ASCE 7-22 Wind: Exposure C and Topographic Factor Kzt

Example

A warehouse in open terrain (Exposure C), 50 ft × 100 ft, 25 ft height. Basic wind speed 120 mph. Site on 2D ridge: hill height H=100 ft, Lh=200 ft, building at crest. Determine qh with Kzt. Exposure C and Kzt increase pressure vs suburban.

How StructSuite solves this

StructSuite's free ASCE 7 wind load calculator. Step 3: Select Exposure C (open terrain). Step 3 Kzt: enable Table method, enter H, Lh, x, z, hill shape (2D Ridge), upwind/downwind. Kzt from ASCE 7-22 Figure 26.8-1. qh = 0.00256×Kh×Kzt×Ke×V². Exposure C yields ~30% higher Kh than B; Kzt at crest can double pressure.

Steps

1. Step 1: Determine risk category of building

   Design consideration: Warehouse Risk II. 120 mph—open terrain often in rural/farmland. Wind contours vary; open exposure can occur in buffer between suburban and coastal. Higher V and Exposure C compound.

   In StructSuite: In Step 1, use the Risk Category dropdown to select I, II, III, or IV (e.g., II for offices, residential). In the Basic Wind Speed (mph) input box enter 120. Per ASCE 7-22 §26.2, Figure 26.5-1. If using address lookup, use the address search to auto-fill wind speed.

2. Step 4: Velocity pressure exposure coefficient

   Design consideration: 25 ft height in Exposure C: Kh≈0.94 vs 0.70 in B—34% higher qh before Kzt. Low-rise but open terrain = significant pressure. Plan dimensions affect zone size for MWFRS.

   In StructSuite: In the Building dimensions section, in Building Length (ft) enter 50; in Building Width (ft) enter 100; in Mean Roof Height (ft) enter 25. These drive Kh from ASCE 7-22 Table 26.10-1.

3. Step 3: Wind load parameters

   Design consideration: C: open country, obstacles <30 ft. Kzt from Figure 26.8-1: H (hill height), Lh (horizontal), x (building position). Crest (x=0): Kzt max. 2D ridge vs 3D hill: different multipliers. H/Lh=0.5, x=0: Kzt can reach 2.0.

   In StructSuite: In Step 3, use the Exposure dropdown to select B, C, or D. If ground elevation > 1000 ft, use Table 26.9-1 for Ke. For topographic features (hills, ridges), use the Kzt section and enter parameters from ASCE 7-22 Figure 26.8-1. Kd = 0.85 for MWFRS is typical.

4. Step 5: Velocity pressure at mean roof height

   Design consideration: qh×Kzt: at crest Kzt=2 doubles design pressure. Rooftop equipment, parapets, canopies see amplified load. Document H, Lh, x, shape on structural drawings. Downwind (x>0) Kzt decreases; upwind (x<0) also elevated.

   In StructSuite: In Step 5, the velocity pressure qh is auto-calculated from ASCE 7-22 Eq 26.8-1. Review the displayed value. Open Step 6 for external pressure coefficients (load cases 1–4); open Step 7 for design pressure results by zone.

Live design (pre-filled)

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