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Special Moment Frame — Base Fixity

Strong Frame Special Moment Frame

Moment Frame Design Requirements and Assumptions

Base Fixity

Column base fixity has a considerable effect on the performance of moment frames. Currently engineers assume either a fixed-base connection (Figure 1) or a pinned-base connection (Figure 2) in the analysis of moment frames. In reality, the performance of the connection is in between the two limits. Figure 3 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.

SMF Base Fixity, Figure 1 — Fixed-Base Connection in AISC Seismic Design Manual

Figure 1 — Fixed-Base Connection in AISC Seismic Design Manual

SMF Base Fixity, Figure 2 — Pinned-Base Connection in AISC Design Guide #1

Figure 2 — Pinned-Base Connection in AISC Design Guide #1

SMF Base Fixity, Figure 3 — Connection Classi cation per AISC 360-10

Figure 3 — Connection Classi cation per AISC 360-10

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

SMF Base Fixity, 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.