RAM Concept Conflicting Audit Strip Results

I have modeled a 1-way PT slab and beam system starting with RSS and importing into Concept. At the lowest level I have several columns that need to jog. The audits on the strips at those locations produce a couple sections with errors that suggest removing 100% of the PT but adjacent sections may tell me to increase the amount of PT by a more reasonable percent. 

Is that a function of the forces induced by the column jog, or am I missing something? 

I have uploaded my file with the post URL. When opening uncheck all of the import options from RSS as I have made some geometry and load modifications to the model that would be overwritten. 

  • I have your file but I'm not sure what strip you are inquiring about. I think I can answer in general terms, however. 

    The program is evaluating the strip stresses and also a balanced load percentages. For details on how this is done, see the program manual, section 54.8.5 "Calculating the balanced load percentages" and chapter 34, "Using the Auditor".

    In the audit, if the section fails then it will make a suggestion. Here are some details form the manual on that: 

    For an unusual span definition or PT layouts I think you will want to treat the balanced load percentages and the audit suggestions with a grain of salt. Review the stress diagrams, as well as the deflection (e.g. due to the All Dead + Balanced condition) to double-check for over or under balancing and make a judgment call. It may very well be that you need to adjust the tendon drape, or the design strip layout, rather than the number of strands. 



    Answer Verified By: Max Karr 

  • The particular strips would be 17-20 & 22 C-1 at locations where the upper floor columns jog plan South of the lower columns. 

    I have gotten some good results in other areas by modifying the drape as opposed to changing the quantity. I will dig into what you suggest and report back. 

    For clarification I believe it is section 55.8.5.

  • Having a very large point load from the transfer column close to the end of the span is going to create a large stress that is unlike what you get from uniform load. You can see that in the plot for top stress, for example. It looks unlike a typical span condition.

    You will probably want to reposition the tendon low point to better balance the eccentric loading, and you may ultimately want to add more strands too. 

    Note, the eccentricity here is so small that the columns physically overlap, but the program considers a column and loads from a column above at a single point. You could stiffen the slab in the column area to make this joint more rigid.  



    Answer Verified By: Max Karr 

  • Ok, I finally had some time to jump back into this this morning. 

    Starting with the tendon balancing you previously suggested:

    Since my last big PT project I had forgotten that you can view the balanced loads in the design strip views . I looked at line 20C and had DL balancing %s of 330, 170, 360, & 160. I was able to finally understand how to use the "Calc Tendon Profile" tool using the %=-100(Wb/Wl) equation to get all my DL balances down to 100% which got me down to similar status results as I got by blindly manipulating my drapes. With only the one line changed I am not noticing much difference in the program bars, but this should help me get closer to a more reasonable amount of mild steel. 

    Changing the slab mesh did produce some results. I looked at 4 cases at strip 20C-1 (no add'l mesh, bottom col mesh, top col mesh, and both col mesh). In all cases sections 8 & 9 suggest decreasing PT by 100%. All cases also had failures in sections 6 & 7, with the exception of bottom col only meshed, which suggested increasing tendons by varied percentages. Bottom col only had section 7 failing but only required a 39% tendon increase which seems reasonable. 

     In the image you posted you show a factor of 20, but the program doesn't allow me to go past 10 and the following link shows 10 as the max factor. 

    https://communities.bentley.com/products/ram-staad/w/structural_analysis_and_design__wiki/53257/rigid-support-zones-in-ram-concept

    For columns along this E-W grid I think the jog is small enough to argue that we can detail the columns in a way that I can likely ignore sections 8 & 9, but there are a couple other locations with a higher eccentricity that will require more thought.

    Thank you for your help Seth.

  • One more follow up on balancing; over on the right side of the building at where 24C-2 and 25C-1 diverge. Our intention for the forked beams is really to detail that entire wedge as one wide beam as opposed to a forked beam. Since I set this model up in RSS there is an obvious modeling issue here as in the case of this one way slab system I can't have unsupported slab edges. 

    Currently the beam with section 25C-1 has a DL balance of -520% which creates some PT drape issues. While the 24C line has realistic balance values. I can determine that the program doesn't consider 25C as a part of 24C.

    So my questions are:

    Should I keep this forked beam system that I have, or should I delete the beams out and model a thick slab in concept to analyze more closely to how we intend to build it? 

    If keeping the beam system, is there a better way to define the design spans such that the program understands the intention?

    If changing to the thickened slab approach would that cause some confusion when exporting back to RSS?