Analysis - cut hole thru existing two way slab

[blocko]

Our project is to design a dumbwaiter installation into an existing building. The building is most likely from the 20's or 30's due to the two-way slab being a flat slab with drop panels and capitals at the columns. The flat slab is 9" thick with 1 1/2" topping. The reinforcement layout can only be assumed at this point. The capital, at the widest dimension, is 48"x48" and tapers down to the column, which is 20"x20". The building was built for heavy industrial telephone equipment, but is now office space. I am assuming that the slab was designed for a load in excess of 100psf.
Our design involves cutting a 26"x28" hole through the existing flat slab. The hole will be cut starting at one corner of the capital...both cuts going away from the capital.
My question is "how does one complete an engineered analysis to determine if the integrity of the slab will be compromised by cutting the hole?"...especially since we are creating this hole so close to a support. Obviously, our load is much less than the slab was designed for. Possibly, someone with extensive concrete design experience would look at this scenario and simply state that it will be fine and doesn't require any engineering analysis, due to the small loads being applied.
Lastly, since we are constructing a hoistway, we will have walls spanning between floors, thus we could provide support all the way around our cut-out, however, I would like to know how to design a hole in a two-way slab for future reference and to determine if the walls for this application need to be structural.
Thanks in advance for your help.

 

[sdz]

I think you will have to do an analysis using something like the strip method of design. (e.g. Hilleborg), or finite element. That's what you would do if designing from scratch.

Your bigest problem seems to be that you don't have much information on original design loads or reinforcement layout.

 

[austim]

blocko, you are really asking for the impossible.

You don't need any analysis to determine whether "the integrity of the slab will be compromised by cutting the hole". Cut any hole, the structure is weaker. That is about the total analysis that you can do with your current information.

Suppose you get a dozen responses from (anonymous) people claiming "extensive concrete design experience", what do you do then? Surely you cannot just say to yourself - "Well, all these people that I don't know, working I know not where, who claim experience that I cannot verify, and who have not seen the situation for themselves, say it will be OK. I will go ahead, and tell my client that I have satisfied myself, as the responsible engineer, that all will be well". I can hear the lawyers rubbing their hands in glee already.

My gut feeling is that you may well be able to cut the proposed hole without any resulting problems. I am saying this on the understanding that the hole will not affect the shear capacity of either of the adjacent drop panels (which presumably form the bulk of the column strips), and that most of the effective negative bending steel in each direction will be within the drop panels.

But were I responsible, I would be making a serious attempt at determining how much reinforcement there is in both the drop panels and the basic slab, and then doing the best analysis that I could. In the absence of any contemporary documentation, I definitely would not rely on any assumption regarding the original design loading.

 

[danTES]

I am doing something similar on a building from the 40's. It is a 3.5" slab and we are cutting a hole for a new make-up air unit. My question is simple, as I am really a beginnner at ths type of thing:

How do I analyse the shear and moment capacity of the two-way slab in order to determine whether or not the slab can handle the andditional load (ignoring the hole)? I am used to treating slabs as REALLY wide beams, and checking capacity based on reinforcement in one direction. However, my boss told me that the rebar is too closely spaced, and that the slab will have two-way effects.

My plan was to do worst case scenarios: check simply-supported distributed load in one direciton, then simply supported distributed load in the other direction. The moment distributions are easily taken from page 1-39 in the CPCA Concrete Design Handbook... ps, I'm working under CPCA Code.

Reply instantly and I don't get fired.

 

[sdz]

danTES,

Time to admit ignorance and find someone who can help you face to face. A forum like this can't give you the help you really need.

 

Have you considered reinforcing the cutout using externally bonded carbon fiber?

 

[drgeers]

The hole that you are proposing to cut is not that large. Of course the strength of the slab will be reduced, but I do not see why there would be a problem redistributing the load to other slab strips. The main problem is the reinforcing ratio. You will probably have to assume the min. reinforicing allowed at the time of construction for the slab in question.