Updated: Jan 19, 2022
Openings in slab is some thing that is not discussed as much as it needs to be.
Most Engineers,Architects and clients take slabs for granted and abuse is it during and after construction by providing cutouts and overloading and so on. However if you are a professional there are a few technical points that you will have to consider to ensure every thing is allright
Here are 4 tips for you to understand and consider while deciding and providing openings in slabs.
1. Puncturing slabs after construction
Drilling openings in slabs is usually done to run pipes vertically down the building.These openings are drilled to run electrical,plumbing and irrigation pipes.Though drilling is not prefered some times it is not avoidable as planning of every opening is some times difficult.All I can say is avoid drilling after construction as much as possible.However drilling a slab is slightly better than drilling a beam or column.If you want to read about my blog on sleeves in beams, you can see HERE
Any large opening in slabs after constructuon shall be any way strictly avoided. All openings shall be planned ahead of construction.
2. Tip if you want to plan opening ahead of construction
Well! Yes we can plan ahead of construction in the planning stage, if all stakeholders including architects and engineers work together.
Structurally this can be done in 2 ways.
a) Include the opening in slab analysis and design for it.There will be some additional stresses around the openings that needs to be attended to. You can see this video below that explains how to do openings in slabs in ETABS.
b) You can go by conventional RC detailing rules to take care of this additional stresses developed.
This is also mentioned in the same video above.Basically it is about providing additional rebards around the edge and corner of the cutouts as per SP 34
The additional rebars take care of the additional stresses and provide load path around it by bridging the load. This is explained in the video. Though it sounds a little thing,proper detailing will ensure we dont end up having undesirable cracks and weaknesses.
For smaller openings this additional rebars would do the job where as for large openings, one will have to attend it to it in the analysis. More over,if the openings are large, then there is another problem related to seismic analysis. This is mentioned in point 3 below.
3. Large openings in slabs
Large openings in slabs is a problem as it induces irregularity in your building. This can result in excess seismic demand for the building and attending to it can result in un economy.
Even if it is attended to,there can be issues with the performance of the building during a seismic event. This horizontal irregularity is mentioned in a video in our youtube.Its a 13 minute video that explains all kind of horizontal irregularity.If you are in a hurry, you can skip to the 9th minute of this video HERE
4. Openings in slabs after construction
All the 3 points above spoke about having openings in slabs before construction. Though I mentioned it is not good to do it after construction,many times users want to do it for renovation and extensions. The clients are alwyas confused on this aspect on how to do it and if they can do it. This topic needs a bit of elaborate explanation and I will be covering this in one of my next blog or webinars. Please ensure you are in my structural group.
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Openings are needed in slabs and even in beams at times to run the services pipes. Care shall be taken while incorporating them.Clients look forward for openings in slabs for horizontal shafts for services.Some are before construction which can be planned and some are after construction.Some are possible some are not.Even where possible,special attention is needed