Seismic Base Shear for Residential Building in SDC D per ASCE 7-22

Example

A two-story residential building in SDC D has 80 kip total seismic weight (40 kip per floor, 10 ft story height each). Wood light-frame with R = 6.5 (Table 12.2-1, Light-frame wood walls). Site Class D. hn = Σhx = 20 ft. Table 12.8-2 structure type for wood: "All other structural systems", Ct=0.02, x=0.75. Determine base shear V and vertical distribution Fx per ASCE 7-22 §12.8.

How StructSuite solves this

StructSuite's free structural design software. Step 1: enter Level, wx (lb), hx (ft) for each story. hn = Σhx used in Period step. Step 4: select Table 12.2-1 row (e.g., id 16 wood light-frame); Table 12.8-2 row auto-selects for Ta. Step 5: hn and Ct, x read-only. Step 6: Cs and V. Step 7: Fx, Vx distribution.

Steps

1. Step 1: Building geometry and weights

   Design consideration: Weight W (Σwx) drives base shear directly: V = Cs×W. Height hn (Σhx) drives period Ta = Ct×hn^x—taller buildings have longer periods. Longer T can reduce Cs when SD1-controlled (upper bound Cs ≤ SD1/(T×R/Ie)). For two-story residential, 40 kip/floor (35–50 psf) typical; 10 ft story height common. Weight at roof (snow, mechanical) affects Fx distribution.

   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 and SD1 from USGS or site-specific study. Site Class D (stiff soil) is default; Class E or F (soft soil) amplifies ground motion—SDS/SD1 can increase 30–50%. Higher SDS = higher base shear. SDC D typically SDS 0.5–1.5g, SD1 0.2–0.6g. Coastal CA, Pacific Northwest, central US have high values.

   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 Table 1.5-2: Risk II = 1.0, III = 1.25, IV = 1.5. Cs = SDS/(R/Ie)—higher Ie raises seismic force. Residential = Risk II. Schools, assembly, essential facilities = III or IV.

   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: R reduces base shear: V ∝ 1/R. Wood light-frame R=6.5; steel MRF R=8; concrete shear wall R=5. Higher R = lower V but requires more ductile detailing (special moment frames, hold-downs). Cd amplifies drift; Ω0 for overstrength in load combinations. Wood R=6.5 balances economy and ductility for residential.

   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: Ta = 0.02×hn^0.75 for wood ("all other systems"). Longer T → lower Cs upper bound when SD1 governs. For hn=20 ft: Ta ≈ 0.23 s. Steel MRF (Ct=0.028, x=0.8) gives longer Ta for same hn. Period affects both base shear and drift.

   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: Cs = SDS/(R/Ie) with upper bound SD1/(T×R/Ie) and lower bound 0.044×SDS×Ie. For 80 kip, R=6.5, SDS=1.0: Cs≈0.154, V≈12 kip. Upper bound often governs for stiff, short-period buildings; lower bound for long-period.

   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.

7. Step 7: Vertical and horizontal distribution

   Design consideration: Fx concentrates force at top (whiplash effect). Roof gets largest Fx; F1 smaller. ΣFx = V. Story shear Vx = cumulative from top—used for diaphragm design. Heavier upper floors increase roof Fx.

   In StructSuite: In Step 7: Vertical and Horizontal Distribution, story forces Fx are computed per ASCE 7-22 Eq 12.8-11, 12.8-12. Story shear Vx is shown for diaphragms and shear walls. Review the distribution table.

Live design (pre-filled)

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