notice that the secondary beam will need to move up by (450/2) – (300/2) = +75 mm. Note that the same effect will be achieved if the primary beam comes down by -75mm.

Beam 2

Also, if we want to the secondary beam to rest on top of the primary beam, then the secondary beam will need to move up by (75 mm + 300 mm = 375 mm). Note that additional internal stresses will be induced in the members due to the eccentricities in the connection.

Step 2: Input your values

After modelling your structure, go to GENERAL → SPEC → BEAM → OFFSET

Depending on the arrangement of the structure, you can offset in any direction you wish at the start and end of each member. Assign the the command to the beam in question.

OFFSET COMMAND

Step 3: Load the structure and analyse as usual

You can now apply the loading on the structure, and analyse as appropriate.

Watch a sample video tutorial below;

Cracking Due to Edge Restraint and Early Thermal Effects in Concrete

By
Peace Ikpotokin
-

Crack width is calculated by multiplying the crack inducing strain by the crack spacing (i.e. the movement over a length equal to the crack spacing). Crack inducing strain εcr is calculated based on whether the element is subjected to edge restraint (which can be early or long term thermal effects), end restraint, and flexure/direct tension. In this article, we are going to shown how to calculate the crack width of a structural element subjected to edge restraint and early thermal cracking.

Edge restraint occurs where the young concrete section (say a wall) is cast on a hardened concrete base. This means that restriction is only in one direction, and there is interaction between the old and new concrete in terms of distribution of cracks. Edge restraint is different from end restraint because the crack width is a function of restrained strain rather than the tensile capacity of the concrete.

At the early age of freshly poured concrete (within 3 days), the crack inducing strain due to edge restraint in concrete element is given in equation (3.6) of CIRIA C660 which is shown below.

εcr = K[αcT1 + εca] R1 – 0.5 εctu

where;
K = allowance for creep
= 0.65 when R is calculated using CIRIA C660
= 1.0 when R is calculated using BS EN 1992-3.

αc = coefficient of thermal expansion
T1 = difference between the peak temperature of concrete during hydration and ambient temperature °C (See Table 1).
εca = autogenous shrinkage strain – value for early age (3 days, see Table 2)
R1 = restraint factor from Figure L1 of BS EN 1992-3 for the short-term situation
εctu = tensile strain capacity of the concrete (see Table 3)

Table 1: Typical values of difference between the peak temperature of concrete during hydration and ambient temperature (Narayanan and Goodchild, 2012)

Values of T1

Table 2: Typical Values of Autogenous Shrinkage Strain

TYPICAL VALUES OF AUTOGENOUS SHRINKAGE STRAIN

Table 3: Values of Tensile Strain Capacity of Concrete (CIRIA C660)

TENSILE STRAIN CAPACITY OF CONCRETE

Calculation Example

Calculate the early thermal crack width of concrete wall cast in Nigeria with the following data;

Thickness of wall = 400 mm
Reinforcement provided = H12@150mm c/c on both faces
Concrete cover = 50 mm
Type of restriant = Edge restraint
Concrete grade = C30/37
Type of coarse aggregate = Granite

Solution

Crack width wk = sr,max εcr

The maximum crack spacing sr,max = 3.4c + 0.425 (k1 k2ϕ/ρp,eff)

To see the definition of these terms, see the post below;

Crack width and crack spacing calculation in concrete

c = 50 mm to outer face
k1 = 0.8
k2 = 1.0
ϕ = 12 mm
ρp,eff  = 754/{1000 × min[400/2; 2.5 × (50 + 12/2)]}
ρp,eff  = 754/(1000 × 140) = 0.00538

sr,max = 3.4 × 50 + 0.425 (0.8 × 1.0 × 12 / 0.00538) = 170 + 758.36 = 928.36 mm

Early age crack-inducing strain, εcr = K[αcT1 + εca] R – 0.5 εctu

Using CIRIA C660 the following parameters can be determined;

K = 0.65
αc = 10 × 10–6 (granite coarse aggregate, see coefficient of thermal expansion of concrete)
T1 = Using 35 °C (for a 400 mm thick wall cast in Nigeria, assuming Class N cement, 18 mm marine plywood, concrete grade C30/37 and cement content not less than 360 kg/m3, see Table 1)
εca (autogenous shrinkage strain for grade 30/37 concrete at 3 days) = 15 × 10–6 (see Table 2)
R = Restraint factor Rj =  1/(1 + EnAn/EoAo)

Where;

En and Eo are the elastic modulus of new and old concrete respectively
Assume En/Eo = 0.80 (CIRIA C660)
An and Ao are the areas of new and old concrete respectively (use An/Ao = hn/2ho assuming that the wall is cast remote from the edge of slab) = 0.4/2(0.4) = 0.5

R = 1/[1 + (0.8 × 0.5)] = 0.714

εctu = 76 × 10–6 (for early age thermal cracking, see Table 3)

εcr = K[αcT1 + εca] R – 0.5 εctu = 0.65 [(10 × 10–6 × 35) + 15 × 10–6] 0.714 – 0.5(76 × 10–6) = 1.3139 × 10–4

Therefore the early age crack width = wk = sr,max εcr = 928.36 x 1.3139 × 10–4 = 0.121 mm

Since this crack width is less than 0.2mm, the early crack width can be considered acceptable for water retaining structures.

Coefficient of Thermal Expansion of Concrete

By
Ubani Obinna
-

Coefficient of thermal expansion is defined as the change in unit length per degree of temperature change. In a concrete element, it is therefore a measure of the free strain produced in concrete subject to a unit change in temperature and is usually expressed in microstrain per degree centigrade (με/°C). It is a very important property of concrete which influences its behaviour under thermal actions. Thermal actions in concrete can come from the environment, stored materials, or during hydration reaction.

Concrete as a material will expand and contract when exposed to temperature change, and when this is not catered for in design, there will be cracks in the concrete element. The coefficient of thermal expansion of concrete largely depends on the aggregate, but a conservative value of 12 × 10–6/°C can be used in the absence of data in the UK. Eurocode states that a value of 10 × 10–6/°C but this value is deemed not to be conservative. The range of coefficient of thermal expansion of concrete ranges from 7 to 13 × 10–6/°C.

Factors such as cementitious material content, water-cement ratio, temperature range, concrete age, and ambient relative humidity can also influence the thermal properties of concrete. However, the nature of aggregates is the principal factor in determining the coefficient of thermal expansion, and the resistance of the concrete to fire since they make up about 70% of concrete. In design αc is assumed to be constant for a particular concrete, in fact it varies with both age and moisture content. Semi-dry concrete has a slightly higher coefficient of thermal expansion than saturated concrete.

Where the type of rock group of the coarse aggregate is known and can be guaranteed to be used, the appropriate value of the coefficient of thermal expansion from the table below may be used e.g. 10 × 10–6/°C for granites and 9 × 10–6/°C for limestones.

Table 1: Design values for coefficient of thermal expansion

Coefficient of thermal expansion of different types of concrete aggregates

There is no standard method for measuring the coefficient of thermal expansion for concrete in CEN, ISO or ASTM although a method for repair materials is provided in BS EN 1770. However, in-house methods can be used for laboratory mesurement. Typically, measuring points would be fixed to a concrete specimen that is placed on roller bearings in a water tank. The specimen is left in the water until there is equilibrium of temperature, and a set of length readings taken. The specimen is then heated to, say, 80°C and kept constant until this temperature is achieved throughout the specimen depth. A second set of readings is taken and the coefficient of thermal expansion calculated.

Thank you very much for reading, and God bless you.

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