Hello,
I am designing the shear reinforcement in a straight pier cap for a multi-column pier. The issue I am having is that whenever I increase the number of legs for my stirrups, the area of steel required also increases to the point where increasing the number of legs is just not advantageous. I'm designing the cap for shear and torsion along with magnified moments. I have attached a picture of the output for the shear and torsion design. On the left is the result from using stirrups with 2 legs and on the right is results from using 4 legs. The same bar size and spacing was used.
Is there any particular reason this is happening? I've made multiple stand-alone substructure files thinking that a new file would solve the issue, but they've all had this issue. Any help will be greatly appreciated.
Rebecca,
I believe this is due to having the "Shear Force is Equally Distributed Among Stirrups" option checked under the A/D Parameters > Shear and Torsion Calculations. When this is unchecked, the program allows any stirrup legs to be used for the shear demand and the outer legs are used for torsion demand. When you have the option checked and you have more than 2 legs defined, the legs that are not on the outside of the cap will be under utilized (i.e. would have additional steel area that is not being used for either shear or torsion demand).
To try to explain this with your screenshot example, look at the 4 leg case that shows the shear check is not working. You need 1.49sq in for the shear demand, and if the shear force is distributed equally, each of the 4 legs will use 1.49/4 = 0.3725 sq in. For the torsion demand, each outer leg requires 0.51sq in of steel so that must be added to the previous number...0.3275 + 0.51 = 0.8825sq in of steel is required for just one outer leg. I think the program assumes that all of your legs of steel will be the same size so it says you need 0.8825 x 4 legs = 3.53sq in (slight rounding difference compared to the 3.54 in your screenshot).
The bottom line is that I think the option that is checked is what is making the required reinforcement go up each time you increase the number of legs of steel, and hopefully my explanation helps.
Thanks