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Thursday, February 28, 2019

Tuesday, February 26, 2019

February 26, 2019

How to Calculate Crackwidth Due to Bending According to EC2 (Download Excel Spreadsheet)


Cracking is normal in reinforced concrete structures subjected to bending, shear, twisting, axial tension, and restraint from movement. This is mainly due to the low tensile strength of concrete. Cracking is usually a serviceability limit state problem, but apart from ruining the appearance of the concrete surface, it also posses durability issues, and leakage problem in water retaining structures.

Saturday, February 23, 2019

Saturday, February 16, 2019

Thursday, January 10, 2019

January 10, 2019

Transfer Structures: Design Example on 5-Storey Building


Transfer structures can be described as structures in which the loads from above (usually from columns or walls) are transferred to other structures (such as beams or plates) for distribution to another supporting structure which can resist the load. The prominent issue in transfer structures is that the load path is unconventional, and this is usually found in high rise buildings where floor arrangements differ.

Saturday, December 29, 2018

December 29, 2018

Uplift Verification for Underground Structures: Solved Example

It is widely recognised that an object will float in water, if the weight is less than the upthrust. Upthrust is an upward force exerted by a fluid that opposes the weight of an immersed object. This also applies to structures that are buried under the ground, and subjected to ground water action. Structures such as basements, foundations, underground tanks, and swimming pools are at risk if the dead weight is less than the upthrust, especially when the structure is empty.

Monday, December 24, 2018

December 24, 2018

Merry Christmas from all of us at Structville


Christmas is a special season of the year that offers us an atmosphere filled with love and happiness. From all of us at Structville, we wish you and your family a Merry Christmas filled with happiness, love, and fulfilled living.

Merry Christmas and Happy new year in advance. 🎊🎊🎊🎉🎉🎉🎄


Tuesday, December 18, 2018

Saturday, December 15, 2018

December 15, 2018

Plastic Analysis of Framed Structures


The fully plastic moment of a section (Mp) is the maximum moment of resistance of a fully yielded cross-section. The yielded zone due to bending where infinite rotation can take place at constant plastic moment (Mp) is called a plastic hinge. In order to find the fully plastic moment of yielded section, we normally employ the force equilibrium equation by saying that the total force in tension and compression at that section are equal.
December 15, 2018

How to Analyse Retaining Walls for Trapezoidal Load

In the analysis of retaining walls subjected to earth pressure, it is very common to observe trapezoidal load distribution on the walls. Normally, earth pressure on a retaining wall is assumed to adopt a triangular load distribution, but due to surcharge which is usually assumed to act on the ground surface, the top of the wall experiences some degree of lateral pressure.

Tuesday, November 27, 2018

November 27, 2018

Structural Design of Swimming Pools and Underground Water Tanks



I spent a large part of the last few months developing the contents of this booklet on 'Structural Design of Underground Water Tanks and Swimming Pools' (According to the Eurocodes). Water is necessary for survival of mankind, but in one way or another, water is relatively scarce. We all know that rain does not fall continuously, and for water to be available for usage in homes, it will have to be fetched/pumped from the stream, or harvested during rainfall, or dug up from the ground. As a result, survival instincts made man to create different means of storing water in order to face the periods of scarcity.

Tuesday, August 21, 2018

August 21, 2018

Is This Ultimate or Serviceability Limit State Failure?

Kindly look at the image carefully, and lend your professional opinion if the failure of the building will be categorized under ultimate or serviceability limit state. By posting and discussing your opinion on the comment section, I am very certain that knowledge and deeper understanding of this topic will be enhanced.

Saturday, August 18, 2018

August 18, 2018

Aspects of Modelling of Shear Walls

Shear walls are structural elements usually employed in tall buildings to assist in resisting lateral loads. Shear walls can be solid or pierced (coupled), depending on their location in the building. In the design of  tall buildings, structural engineers normally throw the entire lateral load (say wind action) to the shear walls, which means that the columns will not be relied on for lateral stability. In a more practical scenario however, the shear walls and columns interact in resisting lateral loads, which can be taken into account.

