Column Shortening
Column Shortening
(OP)
In performing design of X-braced frame, it is usual to ignore that column shortening will cause compression in the brace member. When performing a computer analysis in STAAD, this load is included in the brace. For a tall heavily loaded structure this could add substantially to the brace design forces, where compression bracing is used. Does anyone have any thoughts or know of any guidelines for this?






RE: Column Shortening
RE: Column Shortening
RE: Column Shortening
Elastic shortening is a true phenomenon, but rarely as signficant as theory will predict.
RE: Column Shortening
RE: Column Shortening
RE: Column Shortening
RE: Column Shortening
Also, the construction sequence will effect what the braces take, (normally I see the columns and braces installed before floors are poured). So your braces may be taking axial with the columns anyway, even if the columns can take the full gravity themselves.
RE: Column Shortening
Recommended reading in my opinion.
Regards,
-Mike
RE: Column Shortening
RE: Column Shortening
I just read you are expecting the bracing to act in compression and tension to resist lateral forces so that's not much help.
Your method of making the bracing inactive for gravity loads seems good. I would definitely ensure that the column can support all of the load.
RE: Column Shortening
JAE's comment is exactly how it gets done sometimes (bolts left loose, pulled up tight later....hopefully! Don't forget to make them ream the holes for the slight misalignment instead of using a torch to make all the holes the same size.)
haynewp is on target with his comment about designing for every condition...we can't...put a comment on the drawing about removal.
RE: Column Shortening
RE: Column Shortening
RE: Column Shortening
If the effects are significant you can precamber the members so there is zero load in the bracing under full dead load. This might involve fabricating the columns a bit longer and forcing the bracing into place during construction. In your analysis you can put temperature loads or prestressing to simulate the shortening. We do this in arch bridges and cable stayed bridges.