Special Moment Frame – Drift Check
Moment Frame Design Requirements and Assumptions
D1. Drift Check
Drift Check for Seismic Loads
ASCE 7-16 Section 12.12.1 states that design story drift of a structure shall not exceed the allowable drift limit listed in Table 12.12-1. For seismic applications, the story drift limitation not only serves as a serviceably check but is an inherent ductility requirement for seismic design related to the Response Modification Coefficient (R-value) as well as structural stability.
Figure D1.1 — Drift and Ductility Relationship
In the current seismic design philosophy, structures do not have to be designed for the Maximum Considered Earthquake (MCE) forces. Reduction in design forces is primarily related to the R-value in lateral force-resisting systems. The R-value for each lateral system is related to ductility and design codes have taken this into consideration when assigning higher R-values to more ductile systems. Reduced design forces used for drift check should be at strength level (LRFD) (ASCE 7-16 Section 12.8.6), and the deflection amplification factor (Cd) used shall correspond to the R-value used for the lateral forceresisting system. Please note, for drift check, ρ shall be taken as 1.0 per ASCE 7 Section 12.3.4.1. In addition, drift check need not include overstrength combinations since the ultimate displacement calculation already includes the Cd factor.
Drift Check for Wind Loads
Currently, there are no drift limit requirements for wind design. However, there are some recommendations for serviceability considerations, such as Appendix C in ASCE 7 and AISC Design Guide 3, Serviceability Design Considerations for Steel Buildings.
Strong Frame Special Moment Frame Drift Check
Because the Strong Frame SMF connection is considered a partially restrained (PR) connection, modeling and analysis is more involved than for a traditional moment frame connection. When designing and analyzing PR connections, the strength and stiffness of the connection need to be considered. A detailed step-by-step procedure to calculate the axial Yield-Link® or rotational Yield-Link parameters for Strong-Frame moment connection is documented in Chapter 12 of the AISC 358-16. Once the PR connection is modeled, frame drift can be calculated similar to the traditional fully restrained (FR) connections. For pushover or nonlinear time history analysis, a full nonlinear axial Yield-Link or rotational Yield-Link model is required (see Figure D1.2). Design tools for calculating the Yield-Link parameters can be obtained from Simpson Strong-Tie at strongtie.com.
Case 1: Link Axial Force vs. Link Axial Deformation
Case 2: Connection Moment vs. Rotation
Figure D1.2 — Simpson Strong-Tie Yield-Link Modeling Parameters Connection Moment (Ref: AISC 358-16, Chapter 12)
Drift Check Options in Strong Frame Selector Software
The Simpson Strong-Tie Strong Frame selector is a software tool developed to assist designers to size moment frames for their projects. The adjacent table lists the various selections available within the Strong Frame selector for considerations of drift for seismic and wind design. These are provided from least restrictive to more restrictive as you move down the table. The appropriate drift selection may depend on building code and/or material requirements such as Structure Type, Risk Category, Finish Materials or various other considerations in order to accommodate the story drift. For other drift/deflection requirements not listed here, contact Simpson Strong-Tie to assist with providing a tailored design to meet your specific requirements.
