According to Section 12.2.3 of ASCE 7-10, when a moment frame is combined with other lateral systems in the horizontal direction, the R-value used for design in the direction under consideration shall not be greater than the least value of R for any system in that direction (i.e., when combining a wood shearwall with R = 6.5 and a steel SMF with R = 8.0, R = 6.5 shall be used for the design of the SMF).
However, there is an exception if the following three conditions are all met:
If the above three conditions are met, then lateral-resisting elements are permitted to be designed using the least value of R found in each independent line of resistance. For example, if a wood shearwall with R = 6.5 is used at the interior wall of a garage and a steel SMF is used at the front of the garage parallel to the interior shearwall, then the SMF can be designed using an R-value of 8.
For vertical combinations of lateral system, according to ASCE 7-10 Section 188.8.131.52, where the lower system has a lower R-value compared to the upper system, a higher R-value can be used for the upper system. In other words, when combining an OMF (R = 3.5) at the first level and a wood shearwall (R = 6.5) at the upper level, the design of the shearwall above can use an R = 6.5. However, the lower system shall be designed using the lower R-value (i.e., R = 3.5 for the OMF). In addition, force transferred from the upper system to the lower system shall be increased by multiplying by the ratio of the higher R-value to the lower R-value (in the OMF and shearwall example, this ratio would be 6.5/3.5).
When the upper system has an R-value lower than that of the lower system, the R-value of the upper system shall be used for both systems (i.e., when a SMF [R = 8] is used at the lower level and a wood shearwall is used at the upper level, R = 6.5 shall be used for the design of both systems. When it comes to retrofits with moment frames, the International Existing Building Code (IEBC) allows the use of moment frames with a higher R-value at the base regardless of the existing lateral system at the top of the frames. Check with your local building official for additional requirements.
The following are items a Designer should consider when modeling and designing steel moment frames. We’ll discuss these in more detail later in this design guide.