ppt asce 7-10snowloadprovisionsjuly2012

ASCE 7-10 Snow Load Provision

ASCE WebinarJuly 19 , 2012

Michael ORourke PE , Ph.D.E-mail: [email protected]

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Objectives I t d h i th ASCE 7 10 Introduce changes in the ASCE 7-10

Snow Load provisions P t i b hi d h Present reasoning behind changes Illustrate with design examples as

d dneeded

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Outline Mi i R f S L d Minimum Roof Snow Load Thermal Factor Unbalanced Load Drift Load on Adjacent Roof Parapet Wall & RTU Drift Sliding Load on Adjacent Roof Sliding Load on Adjacent Roof Ponding

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Minimum Snow LoadLow Slope Cl ifi ti Clarification Scenario- roof

l d i ht ft load right after heavy snow w/o wind wind

No time for thermal no windthermal, no wind

Roof load Pr= Pg4

Minimum Snow Load-Low SlopeP I P P < 20 fPm = Is Pg Pg < 20 psf

Pm = 20 Is Pg > 20 psf

The 20 psf value is our estimate of the maximum size of a single heavy g ysnow storm

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Minimum Snow Load-Low SlopeAft th i l h t After the single heavy snow storm -

eventually the wind blows , thermal effects have time to act and we then effects have time to act, and we then get Ps on the roof.

This minimum roof load is a separate This minimum roof load is a separate uniform load case. It need not be used in determining or in combination used in determining or in combination with drift, sliding, unbalanced or partial loads partial loads

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Thermal FactorU ll th f l d / d ifti Usually the roof snow load w/o drifting

is less than the ground snow load , but with special circumstances p >pbut with special circumstances pr>pg

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Thermal FactorI th SEAW t bi t diff In the SEAW report, biggest differences

were for Freezer buildings going from hot to coldfrom hot to cold

Roof Heated Bldg hot air below Roof Heated Bldg- hot air below Ground- warm earth below Roof Open Air Bldg- ambient air below Roof Freezer Bldg- cold air below

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Thermal Factor I ASCE 7 10 h C In ASCE 7-10 we now have a new Ct

factor U h t d d i C 1 2 Unheated and open air Ct = 1.2 Structures intentionally kept below

f i C 1 3freezing Ct = 1.3 As a result , for freezer w/ Is=1.0 and

Ce = 1.2 , flat roof load > pg

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Outline Mi i R f S L d Minimum Roof Snow Load Thermal Factor Unbalanced Load Drift Load on Adjacent Roof Parapet Wall & RTU drift Sliding Load on Adjacent Roof Sliding Load on Adjacent Roof Ponding

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Unbalanced Loads-Gable Roof U Li it R f Sl Upper Limit Roof Slope Lower Limit Roof Slope Small Eave to Ridge Distance

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Unbalanced- Upper Limit Slope I 7 05 In 7-05 upper

limit slope based on C charton Cs chart

Unbalance load for roof slope up for roof slope up to 70

Angle of repose Angle of repose for drift same as fresh fallen snow?fresh fallen snow?

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Unbalanced-Upper Limit Slope Ob ti b Observations by

TTEA- unbalance for 6 on 12 & lessfor 6 on 12 & less

Consistent with max slope of roof max slope of roof step drifts 1V:2H

Seems drifted Seems drifted snow has smaller angle of reposeangle of repose

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Unbalanced-Upper Limit SlopeI ASCE 7 10 bit tiIn ASCE 7-10 we were a bit conservative

For hip and gable roofs with slope exceeding 7 on 12 (30.2)unbalanced

l d i d b snow loads are not required to be applied

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Unbalanced Loads-Gable Roof U Li it R f Sl Upper Limit Roof Slope Lower Limit Roof Slope Small eave to Ridge Distance

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Unbalanced-Lower Limit Slope I 7 05 l li it li t d In 7-05 lower limit was complicated -

slopes less than larger of 70/W +0.5 and 1/2 on 12and 1/2 on 12

Based upon observed occurrence

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Unbalanced-Lower Limit Slope V ti l li 12 li it Vertical line - on 12 limit Horizontal line - roof too small to care? Transition curve fit ?

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Unbalanced-Lower Limit Slope 12 t on 12 seems to

be a physical limit V t i t b h Venturi tube has

angle 4 separation > 4 separation , wind shadow & driftdrift

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Unbalanced-Lower Limit SlopeI ASCE 7 10 l li it l ti In ASCE 7-10 lower limit relation

simplified

For hip and gable roofs with a slope l h 2 38 (1/2 12) less than 2.38 (1/2 on 12) unbalanced snow loads are not required to be appliedrequired to be applied

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Unbalanced Loads-Gable Roof U Li it R f Sl Upper Limit Roof Slope Lower Limit Roof Slope Small Eave to Ridge Distance

