AISC seismic provisions

[rfd23]

Hi,

Seismic provision(2005) clearly states that it is applicable to systems for R>3.

I am working on OCBF system. As we know this structures has beam, col, horizontal brace, vertical brace and struts (horizontal beam that may carry axial force with combination of vertical brace).

I am confused with table-8-1 pg.25/26.

My question For OCBF : We should check vertical brace width thickness ratio or other members too.

If yes, why.

We know that col and struts are also main members of SLRS. How about horizontal bracing.

I would greatly appreciate all you guys.

Thanks

 

[kevo_55]

rfd23,

When looking at the code, only the brace need meet "seismic" slenderness if in a K, V, or inverted V configuration.

The seismic provisions are quite confusing to say the least. What works for me is that you must start from what MFRS you are choosing and go from there. For OCBF (chapter 14) you will be told if the beam, column, brace, connection(s) need to have special attention.

With that being said, you need to be slightly careful about the brace and brace connection for your case. The beam will actually act as a drag strut for your floor diaphragm, and will also only be a "beam" for gravity loads. For lateral loads, beams & columns act as compression & tension members in CBF applications.

I hope this helps!

 

[rfd23]

kevo,

Thanks for your feed back.

I still have confusion about frames mmebers.

For brace frame or moment frame, let's suppose we have 10 bays in any grid line(elevation). We choose to have only one brace bay or two and similarly for MF (either one or two bays moment frame connection).

For OCBF

My SLRS is only these brace bays (beam/col/brace), not all columns and beams on this elevation. Technically I need to satisfy seismic provision for only these columns/beams(i.e these members must develope min. plastic moment of connecting brace) that are connected to braces. This translate to have heavier members than this brace memeber(fuse). Usually we need heavier member for larger plastic moment.

In reality all beams in this elevation will carry horizontal force and some how col. may participate also. Now if I choose to select heavier beam/col (stronger than brace plastic moment) than existing brace for this complete elvation, weight of building is much high as compared to if i concetrate on only on braced bay.

Please comment. I really appreciate your input and your valuable time.