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## Q. 1.2

A three-story office building located on Main Street in Nyack, NY, in close proximity to the Hudson River (Figure 1.8) has an upwind direction from the river. In order to calculate the at roof snow load for the building, we need to determine the variables in Equation 1.2.

$p_{f}=0.7C_{e}C_{t}I_{s}p_{g}$   (1.2)

## Verified Solution

Step 1: Obtain the ground snow load ($p_{g}$ ) from Figure 7.1 in ASCE 7-10. The ground snow load for Nyack, NY is 30 psf, ($p_{g}=30 psf$).

Step 2: Determine the roof exposure. The exposure factor, $C_{e}$ , is based on the wind exposure of the building and the surface roughness. The building, as shown in Figure 1.8, is 40 ft tall, and has a surface roughness in compliance with that of surface roughness B. The surface roughness prevails for a distance of approximately 2200 ft, which is less than the 2600 ft required to satisfy the condition for exposure category B. Additionally, exposure category D is not satisfied and consequently exposure category C prevails. Hence, use terrain category C in Table 1.3, and for a fully exposed roof,$C_{e}$ =.9.

Step 3: Determine the thermal factor, $C_{t}$ , which is based on the thermal condition of the building described in Table 1.4, hence $C_{t}$=1.0

Step 4: Determine the importance factor, $I_s$, which is based on the risk category assignment of the building in Table 1.5-1, in ASCE 7-10. Risk categories I, II, III, and IV are based on the potential loss of life during a catastrophic failure. According to Table 1.5-1, an office building has a risk category of II. From Table 1.5, a risk category II has an importance factor, $I_{s}$= 1.0.

Step 5: Finally, the at roof snow load for the building in Figure 1.8 is found from Equation 1.2

$p_{f}=0.7C_{e}C_{t}I_{s}p_{g} \\p_{f}=0.7(.9)(1.0)(1.0)(30 psf)=18.9 Ib/ft^2$
 Table 1.3 Exposure factor, $C_e$ Terrain category Exposure of roof Fully exposed Partially exposed Sheltered B 0.9 1.0 1.2 C 0.9 1.0 1.1 D 0.8 0.9 1.0 Above tree line in mountainous areas 0.7 0.8 N/A
 Table 1.4 Thermal factor, $C_t$ Thermal condition $C_{t}$ All structures except as indicated below 1.0 Structures keep just above freezing and others with cold, ventilated roofs in which the thermal resistance (R-value) between the ventilated space and the heated space exceeds 25°F × h × ft/Btu 1.1 Unheated and open air structures 1.1 Structures intentionally kept below freezing 1.2 Continuously heated greenhouses with a roof having a thermal resistance (R-value) 1.3 ess than 2.0°F × h × $ft^2$/Bt 0.85
 Table 1.5 Important factors by risk category of buildings for snow, ice, and earthquake loads Risk category from Table 1.5-1 Snow importance factor, $I_{s}$ Ice importance factor–thickness, $I_{i}$ Ice importance factor–wind, $I_{w}$ Seismic importance factor, $I_{e}$ I 0.80 0.80 1.00 1.00 II 1.00 1.00 1.00 1.00 III 1.10 1.25 1.00 1.25 IV 1.20 1.25 1.00 1.50