Updated: Jan 19
Openings are needed in a building for various reasons. These are mostly needed for services reasons. The openings can be vertical shafts which are mostly in slabs. Vertical routing of services also will be needed and for this, there will be openings needed in beams and walls and basement walls.
This blog specifically discuss the attention needed structurally on openings provided in beams.
Why openings in Beams :
Openings are required for providing pipes for plumbing and other services.Some times the sleeve needed will be very small and this can be provided with out much structural attention.Please read the design considerations in the below section to understand more about sleeves in beams.
Why pipes cannot be under the beam:
Ideally services pipes shall be run under the beams.However clients and services consultants do not prefer this many times.Pipes will have to bent, Floor height will reduce.You can think of a 20 floor building and the builder getting approval for 60m height for building.a 150mm pipe in all levels will eat up 3m of the total building and this means he looses 1 floor and this means the builder loose an opportunity to sell a floor.
Also note that, manytimes, builders and Architects prefers flat plates or flat slabs in situations where the height has a restriction. This helps in avoiding sleeves and also achieve the number of floors as desired. However this will have some issues for the sesismic resistance of the building.
You can read more about this topic here.
Options for openings:
Core cutting – Drilling after the beam is concreted. This should be avoided strictly as it weakens the beam and also may affect the provided rebars.This is some thing that we should never recommend or allow at site.Hard reality is that many do this with out planning the openings properly.
What is a sleeve? :
A planned and designed opening is called a sleeve. It is provided prior to concreting by leaving a pipe during concreting. Also structurally, the beam is designed for the opening and additional rebars are provided to take care of the loss of strength if any due to the introduction of opening in the beam.
Small square or circular openings may be placed in the mid-depth of the web.
Cover to main bars & links shall be satisfied all around the sleeve.
Clear distance between such openings, shall not be less than 150 mm.
The area of small openings shall not exceed I 000 mm2 for members with an effective depth less than or equal to 500 mm, or 0.004 d2 when the effective depth is more than 500 mm.
The opening shall not be in compression face. Near columns and supports, bottom region of beam is compression zone and near span, top part is compression zone.
The basic principle of this above point is that, concrete is never considered to take tension in a beam and hence we can afford to have sleeves in tension zone.Only rebar is taking the tension. You can read this blog here on filler slab which has the same principle where we remove concrete from the tension zone.I have explained this principle better there.
Whenever the largest dimension of an opening exceed one-quarter of the effective depth of the member, it is to be considered large. In no case shall the height of the opening exceed 0.4 d nor shall its edge be closer than 0.33 d to the compression face of the member Main bars & stirrups shall be placed in the compression side of the web to resist one and one-half times the shear across the opening.Stirrups, extending over the full depth of the web, shall be placed adjacent to both sides of a large opening over a distance not exceeding one-half of the effective depth of the member to resist twice the entire design shear across the opening. More info : Refer SP34
In case you want to read about openings in slabs and the point that you need to consider structurally, read my other blog here.
If you have any thing to ask me on this topic,write in the comment box here
You can also join there in the above link to be in my webinar group.
Core cutting of beam is a strict no.You should not be doing it.Planned sleeve is better if absolutely required.The best way would be to run the pipes below the beam with out sleeves.However this cannot be a structural decision alone.Try adopting the design practices mentioned above fully and correctly for better performance of the building.