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
Minimum Roof Snow Load
Clarification
Scenario- roofload right afterheavy snow w/owind
No time forthermal, no wind
Roof load Pr= Pg
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Minimum Roof Snow Load
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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Minimum Roof Snow Load
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
Drift Load on Adjacent Roof
Parapet Wall & RTU Drift
Sliding Load on Adjacent Roof
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
Minimum Roof Snow Load
Thermal Factor
Unbalanced Load
Drift Load on Adjacent Roof Parapet Wall & RTU drift
Sliding Load on Adjacent Roof
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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Unbalanced Loads-Gable Roof
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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Unbalanced-Lower Limit Slope
Vertical line - ½ on 12 limit
Horizontal line - roof too small to care?
Transition – curve fit ?
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Unbalanced-Lower Limit Slope
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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Unbalanced-Lower Limit Slope
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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Unbalanced Loads-Gable Roof
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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Unbalanced-Small Width
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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Unbalanced-Small Width
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
Minimum Roof Snow Load Thermal Factor
Unbalanced Load
Drift Load on Adjacent Roof
Parapet Wall & RTU Drift
Sliding Load on Adjacent Roof
Ponding
FAQ’s25
Drift Load on Adjacent Roof
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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Drift Load on Adjacent Roof
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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Drift Load on Adjacent Roof
Windward drift ifs < 20’
Truncated drift
hd windwarddrift heightbased on fetchfor lower roof
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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’s30
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Parapet Wall & RTU Drift
In ASCE 7-05upwind fetch forparapet wallclear
In ASCE 7-05upwind fetch forRTU unclear
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Parapet Wall & RTU Drift
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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Parapet Wall & RTU Drift
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
Minimum Roof Snow Load
Thermal Factor
Unbalanced Load
Drift Load on Adjacent Roof
Parapet Wall & RTU Drift
Sliding Load on Adjacent Roof
Ponding
FAQ’s34
Sliding Load on Adjacent Roof
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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Sliding Load on Adjacent Roof
New provision
Sliding load onadjacent if ss (45ºsliding shadow)
Load pro-rated0.4pf W(15-s)/15
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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’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
Minimum Roof Snow Load
Thermal Factor
Unbalanced Load
Drift Load on Adjacent Roof
Parapet Wall & RTU Drift
Sliding Load on Adjacent Roof
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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FAQ # 2 Combined Loads
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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FAQ # 2 Combined Loads
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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FAQ # 4 Odd Drift Geometries
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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FAQ # 4 Odd Drift Geometries
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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FAQ # 4 Odd Drift Geometries
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