6 Structural Design tips for Civil Engineers to take correct & better site decisions

Every one knows Structural engineering as a specialisation of civil engineering. However it is not the specilisation degree alone that matters but the experience and the ability to appreciate the basic principles of structures.This ability to appreciate structural behavior is handy not only for

structural design engineers but also for all civil engineers.


For many,design is an office job and young engineers think learning ETABS Tutorials and Revit Structures alone makes them independent.But the fact is that structural design is not limited to modelling,analysis and design of structures.It also involves a thorough understanding on how structural designs are executed at site.

The same way it is required that the construction engineers understand the basic structural design principles to appreciate why and how to construct what is designed.


Here are 6 tips that is significant even if you are not a structural design engineer but a building site engineer or a civil engineer.


Many appreciates the role of a structural engineer as only ensuring strength. In reality,it also includes stability,servicebility,buildability and much more.Ensuring economic design is also a huge responsibility as all clients would expect to cut down the cost especially in the current times.

To achieve certain requirements and economy,very often, structural engineers rely on construction sequence and situations to create certain design conditions that allows improvisations. It is very important for the site engineers to appreciate these conditions and follow the steps of constructions. It is also very important for structural engineers to convey this to site. The ideal world will be site incharge himself being a structural engineer. This is quite the case in many large scale projects and it is great if all civil & structural engineers starts appreciating the complimentary role of each and master the required skill set!


A few very general conditions are mentioned below.


1. Effects of Construction Sequence on Design


a) Stability of certain elements


i) Lintels

Let us consider a framed building where we have columns,beams and slanbd. In such cases,mostly the lintels will be just over the window and not continuos and connecting to columns.

Now the cantelever sunshade do induce an over turning moment and this is not going to be balanced by the weight of wall alone (as the wall is too short and light and the lever arm is far too less to offer any balancing moment) Here the tightness of the brick packing is maintaining equilibrium and hence the construction sequence has to be precise.

Sequence 1 - The frame is up and constructed

Sequence 2 - The wall up to the lintel is constructed

Sequence 3 - The lintel and sunshade is concreted and supported by a forwork

Sequence 4 - The wall above the lintel is built

Sequence 5 - De shuttering of the sunshade is done


Here ,the deshuttering of the sunshade shall not be done prior to sequence 4 as the tightness of the packing offers stability. Structural engineer would have stated this as a note most likely. However a bit of understanding of this helps at site to avoid any kind of issues like over turning of the lintel and sunshade.

It is also worth to point out that the responsibility to ensure this is a shared one and the structural drawings should have a note and the consultant is supposed to convey the design consideration as appropriate so that same is achieved during construction.


If you want any additional videos on this point,do this free course on stability which fully explains it.


ii) Propped Cantelever retaining walls

Many times when you have a retaining requirement near to drive way in an apartment ,there may be a RCC cover slab for the drive way and this slab is infact propping the cantelever and changing the behavior of the retaining wall.This change in behavior would have been taken in to account in design and the steel and thickness would have been taken care of appropriately. However the same should be executed at site. If dowells are provided and if the roof slab is being concreted at a later date,care shall be taken to ensure that the back filling of retaining wall is done only after the slab is in place as a prop.

Same way many times the sliding of the retaining walls are controlled by the gradeslab monolithically connected to the base of the retaining wall or the roof slab propping it and the base length would have been designed lesser to suit thes design considerations. If this is not taken care at site, there can be serious issues to the safety.


The same prop considerations are often employed in multi basement design where the touch piles/tangent piles are considered propped during the construction and a truss system may be needed at site to ensure the design considerations are replicated at site till the roof of basement is constructed/connected. There was a 3 basement structure failing in the city of Cochin,a couple of years back and its worth looking it as a case study.


