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Thursday, May 16, 2019

May 16, 2019

Structural Design of Flat Slabs to Eurocode 2

Flat slabs can offer economical solutions to wider floor spans in a reinforced concrete building. Flat slabs are slabs that are supported directly by columns without floor beams. The columns may or may not have drops. There are many advantages of flat slab such as increased head room, easier flow of mechanical and electrical services, ease in construction of form work, faster construction etc.

Saturday, May 4, 2019

May 04, 2019

Simplified Design of Welded Connection to Eurocode 3

In our previous post, we were able to present an example on the design of fillet welds for truss members using two different approaches. In this post, we are going to simplify it further by presenting an example on how the strength of fillet welds can be easily verified, according to the requirements of Eurocode 3.

Tuesday, April 30, 2019

April 30, 2019

Example on Structural Design of Waffle Slab

Waffle slabs can be described as the equivalent of 2-way solid slabs especially when the spans are large, and ribbed system is to be adopted. The design of waffle slabs is the same as that of ribbed slabs, with the difference being that waffle slabs have ribs spanning in both directions, and the coefficients used for analysing the slab is similar to those used for two-way restrained slab. Waffle slabs are supported on beams or columns, where the support zones are made to be uniformly thick.

Friday, April 26, 2019

April 26, 2019

8 Top Civil Engineering Questions (Part 2)

(1) For the structure shown in the figure below, which of the following is the most likely bending moment diagram considering linear first order elastic analysis?


(2) For the structure shown in the figure below, what is the most likely shear force diagram?


(3) A building is generally divided into how many parts?

(A) Four
(B) Three
(C) Two
(D) Six

(4) What is the recommended maximum spacing of shear links in a beam according to British and European codes?

(A) d
(B) 1.5d
(C) 0.75d
(D) Width of beam

(5) What is the recommended minimum compressive strength for water retaining structures according to the Eurocodes?

(A) C20/25
(B) C25/30
(C) C30/37
(D) C35/45

(6) The process through which water reacts with free cement after hydration reaction in a concrete structure to seal up cracks is known as?

(A) Post-hydration reaction
(B) Consolidation healing
(C) Efflorescence
(D) Autogenous healing

(7) How many percentage of 28 days strength is expected of concrete after 7 days?

(A) 50%
(B) 65%
(C) 30%
(D) 75%

(8) A beam of span L is subjected to a uniformly distributed load w with support conditions pinned and fixed at either ends. What is the bending moment at the fixed end?

(A) wL2/8
(B) wL2/12
(C) wL2/16
(C) wL2/24

Thursday, April 25, 2019

April 25, 2019

Use of Polystyrene in Ribbed Slabs: Structural Design Example

For long span slabs in a building subjected to light load, ribbed slabs are more economical than solid slabs. In ribbed slab construction, reduction in volume of concrete  is achieved by removing some of the concrete below the neutral axis of the section, under the assumption that the tensile strength of concrete is negligible. Two major forms of construction of ribbed slab are;

Sunday, April 21, 2019

April 21, 2019

Advise on the Stability of this Structure

As a structural engineer, advise the client/architect on the stability of the structure, distinguishing between transient and permanent situation.

Height - 11.28 m
Length - 20.00 m

Note: This is an existing observatory structure in Tielt-Winge Belgium. Report has it that it was recently vandalised and is looking to be rebuilt...
April 21, 2019

Comparative Analysis of Cylindrical Water Tanks

Water tanks are usually rectangular or cylindrical in shape. Cylindrical tanks are usually employed for surface or elevated water tanks. Some of the challenges involved with cylindrical tanks are the difficulty associated with construction such as setting out, formwork preparation/installation, reinforcement installation, increased labour cost, etc.

Cylindrical tanks are subjected to radial pressure from the stored water, and/or from the retained earth when they are buried under the ground. Just like rectangular water tanks, the analysis of cylindrical tanks is easily done with the use of coefficients picked from already made tables (see Fig 2.0). The values of the coefficients are usually based on the support condition of the wall relative to the base.

The internal forces normally analysed for are;

  • Circumferential tension (hoop tension)
  • Radial shears
  • Vertical moments

If the wall is supported on the base in such a way that no radial movement can occur, radial shear and vertical bending result, and the circumferential tension is always zero at the bottom of the wall. This is usually referred to as the fixed joint condition, and has been considered in this publication.

Table 1.0: Coefficients for Analysis of Cylindrical Tanks Fixed at the Base (Reynolds and Steedman, 2008)
Analysis Example
Determine the maximum service values for circumferential (hoop) tension, vertical moment and radial shear in the wall of a cylindrical tank that is free at the top edge and fixed at the bottom. The wall is 400 mm thick, the tank is 4 m deep, the diameter is 8 m, and the water level is taken to the top of the wall.

Hydrostatic pressure at the base of the tank (n) = 10 kN/m3 × 4 m = 40 kN/m2
From Table 1.0;
lz2/Dh = 42/(8 × 0.4) = 5
Maximum hoop tension (t) = αtnr = 0.477 × 40 × 4 = 76.32 kN/m

Maximum vertical negative moment (outside face) = αmnlz2 = 0.0059 × 40 × 42 = 3.776 kNm/m
>Maximum vertical positive moment (inside face) = αmnlz2 = 0.0222 × 40 × 42 = 14.208 kNm/m
Radial shear V = αvnlz = 0.213 × 40 × 4 = 34.08 kN/m

When analysed on Staad Pro as a concrete cylindrical tank with poisson ratio of 0.2, fixed at the bottom, and subjected to hydrostatic pressure of 40 kN/m2 the following results were obtained;

Fig 2.0: Finite Element Meshing of Cylindrical Tank

Fig 3.0: Horizontal Moment on the Tank Walls

Fig 4.0: Vertical Bending Moment on the Tank Walls
Table 2.0: Analysis Result from Staad Pro

Maximum vertical bending moment = 10.714 kNm/m
Maximum vertical shear stress = 0.073 N/mm2 = 29.2 kN/m
Maximum hoop stress (membrane) = 0.195 N/mm2 = 78 kN/m
Let us compare the results from Staad Pro with results from the use of coefficients (see Table 3.0).

Table 3.0: Comprison of Analysis Result from Staad Pro and use of coefficients

Thank you for visiting Structville today, and God bless you.

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

February 23, 2019

How to Design RC Beams for Heavy Shear Load

Shear failure usually occurs in form diagonal cracks, and must be checked under ultimate limit state during the structural design of buildings. In reinforced concrete buildings, shear is resisted using links (stirrups). When a design is failing in shear, the following solutions can be adopted;

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

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.

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