How to reinforce an existing reinforced concrete slab due to a creation of a future opening
How to reinforce an existing reinforced concrete slab due to a creation of a future opening
(OP)
Hello all. I am currently working on a project whereby we are required to evaluate and potentially reinforce an existing concrete slab due to the creation of a future 4 feet by 4 feet future opening for electrical conduits. We are dealing with a very old building that is in good shape (built in 1930). The challenge here, is that the rebar has been placed both orthogonally and diagonally (I am not sure what the reason is, we do not have any existing calculations). The future opening would therefore cut through what seems to be the main bottom bars. Can anyone please provide consultation on what should be done to reinforce the slab, and confirm that the diagonal bars are in fact the main column strip? Please see attached for the plan view snippet and the anticipated opening will be.
Eagerly awaiting your comments.
Thanks!
Eagerly awaiting your comments.
Thanks!
RE: How to reinforce an existing reinforced concrete slab due to a creation of a future opening
If it were me I'd be trying to talk them into any of these locations. And also maybe smaller openings, even if it is more of it.
RE: How to reinforce an existing reinforced concrete slab due to a creation of a future opening
Thanks in advance
RE: How to reinforce an existing reinforced concrete slab due to a creation of a future opening
Whatever you do (carbon fiber, beams, steel members, etc.), it's vitally important that they're carrying the dead loads before the opening is cut.
RE: How to reinforce an existing reinforced concrete slab due to a creation of a future opening
I feel this is one of those times. At least I would be raising a stink until my boss told me to back off.
RE: How to reinforce an existing reinforced concrete slab due to a creation of a future opening
Also, I believe that what is shown in the existing drawing is an old design system termed the "CAP" system, whereby there are 4 strips (orthogonal and diagonal) each acting as a cantilever over the column in order to transfer the load back the column. I am wondering if that would change anything to the reinforcement method you are proposing?
Thanks in advance.
RE: How to reinforce an existing reinforced concrete slab due to a creation of a future opening
I've had electrical guys re-configure their floor layouts and conduit layouts to suit structural. Especially on retrofit jobs like this. Everyone understands it's significantly different than new design.
RE: How to reinforce an existing reinforced concrete slab due to a creation of a future opening
RE: How to reinforce an existing reinforced concrete slab due to a creation of a future opening
Now, if I were to do this, I would be putting beams on either side of the holes going from column to column. I would want to re-support the entire thing.
To me, it looks like you're taking out a significant portion of important steel.
If it wasn't required, why would they have put it in there to begin with.
I'm going to say it once more, and then I'm unfortunately going to bow out and let other people run with this if they want to pursue it. Don't do this! Find another way to get the conduits through the floor that doesn't require severing 20 bars.
RE: How to reinforce an existing reinforced concrete slab due to a creation of a future opening
I hope to hear other people`s ideas for how to detail the reinforcing system required for such an opening.
Thanks in advance guys.
RE: How to reinforce an existing reinforced concrete slab due to a creation of a future opening
Building on the idea of scanning (x-ray or covermeter) mentioned above, with shoring to the underside of the slab around the perimeter of the proposed hole, it may be possible to jackhammer our the concrete in the 4'x4' area similar to a through slab concrete repair. Do not cut any of the steel reinforcement. Then have the electrical contractor use the spaces between the existing rebar to thread the conduit through. If they need to cut one or two rebar, then the structural repairs are simpler than cutting all the rebar. When completed, repour the concrete using standard concrete repair techniques and then remove the shoring. Keep in mind that depending on the total amount of conduit that passes through the opening, you still may need to provide some structural strengthening, just much less than would be required if you cut all the rebar out.
RE: How to reinforce an existing reinforced concrete slab due to a creation of a future opening
The desired location for the opening is the best place for an opening in a flat slab, only complicated by the orientation of the reinforcement. But the slab doesn't know the direction of the reinforcement, but rather it will just follow the stiffness, and span primarily in the shortest direction. There was mention that this old system relies on cantilever action in all directions. If that is the case, cutting an opening in the M-M strip will not materially affect the cantilever capacity, but in fact just remove some dead load.
RE: How to reinforce an existing reinforced concrete slab due to a creation of a future opening
Slab span appears to be 27'8". What is the slab thickness?
And what is the existing/proposed floor loading?
This appears to be a C.A.P Turner Mushroom System of Construction. A somewhat infamous system dated back to the early 1900's. Early pioneering flat slab design was somewhat controversial, right up to the 1970's. Many competing systems were patent-protected and design methods were held closely.
The 'cantilever' reference you refer probably is in reference to what Turner called "mushroom" cantilever shear head:
Turner's 4-way 'belt' reinforcement system was based on extremely low total design moments:
Interestingly, many of his floor systems were load tested to very significant over-loads:
I think it was the 1963 edition of ACI 318 code that deleted the previous requirement where you could design for 80% of the total static design moment in 2-way flat slabs.