Thursday, August 16, 2018

August 16, 2018

A brief Presentation on Flexural Buckling of Columns

Introduction
When structural members are subjected to compressive forces, the members may fail before the compressive resistance (A.fy) is reached. This premature failure is usually caused by secondary bending effects such as imperfections, eccentricity of loading, asymmetry of the cross-section etc. In such cases, the failure mode is normally buckling, and this is unlike when the member is subjected to tensile forces, where the member will generally fail when the stress in the cross-section exceeds the ultimate strength of the material. Members subjected to tensile forces are inherently stable.

Tuesday, August 14, 2018

August 14, 2018

Structural Design of Cantilever Slabs - Solved Example



Cantilever slabs are common features in buildings due to the need to have bigger spaces at upper floors. To achieve this, architects normally extend the slab beyond the ground floor building line, thereby forming a cantilever. In this post, we are going to show how we can analyse and design cantilever slabs subjected to floor load and block work load.

Wednesday, August 8, 2018

August 08, 2018

How to Calculate the Quantity of Mortar for Laying Blocks


Engineers, site managers, and quantity surveyors are always faced with the challenge of specifying as accurately as possible, the quantity of materials needed to execute a specific item of work. In this post, we are going to explain how you can estimate the quantity of mortar (cement and sand) needed to lay blocks per square metre of wall.
August 08, 2018

Analysis and Design of Pedestrian Bridge Using Staad Pro


Pedestrian bridges (footbridges) are structures designed to enable human beings cross over obstacles such as busy highways, water bodies, gullies, etc. There are several variations of foot bridges based on structural configuration and materials. Modern footbridges are increasingly becoming elements of street beautification, with a view on sustainability and environmental friendliness. In this post, a simple pedestrian bridge has been modelled on Staad Pro software, and the result of internal stresses due to crowd load on the bridge presented.

Saturday, August 4, 2018

August 04, 2018

Bonding of Old and New Concrete


In construction, there always comes a time when there is need to bond old hardened concrete (substrate) with fresh concrete topping/overlay. This post aims to explain how to bond old and fresh concrete successfully, and also review the strength of  interfacial bond between old and new concrete based on already carried out experimental works.




Bonding is very important for adequate performance of finished concrete when fresh concrete topping is used to overlay an existing hardened concrete. This construction feature is usually found during bridge deck construction, concrete pavement, precast filigree slab, pile caps (in some cases) etc. The truth is that adequate bonding is not always guaranteed between the two layers, unless adequate precautions are taken.

For adequate bonding, it is very important to prepare the surface of the substrate adequately. The preparation of the surface usually involves roughening the surface, and removal of all dirt, oil, grease, loosened or unbonded portions of the existing concrete. By implication, the surface of the substrate should be hard, firm, clean, and free from loosened particles. This can be achieved by the use of chipping hammers, wire brushing the surface etc. After this is done, the exposed concrete surface can be cleaned by using pressurised clean water, air, etc. The man hours involved depends on the area of the surface, location, and the ease of cleaning (e.g reinforcement interference).

Precast Filigran Slab





After surface preparation, there is usually need to apply bonding agent on the surface of the existing concrete in order to facilitate the bonding. Epoxy based bonding agents are very popular for such operations. It is recommended that bonding agent is applied prior to casting the fresh concrete.  In essence, the procedure should be 'wet-to-wet' as the bonding agent should not be allowed to dry before the fresh concrete topping is placed.

Hardened Concrete With Bonding Agent Ready for Topping/Overlay
In a research carried out by Vandhiyan and Kathiravan (2017), the compressive strength of monolithic and bonded concrete was compared using 150mm x 150mm cube specimens at 28 days. With epoxy based bonding agent, the compressive strength of the bonded concrete was about 5% less than the monolithic strength, while without bonding agent, the compressive strength was about 28% less than the monolithic compressive strength.

Research has also shown that the moisture condition of the substrate affects the shear bond strength of bonded concrete. Shin and Wan (2010) investigated the interfacial bond strength of old and new concrete considering saturated surface dry (SSD) and air dry conditions. Saturated surface dry is  condition that can be described as the concrete containing moisture that is equal to its potential absorption, without the surface being wet or damp. At water/cement ratio of 0.45 (for the topping concrete), the shear bond strength at the interface was about 44% greater when the substrate was at SSD condition than when it was air dry. At water/cement ratio of 0.6 for the topping layer, an increase in shear bond strength was recorded, but there was a reduction in the compressive strength of the concrete.

