Program reinforcing almost doubling for same loading conditions

Hello,

I’m working on a 1-way PT slab design in Ram Concept, and I’m running into some strange bottom reinforcing that the program adds for strength, and I was hoping someone had some advice.  I had the slab working, both reinforcing and PT, but we’ve had to push the PT (maintaining the same total drape) and top steel down in the section for top cover requirements.  It was during this change that some of my design strips started adding almost twice the amount of bottom mild steel than had been originally provided.  I’ve narrowed it down to being related to skip load patterns.  If I take the patterns away, that program added steel goes away.  It seems strange because only some bays have the added bars and the bay over (with the same total strip positive moment demand) does not, or even the design strip down in the same bay doesn’t have the added bars (see below, I turned the added bars on in yellow).

   

Has anyone had this happen before and figured out why the strength detailer added so much reinforcing?

Please let me know if I can provide any more information, thank you!

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  • The file has been posted, thank you for the quick reply!

  • I shared this model with the lead developer for RAM Concept. He replied back with the following:

    "The sensitivity is being caused by the large hyperstatic axial tension and the effect it can have on the flexural-axial design.  Concept first does a flexural design, which ensures that the flexural capacity is greater than the flexural demand at a given axial force level. However, there are rare situations, usually with net tension, where this is not adequate.  If you think about a P-M diagram with a tension tip that is offset from the origin, it is possible to have a demand point that is lower than the capacity points and be outside the interaction curve.  It is this second design that is kicking in all that steel that is being supplied because the flexural-axial design can be sensitive with large tensions.

    As for the cause of the hyperstatic tension, most of it is resulting from the long walls running along the longitudinal axis of the building diverting the prestress.  If this rigidity will really exist (in the multistory model) then this effect is real and that sensitivity exists.  If the restraint will be lower the hyperstatic tension will be overstated in the model and would not be as much of an issue.  You can observe how this effects the behavior by turning off the “shear wall” check box on those 4 sets of walls and then running the model again.  All that bottom steel we were looking at goes away.".



    Answer Verified By: Addie Lederman