Column Shortening

[DTJ]

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?

 

[haynewp]

Why is the column shortening?

 

[DTJ]

The column is shortening due to the axial load present (Delta=PL/AE). For tall structures, this cumulative shortening from floor to floor could be significant.

 

[Ron]

DTJ...be careful that you're not measuring with a micrometer what will be marked with a crayon and cut with an axe during construction.

Elastic shortening is a true phenomenon, but rarely as signficant as theory will predict.

 

[DTJ]

I should also clarify, that this only occurs when there are for example, three bays and the bracing works up from the outside to the middle column. The opposing braces prevent rotation from occuring and therefore the result is force in the bracing and the resulting vertical load in the middle column is reduced.

 

[haynewp]

So you mean it is just sharing the gravity load with the column. This is normally accounted for in my models also.

 

[DTJ]

Thats what I was getting at. The only problem I have with this is that if the client came back and wanted to relocate the brace later on, the column would not be sufficient to carry all of the gravity loads. In my mind, I always want the columns to have the ability to take the full gravity. What I have done in the past is make my bracing inactive for gravity loads to design the columns and "reactivate" the braces under lateral.

 

[haynewp]

Owners that move around braces remind me of the owners that choose to remove braces in metal buildings so they can put in a new door. They shouldn't be doing that unless you have authorized this on your drawings, in my opinion. We can't engineer for every imaginable stupid decision. If it has been requested to have this flexibility then it is different.

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.

 

[mrMikee]

It is possible to minimize gravity loads getting into diagonal bracing by making the right selection in the bracing scheme, provided there aren't too many other customer or political considerations getting into the picture. Pages 163 and 164 in the text "Loads and Load Paths in Buildings, Principles of Structural Design" by Narendra Taly is an interesting discusion of this subject.

Recommended reading in my opinion.

Regards,
-Mike

 

[JAE]

Just hook up the X braces after all the other framing and dead loads are in place!!! (sounds of OSHA secret police knocking down my door).

 

[apsix]

For X-bracing my software allows me to specify the members as tension-only. If the loading tends to put the member in compression, the member is ignored.
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.

 

[Ron]

DTJ...I also agree with your 2 part approach. It's a bit more analysis, but it does solve the issue.

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.

 

[haynewp]

DTJ, do you also design your beams to take full gravity load where you have chevron braces? I do not, I consider the braces as integral gravity carrying members unless there is a special circumstance.

 

[DTJ]

In certain situations, in the case of chevron bracing, I design the beam to take the full gravity load. There are some situations where this is a code requirement. (i.e Special Concentrically Braced Frames)

 

[graybeach]

DTJ,
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.