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

Example

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.

How StructSuite solves this

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.

Steps

1. Step 1: Building geometry and weights

   Design consideration: 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.

   In StructSuite: In Step 1: Building geometry and weights, add a row for each story. Enter Level, Weight wx (lb), and Height hx (ft) for each level. hn = Σhx (sum of story heights) is used in Step 5 Period Determination for Ta = Ct × hn^x. Click + Add story for additional levels.

2. Step 2: Site classification and spectral parameters

   Design consideration: 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.

   In StructSuite: In Step 2: Site Classification and Spectral Parameters, enter the project address (StructSuite fetches Ss and S1 from USGS) or manually enter Ss and S1. Use the Site Class dropdown to select A, B, C, D, E, or F. StructSuite computes SDS and SD1 per ASCE 7-22 §11.4.

3. Step 3: Risk category and importance factor

   Design consideration: 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.

   In StructSuite: In Step 3: Risk Category and Importance Factor, use the Risk Category dropdown to select I, II, III, or IV. The Importance Factor Ie populates per ASCE 7-22 Table 1.5-2.

4. Step 4: Seismic force-resisting system

   Design consideration: Steel MRF R=8 gives Cs = SDS/8 = 0.125 (before bounds). Ct=0.028, x=0.8 → Ta longer than wood (0.02, 0.75). Higher R reduces V but needs special moment connections. Cd=5.5 for drift; Ω0=3 for overstrength. Concrete MRF R=8, Ct=0.016—stiffer, shorter T.

   In StructSuite: In Step 4: Seismic Force-Resisting System, select a row from Table 12.2-1 (e.g., Light-frame (wood) walls sheathed with wood structural panels, id 16 for R=6.5). This selection also determines the Table 12.8-2 structure type used for the approximate period Ta = Ct × hn^x—wood light-frame maps to "All other structural systems" (Ct=0.02, x=0.75).

5. Step 5: Period determination

   Design consideration: Steel MRF: Ta = 0.028×65^0.8 ≈ 0.9 s. Moment frame formula (0.1N for ≤12 stories): Ta = 0.5 s. Longer Ta from approximate often governs for 5+ stories. Upper bound Cs = 0.6/(0.9×8) ≈ 0.083—reduces V vs short-period. Analytical T from model can be used if T ≤ Cu×Ta per ASCE 7-22 §12.8.2.

   In StructSuite: In Step 5: Period Determination, select Approximate method. hn (ft) is read-only—it comes from Step 1 (Σhx). Table 12.8-2 structure type is derived automatically from your Step 4 selection. Ta = Ct × hn^x; the row is highlighted. Period T affects the Cs upper bound.

6. Step 6: Seismic response coefficient Cs

   Design consideration: For T=0.9s: Cs = min(0.125, 0.083) = 0.083. V = 0.083×800 = 66 kip. If T=0.5s: Cs=0.125, V=100 kip. Period choice can change base shear 30–50% for mid-rise. When SD1 governs, taller and more flexible = lower V.

   In StructSuite: In Step 6: Seismic Response Coefficient Cs, review Cs and base shear V = Cs × W. W comes from Step 1 story weights (Σwx). StructSuite calculates Cs per Eq 12.8-2. Review upper and lower bound limits per §12.8.1.1.

Live design (pre-filled)

The form below is the real StructSuite module with example data loaded. Display only—values cannot be changed.