Civil Graduate 2.0

Hi, I am Himanshu Mishra, the man behind civil graduate [dot]com.
In last two years, I and my team have written hundreds of articles on various civil engineering topics. The articles have been read by thousands of people throughout the world.

Now I am feeling something strange. The information we shared in our articles is like a body without spirit. This is a blog and does not mean to be a textbook.

Now the blog is going to be more like 'Diary of a Civil Engineer' rather than an information sharing website.
In Civil Graduate 2.0, the theme of articles will be ranging from civil engineering articles (with a personal touch), test preparation strategies to life lessons.

I am grateful to the interns who put their faith in me and gave strengths to my vision, to friends who did not leave any stone unturned to make the blog popular.
Last, but not the least, I am thankful to the readers who have been my constant source of encouragements. I shall try my best to reach you every weekend.

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Prestressed Concrete

What is Prestressing?

Prestressing is the application of an initial load on a structure, to enable it to counteract the stresses arising from subsequent loads during its service period.

  • Prestressing denotes a method in concrete construction in which the member is subjected to initial compression at the time of manufacturing itself. Thus, prestressing induces compression on the lower side of a beam and when the load is applied, tension so produced neutralize the compression already set up by prestressing.

Advantage of using Prestressed Members: 

1. In the case of RCC, the section below the neutral axis is ineffective (as all the tensile stresses are taken by steel). But in the case of prestressed concrete, the entire section is under compression and thereby increasing the load carrying capacity of the member.

2. Because of the entire section being in compression, the structure is crack free.

3. As the self-load is taken care of by additional eccentricity, the concrete required is about one-third the quantity required for corresponding RCC structure.

4. Thus because of reduced section and reduced weight, the cost of foundation etc. is less.

5. Most of the large span prestressed concrete structures are prefabricated under controlled conditions, the cost of shuttering and centering is considerably reduced.

Limitations in the use of Prestressed Structures:

The main limitation in the use of prestressed concrete is the comparatively expensive equipment and skilled operation.The method is, therefore, economical in large projects.

The grade of concrete used: M35 and above concrete.

Methods of Prestressing:

1. Pre-Tensioning:

In this method, the tendons are stretched to the desired amount. One end of the tendon is secured to an abutment, while the other end is stretched out with the help of prestressing jack. The formwork is then erected around the tensioned tendons and fresh concrete is poured, compacted and properly cured. At the end of the curing period, concrete is hard enough to sustain the compression passed through the bond. The projecting steel wires are then cut. The method is particularly adapted in precast beams, posts and simply supported slabs.

2. Post Tensioning: 

In this method, the structural member is cast, leaving ducts inside for subsequent insertion of prestressing tendons. The ducts are formed by placing CGI sheets inside the formwork or by keeping steel spirals or sheet metal tubes or any other suitable methods. When the concrete has been fully cured and hardened, the tendons are inserted inside the ducts. One end of the duct tendon is fixed while the other end is attached to prestressing jacks. When the required stretch has been given, the jack end of the tendon is also anchored and the duct is grouted with cement.

Thumb Rules to achieve economy in RCC construction

The following are some of the rules of thumb which will be useful to the practicing engineers:-

Minimize floor-to-floor height: 

By minimizing the floor-to-floor height, the cost associated with mechanical services, stairs, exterior building cladding can be significantly reduced. The limiting factor will be deflection considerations.

Use repetitive formwork

The cost of formwork may be very high and is not given due consideration by the designers. The cost can be reduced when the framing system is used repetitively (10 or more times) on a structure.

Use standard column size 

This can be achieved by varying the amount of reinforcing steel and the concrete strength within the column. This will allow for a single column form and will minimize the number of variations to meet beam or slab forms.

Adopt uniform column layout

Uniform column layout results in simple formwork, which can be used repetitively from floor-to-floor. Similarly, regular shaped buildings will be more economical than irregularly shaped buildings with L- or T-shaped columns.

As far as possible, use the same depth for beams 

The saving in formwork and shoring costs will exceed any additional costs for concrete and reinforcing steel. This will also provide a uniform ceiling elevation and minimize mechanical service installation difficulties.

Use high strength concrete in columns 

The high strength may reduce the column size or the amount of reinforcing steel required for the column.

Use high early strength concrete

This will allow for earlier form stripping and will reduce total construction time.

Specify self-consolidating concrete 

Heavily reinforced columns and beams can be very congested with rebar, which prevents the proper placement of the concrete. SSC maximizes concrete flowability without harmful segregation and dramatically minimize honeycombing and air pockets.

Specify locally available materials 

The use of local aggregates and recycled materials in concrete makes it a 'green' product, which is requested by environmentally responsible owners.

Consider accidental loads for important buildings

for high-risk facilities such as public and commercial tall buildings, the accidental load such as bomb blast or high-velocity impact should be considered.

Use commonly available size of bars and spirals 

For a single structural member, the number of different sizes of bars should be kept to a minimum.

Use the largest bar size that satisfies the design considerations

Use larger size bars in columns and smaller size bars in slabs. Larger diameter bars reduce the number of bars that must be placed and minimize installation costs.

Eliminate bent bars where possible 

Bent bars increase fabrication costs and require greater storage area and sorting time on the job site.

Avoid congestion of steel 

Congestion of bars should be avoided, especially at beam-column joints, so that all reinforcements can be properly placed.

