concentric braces 1

[casasf]

I have a design issue and wonder who can assist:
I have designed a building with steel concentric ordinary braced frame (diagonal). If I need to move the brace away from the column beam connection, there will be an eccentricity on the connection. How far(maximum distance) can I go before considering the brace as an eccentric brace? I am not changing the R values 8 vs 10. The building is designed per UBC 1994. The code is not clear: how far. It only says short distance from the column beam joint. How much is short distance? It does not say.

Thank you for your help in advance.

 

[BAretired]

The brace is eccentric if it does not align with the center of gravity of the members meeting at a joint. There is no specific distance you can go before considering the brace eccentric, but small eccentricities do not contribute significantly to secondary moments.

If your brace is connected only to the beam, then you must consider the vertical component of the brace when designing the beam/column connection. If your brace is connected only to the column, you must consider the horizontal force imparted to the beam by the column.

I am not changing the R values 8 vs 10.

I don't know what you mean by that comment. Kindly elucidate.

BA

 

[ishvaaag]

Isn't there any other bay available to place the braces? Exposing the question to the architect and owner may give an alternative.

Packer and Henderson on secondary moments...
"can generally be ignored for both members and connections, provided that there is deformation and rotation capacity adequate to redistribute stresses after some local yielding at the connections.

Eccentricity is positive when measured towards the outside of a chord.

Limits of validity:

For all cases of connections considered:

Nodal eccentricity limits are -0.55 and +0.25 of the chord height.

Design guide for hollow structural section connections
J.A. Packer
J.E. Henderson
Canadian Institute of Steel Construction 1992