Seismic Period: Approximate vs Moment Frame vs Shear Wall per ASCE 7-22

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Seismic Period: Approximate vs Moment Frame vs Shear Wall per ASCE 7-22

Summary: Compare different period calculation methods and their effect on base shear: approximate Ta, moment frame formula, shear wall formula, and direct period…

Overview

Compare different period calculation methods and their effect on base shear: approximate Ta, moment frame formula, shear wall formula, and direct period input. Free ASCE 7 load combination calculator.

Problem statement

A five-story steel moment frame building, 65 ft height, 800 kip weight. SDS=1.0, SD1=0.6, R=8. How does period method affect Cs and base shear? Approximate Ta vs moment frame formula vs direct T.

Workflow in StructSuite

StructSuite Step 1: enter story weights wx and heights hx. Step 4: select seismic force-resisting system (e.g., steel MRF). Step 5: Period determination—Approximate: Ta = 0.028×hn^0.8 (steel MRF per ASCE 7-22 Table 12.8-2). Moment frame: Ct=0.028, x=0.8. Shear wall: 0.0019/√Ac. Direct: user T. Longer T increases Cs upper-bound check; may reduce or cap Cs. StructSuite shows which bound governs.

Design considerations (excerpt)

Five-story 800 kip building: enter Level 1–5, wx (lb) and hx (ft) for each story. hn = Σhx = 65 ft drives period Ta. Steel MRF typically has longer period than wood—Ct=0.028, x=0.8 vs wood Ct=0.02, x=0.75.

SDS=1.0, SD1=0.6 typical for SDC D. Period T directly affects Cs upper bound: Cs ≤ SD1/(T×R/Ie). Longer T → lower Cs when SD1-controlled. Site Class E/F amplifies; stiff soil (D) is baseline. Ts = SD1/SDS = 0.6 s—periods above Ts hit upper bound.

Ie (Importance Factor) from ASCE 7-22 Table 1.5-2: Risk II = 1.0, III = 1.25, IV = 1.5. Cs = SDS/(R/Ie)—higher Ie raises seismic force. Office/residential = Risk II.

Steel MRF R=8 gives Cs = SDS/8 = 0.125 (before bounds). Ct=0.028, x=0.8 → Ta…

Related terms

  • StructSuite
  • structural engineering
  • design example
  • seismic
  • ASCE 7-22
  • period methods comparison

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