UBC 97 Orthognal Effects 1

[dawn836]

Hi all

When the requirment of orthognal effects shall be considered in the analysis of a building as stated in UBC 97(Section 1633)?

 

[EBF]

I believe that orthogonal effects need to be considered only when certain types of irregularity exist or for columns that are part of two perpendicular frames. According to UBC Table 16-M, the irregularities that require consuideration of orthogonal effects are torsional irregularity and non-parallel systems.

 

[dawn836]

well Thanks so much EBF
another question, should i consider Orthognal effects when i am using either Equivalent Static Load Method or Response Spectrum Analysis?

 

[EBF]

My understanding is that you would need to consider orthogonal effects in both types of analysis for the condiitons that I mentioned previously. So it doesn't matter if you are doing static or dynamic analysis, if you have those kinds of irregularity or you have columns that form part of 2 intersecting frames, then you would need to consider the orthogonal effects. Keep in mind that the plan irregularities will probably also mean that you have to use the dynamic procedure.

 

[dawn836]

well thanks so much EBF
i checked UBC97 again and found the following defination for Orthognal effects.

ORTHOGONAL EFFECTS are the earthquake load effects on structural elements common to the lateral-force-resisting systems along two orthogonal axes.

and the code does not mention anything about irregularities that require consuideration of orthogonal effects.

So does this statemnt means i should only consider Orthognal effects for columns that are part of two perpendicular frames?

 

[mrMikee]

dawn836,

The definition you found describes when orthogonal effects could become significant. For example, if you have two x-brace systems in a structure, one for the N-S direction and the other for the E-W direction that are connected to a common column, then the column could carry seismic forces from both the N-S or E-W directions simultaneously. The IBC does not require orthogonal effects be considered for Seismic Design Category A and B however, and I think the old versions of the UBC had similar requirements but I don't really remember. It's been a long time. Sorry, I haven't read UBC87.

The Equivalent Static Method is a considerable simplification of the dynamic response of a structure. In my opinion the irregularities are discussed mostly to identify structures where the static method would not give a good answer and a dynamic analysis is required.

Regards,
-Mike