2011 Presentation on Snow Loads

8/17/2019 2011 Presentation on Snow Loads

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ASCE 7-10 Snow Load Provision

SEAoO Conference

September 2011

Michael O’Rourke PE , Ph.D.

Rensselaer

Objectives

Introduce changes in the ASCE 7-10Snow Load provisions

Present reasoning behind changes

Answer Frequently Asked Questions

Answer audience questions (hopefully)

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Outline

Minimum Roof Snow Load

Thermal Factor

Unbalanced Load

Drift Load on Adjacent Roof

Parapet Wall & RTU Drift

Sliding Load on Adjacent Roof

Ponding

FAQ’s3


Clarification

Scenario- roofload right afterheavy snow w/owind

No time forthermal, no wind

Roof load Pr= Pg

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Pm = Is Pg Pg < 20 psf

Pm = 20 Is Pg > 20 psf

The 20 psf value is our estimate of themaximum size of a ‘single’ heavysnow storm

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After the single heavy snow storm -eventually the wind blows , thermaleffects have time to act, and we thenget P

son the roof.

“ This minimum roof load is a separateuniform load case. It need not beused in determining or in combinationwith drift, sliding, unbalanced orpartial loads”

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Outline

Minimum Roof Snow Load Thermal Factor

Unbalanced Load




Ponding

FAQ’s7

Thermal Factor

Usually the roof snow load w/o driftingis less than the ground snow load ,but with special circumstances pr>pg

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Thermal Factor

In the SEAW report, biggest differenceswere for Freezer buildings – goingfrom hot to cold

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

In ASCE 7-10 we now have a new Ctfactor

Unheated and open air Ct = 1.2

Structures intentionally kept belowfreezing Ct = 1.3

As a result , for freezer w/ Is=1.0 andCe = 1.2 , flat roof load > pg

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Outline


Thermal Factor

Unbalanced Load

Drift Load on Adjacent Roof Parapet Wall & RTU drift


Ponding

FAQ’s11

Unbalanced Loads-Gable Roof

Upper Limit Roof Slope - change

Lower Limit Roof Slope - simplify

Small Eave to Ridge Distance -change

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Unbalanced- Upper Limit Slope

In 7-05 upperlimit slope basedon Cs chart

Unbalance loadfor roof slope upto 70º

Angle of reposefor drift same asfresh fallen snow?

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Unbalanced-Upper Limit Slope

Observations byTTEA- unbalancefor 6 on 12 & less

Consistent withmax slope of roofstep drifts 1V:2H

Seems driftedsnow has smallerangle of repose

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Unbalanced-Upper Limit Slope

In ASCE 7-10 we were a bit conservative

“For hip and gable roofs with slopeexceeding 7 on 12 (30.2º)…unbalancedsnow loads are not required to beapplied”

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Upper Limit Roof Slope

Lower Limit Roof Slope

Small Eave to Ridge Distance

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Unbalanced-Lower Limit Slope

In 7-05 lower limit was complicated -slopes less than larger of 70/W +0.5and 1/2 on 12

Based upon observed occurrence

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Vertical line - ½ on 12 limit

Horizontal line - roof too small to care?

Transition – curve fit ?

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½ on 12 seems tobe a physical limit

Venturi tube hasangle < 4º avoidsseparation

½ on 12 has angle> 4º separation ,wind shadow &drift

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In ASCE 7-10 lower limit relationsimplified

“ For hip and gable roofs … with a slopeless than 2.38º (1/2 on 12)unbalanced snow loads are notrequired to be applied”

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Upper Limit Roof Slope

Lower Limit Roof Slope

Small Eave to Ridge Distance

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Unbalanced-Small Width

Fig 7-9 originallyfor roof steps

lu restriction not aissue for steps

Fig 7-9 now alsoused for gables

lu=25 ft seemedarbitrary

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The question of whether the lu=25 ftshould apply to gable roof drifts iscomplicated by the following issues

Theoretical issue- Fig 7-9 is empiricalrelation based on case histories witha mean value of lu=172 ft

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

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For bldg’s with small W , JC/MOR methodwas used to simulate max annual driftsfor a # of locations & winters

Big differences between upper Midwest &Pacific NW

However results suggest that …“For Wless than 20 ft, use 20 ft in Fig 7-9”

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Outline


Unbalanced Load




Ponding

FAQ’s25


In ASCE 7-05 atruncated driftrequired if loweradjacent roof within 20 ft. ofhigher level roof

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

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Drfit Load on Adjacent Roof

In reality driftonly if lower roofin wind shadow ofupper roof

In ASCE 7-10 weassume a 1(V) to6(H) wind shadowafter Tabler’s workon snow fences

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Leeward drift ifs < 20’ & s < 6h(in wind shadow)

Drift heightsmaller of hd and(6h-s)/6

Drift lengthsmaller of 6hdand (6h-s)

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Windward drift ifs < 20’

Truncated drift

hd windwarddrift heightbased on fetchfor lower roof

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Outline


Thermal Factor

Unbalanced Load

Drift Load on Adjacent Roof Parapet Wall & RTU Drift


Ponding

FAQ’s30


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In ASCE 7-05upwind fetch forparapet wallclear

In ASCE 7-05upwind fetch forRTU unclear

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In reality for North wind –Drift North ofRTU is windward drift w/ fetch = LNDrift South of RTU is leeward drift w/effective fetch < LN

