TR-34 Concrete slab on grade design for racking loads

[Once20036]

A different post here a couple days ago referenced slab design in accordance with the Eurocode and TR-34. I wasn't familiar with the document, so I wanted to check it out and see how the results compared to other design methods. I've been doing a lot of slab on grade design recently and I haven't been very satisfied with Ringo, PCA, or the controversial "Structural Magazine" article by Azzi.

I made up spreadsheet with the TR34 equations and solved dozens of arbitrary slab loadings to determine allowable loads.

Methods provided by Ringo, PCA, and TR34 all produced relatively similar results, but I noticed a number off odd things in TR-34:
1) I cannot figure out any combinations of loads, base plates, spacings, etc that make the flexural calculations matter. Everything is always shear controlled.
2) Punching shear is checked at 2d from the base plate, rather than d/2 per ACI.
3) Punching shear does not include any contribution from the soil "pushing back". This is specifically noted and done intentionally.
4) Shear equations never take the sqrt of concrete strength, per US codes.
The Azzi article allowed 3-4 x more than the loads calculated with any of the other methods.

Unrelated to design of plain concrete slabs, there are some add'l things I noticed:
5) It appears that their gamma-c factors are missing in a number of places (eq'n 9.32 and 9.34)
6) It appears that adding steel fibers decreases the flexural strength of the concrete. All of the equations are based on Mp+Mn, and steel fibers reduce the Mp equation by Rc3.

Mostly it's item 1 that seems off to me. What's the purpose of all the calculations for flexure if they're never going to control.
Has anybody else run across this before?
Does anybody have some design examples of slab on grade design w/TR-34 I can puruse and see where I`m off?

 

[ukengineer58]

Your comparing two codes. the punching at 2d is a eurocode check. why compare with aci and expect it to be the same?

 

[Once20036]

It's item #1 that lead me to believe there was something wrong in my calculations. If you have experience with slab on grade design per TR-34, have you found situations where the flexural strength controls?

With items 2&4, i meant to note the differences in case I was mis-understanding. It does seem a little strange to me that two groups would independently research the shear strength of concrete with one group determining shear strength is proportional to sqrt(f'c) and the other determine that it's a linear relationship, but those are the codes and it is what it is.

With item #6, I assume that this is a typo in the code. The text indicates that providing steel sibre reinforcing, "...provides residual (i.e. post-cracking) positive bending moment capacity..." Because the equations are based on the cracking moment, it seems to me that the positive moment capacity should increase, however, none of the equations nor the design examples reflect this increase.

Unfortunately, I couldn't find any add`l solved examples (except the ones included in the code) to confirm any of these items.