ASCE 7-05 table 12.8-2 1

[SteelPE]

I design mostly small steel framed buildings (less than three stories in height) nothing glamorous. When checking seismic loads I typically use a R=3 systems to avoid the seismic requirements (mostly the connection forces). Recently I designed a 3 story office building that required the use of an eccentric braced frame system. Again, to avoid the seismic requirements I used a R=3 system. My question is in relation to table 12.8-2 of ASCE 7-05. As I read this table, I could use a Ct = 0.03 for this system as there is nothing in the table that says the Eccentrically braced steel frames must be in compliance with the seismic provisions. Is this interpretation correct?

 

[Ron]

Based on Table 12.2-1 (B1 or B2), R is 7 or 8 for your framing system. Why R=3?

 

[SteelPE]

R=3 so we can opt out of the seismic provisions (as indicated in the OP). So in this instance we have a system utilizing R=3 but the structural system is eccentrically braced frames. I selected this system such that I don't have to be in compliance with the detailing requirements of the seismic provisions. I am questioning if the framed need to be detailed in accordance with section 14.1 in order to use Ct=0.03 in table 12.8-2.

 

[MotorCity]

Check to see if your lateral system can be classified as "System not specifically detailed for seismic resistance" (check the exact wording, but I believe that's what its called). In that case, your braced frames or moment frames can take any configuration, even if it happens to be an EBF and you can use R=3. However, if you DETAIL it according to the seismic provisions, than you need to use the appropriate R=7 or 8.

 

[Ron]

Here is the paragraph to which MotorCity refers:

14.1.2 Seismic Design Categories B and C. Steel structures assigned
to Seismic Design Category B or C shall be of any construction
permitted by the reference documents in Section 14.1.1.
An R factor as set forth in Table 12.2-1 is permitted where the
structure is designed and detailed in accordance with the requirements
of AISC 341 for structural steel buildings and AISI Lateral
for light-framed cold-formed steel construction. Systems not detailed
in accordance with AISC 341 or AISI Lateral shall use the
R factor designated for “Structural steel systems not specifically
detailed for seismic resistance” in Table 12.2-1.

If you satisfy this paragraph then R3 could be used and yes, I would agree with your interpretation of Table 12.8-2

 

[SteelPE]

Ron,

Thanks, the SDC for this particular structure is B with a height of approx 44 feet. So I am justified in using a system with R=3.

 

[WillisV]

The intent of the table is for the eccentrically braced frame to meet seismic detailing provisions. R=3 eccentric braced frames should use a Ct of 0.02 for an "All other structural systems." This was clarified in ASCE 7-10 where the exact same table now says "steel eccentrically braced frames in accordance with Table 12.2-1 lines B1 and D1" which are the higher R seismically detailed eccentric systems.