sgs114
Structural
- Oct 7, 2013
- 33
Hello,
I am working on a building that is a 6-story office building. The lateral system for levels 2-6 are Buckling Restrained Braced Frames and the lateral system for the base level is CMU walls around the perimeter. Working through the design, I have encountered various items that appear (to me at least) to be somewhat ambiguous in the code:
1. The steel columns for the braced frames will sit on concrete pilasters at the base level. I would not consider the concrete pilasters to be members common to the different framing systems (they do not provide lateral resistance at the base level), therefore I would design these concrete pilasters based on the seismic load combinations including over strength factors and not need to multiply the forces by the ratio of the R-factors (8/5). Agree or disagree?
2. Uplift on the footing: Due to the magnitude of the forces, I have fairly large uplift on the BRBF columns. I do not consider the foundation an element which is supporting a discontinuous frame, therefore I do not need to apply overstrength factors to the foundation design (net uplift). Additionally, I do not consider the foundation a different lateral system, therefore I do not need to magnify my uplift by the ratio of R-factors (8/5). Agree or disagree?
3. (3) of the braced frames around the perimeter do not directly align with the CMU wall below. The centerline of the brace is approximately 8" from the centerline of the wall. I cannot find any indication in the code about what kind of offset is acceptable without kicking you the the overstrengthed design forces. Would you consider this to be a Horizontal Irregularity Type 4? Per the BRBF provisions I already have to design my columns to have a required axial strength equal to the forces including the overstrength factor, so I could just continue that to the foundation level, which may make the most sense. Any thoughts?
Thanks,
SGS
I am working on a building that is a 6-story office building. The lateral system for levels 2-6 are Buckling Restrained Braced Frames and the lateral system for the base level is CMU walls around the perimeter. Working through the design, I have encountered various items that appear (to me at least) to be somewhat ambiguous in the code:
1. The steel columns for the braced frames will sit on concrete pilasters at the base level. I would not consider the concrete pilasters to be members common to the different framing systems (they do not provide lateral resistance at the base level), therefore I would design these concrete pilasters based on the seismic load combinations including over strength factors and not need to multiply the forces by the ratio of the R-factors (8/5). Agree or disagree?
2. Uplift on the footing: Due to the magnitude of the forces, I have fairly large uplift on the BRBF columns. I do not consider the foundation an element which is supporting a discontinuous frame, therefore I do not need to apply overstrength factors to the foundation design (net uplift). Additionally, I do not consider the foundation a different lateral system, therefore I do not need to magnify my uplift by the ratio of R-factors (8/5). Agree or disagree?
3. (3) of the braced frames around the perimeter do not directly align with the CMU wall below. The centerline of the brace is approximately 8" from the centerline of the wall. I cannot find any indication in the code about what kind of offset is acceptable without kicking you the the overstrengthed design forces. Would you consider this to be a Horizontal Irregularity Type 4? Per the BRBF provisions I already have to design my columns to have a required axial strength equal to the forces including the overstrength factor, so I could just continue that to the foundation level, which may make the most sense. Any thoughts?
Thanks,
SGS