Factors Affecting Shear Strength of Concrete

 Shear strength of any material is the maximum shear load a body can withstand before failure occurs divided by its cross sectional area.

Shear strength of concrete is affected by following factors:

1.Tensile strength of concrete: The inclined cracking load in shear is a function of the tensile strength of concrete.
2.Longitudinal reinforcement ratio: The shear strength of the RC beams is found to drop significantly if the longitudinal reinforcement ratio is decreased below 1.2–1.5 per cent.

3.Shear span to effective depth ratio: Its effect is pronounced when av /d is less than two and has no effect when it is greater than six.

4.Lightweight aggregate concrete: They reduce tensile strength than concrete with normal aggregates.

5.Size of beam: As the depth of the beam increases, the shear stress at failure decreases.

6.Axial forces : Axial tension decreases the inclined cracking load and the shear strength of concrete, whereas axial compression does just the opposite.

7.Size of coarse aggregate: Increasing the size of coarse aggregates increases the roughness of the crack surfaces, thus allowing higher shear stresses to be transferred across the cracks.

Read: Types of Shear Reinforcements

Types of Shear Reinforcements

Shear or web reinforcements, called stirrups, links, or studs, may be provided to resist shear in several different ways such as the following:

1. Stirrups perpendicular to the longitudinal flexural (tension) reinforcement of the member, normally vertical.

2. Inclined stirrups making an angle of 45° or more with the longitudinal flexural reinforcement of the member.

3. Bent-up longitudinal reinforcement, making an angle of 30° or more with the longitudinal flexural  reinforcement.

4.Welded wire mesh, which should not be used in potential plastic hinge locations. They are used in small, lightly loaded members with thin webs and in some precast beams


6.Combination of stirrups and bent-up longitudinal reinforcement

7.Mechanically anchored bars (head studs) with end bearing plates or a head having an area of at least 10 times the cross-sectional area of bars

8.Diagonally reinforced members

9.Steel fibres in potential plastic hinge locations of members.

Also read: Factors Affecting Shear Strength of Concrete

One Way and Two Way Slabs


                                        One-way Slab

  • One-way slabs, supported by parallel walls or beams, bend in only one direction and transfer their loads to the two opposite support walls or beams.

  • Even when a rectangular slab is supported on all the four edges, the slab may be considered as a one-way slab if the length-to-breadth (L/B) ratio of the slab is equal to or greater than two.

  • A one-way slab is designed for the spanning direction alone; the main tension reinforcing bars of such slabs run parallel to the span. For the transverse direction, a minimum amount of shrinkage reinforcement is provided.

  • One-way slab action is assumed in a ribbed floor (slab with joist beams) made of precast double-tee sections, in a ribbed floor with integral beams, and also in hollow-block or -cored slabs.

Plan view of one-way slab (a) Supported on two opposite edges
(b) Supported on all edges (L/B > 2)

Two-way Slab

  • When the ratio of long side to short side of a slab is less than two, it is called two-way slab.

  • The panel will deflect in a dish- or saucer-like form under the action of external load and its corners will lift if the slab is not monolithically cast with the supports.

  • Two-way slabs are designed to transfer their loads to all the four support walls. A slab supported on three edges or two adjacent edges may also be considered as a two-way slab. The load gets divided in the two directions, depending on the ratio of the sides.

  • Two-way slab behaviour is assumed in a waffle floor and in a waffle floor with integral beams. In waffle slabs, also called two-way ribbed slabs, ribs are provided in both directions of the span. The hollow block floor is constructed with blocks made of clay tile or lightweight concrete blocks.
Two-way slabs bend and deflect in double curvature

10 things to know about Designing a Multi-storey Building

1. Most of the buildings in Indian cities have ground plus three floors. The recent trend is to construct buildings with at least 10–15 storeys so that the massive housing and commercial needs are satisfied.

2. Most of these buildings are made of reinforced concrete (RC). Mass of concrete floor slabs, beams, and columns of RC construction is higher than that of comparable steel construction.

3. This results in larger earthquake-induced bending moments and axial forces. As the height of building increases, the behavior of the structure becomes more complex.

4. Such buildings are more sensitive to wind and earthquake loads and hence, need to be very carefully designed and detailed.

5. The Wind influences the design of the structural system of tall buildings as well as their shape and form.

6. Since the day-lighting design has a large impact on the sustainability of the design, the facade may be one of the most important factors in controlling the daylight and shadow that enters a high-rise structure.

7. The latest trend is the use of double skin, and occasionally triple skin, facade with ventilation systems. Double glazing with argon-filled cavities, triple-glazing, and glass coating can increase U-values.

8. With careful design, high-rise structures can be aerodynamically designed to resist high wind speeds and to simultaneously utilize them, by using strategically placed wind turbines, thus producing more energy with no risk to the safety of the building’s users.

9. Multi-storey buildings are often analysed, designed, and detailed using a commercially available software. The commercial software packages available in the market include STAAD.Pro, SAP 2000, ETABS, SAFE, Nastran, Midas NFX, ANSYS, and STUDS. Many of these programs have analysis and design capabilities.

10. Linear analysis is considered sufficient for buildings having 15 storeys, whereas dynamic analysis is preferred for regular buildings in high seismic zones and having a height greater than 40 m, irregular buildings, and slender buildings subjected to heavy wind loads.

BONUS: AutoCAD is the most preferred detailing tool.