If you are a beginner and want to get more explanation on these points,or want some detailing understanding,Contact us from the contact page of the website mentioning clearly what is the clarification needed or write to contact@civilera.com


b) Forces in certain elements

i) Over Head Tank with cover slab

Similar to the propped cantelevers explained above,OHT having a monolithic cover slab also is idealised often as a propped cantelever. This helps in reducung the side wall thickness and the steel.If the inside side face steel of side wall is much more than the outside face at the base, then it is probably a cantelever wall and if the outside face steel of wall at midspan is not nominal , then it is probably considered as a propped cantelever in the design.If the water tank is designed as propped cantelever, care shall be taken to ensure that the tank is filled with water only if the cover slab is concreted and connected with the walls.

Note : 1 Some times the the height of the tank may be less or due to some reasons both face steel may be the same.Always understand the design 100% if you are taking a decision on these aspects.

Note 2 In case of sump tanks which is under the ground, there will be earth pressure too and the pattern of steel I mentioned above will not be the case.The inside face steel at the mid point might be more than the outer face because of the earth pressure at empty condition. If you have any questions,Contact us from the contact page of the website mentioning clearly what is the clarification needed or write to contact@civilera.com


2. Anchorage of Rebars

Most site engineers follows the drawings provided at site and this is a requirement to adhere.However there are many reasons that it will be better if they appreciate calculating this value. A thorough knowledge on this point than just knowing 50 times dia of rebars as the anchorage length is useful. Also note that it is not 50 times dia of rebars all the time. It depends on grade of concrete and grade of rebars and a few other parameters. There are more important points on bents and hook and its significance.Some times by accident, one may end up having slight shortage of anchorage length. What do you do in such situations?Untie the rebars and redo the details at site?or do a welding? A basic understanding of how anchorage is calculated will come to your help.You can read about anchorage in my separate blog on ANCHORAGE


3. Where to provide sleeves in a beam

This is a design item and the structural design engineer has to decide it.However some times a smaller sleeve may have to be incorporated at the last minute just before construction and if the site engineer knows the fact that sleeves preferably shall not be in the compressive zone of a beam,can help avoiding mistakes. I am not elaborating this topic here as I have already covered this topic in another blog exlusive on sleeves in beams.


4. A basic understanding of stiffness


I can explain this point only with an example.See the Fig

There are 2 beams which are meeting.One is the main beam and the other is a secondary beam.Which one?

Before that,why should a site engineer even know which is primary beam!

Many times, details issued to site will be conveying only the design intent and the exact location of laps may not be indicated. An understanding of this point helps to avoid lapping of bars at undesirable locations- say at the midspan of main beams

So in this case, if the spans are equal, it is required to know if B1 is main or B2. The main beam bottom rebar shall not be lapped at the mid point where both meets as thats the point where you have maximum bending in primary beam.

I will be writing a separate blog on this a little latter.However these points are covered in our course ETABS Tutorials for Design of Building Superstructure


5. Curtailment of rebars

Where to curtail reabars is always indicated in the structural drawings.Some times the understanding where the approximate point of contraflexure is, will help you to pick mistakes in curtailment distances and co-ordinate with consultants. Some times you will be able to take decisions on certain things. If your site engineering experience is supported by understanding of structural basics,that would be the best combination.


6. More cover to rebar is not always great

Cover to rebars are mainly decided based on durability requirement.More the cover,more the durability as lesser the chances to be in contact with water,air or any corrosive element. The minimum required cover is specified in code of practices and in the case of India,it is specified in IS-456. Though a site engineer can read and interpret this,generally the cover is specified in the drawings by the structural engineer. The reason to this is that effective depth/section decides the capacity of the members.This means that more cover might be good for durability but this reduces the strength/capacity.There fore it is essential that this is specified by the designer and not by the site engineer. This understanding helps the engineer to stick to the requirement and not exceeding the cover requirement.


Summary

There are many more structural design points that will be handy for a site engineer. In reality all civil engineers should be knowing structures and all structural engineers should be knowing the basic construction related aspects including constructibility issues and the sequences. Here I have indicated very basic points as I expect the audience to be fresh civil engineers.


In my view,there is a need to narrow down this gaps in understanding. Structures and site engineering go hand in hand and are inseparable.A good site engineer has to possess sound structural knowledge and vice versa.

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