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Unbalanced-Small Width Fi 7 9 i i ll Fig 7-9 originally

for roof steps l t i ti t lu restriction not a

issue for steps Fi 7 9 l Fig 7-9 now also

used for gables lu=25 ft seemed

arbitrary

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Unbalanced-Small Width Th ti f h th th l 25 ft The question of whether the lu=25 ft

should apply to gable roof drifts is complicated by the following issuescomplicated by the following issues

Theoretical issue- Fig 7-9 is empirical relation based on case histories with relation based on case histories with a mean value of lu=172 ft

Practical issue relation gives negative Practical issue-relation gives negative values for low Pg and small W=luhence some limit neededhence some limit needed

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Unbalanced-Small Width F bld ith ll W JC/MOR th d For bldgs with small W , JC/MOR method

was used to simulate max annual drifts for a # of locations & winters for a # of locations & winters

Big differences between upper Midwest & Pacific NWPacific NW

However results suggest that For W less than 20 ft use 20 ft in Fig 7 9less than 20 ft, use 20 ft in Fig 7-9

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Drift Load on Adjacent Roof I ASCE 7 05 In ASCE 7-05 a

truncated drift required if lower required if lower adjacent roof within 20 ft of within 20 ft. of higher level roof

In ASCE 7-05 In ASCE 7-05 roofs A,B & C all get drifts get drifts

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Drfit Load on Adjacent Roof I lit d ift In reality drift

only if lower roof in wind shadow of in wind shadow of upper roof

In ASCE 7 10 we In ASCE 7-10 we assume a 1(V) to 6(H) wind shadow 6(H) wind shadow after Tablers work on snow fenceson snow fences

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Drift Load on Adjacent Roof L d d ift if Leeward drift if

s < 20 & s < 6h (in wind shadow)(in wind shadow)

Drift height smaller of h and smaller of hd and (6h-s)/6

Drift length Drift length smaller of 6hdand (6h s)and (6h-s)

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Drift Load on Adjacent Roof Wi d d d ift if Windward drift if

s < 20 T t d d ift Truncated drift hd windward

d if h i h drift height based on fetch for lower rooffor lower roof

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Drift Load on Adjacent RoofE l d t i th l d d ift Example: determine the leeward drift

on adjacent roof (Elev. 90), upper roof (Elev 100) length=120 roof (Elev. 100 ) length=120 , separation distance 8, Pg = 40 psf

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Drift Load on Adjacent RoofS l tiSolution:S=8


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Parapet Wall & RTU Drift I ASCE 7 05 In ASCE 7-05

upwind fetch for parapet wall parapet wall clear

In ASCE 7 05 In ASCE 7-05 upwind fetch for RTU unclearRTU unclear

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Parapet Wall & RTU DriftI lit f N th i d D ift N th f In reality for North wind Drift North of

RTU is windward drift w/ fetch = LNDrift South of RTU is leeward drift w/ Drift South of RTU is leeward drift w/ effective fetch < LN

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Parapet Wall & RTU DriftASCE 7 10 l ifi d i lifi th ASCE 7-10 clarifies and simplifies the

RTU case by specifying windward drift for both sidesfor both sides

For roof projections ,lu shall be taken equal to the greater of the length of equal to the greater of the length of the roof upwind or downwind of the projectionprojection

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Sliding Load on Adjacent Roof Slidi l d Sliding load on

lower roof in 7-05 S h t k Surcharge taken

as 0.4pfW A li l Applies to slopes

greater than on 12(slippery) or on 12(slippery) or 2 on 12(non-slip)

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Sliding Load on Adjacent Roof N i i New provision Sliding load on

dj t if 15 adjacent if ss (45 sliding shadow)sliding shadow)

Load pro-rated 0 4p W(15 s)/150.4pfW(15-s)/15

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Sliding Load on Adjacent RoofE l d t i th lidi l d Example determine the sliding load

on the adjacent lower roof , W=30 ft, h=12 ft s=8ft upper non slippery h=12 ft, s=8ft , upper non-slippery slope = 3 on 12 , pf = 25 psf

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Sliding Load on Adjacent RoofS l ti 3 12 > 2 12 lidi Solution 3 on 12 > 2 on 12 sliding occurs s=8s=8 close & in shadowHorizontal extent=15-s=15-8=7Load /ft = 0.4(30)(25)(15-8)/15=140plf

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Ponding I ASCE 7 05 In ASCE 7-05 a

ponding analysis was required was required only for roof slopes less than slopes less than on 12

Envisions a free Envisions a free draining eave

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Ponding N i i i New provision in

7-10 account for impounded water impounded water in susceptible bays w/ anybays w/ anyslope

Problems arise Problems arise w/o SE/ME/Arch interactioninteraction

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ASCE 7-10 Snow Load ProvisionAdditi l ti ith Additional questions either

Contact M. OR at [email protected]

Buy Snow Loads A Guide to the Snow Load Provisions of ASCE 710 ASCE Press

44

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ppt asce 7-10snowloadprovisionsjuly2012

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