Waffle Slab
Waffle Slab
(OP)
I am checking a waffle slab building built in 1956, in particular the shear strength of the waffle ribs. Does anyone know answers to any of the following:
a) the ACI permissible working strength design shear stress for the ribs (i.e. similar to joist ribs)?
b) was the shear checked at the effective depth from the support face (which for the ribs would be "d" from the face of the solid area of slab surrounding the supporting column)?
c) were there CRSI table available in 1956; if so, does anyone have them?
I have the National Building Code of Canada 1960, so I know what was done then (90 psi permissible shear stress) and check "d" from support face), but that may have been different from what was in 1956, and in any event I suspect the waffle design would have been selected from the CRSI Tables, if there were CRSI Waffle Slab Tables in 1956.
a) the ACI permissible working strength design shear stress for the ribs (i.e. similar to joist ribs)?
b) was the shear checked at the effective depth from the support face (which for the ribs would be "d" from the face of the solid area of slab surrounding the supporting column)?
c) were there CRSI table available in 1956; if so, does anyone have them?
I have the National Building Code of Canada 1960, so I know what was done then (90 psi permissible shear stress) and check "d" from support face), but that may have been different from what was in 1956, and in any event I suspect the waffle design would have been selected from the CRSI Tables, if there were CRSI Waffle Slab Tables in 1956.






RE: Waffle Slab
Inside, it says:
So I'm not sure if my copy is the same as what was available in 1956 or not.
BA
RE: Waffle Slab
Tabulated safe loads are the maximum obtainable within the stresses and factors of the 1956 ACI "Building Code Requirements for Reinforced Concrete" and should be used only by one familiar with reinforced concrete design. No increase above the tabulated values should be made.
BA
RE: Waffle Slab
RE: Waffle Slab
I am looking at an old textbook "Reinforced Concrete Design" by Sutherland and Reese, Copyright 1943, which was used at the University of Alberta in 1953 for concrete design. It refers to ACI "Building Regulations for Reinforced Concrete" (1941). During the war, I don't suppose there was much research done on reinforced concrete as there were more pressing matters to attend to.
The book goes through calculations for tee beams and ribbed slabs but not waffle slabs. I have not perused the book sufficiently to answer all of your questions about shear, but the book presents fairly detailed calculations and might be useful for your present purpose.
BA
RE: Waffle Slab
1947- Building Code Requirements for Reinforced Concrete (ACI 318-47)
1951- Building Code Requirements for Reinforced Concrete (ACI 318-51)
1956- Building Code Requirements for Reinforced Concrete (ACI 318-56)
I have a copy of the 1947 code and the allowable shear stress is as follows:
And here is a part copy of Section 8 re shear for beams/joists:
I am not sure if this of assistance.
“…structural engineering isn’t rocket science. Evidently, it is considerably more difficult.”
Norbert J. Delatte, Jr., PhD, PE. 2009. Re Harbour Cay Condominium Collapse, March 27, 1981.
RE: Waffle Slab
Wow! That is exactly what I was looking for, and gives the complete picture of allowable stress and how to calculate it. Thanks for going the extra mile to find it and post it here. Much indebted to you. Interestingly, the allowable 1951 ACI shear stress of 60 psi for 3000 psi concrete was much less than the 1965 National Building Code of Canada's (NBC) 90 psi. The current NBC says for permissible factored shear:
vc = 0.19 phi root f'c (metric)
which translates into unfactored imperial as
0.19 x 0.65 x root (3000/145) x 145 / 1.35 = 60 psi
where 1.35 is the usual average load factor of the 1.25 and 1.5 dead and live load factors respectively. Current ACI has different factors but I believe the end result is similar.
I note that the 1951 ACI did not have the 10% increase for joists that later codes have, including the current NBC.
So current Code seems to be essentially the same as 1951 ACI, except for the 10% increase for joist ribs.
Just wondering how did you get access to this information?
To BARetired: Thanks for that info. We have in the office texts going back to 1912 as the precursor of our current firm started in 1921. I am now generally working at home, but am going into the office today and will have a look at some of these old books. They are generally very well written to cover the technology of their day. I have the 1963 Toronto Code at home and it gives 90 psi, with no allowance for increased shear for joists.
So concrete designed within a certain time period of time (1965 to ?)is probably deficient in shear.
Thanks again for your help.
RE: Waffle Slab
RE: Waffle Slab
You are welcome.
Just to be perfectly clear, the tabulated info above is from 1947 NOT 1951.
Over the years I have been adding to my digital library and this is one of the documents in it. I have hard copies of all ACI-318 dating back to 1963. Some interesting info: the 1947 code was 64 pages (in small book format) and cost 50 cents, or 40 cents for members. Oh, how times have changed - ACI318-14 is 520 pages and $249.50 or $149 for members.
Hokie93,
I don't have the USB of the "318 Historical Building Code Requirements (1908-2005)" but I have been meaning to purchase a copy for some time, and at $119 member price it is great value. Can you tell me if the files are regular PDF files, that you can cut and paste, or are they formatted under a proprietary reading system?
“…structural engineering isn’t rocket science. Evidently, it is considerably more difficult.”
Norbert J. Delatte, Jr., PhD, PE. 2009. Re Harbour Cay Condominium Collapse, March 27, 1981.
RE: Waffle Slab
ACI 318-47 indicates two values for allowable shear, namely 60 psi without special anchorage of the longitudinal steel and 90 psi with special anchorage.
BA
RE: Waffle Slab
RE: Waffle Slab
Many thanks for the reply. SECURED individual PDF is okay, I just take issue with proprietary PDF secured-reader files that are a pain, like the Post-Tensioning Institute has done recently with electronic copies of their documents using LockLizard. I understand PTI want to protect their IP, but is cumbersome for the user.
I shall now purchase the ACI-318 historical bundle - thanks for the info.
Sorry to hijack your thread, ajk1.
RE: Waffle Slab
Also, I erred in saying I was quoting from 1965 NBC; it was actually from 1960 NBC (but I had it in a binder cover that was from the 1965 NBC, thereby misleading myself. I have now removed it from there).
It would be useful to have a table summary of the permissible shear stresses from the various editions of ACI (and from NBC for Canadians) for beams, joists, and flat slabs, and whether the critical shear periphery for flat slabs was at d or d/2 from support face. Then when it comes to do repairs, or review floor strength design, one would have an indication early on if it would likely not meet current Code. Maybe if I ever get nothing to do, I will try to make such a summary. I wonder if ACI has such a summary?
Hokie93 -thanks for bringing the ACI historical building Codes document to my attention. I wonder if a 51 year member of ACI like me might get an additional discount. I think I shall get it.
BARetired - thanks for pointing out the increased allowable shear if longitudinal bars have special anchorage. I don't see that wording in my 1963 edition of ACI. Do you know what they mean by "special anchorage"?
RE: Waffle Slab
From ACI-318-47, Section 802(a)(4)(page 33 below), references to Section 804(f), on page 34, as follows:
Page 33:
Page 34:
RE: Waffle Slab