Determination Of Column Base Reactions Atop Piers

[Pcacciat]

I have run into the same issue on multiple projects lately, so I though I would ask here and see if anybody else has come up with a good methodology to address this problem.

I have a 6 story structure with a 1 story basement modeled in RAM, and grade is at level 1, at the top of the basement. The lateral system consists of chevron braced frames - in the center of the building the frames continue down to the foundation level, but along the exterior walls the steel frames stop at the ground level and have baseplates on concrete piers which are integral with the basement walls.

My question is, what is the best way to get the vertical and lateral (shear) reactions at the base of those columns on the exterior wall in RAM where braces frame in at the bottom of column? RAM Frame does not report reactions at that point because it considers the concrete pier below to be the part of the column, and instead reports the reaction at the base of the pier. The reaction from the pier is a reasonable approximation for the vertical load (minus the self weight of the concrete) but not the shear.

I am sure I am not the first person to run into this issue, how do you all approach this? If possible I would like to avoid using the concrete module of RAM, since we are not using it at all otherwise so that's a bunch of extra work to set up load combinations properly and such. I tried removing the basement from the model and re-running it, but that greatly affect the axial loads in the braces so I do not believe that the base reactions I get will be reasonable.

Thanks for your help!

 

[SethGuthrie]

You have a couple of options, none great.
1. Where the outer frame set on walls, don't model those walls at all. The bottom of the brace will then become the external support point at the first above ground story and reactions provided. Obviously this is no good if the wall also needs to support gravity beams or if the ground level remains at the Base in Ram Frame.
2. Just use trigonometry on the brace and column forces to get the resultant reaction. This is the labor-intensive approach, but probably the best.
3. Adjust the brace to intercept the column 6" above the top of the wall, then use the column bottom forces directly. This changes the work point and thus the overall model stiffness, but may be preferred over option 2.