So the recommendation in this article is that when casting a topping layer of fresh concrete on old concrete, adhere to the following guidelines;

(1) Prepare the surface properly
(2) Make sure that the substrate is at saturated surface dry condition
(3) Use a bonding agent and follow the manufacturer's technical recommendation properly.

Thank you for visiting Structville today, and God bless.

References
Vandhiyan R., Kathiravan M. (2017): Effect Of Bonding Chemical On Bond Strength Between Old And New Concrete. SSRG International Journal of Civil Engineering- (ICRTCETM-2017) - Special Issue – April 2017 ISSN : 2348 – 8352 pp 129-134

H-C. Shin,  Z. Wan (2010): Interfacial shear bond strength between old and new concrete. Fracture Mechanics of Concrete and Concrete Structures - Assessment, Durability, Monitoring and Retrofitting of Concrete Structures- B. H. Oh, et al. (eds) ⓒ 2010 Korea Concrete Institute, Seoul, ISBN 978-89-5708-181-5 pp 1195 - 1200



Thursday, August 2, 2018

August 02, 2018

Meet the Winners of Structville Design Competition

In the month of May, we announced the commencement of Structville Design Competition, where civil engineering students and serving NYSC members from various universities and polytechnics in Nigeria competed for small tokens in the design of reinforced concrete structures. The exercise was aimed at developing the interest of students in Structural Design, and preparing them for excellence in the field of structural engineering. You can view the details of the competition below;
August 02, 2018

A simpler Approach to the Design of Ground Beams Using Staad Pro


Ground beams are employed in reinforced concrete substructures for a lot of reasons. Ground beams can be differentiated from plinth beams due to a slight variation in the purpose of their construction. Plinth beams are used to connect (chain) separate pad bases together, and blockwork can be built off from the plinth beams. On the other hand, ground beams are designed mainly for the purpose of receiving load from ground floor slab or raft, alongside other functions as envisaged by the designer.

Monday, July 23, 2018

Friday, July 20, 2018

July 20, 2018

Comparative Design of Staircase Using StaadPro and Manual Calculations


Staircases provide simple solutions for vertical circulation in a building. In this post, we are going to model a simple  staircase using finite element plates, and compare the answer with manual calculations in which we assume the staircase to be a simply supported beam. The aim of this post is to verify the results obtained from the different procedures, and give reinforced concrete designers a little idea about the results to expect from their assumptions.

Tuesday, July 3, 2018

July 03, 2018

Structville Design Challenge Results (1st Edition)


On 14th of May, 2018, Structville Integrated Services announced the commencement of Structville Design Challenge for civil engineering students in Nigeria. (If you missed it, you can read post HERE). By the special grace of God, the competition has come and gone, and I wish to appreciate everyone who participated in the exercise.




The price money goes as follows;
1st position - NGN 10,000
2nd position - NGN 3,000
3rd position - NGN 2,000

Competition Details
Total number of successful applicants = 20
Total number of scripts submitted by dead line = 14
Total number of accepted scripts = 12
Total number of assessed scripts = 12

Rationale
Human capacity building in Nigeria has become a necessity, and we must all wake up to that fact. The motivation to start up something is one thing, but staying motivated to finish it up is another thing. The main aim of this competition was to steer the younger generation to a path of creativity, curiosity, technical capacity, problem solving, and tenacity. It was also designed to make them optimistic and look forward to a wonderful career in structural engineering.

While assessing the scripts, I made a lot of observations, and I will briefly summarise them using the points below;



(1) A lot of participants disregarded the first instruction of the exercise which was to pay attention to details. A lot of people lost marks by assuming weight of finishes, when the details of the finishes were clearly specified. Other people quoted values and formulars without properly referencing them. This affected a lot of people.

(2) In most cases, there was poor reading and interpretation of the architectural drawing. All those who modified the structure significantly lost a lot of marks.  However, I saw a lot of brilliance in some people with the way they managed the complexities of the architectural drawing, and produced a very good design. Some others came up with interesting GA's that are stable and buildable, but not very economical, so they lost marks in that aspect. For some others, they came up with GA's that are good, but did not reflect it properly in their analysis.

(3) No single person from South-East or South-South part of Nigeria participated in the exercise. I hope to see more of them next time.

(4) Finally, structural design is not about evaluating  M/fcubd2 and providing 2Y16, but it is more about the processes that led to the result, and the ability to execute the design economically, with adequate reliability.