RE: How to reinforce an existing reinforced concrete slab due to a creation of a future opening
Thanks for your feedback. This is indeed the C.A.P. Turner mushroom system described by @Ingenuity! To answer your question, the slab thickness seems to be 12 inches, and the intended live load would be 50 PSF. The dashed lines on the plan snippet represent bottom steel and the solid lines represent top steel. The bars are detailed as "boat-shaped" whereby longitudinally they shift from top to bottom and then from bottom to top via diagonal sections (as per all reinforcing bars during that era).
From my understanding the C.A.P. system has 4 directions instead of the modern 2 directions we are used to dealing with, which is what causes the confusion. The intended opening is planned in the intersection of the bottom steel of the diagonal bands, which would normally be the intersection of the middle strip in a modern slab system and therefore usually not impose much of a problem... However, I also agree with @hokie whereby a slab does not see the direction of rebar, it only sees the rigidity defined by the column locations, and would want to span in the smallest direction (i.e. orthogonally, not diagonally). Also, in adding the opening, we are removing any load that would otherwise be there.
All that said, would you gents agree with the following: Doing a finite element analysis of the slab with the opening input, and verifying the redistribution of stresses. From there, I can verify if the existing remaining rebar is capable of resisting the moments that the program gives me, in order to see if strengthening is required?
Finally, if there is any strengthening to do, what would be the best way to do it without being too destructive (i.e. since the structure is already in great shape)?
Thanks in advance, everyone has been a great help so far.
RE: How to reinforce an existing reinforced concrete slab due to a creation of a future opening
What is the reason for such a big opening ? I fully agree with JAYROD. In past, i experienced that MEP people , architects demanding "exaggerated" openings which are compromising the behavior and strength of the structure. I always simply rejected .
If the opening is for future electrical conduits, drilling of several small bore holes should do the work..
I will suggest you to speak with electrical people and try to see the real demand for the no. of holes rather than compromising the behavior of 90 years old structure .
RE: How to reinforce an existing reinforced concrete slab due to a creation of a future opening
RE: How to reinforce an existing reinforced concrete slab due to a creation of a future opening
My suggestion will be,
i) Shoring of the subject slab with props and lifting about equal to elastic deflection,
ii) Braking of the opening with dimensions 5'X 5',
iii) Prepare a heavy steel frame of L sections with interior dimensions 4'X 4' , and weld the cutted rebar ends to the flange of L frame ,
iv) Fill the perimeter opening with high strength repair grout,
v) Take out the props after new concrete get hardened .
I want to express that , still I suspect for the dimensions of such an opening..
Good Luck..
RE: How to reinforce an existing reinforced concrete slab due to a creation of a future opening
I don't see that something like HTURKAK's frame or the curb suggested by others, just around the opening, does much good.
RE: How to reinforce an existing reinforced concrete slab due to a creation of a future opening
Thank you all for your advice so far. I am eager to hear other ideas for reinforcement of such an opening.
If reinforcement is required based on analysis, how would we detail an upturn beam or frp strips? In other words, where would they start and end? Would we simply only extend such reinforcements from the perimeter of the opening to the column strips? Or would we have to find a way to bring them all the way to the columns and connect into the columns? See my image below for my first option. The blue lines represent either upturn beams or frp. Awaiting your feedback
Thank you all un advance, much appreciated
RE: How to reinforce an existing reinforced concrete slab due to a creation of a future opening
Does the project really need the floor space directly adjacent to the proposed opening?
If not, you could remove whole middle area of the slab completely (the 13'-10" wide zone) and only keep the area with orthogonal bands / column strips. Then the column strips only need to carry themselves plus the loading directly above from column to column (which they are probably able to do), since there are no redirected loads from adjacent areas to take on any more. Lots of space for lots of cables and ducting
Some things to take into account if you want to take this route:
RE: How to reinforce an existing reinforced concrete slab due to a creation of a future opening
RE: How to reinforce an existing reinforced concrete slab due to a creation of a future opening
Additionally, consider that in the 27'8" E-W long-span direction there is only 7-5/8"Ø bars @ 7" in the half column-strip width of 5'3" (presumably 14-5/8"Ø @ 7" in 10'6" full column strip), however in the 22'4" N-S short-span direction column strips there are 15-5/8"Ø bars @ 9-1/2" to 9-3/4" centers over the full 11'2" column strip.
By more current design standards/methods, the total static panel design moment [L12*L2 vs L22*L1] is about 125% more in the long-direction than the short-direction, but less rebar!
If you have not already done so, a site visit is worthwhile to review the slab soffit just to make 100% sure if there are soffit thickenings BEFORE you undertake further analysis or strengthening proposals.
RE: How to reinforce an existing reinforced concrete slab due to a creation of a future opening
However, if I do the reinforcement of FRP or upturn beams on top of the slab, then we whould not interfere with the drop. Do you gents think that FRP stripes is a better solution than upturn beams? I am not sure how I would detail an upturn beam wihtout intervening destructively into the existing slab in order to have a monolithic connection?
Thanks in advance.
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RE: How to reinforce an existing reinforced concrete slab due to a creation of a future opening
RE: How to reinforce an existing reinforced concrete slab due to a creation of a future opening
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