Updated: Apr 21, 2021
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
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 firstname.lastname@example.org
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 t