So this is the result of the challenge;

I wish to say a very big congratulations to the winners;

1st Position - USMAN UMAR (Ahmadu Bello University, Zaria)
2nd Position - Ogungbire Adedolapo (Osun State University, Osogbo)
3rd Position - Olajide Bukoye (Federal Polytechnic Offa, Kwara State)

We will be celebrating them with their certificates and prize money in our next post. Structville will engage all the participants with corrections, recommendations, and discussion on all aspects of the design. Thank you, and God bless you.


Saturday, June 30, 2018

June 30, 2018

Design of Precast Seating Decks for Stadium


One of the most common construction concept of sports stadiums today normally involves having precast concrete terrace units (seating decks) span between inclined (raker) steel or reinforced concrete beams and rest on each other, thereby forming a grandstand. The raker beams are usually formed in-situ with the columns of the structure, and forms part of the structural frame of the grandstand. It is also feasible to construct precast raker beams as was done in the Corinthians Arena Sao Paolo, Brazil for the 2014 FIFA world cup.




Fig 2: Double L Precast Seating Deck


Fig 3: Structural Section of a grandstand

Precast seating decks are usually of L-shaped reinforced concrete units of length usually between 7-8 meters spanning between the raker beams. The seating decks also rest on each other. The role of the third (resting support) is to stop the units from undergoing excessive twisting, and in general, provide extra stability. Seating units are used to span between raker beams, and form the exposed surface which the seats are bolted onto. The seating units are fabricated in moulds depending on the length of the span, angle of inclination/curve, and support conditions.

Fig 4: Triple L Seating Deck being installed at Cape Town Stadium SA

Seating units can be easily installed on site, and when the joints between units have been sealed, form an effective barrier against external elements. Precast seating units can also be easily installed in steel structures.

Design Example
Let us design a 6m long precast seating deck for a stadium with a section shown below;


Fcu = 35 N/mm2; Fyv = 460 N/mm2; Fy = 460 N/mm2
Concrete cover = 30 mm
Unit weight of concrete = 24 kN/m3

Loading Analysis
Load type = uniformly distributed loading

Dead Load
Self weight of the unit = (24 × 0.15 × 0.25) + (24 × 0.15 × 0.95) = 4.32 kN/m
Make allowance for stair units and railings = 2 kN/m2

Live Load
For grandstands with fixed seating = 4 kN/m2
Making allowance for dynamic magnification = 5 kN/m2

At ultimate limit state;
n = 1.4gk + 1.6qk
n = 1.4(6.32) + 1.6(5) = 16.848 kN/m


Design Moment Mmax @ 3.0m = (ql2)/8 = (16.848 × 62)/8 = 75.816 KN.m
End shears = ql/2 = (16.848 × 6)/2= 50.544 KN

Design of the section to resist the applied moment
M = 78.816 KN.m
Effective depth d = h – Cc - ∅⁄2 - ∅links

Assuming Y16mm for main bars and Y8mm for links
d = 400 – 30 – 10 – 8 = 352 mm

b = bw = 150mm (since the flange is at the bottom)
k = M/Fcubd2 = (78.816 × 106)/(35 × 150 × 3522) = 0.121

la = 0.5 + √[0.25- 0.121/0.9] = 0.8399

ASreq = M/(0.95Fy.la.d) = (78.816 × 106)/(0.95 × 460 × 0.8399 × 352) = 610 mm2

In the web, provide 2Y16mm + 2Y12mm (ASprov = 628 mm2)
Provide 2Y12mm (Asprov = 226 mm2) in the compression zone.

Spread the As,req also along the width of the thread
Provide Y12 @ 175mm c/c Top and Bottom (Asprov = 646 mm2/m)

Distribution bars
Provide Y10 @ 200mm c/c as closed links




Check of Deflection
Basic span/effective depth ratio = 16 (for simply supported beams of b/ b < 0.3)

In this case bw/bf = (0.15)/(0.95) = 0.157

Modification factor for tension reinforcement
Service stress F.S = (2FyAsreq)/(3Asprov) = (2 × 460 × 610)/(3 × 628)
f.s = 297.9 N/mm2
m.f = 0.55 + (477 - Fs) / 120(0.9 + M/bd2)
m.f = 0.55 + (477 - 297.9) / 120(0.9 + 1.1445) = 1.28

