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Load Calculations

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

Ground snow load:

Ground snow load, pg, shall be according to ASCE 7 Figure 7-1. or IBC Section 1608)

pg, can be obtained from

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

Flat roof snow load:

pf = 0.7 Ce Ct Is pg

  • Ce is exposure factor.
  • Ct is Thermal factor.
  • Is is important factor.

Exposure factor, Ce:

Exposure factor, Ce is based on wind exposure of structure.

  • For fully exposed structure, Ce = 0.9 for Exposure B & C, 0.8 for Exposure D.
  • For partially exposed structure, Ce = 1.0 for exposure B & C, 0.9 for Exposure D.
  • For Sheltered structure, Ce = 1.2 for Exposure B, 1.1 for Exposure C, 1.0 for Exposure D.

Thermal factor: Ct:

  •  Ct = 1.0 for all structure except the condition below.
  • Ct = 1.1 for all structure keep just above freezing.
  •  Ct = 1.2 for all untheated structure
  • Ct = 1.3 for all structure inteintionally keep below freezing.

Important factor, Is:

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

Miminum snow load for Low-slop roofs, pm:

pm = Is pg for pg is 20 psf or less

pm = Is  (20 psf) for pg exceeds 20 psf or less

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

Snow example 1

Sloped roof snow loads, ps

ps = Cs pf

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

It  can also obtained from the equation below.

Slope roof eq 2

Roof slope and factor 

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

Slope roof example 2

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.

Snow drift drawing

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

snow drift eq

Snow density is

snow density

but it does not need to exceed 30 psf

Snow drift length (height of triangle):

Drift height in leeward direction, hdl is

drift height eq

where Lup is length of upper roof. If Lup is less thatn 20 ft, use 20 ft.

Drift height in windword direction is

drift height windward

where Lul is length of lower roof. If Lul is less thatn 20 ft, use 20 ft.

The design drift height hd shall be the higher value of the two.

If snow drift height hd is higher than the drop between upper and lower roof, hc. then, the hd = hc.

Snow drift length, W (length of triangle):

W = 4 hd     If snow drift height hd is lower, hc,

W = 4 hd2/hc     If snow drift height hd is lower, hc.

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

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

Snow example 3R