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

Summary: Calculate velocity pressure for building in open terrain (Exposure C) and on hillside per ASCE 7-22 Section 26.7, Section 26.8 (Kzt from Figure 26.8-1),…

Overview

Building / scenario

• Geometry / size: 50×100 ft, h=25 ft

Problem statement

A warehouse in open terrain (Exposure C), 50 ft × 100 ft, 25 ft mean roof 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.

Workflow in StructSuite

In StructSuite Wind Loads, Step 3: Select Exposure C (open terrain). Kzt: enable Table method, enter H = 100 ft, Lh = 200 ft, x, z, hill shape 2D Ridge, upwind/downwind. Step 4: Enter h = 25 ft and confirm Kh; after Step 6 geometry, Step 4 shows roof angle. Step 5: qh = 0.00256×Kh×Kzt×Ke×V². Exposure C yields higher Kh than B; Kzt at crest can increase pressure substantially.

Design considerations (excerpt)

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.

Step 4 sets h and Kh; Step 6 sets L, B, θ. Step 4 accordion lists Kz at each z for Chapter 27 Directional. 25 ft height in Exposure C: Kh≈0.94 vs 0.70 in B—34% higher qh before Kzt.

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.

qh×Kzt: at crest Kzt can greatly increase design pressure. Chapter 28 MWFRS Step 7 includes minimum design wind…

Related terms

• StructSuite
• structural engineering
• design example
• wind
• ASCE 7-22
• exposure topographic

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