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Snow load: (ASCE 7-05, 10, IBC 2006, 2009, 2012)

Ground snow load:

Ground snow load, *p*_{g}, shall be according to
ASCE 7 Figure 7-1. or IBC Section 1608)

*p*_{g}, can be obtained from

ICC free website: http://publicecodes.cyberregs.com/icod/index.htm

**Flat roof snow load:**

*p*_{f}^{ }= 0.7 C_{e} C_{t}
I_{s} *p*_{g}

- C
_{e}is exposure factor. - C
_{t}is Thermal factor. - I
_{s}is important factor.

Exposure factor, C_{e}:

Exposure factor, C_{e} is based on wind exposure of
structure.

- For fully exposed structure, C
_{e}= 0.9 for Exposure B & C, 0.8 for Exposure D. - For partially exposed structure, C
_{e}= 1.0 for exposure B & C, 0.9 for Exposure D. - For Sheltered structure, C
^{e}= 1.2 for Exposure B, 1.1 for Exposure C, 1.0 for Exposure D.

Thermal factor: C_{t:}

- C
_{t }= 1.0 for all structure except the condition below. - C
_{t }= 1.1 for all structure keep just above freezing. - C
_{t }= 1.2 for all untheated structure - C
_{t }= 1.3 for all structure inteintionally keep below freezing.

Important factor, I_{s}:

- I
_{s}= 0.8 for Risk Category I (low risk to human life) - I
_{s}= 1 or Risk Category II (all structures except in I, III, and IV) - I
_{s}= 1.1 for Risk Category III (substantial risk to human life) - I
_{s}= 1.2 for Risk Category IV (Essential facilities)

Miminum snow load for Low-slop roofs, p_{m}:

*p*_{m} = I_{s }pg for *p*_{g}
is 20 psf or less

*p*_{m} = I_{s } (20 psf) for *p*_{g}
exceeds 20 psf or less

Example 1: an office building in downtown Peoria, IL with a flat roof that is partial exposed to wind.

**Sloped roof snow loads, ***
p*

*p*_{s} = C_{s} *p*_{f}

C_{s} is depended on roof slope, surface roughness, and
termal factor. C_{s} shall be obtained from ASCE 7,
Figure 7-2.

It can also obtained from the equation below.

Example 2: an office building in downtown Peoria, IL with a 5/12 slope slippery metal roof that is partial exposed to wind.

**Snow drift from upper roof to lower
roof**

Snow draft load from upper roof to lower roof depends on snow density, length of upper and lower roof, and height between two roofs.

Snow drift load is in a triangular shape with maxim load pd at edge between upper and lower roof and decrease to zero for a length W.

The drift load shall be added to normal roof snow load. The maximum drift load is

Snow density is

but it does not need to exceed 30 psf

Snow drift length (height of triangle):

Drift height in leeward direction, h_{dl }is

where L_{up} is length of upper roof. If L_{up}
is less thatn 20 ft, use 20 ft.

Drift height in windword direction is

where L_{ul} is length of lower roof. If L_{ul}
is less thatn 20 ft, use 20 ft.

The design drift height h_{d} shall be the higher value
of the two.

If snow drift height h_{d} is higher than the drop
between upper and lower roof, h_{c}. then, the h_{d}
= hc.

Snow drift length, W (length of triangle):

W = 4 h_{d} If snow drift height
h_{d} is lower, h_{c},

W = 4 h_{d}^{2}/h_{c}
If snow drift height h_{d} is lower, h_{c}.

Note: If W > L_{ul}, then, W = L_{ul}, and height
of snow drift at end of L_{ul} shall be prorated based on
length of L_{ul}.

Example 3:

Ground snow load: 20 psf

Length of upper roof: 30 ft

Length of lower roof: 20 ft

Height between upper and lower roof: 4 ft