The enclosed building shown in Fig. 1–14a is used for agricultural purposes and is located outside of Chicago, Illinois on flat terrain.When the wind is directed as shown, determine the design wind pressure acting on the roof and sides of the building using the ASCE 7-05 Specifications.

Question

The enclosed building shown in Fig. [latex] 1-14 (\mathrm{a})[/latex] is used for agricultural purposes and is located outside of Chicago, Illinois on flat terrain. When the wind is directed as shown, determine the design wind pressure acting on the roof and sides of the building using the ASCE 7-05 Specifications.

Accepted Answer

First the velocity pressure will be determined using Eq. 1-2. From Fig. 1-12, the basic wind speed is [latex]V=90 \mathrm{mi} / \mathrm{h}[/latex], and since the building is used for agricultural purposes, the importance factor is [latex]I=0.87[/latex]. Also, for flat terrain, [latex]K_{z t}=1[/latex]. Since only wind loading is being considered, [latex]K_{d}=1[/latex]. Therefore,

[latex] \begin{array}{l} q_{z}=0.00256 K_{z} K_{z t} K_{d} V^{2} I\left(\mathrm{lb} / \mathrm{ft}^{2} ight) \ q_{z}=0.613 K_{z} K_{z t} K_{d} V^{2} I\left(\mathrm{~N} / \mathrm{m}^{2} ight) \end{array} [/latex] (1-2)

[latex] \begin{aligned} q_{z} &=0.00256 K_{z} K_{z t} K_{d} V^{2} I \ &=0.00256 K_{z}(1)(1)(90)^{2}(0.87) \ &=18.04 K_{z} \end{aligned} [/latex]

From Fig. 1-14a, [latex]h^{\prime}=75 an 10^{\circ}=13.22 \mathrm{ft}[/latex] so that the mean height of the roof is [latex]h=25+13.22 / 2=31.6 \mathrm{ft}[/latex]. Using the values of [latex]K_{z}[/latex] in Table 1-5, calculated values of the pressure profile are listed in the table in Fig. 1-14b. Note the value of [latex]K_{z}[/latex] was determined by linear interpolation for [latex]z=h[/latex], i.e., [latex](1.04-0.98) /(40-30)=\left(1.04-K_{z} ight) /(40-31.6)[/latex], [latex]K_{z}=0.990[/latex], and so [latex]q_{h}=18.04(0.990)=17.9 \mathrm{psf}[/latex].

TABLE 1–5 • Velocity Pressure Exposure Coefficient for Terrain with Low-Lying Obstructions
z
ft	m	[latex] K_{z} [/latex]
0-15	0-4.6	0.85
20	6.1	0.90
25	7.6	0.94
30	9.1	0.98
40	12.2	1.04
50	15.2	1.09

In order to apply Eq. 1-3 the gust factor is [latex]G=0.85[/latex], and [latex]\left(G C_{p i} ight)=[/latex] [latex]\pm 0.18[/latex]. Thus,

[latex] \begin{aligned} p &=q G C_{p}-q_{h}\left(G C_{p i} ight) \\end{aligned} [/latex] (1-3)

[latex] \begin{aligned} &=q(0.85) C_{p}-17.9(\pm 0.18) \ &=0.85 q C_{p} \mp 3.21 \end{aligned} [/latex] (1)

The pressure loadings are obtained from this equation using the calculated values for [latex]q_{z}[/latex] listed in Fig. 1-14b in accordance with the wind-pressure profile in Fig. 1–13.

Windward Wall. Here the pressure varies with height [latex]z[/latex] since [latex]q_{z} G C_{p}[/latex] must be used. For all values of [latex]L / B, C_{p}=0.8[/latex], so that from Eq. (1),

[latex] \begin{aligned} p_{0-15} &=7.19 \mathrm{psf} & & ext { or } & 13.6 \mathrm{psf} \ p_{20} &=7.81 \mathrm{psf} & & ext { or } & 14.2 \mathrm{psf} \ p_{25} &=8.35 \mathrm{psf} & & ext { or } & 14.8 \mathrm{psf} \end{aligned} [/latex]

Leeward Wall. Here [latex]L / B=2(75) / 150=1[/latex], so that [latex]C_{p}=-0.5[/latex].

Also, [latex]q=q_{h}[/latex] and so from Eq. (1),

[latex] p=-10.8 \mathrm{psf} \quad ext { or } \quad-4.40 \mathrm{psf} [/latex]

Side Walls. For all values of [latex]L / B, C_{p}=-0.7[/latex], and therefore since we must use [latex]q=q_{h}[/latex] in Eq. (1), we have

[latex] p=-13.9 \mathrm{psf} \quad ext { or } \quad-7.44 \mathrm{psf} [/latex]

Windward Roof. Here [latex]h / L=31.6 / 2(75)=0.211<0.25[/latex], so that [latex]C_{p}=-0.7[/latex] and [latex]q=q_{h}[/latex]. Thus,

[latex] p=-13.9 \mathrm{psf} \quad ext { or } \quad-7.44 \mathrm{psf} [/latex]

Leeward Roof. In this case [latex]C_{p}=-0.3[/latex]; therefore with [latex]q=q_{h}[/latex], we get

[latex] p=-7.77 \mathrm{psf} \quad ext { or } \quad-1.35 \mathrm{psf} [/latex]

These two sets of loadings are shown on the elevation of the building, representing either positive or negative (suction) internal building pressure, Fig. 1-14c. The main framing structure of the building must resist these loadings as well as loadings calculated from wind blowing on the front or rear of the building.

Question 1.3: The enclosed building shown in Fig. 1–14a is used for agricu...

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