Special Moment Frame — Base Fixity
Moment Frame Design Requirements and Assumptions
D7. Base Fixity Design
Column base fixity has a considerable effect on the performance of moment frames. Currently, engineers assume either a fixed-base connection (Figure D7.1) or a pinned-base connection (Figure D7.4) in the analysis of moment frames. In reality, the performance of the connection is in between the two limits. Figure D7.2 shows the AISC definition of a fixed, a pinned and a partially restrained (PR) connection in a graphical format. Connections are considered fixed when the moment vs. rotation stiffness is greater than 20 EI/L of the member, whereas a connection is considered pinned (simple) when the stiffness value is less than 2 EI/L.
Figure D7.1 — Fixed-Base (FR) Connection in AISC Seismic Design Manual
Figure D7.2 — Connection Classification per AISC 360-10
Figure D7.3 — PR Base Connection in AISC Design
Figure D7.4 — Pinned-Base (Simple) Connection in AISC Design Guide #1
Table 1 below shows the effects of base fixity on the different performance parameters. Pinned column bases will have a higher drift and a higher k-value for column design. However, they will have lower floor accelerations than columns with a fixed-base connection. A partially restrained base will behave somewhere in between pinned and fixed bases. Compared to a frame with pinned-base connections, a frame with PR bases will have less drift, higher base shear and higher floor accelerations.
Table 1 — Performance Effects from Different Base Fixities
Strong Frame Special Moment Frame Base Fixity
The Strong Frame typical base fixity assumption is a pinned column base. Reactions for a pinned-base connection consist of axial and shear only. If a fixed-base connection is used, then the designer will need to address the moment in the foundation design. For fixed-base connections, we currently use the embedded column approach. Contact Simpson Strong-Tie for available non-embedded options.