Building Codes - Punching Shear

[slickdeals]

The Indian concrete code does not have any information pertaining to the design of stud rails. As a result, we are looking at other building codes which address it such as ACI.

One of the concerns I have is regarding the different load and resistance factors on which the codes are based.

For example, the Indian code ultimate design is based on 1.5 DL + 1.5 LL with material safety factors of 1.15 (steel) and 1.5 (concrete) already incorporated in codal equations.

ACI uses different load factors (1.2 DL + 1.6 LL) and resistance factors (0.75 for shear).

If I were to use the ultimate loads per Indian codes, can I design for stud rails using the ACI code? I am wondering if the phi factor of 0.75 can be equated to a material safety factor of 1/0.75 = 1.33? Will it be an apples to oranges design?

I would appreciate some discussion on this and if any of you have had to do something similar in design?

 

[JAE]

The phi factor is based upon the variability of the material properties, the variability of the dimensions of the members, and the overall reliability of the limit state (highly variable or instantaneous failures get lower phi factors).

The load factors are based upon variability of the loads themselves.

When comparing two different code requirements for phi factors and load factors, I would think it would be a fairly simple procedure to use the ACI stud rail shear capacities and then compare the two codes for levels of safety (Load Factor)/(Resistance factor) = rough overall safety factor.

The capacity of the stud rails is a codified value that was developed by research and vetted through committees, peer reviews, etc. Since it doesn't appear directly in the Indian code, perhaps the Indian code has a provision within it that allows "other reasonable approaches". (see ACI section 1.4).

You then might have to submit the concept/procedure to the governing authority to get approval for this use of non-Indian code provisions.

 

[slickdeals]

Thanks JAE. Is there a good way to compute an "equivalent" load factor. If all dead loads are 65% of overall load, can I use 1.2 (0.65) + 1.6 (0.35) as a equivalent number?

The Indian code has an inbuilt safety factor. The resistance factors are built into the code equations, the overload factor is 1.5.

 

[JAE]

I suppose you could use the weighted load factor approach. Makes sense.

 

[bookowski]

I wouldn't try to equate the two approaches. How are you getting your axial forces and unbalanced moment? If it's from an analysis program why not just run different load combos for ACI (1.2D + 1.6L etc) and then do the punching shear design per ACI? If you are using actual stud rails you can download the free design program from descon, it will greatly simplify your life - the calcs are not fun to do by hand. You could also look to BS for design, they are more conservative with punching shear but follow a similar approach.