Diaphragm Design Forces Fpx per ASCE 7-22 §12.10

Example

A two-story wood building with F1=8 kip, F2=12 kip (roof), story weights w1=40 kip, w2=40 kip. SDS=1.0. Determine Fpx at each level for diaphragm design. Collectors and chords designed for Fpx.

How StructSuite solves this

StructSuite Step 1: enter story weights wx and heights hx. Steps 2–5: site, risk, structural system, period. Step 8: Diaphragm design forces. Fpx = (ΣFi from i=x to n)×wpx/(Σwi from i=x to n) per ASCE 7-22 Eq 12.10-1. Minimum Fpx = 0.2×SDS×wpx. StructSuite computes Fpx for each level. Critical for collectors in openings, chord reinforcement, and drag struts.

Steps

1. Step 1: Building geometry and weights

   Design consideration: Two-story wood building: enter Level 1–2, wx (lb) and hx (ft). w1=40 kip, w2=40 kip; F1=8 kip, F2=12 kip from vertical distribution. hn drives period Ta; story weights drive Fpx formula.

   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 drives minimum Fpx: Fpx ≥ 0.2×SDS×wpx. For SDS=1.0, floor with 40 kip weight has Fpx_min = 8 kip. Minimum often governs for low-rise (1–3 stories); force distribution may govern for taller. Site Class affects SDS—soft soil raises it.

   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 from ASCE 7-22 Table 1.5-2. Residential = Risk II, Ie=1.0. Fpx minimum 0.2×SDS×wpx does not include Ie; full base shear V and Fx do.

   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: Wood light-frame R=6.5 typical for residential. Table 12.2-1 id 16. R affects base shear V and thus Fx distribution.

   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: Approximate Ta = 0.02×hn^0.75 for wood. T affects k in vertical distribution (T < 0.5s: k=1; T > 2.5s: k=2). Fpx uses ΣFi and Σwi from Fx results.

   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: V = Cs×W. Total base shear distributes vertically as Fx. Fpx formula uses ΣFi (forces above level x) and Σwi (weights above level x). Heavier buildings (larger W) and higher SDS increase V and thus Fpx.

   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 at top—roof gets largest share. Cvx = wx×hx^k / Σ(wi×hi^k). k depends on period. For T < 0.5s, k=1 (linear); T > 2.5s, k=2. Weight at roof (snow, HVAC) increases F2 and roof diaphragm force.

   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.

8. Step 8: Diaphragm design forces

   Design consideration: Fpx = (ΣFi)×wpx/(Σwi) with minimum 0.2×SDS×wpx. Collectors at openings (garage, atrium) carry Fpx to shear walls. Chord reinforcement resists bending; drag struts transfer force. Flexible diaphragm (§12.10.1.2) distributes differently—wood often flexible.

   In StructSuite: In Step 8: Diaphragm Design Forces, diaphragm design forces Fpx are computed per ASCE 7-22 §12.10.1.1. Minimum 0.2×SDS×wpx applies. Use these forces for collector and chord design.

Live design (pre-filled)

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