# Determine the external wind pressure on the roof of the rigid-gabled frame of an apartment building shown in Fig. P2.12. The building is located in the Los Angeles area of California, where the terrain is representative of exposure B . The wind direction is normal to the ridge as shown. Use K d = 0.85 for main wind-force resisting systems, and K e = 1. FIG. P2.12

### Structural Analysis

6th Edition
KASSIMALI + 1 other
Publisher: Cengage,
ISBN: 9781337630931

Chapter
Section

### Structural Analysis

6th Edition
KASSIMALI + 1 other
Publisher: Cengage,
ISBN: 9781337630931
Chapter 2, Problem 12P
Textbook Problem
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## Determine the external wind pressure on the roof of the rigid-gabled frame of an apartment building shown in Fig. P2.12. The building is located in the Los Angeles area of California, where the terrain is representative of exposure B. The wind direction is normal to the ridge as shown. Use Kd = 0.85 for main wind-force resisting systems, and Ke = 1.FIG. P2.12

To determine

Find the external wind pressure acting on the roof of the building.

### Explanation of Solution

Given information:

The terrain of the city belongs to Exposure B.

The basic wind speed is denoted by V.

The direction of the wind is normal to the ridge.

The value of the wind directionality factor (Kd) is 0.85for main wind force resisting system.

The value of the ground elevation factor (Ke) is 1 for ground elevation at sea level.

Calculation:

Show the sloped roof of the building as shown in Figure 1.

Refer Figure 1.

Calculate the angle of slope (θ) of the roof using the relation:

tanθ=BCAB=15ft15ftθ=tan1(1)θ=45°

Calculate the mean roof height (h) as follows:

h=40ft+15ft2=47.5ft

Calculate the value of (hL) as follows:

Substitute 47.5ft for h and 30ft.

hL=47.530=1.58

Refer the Table 2.4 “Exposure Categories for Buildings for Wind Loads” in the textbook.

For exposure B.

The values of gradient height is zg=1,200ft.

The value of power law coefficient is α=7.

Show the expression for the velocity pressure coefficient (Kh) as follows:

Kh=2.01(hzg)2α (1)

Substitute 47.5ft for h, 1,200ft for zg, and 7 for α in Equation (1).

Kh=2.01(47.51,200)27=2.01×0.397456=0.79880.8

The value of the topographic factor (Kzt) as 1 for main wind force resisting system.

Refer Figure 2.11 “Basic Wind Speeds for the United States for Risk Category II Buildings” of the book.

The basic wind speed for the Los Angeles is 95 mph.

Show the expression for the velocity pressure (qh) at height h as follows:

qh=0.00256KhKztKdKeV2 (2)

Here, qh is in pounds per square foot

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