snow load basics

7/29/2019 snow load basics

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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


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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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


ps = sloped roof snow load, psf

Cs = Slope factor


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 are created by

wind blowing snow

WINDWIND


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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


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



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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

snow load basics

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