I'm modeling/designing a 2-way transfer slab for a concrete podium building in RAM Concept and the program is adding one-way shear reinforcement around the columns. I was reviewing an audit of the cross section and noticed the following:
Why is the program adding shear reinforcement if the audit indicates they are unnecessary? Is there a setting I can check/update to avoid having the unnecessary shear stirrups added to the slab?
Thanks!
The audit is for a single section there are several envelopes and "design passes" to report, so you might be looking at a pass that is OK, while another requires stirrups, or it might be an adjacent section that controls. This wiki is useful: https://communities.bentley.com/products/ram-staad/w/structural_analysis_and_design__wiki/3094/ram-concept-shear-reinforcement-faq
Seth,
I'm working on this project with Brinn, and we did review the other design passes to verify. The "controlling criteria" pointed us to a strength provision in the code so we dug into the strength design passes and found what Brinn posted above. We also found the article you linked prior to reaching out and went through each of those potential issues (checking the effective depth, running hand calcs on the one-way-shear, etc).
One item we could not reconcile was the effective width of the section taken for shear. It didn't match any ACI calc, column strip width, or design strip width that we could point to to make sense of it. Even with the width presented, the PhiVc was still > Vu so we doubt that's our problem, but it may point us towards another solution.
Thank you again for your time.
The width (bw) for one-way shear is the width of the shear core. See: https://communities.bentley.com/products/ram-staad/w/structural_analysis_and_design__wiki/4514/cross-section-trimming-tn
We have a flat slab at the locations this is ocurring so it shouldn't have any issue calculating the bw unless there is some maximum limit, and our PhiVc is still greater than the Vu even with the smaller bw.
Are there any other settings, Concept specific assumptions, or other conditions for a thicker transfer slab that would do this given we've chased down the suggestions above?
Thank you
Send your file, or log a service request and we can help.
Be sure to indicate what you are checking and your version.
Just uploaded, please note that out workflow is through RAM SS but we have not been updating loads from RAM SS because we have custom load cases and load history calcs that get wiped out with bringing those in.
Ground Floor or Second? Any strip in particular?
Let's consider Second floor, strip 2C. A plot of the shear density is useful to establish which sections require the shear reinforcement.
Here's the audit for the controlling section on the left.
Auditing 2C-3, section 1.html
If you are comparing Vu to Vc remember to incorporate phi (0.75) and recognize the requirement of 9.6.3.1 (* only for beams in ACI 318-14 *):
In this specific case, the phiVc limit is exceeded, so minimum shear reinforcement is required. At the next section it is OK and the next section is 1.45' away which is approximately "d", so I think you would be justified in ignoring this bar set. Adjusting the strip support width is another way to account for this, but since "d" is a moving target and since the sections are also critical for checking flexure, proceed with caution.
Thank you this was helpful but it's brought up a couple more questions.
1. Why is the slab shear being designed with the Chapter 9 BEAMS code provisions? We would assume that 8.5.1.1 (c) governs for two-way slabs. Even table 9.6.3.1 you refernced provides exceptions for beams that behave more like slabs.
2. The bw used for the shear sections in the 2C audit seems to be the column strip width, but the shear load appears to be higher than we would expect for just that section. Does CONCEPT redistribute the shear for the design strip or is this just a function of the finite element design? Would modelling the design strip as "Full Width" instead of "Code Slab" alleviate this distribution? In either case, the shear is averaged over the cross section, but full-width design strips would seem to be more in-step with 8.5.3.1.1.
1. My error. I confess to still being more familiar with the older code layout. For two-ways slabs we are following the chapter 8 sections. In the case above the max shear demand was more than phi Vc.
2. The demand is a result of the finite element analysis. A full width strip would certainly give you a much larger capacity with only a bit larger demand.