Steel Bracing Design 1

[TNeng]

I am designing a steel bldg. with chevron and "x-bracing" using a structural analysis/design computer program.
The bldg. columns have been analyzed with the bracing "inactive" to insure they will support all the gravity load. My question is do I need to design the compression bracing for gravity load in addidtion to the lateral loads? If I were designing the bldg. with tension-only bracing I would not be concerned with "failure" of the compression side brace in, for instance, an x-braced bay, but when I am relying on compression bracing to share the load with the tension brace it seems the compression brace needs to support all load it will see--including gravity loads. Is this correct thinking? Thanks for any suggestions.

 

[Taro]

There are two philosophies for CBFs:

1) The compression braces are not designed to carry gravity loads. The compression braces are expected to buckle in a large seismic event and the gravity loads they were initially supporting will be transferred to the adjacent beams and columns.

2) The compression braces are designed for gravity loads. This is a conservative approach. If used, the beams and columns should still be designed to support the entire gravity load in recognition of the fact that the braces may buckle.

 

[TNeng]

Thanks for your response Taro. The two philosophies you mentioned seem to be the predominate approaches I've run across in talking with others. Some have said that if one were doing the analysis by hand calc's, gravity load in the bracing would not even be condidered, which is true. It just seems to me that if your tension brace will not carry the full lateral load or your foundation cannot support the full shear load at one column, then your compression brace must be designed so that it will not fail under any loading, including gavity load--your compression brace cannot be in a buckled state when it must carry lateral load.

 

[Taro]

Whether your design allows for brace buckling depends on the type of lateral forces. You didn't mention if seismic resistance is required in your design. My previous comments assumed earthquake loading because seismic resistance is now required in most of the United States. If earthquake loads are not considered, the compression braces should be designed to preclude buckling.

If seismic design is required and you follow the code minimum design requirements, your compression braces will indeed buckle in a design-level event. The lateral system is designed with a reduction factor that reflects the ductility and energy dissipation capacity of the system. The design forces would have to be multiplied several times in order to preclude buckling.