Steel Moment Frame Design Requirements and Considerations
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 7-16, 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:
- Risk category I or II building
- The building is two stories or less above grade
- 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 7–16 Section 220.127.116.11, 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 applicable ordinance or additional requirements.
Design Requirements and Considerations
The following two pages include items a designer should consider when modeling and designing Strong Frame steel moment frames. We will 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
A1. Frame Geometry and Space Restrictions
Even though structural analysis and design for the Strong Frame utilizes the member centerline dimensions, in the actual application the designer needs to be aware of the actual frame geometry for the frame specification. Figure A1 below indicates the seven critical dimensions the designer will need to fit the frame within the given wall space and meet the opening requirements. A more detailed explanation of each of the items below is given in the Installer Overview section on p. 70.
- Frame height
- Clear height
- Inside/clear width
- Outside width
- Column centerline
- Beam and column flange widths/depth with wood nailers
- Column extension below slab
A2. Member Geometries
For steel frame used in wood construction, wood nailers are required for the frame to tie into the rest of the wood structure. For Strong Frame® beam and columns, we provide nailers along with the steel shapes. When considering the depth and width of the members, the designer needs to consider the steel members with the nailers attached. Figures A2a and A2b show typical details of what the beam and column look like with wood nailers attached. See Product and Service Offering section for more detailed information on member depth and width with and without wood nailers.
Since the Yield-Link® moment connection is considered a partially restrained (PR) connection, explicit modeling of the Yield-Link moment connection is required for frame analysis and design. There are several ways to model the Yield-Link moment connection: 1) Moment release with partial fixity rotational springs; 2) Equivalent elastic Yield-Link elements; and 3) Pair of axial springs at the beam flange levels to represent the Yield-Link. For our Strong Frame Selector, option 1 is used. For our design frames using SAP2000, option 3 is used. For more information regarding Yield-Link moment connection modeling for Strong Frames, See F-L-YLCDG20.
A4. Base Fixity Modeling
Since moment frame design is typically governed by drift, frame base fixity modeling for the structural analysis model plays a critical role in the analysis and design of moment frames. For typical applications, pinned base is assumed for the Strong Frame® analysis and design. However, we also offer fixed base solutions using: 1) Embedding the column into the concrete footing with a grade beam; 2) Non-embedded rigid base plate (see Figure A4 below). For more information on the effects of base fixity, please refer to the design section D7.
A5. Load Combinations
Strong Frame design calculations including drift check, Yield-Link® moment connection, beam and column design all use LRFD load combinations per ASCE 7 and IBC. Design of the Yield-Link yielding area uses the standard LRFD combinations (i.e., no overstrength/omega combinations). Once the required yielding area is known, the rest of the connection elements are designed for the Yield-Link maximum probable tensile strength (Pr-link). Strong Frame column design uses overstrength load combinations for seismic design. Columns are designed for both moment + axial load from the overstrength demand load combinations; this is more stringent than the AISC 341-16 requirement where only overstrength axial load (ignoring moment) is required. Strong Frame beam design uses overstrength combination demand loads to make sure the beam can develop the Yield-Link capacities at each end of the beam. However, the overstrength beam design moment at each end need not be greater than the Yield-Link maximum probable moment capacity (Mpr-link).