R-Value for Seismic Design of Pipe Racks 1

[JAX91]

I am currently designing a pipe rack in a seismic design category C location. The pipe rack falls under Chapter 15 of ASCE 7-05 as a nonbuilding structure similar to a building. Section 15.4.1-1a says a system shall be selected from Table 12.2-1 or Table 15.4-1. The section then goes on to say, the appropriate R-values from Table 15.4-1 shall be used. This statements contradicts the previous one.

My proposed design utilizes a concentrically braced frame parallel to the pipe rack. The structure is approximately 25'-0" tall. As a design category C, I would like to be able to use an R=3 (no detailing required) system from Table 12.2-1. If I have to use the concentrically braced from Table 15.4-1, I need to use and R-value of 1.5 to keep from providing any seismic detailing. I need to use moment frames perpendicular to the rack, which have a similar problem. To bypass the detailing requirements, the R-value in Table 15.4-1 equals 1. Does anyone know if you are allowed to use the R=3 from Table 12.2-1 for a non-building structure similar to a building?

 

[SrVaro]

I posted a similar questions a few monthes back, with surprising little response.

Our current design philosphy is to use the higher force, thus the lower R, and resist the siesmic loads by brute force as opposed to resisting the seismic loads by detailing whenever possible. Especially in the lower design categories.

We have been finding that the "higher" material costs are being justified by the "lower" fabrication and erection time.

I too am curious how others approach this.

 

[Den32]

I have not done this before, nor am I in a high seismic region, but I looked thru ASCE 7-05 as you referenced and looks like you could use R=1.5 for Ordinary Steel Concentrically BF with unlimited height and R=1 for Ordinary MF with unlimited height - both of these reference AISC 360.

Reading that 'conflicting section 15.4.1 it says to see Tables 12.2-1 and 15.4-1 for SYSTEM SELECTION but use Table 15.4-1 for R-values, etc. So, I think you are stuck with 15.4-1 based on what I am seeing. And Section 15.5.2.1 specifically lists Pipe Racks, so that is what you should use in my opinion.

 

[JoshPlumSE]

The 2010 version of ASCE 7 takes away the contradiction. Instead of referencing the R value and detailing requirement "from table 15.4-1" they use the language "referenced in the selected table". Therefore, if you select your system from table 12.2-1 then you can use the I, R, Cd and detailing requirements referenced by that table.

I would use the 2010 language as clarification of the code intent.

 

[delagina]

there is Seismic guide by Asce for petrochemical facilities that might help. i have done a lot of piperacks but not with high seismic.

 

[JAX91]

I agree with Josh's assessment. If the new code changed the verbage, they must have done it to clarify their intent. There are numerous systems which appear in Table 12.2-1 that do not appear in Table 15.4-1 (R=3 System, Eccentrically Braced Frames, Dual Systems, etc.) The new verbage creates a solution for this contradiction and allows you to use all of these systems. Thank you to all for your advice!