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Determining Snow LoadsDetermining Snow LoadsDetermining Snow Loads
IntroductionIntroduction
Most building codes reference the procedures
in ASCE 7 - Chapter 7
See your handout
Roof snow loads are based on the historical
records for ground snow loads
we dont usually have records of roof snow
levels
Ground snow loads are converted to roof
snow load using the equations in ASCE 7
Most building codes reference the procedures
in ASCE 7 - Chapter 7
See your handout
Roof snow loads are based on the historical
records for ground snow loads we dont usually have records of roof snowwe dont usually have records of roof snow
levelslevels
Ground snow loads are converted to roofGround snow loads are converted to roof
snow load using the equations in ASCE 7snow load using the equations in ASCE 7
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Flat Roof Snow Load calculated by: Flat Roof Snow Load calculated by:
gtef pICC7.0p gtef pICC7.0p
pf= flat roof snow load, psf
Ce = Exposure factor
Ct = Thermal factor
I = Importance factor
pg = ground snow load, psf
pf= flat roof snow load, psf
Ce = Exposure factor
Ct = Thermal factor
I = Importance factor pg = ground snow load, psf
Flat Roof defined by:
< 5 degree slope
pg = ground snow load, psf
ground snow load comes from
maps in ASCE 7
Flat Roof defined by:
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Ground Snow DataGround Snow Data Ground snow data are provided on map of US
in ASCE 7 50-yr recurrence interval = 2% annual
probability of occurrence
Certain areas on the map need case study to
determine ground snow load - designated by
CS
usually in mountainous terrain
parentheses indicate that above that elevation,
a case study is required
many municipalities in mountainous regions
have local snow data
Ground snow data are provided on map of US
in ASCE 7 50-yr recurrence interval = 2% annual
probability of occurrence
Certain areas on the map need case study to
determine ground snow load - designated by
CS
usually in mountainous terrainusually in mountainous terrain
parentheses indicate that above that elevation,parentheses indicate that above that elevation,
a case study is requireda case study is required
many municipalities in mountainous regionsmany municipalities in mountainous regions
have local snow datahave local snow data
Ground Snow LoadsGround Snow Loads
ASCE 7: Figure 7ASCE 7: Figure 7--11
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Know your geography. Dont ask the professor where to find
places on the map.
Know your geography.
Dont ask the professor where to find
places on the map.
4th Secret to Success4th Secret to Success
in this Coursein this Course
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Ce is Exposure Factor
found in ASCE 7, Table 7-2 based on terrain category in 6.5.6
A = large city centers, 50% of
buildings have heights > 70 ft
B = urban or suburban areas
C = flat open country, grasslands,
shorelines in hurricane prone areas
D = flat open areas around
waterways, lakes, etc.
Ce is Exposure Factor
found in ASCE 7, Table 7-2 based on terrain category in 6.5.6
A = large city centers, 50% of
buildings have heights > 70 ft
B = urban or suburban areasB = urban or suburban areas
C = flat open country, grasslands,C = flat open country, grasslands,
shorelines in hurricane prone areasshorelines in hurricane prone areas
D = flat open areas aroundD = flat open areas around
waterways, lakes, etc.waterways, lakes, etc.