Limiting span/effective depth = 1.28 × 16 = 20.48
Actual span/effective depth = 6000/352 = 17.045
Actual < Limiting, therefore deflection is satisfied


Design of the section to resist shear
Critical end shear = 50.544KN
Shear stress v = V/bd = (50.544 × 103) / (150 × 352) = 0.957 N/mm2

0.957 N/mm2 < 0.8 √35 < 5 N/mm2

Concrete resistance shear stress
vc = 0.632 × (100As/bd)1/3 × (400/d)1/4
vc =0.632 × [(100 × 628)/(150 × 352)]1/3 × (400/352)1/4
vc = 0.632 × 1.059 × 1.032 = 0.69 N/mm2

For concrete grades greater than 25 N/mm2
vc = vc(Fcu/25)1/3 = 0.69 × (35/25)1/3 = 0.772 N/mm2

0.772 N/mm2 < 0.957 N/mm2
0.5 vc < v < (vc + 0.4)

provide minimum links with spacing
sv = (0.95AsvFyv)/0.4bv
(Trying 2 legs of Y8mm bar)
sv = (0.9 5 × 107 × 460)/(0.4 × 150) = 735.62mm

Maximum spacing of links = 0.75d
0.75 × 352 = 264m
Provide Y8 @ 250mm c/c links

Detailing Sketches



Sunday, June 24, 2018

June 24, 2018

Concealing of PVC pipes in RC Columns


In modern building construction, PVC pipes (plumbing works) on the surface of buildings is not always very desirable. In a country like Nigeria, PVC surface pipes deteriorate quickly due to weather conditions thereby leading to increased maintenance costs. On the other hand, they are usually not aesthetically pleasing.

Fig 2: Surface piping in a building




To solve this problem, architects normally provide ducts for MEP services (which is the best practice) during the design of a building. Another option that is normally considered is to conceal the pipes in walls or structural members.

Two things are actually involved;

(1) If it is an intentional design, or
(2) if it is an afterthought.

When it is part of the design, the structural engineer takes into account the effect of the pipes before producing working drawings. But when it is an afterthought, there is need to carry out checks and evaluate the effect of the plumbing work on the structural element before signing off.

Fig 3: PVC pipe concealed in masonry wall
We have always seen situations in buildings where structural members are compromised in order to allow pipes and other services pass through. This should not be so because starting from the onset, we should realise that services are part of a building, and should be considered during the planning and design stage. The flow of services in a building should not be an afterthought.

Let us use this example below to highlight the effect of installing PVC pipes in reinforced concrete columns.

Example
What is the effect of passing a 75 mm diameter pipe longitudinally through an axially loaded short reinforced concrete column with the following data?

Size of column = 230 x 230 mm
Grade of concrete = 25 Mpa
Grade  of steel = 410 Mpa
Reinforcement provided = 4Y16
Design axial load on column = 593 kN




Solution
From equation (39) of BS 8110-1:1997;

N = 0.35fcuAc + 0.7fyAsc

From the data provided above;
Asc = 804 mm2 (4Y16)
A=  (230 x 230) - 804 = 52096 mm2
N = [(0.35 × 25 × 52096) + (0.7 × 410 × 804)] = 686588 N = 686.588 kN

686.588 kN > 593 kN (Therefore column is adequate without the pipe)

On introducing the 75mm PVC pipe;
Area of pipe = (π × d2)/4 = (π × 752)/4 = 4417.86 mm2

Hence;
A=  (230 × 230) - 804 - 4417.86 = 47678.14 mm2
N = [(0.35 × 25 × 47678.14) + (0.7 × 410 × 804)] = 647931.725 N = 647.931 kN
647.931 kN > 593 kN (Therefore column is still adequate with the 75 mm pipe passing through it)

However, we should anticipate more complex interaction and verifications when the loading of the column is complex. Apart from the reduction in load carrying capacity of columns, care should be taken to ensure that the concrete is well consolidated to avoid honeycombs especially around the pipes. Also adequate care should be taken to ensure that the pipes are not leaking (inclusive of the joints) by pressure testing before concreting is done. Leakage of pipes might compromise the reinforcements by corrosion.

Summarily, adequate design and planning for pipe network in a building is the best solution - all options available should be evaluated. As far as possible, it is best to let the structural members be.