steel braced frame & Pre cast shear walls 1

[prsconsultant]

Hi all:

I have an existing 6 storied building with lateral system comprising of braced frames (Chevron, diagonal, X-brace)in the upper 3 floors and precast shear walls in the lower 3 floors. Upon evaluating the lateral system I have found that some of the braces and most of the connections between brace members and precast shear walls are overstressed. All the shear walls have adequate lateral capacity.

All the brace members are of square tubular sections. Under seismic loads, typically braces have enough tensile capacity but fails in compression. I am thinking of strengthening braces by infilling the tube steel members with concrete (as it is very easy to do) or by providing some stiffeners to stabilize the brace. Any thoughts on this?

The bigger problem is how to transfer the brace force to Precast shear walls? The existing conections are steel embed plates with headed studs. One thought I have is to pour a stub wall (2'-4' high) over the existing PC shear wall thus encasing the steel brace partially and achieve shear transfer using new shear studs and dowels. Any suggestions on this?

I appreciate any comments / suggestions.

Thanks

 

[ishvaaag]

You can add to what you think paired angles or plates welded to the bottom of the bracing structure, and then anchoring it to the concrete. If necessary, a suplementary plate may be welded in-between. Or maybe more simply, weld a double tee as a bottom member of the brace, this will help to enhance the overstressed connections, and permit additional connection to the concrete. When drilling for the connections to the top of the concrete shear walls, try to keep separations the wider is feasible, to avoid to distroy its head.