text.skipToContent text.skipToNavigation

Steel Moment Frame Design Requirements and Considerations

Strong Frame Moment Frame

ASCE Design Requirements for Moment Frames, R-Value for Horizontal Combinations, R-Value for Vertical Combinations, and the Exceptions

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:

  1. Risk category I or II building
  2. The building is two stories or less above grade
  3. The use of light-frame construction or flexible diaphragms

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 12.2.3.1, 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.

Strong Frame Vertical Combination Diagram
Strong Frame Horizontal Combination Diagram

Design Requirements and Considerations

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.

Analysis and Modeling:

  • A1. Frame Geometry and Space Restrictions A2. Member Geometries
  • A3. Connection Modeling
  • A4. Base Fixity Modeling
  • A5. Load Combinations
Strong Frame Analysis and Modeling

Design:

  • D1. Drift Check (not shown in drawing), p. 40
  • D2. Panel Zone Check, p. 42
  • D3. Strong Column / Weak Beam Check (not shown in drawing), p. 43
  • D4. Moment Frame Connection Design, pp. 44–51
    • D4a. Beam Bracing
    • D4b. Protected Zones
    • D4c. Connection Design
  • D5. Member Design, p. 52
  • D6. Nailer to Steel Beam Connection Design, p. 53
  • D7. Base Fixity Design, p. 55
  • D8. Anchorage Design, p. 56
  • D9. Foundation Design
Strong Frame Design