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ASCE 7-10 clarifies and simplifies theRTU case by specifying windward driftfor both sides

“ For roof projections ,lu shall be takenequal to the greater of the length ofthe roof upwind or downwind of theprojection”

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Outline


Thermal Factor

Unbalanced Load




Ponding

FAQ’s34


Sliding load onlower roof in 7-05

Surcharge taken

as 0.4pf W Applies to slopes

greater than ¼on 12(slippery) or2 on 12(non-slip)

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New provision

Sliding load onadjacent if ss (45ºsliding shadow)

Load pro-rated0.4pf W(15-s)/15

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Outline


Unbalanced Load




Ponding

FAQ’s37

Ponding

In ASCE 7-05 aponding analysiswas requiredonly for roofslopes less than¼ on 12

Envisions a freedraining eave

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Ponding

New provision in7-10 account forimpounded waterin susceptiblebays w/ anyslope

Problems arisew/o SE/ME/Archinteraction

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Outline


Thermal Factor

Unbalanced Load




Ponding

FAQ’s40

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FAQ #1 Elevated RTU

For a new heavy RTU on a large ,existing roof , how can I avoid snowdrift loads adjacent to the unit?

For normal sized RTU’s( not billboards)specify a 2 foot gap between thebottom of the dunnage/framework andthe top of the balanced snow

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FAQ # 2 Combined Loads

For the step sketched below should slidingand drifting loads be combined?


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No – in ASCE 7 both the drift load and thesliding load are intended to be 50 year MRI

events . While it is possible to have somedrift and some sliding simultaneously on aroof , that load combination is notenvisioned in ASCE 7. The return period forthe simultaneous occurrence of the 50 yeardrift and the 50 year sliding load would bemuch larger than the 50 year MRI

envisioned by the ASCE 7 Provisions.

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If the two events were completelyindependent , the return period would be

2500 years. Actually r² neither 1 nor 0.

For a leeward drift (wind from left to right )

, the sliding snow and the drifted snowcome from the same source area- theupper level roof.

For a step , ASCE 7 is clear in that thelarger of the windward and leeward applies– provisions based on observation.

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FAQ # 3 Awing Drift

A design load for a small awing over adoorway in the end wall of a tall gableseems excessive . Should it bedesigned as a roof step ?

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FAQ # 3 Awing Drift

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FAQ # 3 Awing Drift

Yes – failures havebeen observed for

cases where “ h” is moderate ( ~5

to 15 ft.) .

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FAQ # 3 Awing Drift

If ‘h’ is large , oneexpects smallerdrifts due to lowertrapping efficiency

If the horizontalextent of the awingis small – driftlimited by angle ofrepose of driftedsnow.1:4 showntypical – but notconservative


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FAQ # 4 Odd Drift Geometries

ASCE 7 specifies drifts for simplegeometries based on the size of theupwind snow source area ( fetch andground snow )

For more complex geometries, areasonable approach is to match thecross-sectional area of the oddshaped drift to that for a roof stepwith the same fetch and ground load

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Approach used forgable roof drifts in

7-05

Area for roof step

drift is A = 2(hd)²

For a triangulargable drift with a

1:S slope , areasmatch

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Roof step with asloping lower levelroof

hd is drift height forwindward or leewardroof step with samefetch and ground load

Applies for slopes lessthan 3:12 for typicalangle of repose

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Step with non-vertical separationwall

hd is height forwindward or leewarddrift at step with samefetch and ground load

Applies for slopessteeper than 3:12 fortypical angle of repose

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FAQ # 5 Drift at Addition

For an addition adjacent to anexisting lower lever roof , how do Iavoid the roof step drift ?

There are three approaches ; snow bay,

new walls to trap and/or shield snow,and exotic measures. Some work well, others not so.

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FAQ#5 Addition-Snow Bay

Leeward &windward drift on

new roof

Lower head room

for portion ofaddition

From a structuralengineeringstandpoint – this

works


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FAQ # 5 Addition- New Walls

Fixes include new wall @ roof step(trap) or far upwind wall (shield)

Trapping- some snow upwind of thewall is trapped by the wall

Shielding- all of the snow for adistance downwind of the wall,shielded by the wall

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FAQ # 5 Addition- Trapping

Wall or step traps aportion of upwindsnow , even w/excess drift spaceavailable

Trapping efficiencytypically about50% at leewardstep, less atwindward

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FAQ # 5 Addition-Shielding

Wall will shield allsnow within acertain distancedownwind of wall

10 ho rule isconservative for Cefactor

Measurementssuggests 5 ho isconservative forshielding

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FAQ # 5 Wall Implementation

One proposed fixinvolved a numberof shielding walls

Still have windwarddrift

Due to cost , ownerdecided toreinforce existingroofs

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FAQ # 5 Exotic Measures

Reduce space for drift accumulation withlight geofoam blocks or a false roof

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FAQ # 5 Exotic Measures

Set of baffles which redirect wind andminimize leeward drift. May have difficulty

convincing the local code official sinceapproach isn’t codified. Still have windward


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ASCE 7-10 Snow Load Provision

Additional questions either

Contact M. O’R at [email protected]

Buy “ Snow Loads – A Guide to theSnow Load Provisions of ASCE 7–10 ” ASCE Press

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2011 Presentation on Snow Loads

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