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Using RISA To Analyze Seismic Forces on Steel Framing with Equivalent Lateral Force Procedure

Using RISA To Analyze Seismic Forces on Steel Framing with Equivalent Lateral Force Procedure

Using RISA To Analyze Seismic Forces on Steel Framing with Equivalent Lateral Force Procedure

I'm looking for confirmation on my design strategy using RISA 3D. And if this is an acceptable practice.

I'm designing a steel pipe rack located inside a building subject to seismic loads under category D. The rack has 2 levels and runs along the inside perimeter of the building over an aisle. It is about 10,000 feet long with , 15 feet wide, and 20 feet high with a series of 10,000 pound air handing units spaced throughout. Piping is on 1st level. HVAC units are on second level.

Framing type:
Knee-braced systems qualify as OMF per AISC 341.
Pined connections at the column bases. K=2.
Table 15.4-1 Steel ordinary moment frames (permitted for cat. D per table note d) R=3.5 , Cd = 3
Moment connections at the base of the columns were not possible.
Long direction: Knee braces at each level.
Short direction level 1: End plate moment connections with stiffener plates.
Short direction level 2 (top): Column cap plate moment connections with stiffener plates.

To the best of my knowledge, RISA 3D can only apply seismic loads to the self weight of the structure. So I made basic load cases to account for the self weight of the steel rack as well as the point, area, and distributed loads from the equipment and pipes. I copied all dead loads into multiple load cases which represent the seismic loads and changed their directions. Then, in the load combinations, I use factors to reduce those loads (the copied 100% DL) into the seismic loads per ASCE 7-10. In summary, the program takes my equipment loads and applies the seismic factors and directions to them wherever they are in the model per seismic category D orthogonal combination procedure. So for a 2 level pipe rack, the dead loads on levels 1 and 2 simultaneously act laterally. The program does not add up the total dead load of level 2 and apply that laterally to level 1. Also, the seismic load of a columns's self weight turns into a uniform lateral load on that column. In analyzing the results, seismic drift controls per section, using a redundancy factor of 1.0. And most columns have a unity ratio of 0.7, using a redundancy factor of 1.3 per category D.

Based on your experience, would you say this is under designed or over designed due to the application of seismic loads? RISA is the only program available to me for frame analysis. Is there a better way to use RISA for seismic design that is less time consuming? Or do you recommend another program altogether?

Thank you,
Jason, EIT

Edit: Spelling of analyze in subject title

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