Hello,
I am designing a Mat foundation according to ACI 318. The mat foundation has different section with different thickness but the top elevation is the same for all the section. At the transition zone where there is a change in thickness, what is the right cross-section trimming to use and slope limit? I have set the cross-section trimming to none and I set the slope limit to 9999 on the left side of picture 1 (I believe this will not trim the cross-section) and the software designed many unexpected shear bars at the transition zone which I cant explain why, I believe this is due to wrong cross-section trimming. I have read chapter 25 of the manual about shear core and cross-section trimming but it is still slightly confusing to me.
Please see attached pictures. First picture is the cross-section perspective view (left transition zone is set to none and 9999 slope limit while right transition zone is set to slab rectangle and 0.25 slope limit) and the second picture is the shear reinforcement at the same transition zones shown in picture 1. So my questions are,
1) What is the correct cross-section trimming to use at transition zone, where there is a change in thickness in mat slab based on ACI 318?
2) If I use slab rectangle or none with a slope limit of 0.25, I get warnings that the rebars are out of cross-section like the right side of picture 1, is it okay to ignore this warning since in reality or during construction the transition zone with have bent/sloped bars between the areas of different thicknesses
For a design strip that crosses a sudden step, using a very large slope limit means that the section depth is suddenly increased as you have shown at left. This may be unconservative since that free concrete cannot be physically engaged in flexure immediately. Thus we provide the slope limit to allow for more gradual and realistic depth increases. It's a judgment call what slope limit to use, but 0.25 is very gradual and conservative, hence our default.
As you noted, if the sections are only gradually allowed to increase in depth some user defined reinforcing might not be within the section. A warning is presented for that case and the "external reinforcing" is ignored. So long as the rebar is not essential (e.g. it's on the compression face), there is no harm in that. For more on that subject see: https://communities.bentley.com/products/ram-staad/w/structural_analysis_and_design__wiki/18592/ram-concept---reinforcing-bar-out-of-cross-section .
Note, as an alternative to using a very large slope limit one can also model the strips in segments, stopping, and restarting exactly at the step.
For more on why you might have a one-way shear issue, see: https://communities.bentley.com/Products/Structural/Structural_Analysis___Design/w/Structural_Analysis_and_Design__Wiki/ram-concept-shear-reinforcement-faq.aspx
Thank you Seth for your reply. I believe the bottom rebars are essential. The strip started at the transition zone, strip does not cross the step and I am still having issues due to the trimming. It looks like there is no other way to avoid the issue of reinforcement out of cross-section in Ram concept when trimming is done. And without trimming, design is unrealistic at sudden change in thickness area (Transition zone).
I am just thinking, please is there a way to model the mat such that when ram concept trims the section, its like what is shown in picture attached. This way, no reinforcement is out of the cross-section?
This sketch is parallel to the step I take it, that's a little different. Program designed bottom bars would all be in the bottom of the trimmed section as you have shown with dots. It's only user defined reinforcement that you have to "watch out" for. You certainly can shift the user bars to be within the trimmed area and avoid having bars that are out of the section.
Yes the sketch is different, I know the software trims part of the cross-section with larger thickness. I am just trying to envisage if its possible to create a section after trimming that looks like the sketch (which is like adding a notch to the slab with smaller thickness, and not trimming any section of the slab with larger thickness). The construction of the mat will be exactly like my sketch and not like the ram concept trimming, whereby we can take full advantage of the concrete cross-section.
Another question I have is, is it possible to convert SSR designed for punching shear to stirrups? This is because sometimes studs might be difficult to get and one decides to use stirrups instead of SSR. Can this be done?
We can't do a sloped soffit in the program, so at best the chamfered part of your section could be approximated with steps.
We only design SSR for punching shear, see: https://communities.bentley.com/products/ram-staad/w/structural_analysis_and_design__wiki/12218/ram-concept-punching-shear-checks-faq