Waffle Slab

[ajk1]

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.

 

[BAretired]

I have the CRSI Handbook Revised 1961.
Inside, it says:

Copyright, 1952​

By Concrete Reinforcing Steel Institute​

PRINTED IN THE UNITED STATES OF AMERICA​

FIRST EDITION, 1952​

SECOND PRINTING, 1952​

SECOND EDITION, 1957​

SECOND PRINTING, 1957​

THIRD PRINTING, 1959​

FOURTH PRINTING, 1961​

So I'm not sure if my copy is the same as what was available in 1956 or not.

BA

 

[BAretired]

Further to my earlier post, in the Introduction at the bottom of Page 1 is the note:

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

 

[ajk1]

To BARetired - ok, thanks, that is helpful. Now I have to see if I can find a 1952 edition of CRSI, or the ACI Code in effect in 1952. I am curious if the allowable joist shear stress was greater than current Code (after of course making due allowance for the load and phi factors in current Codes. I suspect that the National Building Code of that era would most likely be the same as the ACI, for shear stress.

 

[BAretired]

The first National Building Code of Canada came out in 1941 but had no legal status unless adopted by the jurisdiction in question. Some cities had their own building codes; in Alberta for example, the City of Edmonton had the Edmonton Building Code which referred to the Uniform Building Code for much of its content. It wasn't until about 1981 that the Alberta Building Code which was modeled after the NBC became mandatory throughout Alberta. Any existing codes were abandoned at that time, much to the chagrin of certain building officials. As I recall, the early NBC codes referred to ACI for concrete design.

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

 

[Ingenuity]

Re ACI-318, the following are 3 issue dates around that construction time:

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.

 

[ajk1]

To Ingenuity:

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.

 

[Hokie93]

The information posted by Ingenuity may have come from the ACI publication titled "318 Historical Building Code Requirements (1908-2005)". This document is available from ACI (

http://www.concrete.org)

for $119.00 (member price) or $198.50 (non-member price). I purchased the CD version of this publication several years ago and have found it to be quite useful.

 

[Ingenuity]

ajk1,

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.

 

[BAretired]

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.

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

 

[Hokie93]

Ingenuity: My copy of the document is individual pdf's for each ACI code. Each file has "SECURED" in the filename but I can cut and paste individual sections at will. My copy came on a CD. It has been a good purchase for me, well worth the cost.

 

[Ingenuity]

Hokie93,

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.

 

[ajk1]

Ingenuity - thanks for pointing out that the excerpt is from the 1947 ACI. Also, no need to apologize for piggy-backing this thread. Great to have this discussion and the things it brings to light. Much appreciated.

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"?

 

[Ingenuity]

Do you know what they mean by "special anchorage"?

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:

 

[ajk1]

To Ingenuity - ok, thanks. It seems that by special anchorage of longitudinal steel, they mean what we used to call bent bars, where the bottom steel bends up to become top steel.