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base-fixity

Special Moment Frame — Base Fixity


	<h2>Moment Frame Design Requirements and Assumptions</h2>
	
	<div class="row">
		<div class="col-sm-12">
			<h4>D7. Base Fixity Design</h4>
			
			<p>
				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.
			</p>
		</div>
	</div>
	
	<div style="margin-top: 40px;" class="row">
		<div class="col-sm-12 col-md-6">
<p style="max-width: 80%; margin: auto; text-align: center;">
				<strong>Figure D7.1 &mdash; Fixed-Base (FR) Connection in AISC Seismic Design Manual</strong>
			</p>
			<img style="display: block; margin: 40px auto 20px; max-width: 80%; height: 200;" src="https://embed.widencdn.net/img/ssttoolbox/j6syfvbrls/exact/F-L-SFDG20-40a-2020-fixbasefr-connection.png" alt="SMF Base Fixity, Figure 1 — Fixed-Base Connection in AISC Seismic Design Manual">
		</div>

		<div class="col-sm-12 col-md-6">
<p style="max-width: 80%; margin: auto; text-align: center;">
				<strong>Figure D7.2 &mdash; Connection Classification<br>per AISC 360-10</strong>
			</p>
			<img style="display: block; margin: 40px auto 20px; max-width: 80%; height: 200;" src="https://embed.widencdn.net/img/ssttoolbox/ijhj5dytvf/exact/F-L-SFDG20-40b-connection-classification.png" alt="SMF Base Fixity, Figure 2 — Pinned-Base Connection in AISC Design Guide #1">
		</div>
	</div>
	
	<div style="margin-top: 40px;" class="row">
		<div class="col-sm-12 col-md-6">
<p style="max-width: 80%; margin: auto; text-align: center;">
				<strong>Figure D7.3 &mdash; PR Base Connection in AISC Design</strong>
			</p>
			<img style="display: block; margin: 40px auto 20px; max-width: 80%; height: 200;" src="https://embed.widencdn.net/img/ssttoolbox/vltod5bqmm/829x656px/F-L-SFDG20-40c-2020-prbase-connection.png" alt="Figure D7.3 &mdash; PR Base Connection in AISC Design">
		</div>
		
		<div class="col-sm-12 col-md-6">
<p style="max-width: 80%; margin: auto; text-align: center;">
				<strong>Figure D7.4 &mdash; Pinned-Base (Simple) Connection in AISC Design Guide #1</strong>
			</p>
			<img style="display: block; margin: 40px auto 20px; max-width: 80%; height: 200;" src="https://embed.widencdn.net/img/ssttoolbox/nibgyohssk/exact/F-L-SFDG20-40d-pinnedbase-sample.png" alt="Figure D7.4 — Pinned-Base (Simple) Connection in AISC Design Guide #1">
			
		</div>
	</div>
	
	<div style="margin-top: 40px;" class="row">
		<div class="col-sm-12">
			<p>
				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.
			</p>
			
			<h4 style="max-width: 80%; margin: auto; text-align: center;">Table 1 &mdash; Performance Effects from Different Base Fixities</h4>
			<img style="display: block; margin: 0 auto 20px; max-width: 80%; height: 200;" src="https://embed.widencdn.net/img/ssttoolbox/7bl1pomtow/exact/F-L-SFDG20-41a-table.png" alt="SMF Base Fixity, Table 1 — Performance Effects from Different Base Fixities">
		</div>
		
		<div class="col-sm-12">
			<h4>Strong Frame Special Moment Frame Base Fixity</h4>
			
			<p>
				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.
			</p><br><br>
		</div>
	</div>

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