Hello,
I have modeled a steel framed structure with BRB's as the lateral elements. The building has multiple levels, including intermediate mezzanine and a low roof level. To ensure transfer of lateral forces from the mezzanine and low roof, secondary braces have been added and the connecting members changed to lateral members, as needed.
The model was initially run without the "secondary braces" and their connecting elements with reasonable, or expected results. By adding the "secondary braces" and pinning the major and minor axes of the bases of their respective columns. (See Frames 3, 5, 6, and 8 and their column bases at nodes 126, 127, 128, and (121 and 116), respectively.) By doing so, the columns of all of the frames exhibited rather large DCRs. Some up to 10.0, or larger. These results did not seem to be reasonable.
The model was run again, with the only difference being that the bases of the "secondary brace" columns being fixed for all conditions.By doing so, the DCRs of the frame columns dropped significantly. Only the columns attached to the "secondary braces" exhibited DCRs greater than 1.0. However, they were less than that of the pinned base condition.
Could you please advise as to why there would be such a great disparity in results?
Also, would the "secondary braces" be necessary? Could the levels with no lateral elements have their masses simply added to tone of the adjacent floors?
Thanks in advance,
Joe Logue