Response factor R for strap bracing of steel framed bldg.

[dwray]

I have a 1 story building. Bar joists bearing on steel girders. The steel columns are HSS6x6 and are infilled with 6" light gage studs at 16"oc. The owner wants the lateral bracing to be strap bracing. I want to weld strap x-bracing to the outside face of the columns to take the lateral load. The strap bracing would be welded to the HSS column about 6" above the base plate and to the top of the adjacent column approximately at the working point of the steel beam and column. I am also going to tack weld the strap bracing to each stud. I am also in seismic category 'D' (Charleston, SC). What 'R' value shoul I use for this. I know R=3 is out because of the 'D' factor. Also, what could be used to take out the torsion at the top of the column since the strap bracing is attached at the outside face of the column (3" eccentricity). Thanks for any input.

 

[JAE]

Your system appears to be a tension only system (straps can't take compression) - which is not allowed in high seismic regions per AISC Seismic Provisions (See sections 13.2c. and 14.2c)

You must balance at least 70%-30% tension and compression braces in a single line of bracing.

 

[dwray]

Thanks for the info. What would the 'R' value be if this were in SDC 'C'?

 

[JAE]

Which code are you under?

 

[dwray]

IBC 2000.

 

[JAE]

Here's a thread that discusses the weird combination of IBC 2000 to AISC Seismic:

thread172-89757

IF you design your structure under the AISC Seismic Provisions, then you can choose the appropriate R value from table 1617.6.

BUT - if you do not design per the AISC Seismic Provisions, you must use R=3.

(see IBC 2000, Section 2212)

For your column/beam strap system, I would suggest it is either type 2.C (R=6) or type 2.D (R=5) if you design under the AISC Seismic Provisions.

 

[ajh1]

A tension-only system is available under Section 14 of the AISC Seismic Provisions. See paragraph 14.5. The IBC Table of R factors permits OCBF up to 160'. Paragraph 14.5 says that if you boost your seismic forces by the omega factor, then you get to ignore 14.2 through 14.4.

 

[JAE]

ajh1 - I agree...good point.

 

[dwray]

Yes, I agree also but the Seismic Provisions for Structural Steel Buildings (1997) Supplement #2 dated November 10, 2000 deletes paragraphs 14.2 through 14.5 and replaces it with a single paragrapgh that states the required strength of members and connections.

 

[Taro]

There is no clear answer to this question. It's not really a steel braced frame. AISC Seismic Provisions do not apply to this system because the bracing is light gage metal, which is exluded from the scope of AISC.

The 2003 IBC and 2002 ASCE 7 include a system that is close: Bearing wall system, light-framed wall system using flat strap bracing, R=4. This system is not really what you have either, because it's not a bearing wall and you also have structural steel elements.

What you really have is an undefined system. That leaves you in a tricky situation, depending upon what the building official requires. Per IBC 1617.6, they could require that analytical and test data be submitted that justifies a given R factor. Or they could be more lenient and accept a conservative assumed R factor. It would be prudent to contact the building official to see which way they will go.

For an assumed value, I would probably use R=3. This should be conservative by comparison to the sytems discussed above.

 

[ajh1]

Two points:
1) IBC 2000 only adopted AISC Seismic Provisions through Supplement 1. Supplement 2 does not apply.
2) What happened in Supplement 2 is in effect the deletion of paragraphs 14.2 through 14.4. All that remains is what was previously in 14.5 along with the new requirement that the connection has to be designed for RyFyAg.

I would not disagree with the idea of just using R=3, but as stated it's up to the code official to buy off on that.