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Ct
is Thermal Factor Ct
is Thermal Factor
I is Importance Factor I is Importance Factor
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I is Importance Factor I is Importance Factor
Minimum values for pf:
when pg < 20 psf; pf= I pg
when pg > 20 psf; pf= I (20 psf)
Minimum values of pfapply to low
sloped roofs also
Minimum values forMinimum values forppff::
whenwhen ppgg 20> 20 psfpsf;; ppff = I (20= I (20 psfpsf))
Minimum values ofMinimum values ofppffapply to lowapply to low
sloped roofs alsosloped roofs also
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Sloped Roof Snow Load calculatedby:
Sloped Roof Snow Load calculatedby:
fss pCp fss pCp
ps = sloped roof snow load, psf
Cs = Slope factor
pf= flat roof snow load, psf
ps = sloped roof snow load, psf
Cs = Slope factor
pf= flat roof snow load, psf
Cs is Slope Factor
found in ASCE 7, Figure 7-2
based on thermal condition of roof
Warm roof defined by Ct =1.0
Cold roof defined by Ct > 1.0
Slippery surface allows snow to
slide off roof surface
Cs is Slope Factor
found in ASCE 7, Figure 7-2
based on thermal condition of roof
Warm roof defined by Ct =1.0
Cold roof defined by Ct > 1.0
Slippery surface allows snow to
slide off roof surface
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Cs Slope Factor
for roof with Ct =1.1, interpolate
between values in both figures
Cs Slope Factor
for roof with Ct =1.1, interpolate
between values in both figures
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Curved Roofs
Curved roofs with slopes > 70o
areconsidered free of snow
Cs = 0
Balanced snow loads for curved roofs
are determined by Figure 7-3
Curved Roofs
Curved roofs with slopes > 70o
areconsidered free of snow
CCss = 0= 0
Balanced snow loads for curved roofsBalanced snow loads for curved roofs
are determined by Figure 7are determined by Figure 7--33
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Multiple folded plate, sawtooth,
barrel vault roofs Use flat roof snow load
Cs = 1
Multiple folded plate, sawtooth,
barrel vault roofs
Use flat roof snow load
CCss = 1= 1
Continuous Beam Roofs
Figure 7-4 shows how to distribute snow
on continuous beam roof systems
Continuous Beam Roofs
Figure 7-4 shows how to distribute snow
on continuous beam roof systems
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Unbalanced Snow Loads
Unbalanced snow loads are created by
wind blowing snow
Unbalanced Snow Loads
Unbalanced snow loads are created by
wind blowing snow
WINDWIND
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Unbalanced Snow Loads
The roof structure needs to be designedfor both balanced and unbalanced loads
Section 7.6 shows how to determine
unbalanced snow loads
Hip and gable roofs with slope > 70o or
with slope < (70/W + 0.5)o dont need to
be considered
Unbalanced Snow Loads
The roof structure needs to be designedfor both balanced and unbalanced loads
Section 7.6 shows how to determine
unbalanced snow loads
Hip and gable roofs with slope > 70o or
with slope < (70/W + 0.5)o dont need to
be considered
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Unbalanced Snow Load Terms
Snow density
Unbalanced Snow Load Terms
Snow density
4W
Lfor0.1
4W
L1for
W
L167.033.0
1W
Lfor5.0
4W
Lfor0.1
4W
L1for
W
L167.033.0
1W
Lfor5.0
14p13.0 g 14p13.0 g
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ExampleExample Given:
Fire station in Muscle Shoals, AL
Find:
balanced and unbalanced snow loads
on building
Given:
Fire station in Muscle Shoals, AL
Find:
balanced and unbalanced snow loads
on building
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Flat Roof Snow Pressure Flat Roof Snow Pressure
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Ct
is Thermal Factor Ct
is Thermal Factor
I is Importance Factor I is Importance Factor
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Flat Roof Snow Pressure Flat Roof Snow Pressure
Sloped Roof Snow Pressure Sloped Roof Snow Pressure
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Sloped Roof Snow Pressure Sloped Roof Snow Pressure
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Check unbalanced snow load
consider roof slope
consider building length, width, snow
density, etc.
Check unbalanced snow load
consider roof slope
consider building length, width, snow
density, etc.
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Conclusion:
Balanced snow load 8.4 psf
Unbalanced snow load
2.5 psf / 12.1 psf
Conclusion:
Balanced snow load 8.48.4 psfpsf
Unbalanced snow loadUnbalanced snow load
2.52.5 psfpsf/ 12.1/ 12.1 psfpsf
8.4 psf8.4 psf
12.1 psf12.1 psf2.5 psf2.5 psf