ASCE 7-10 Snow Load Provision -...
Transcript of ASCE 7-10 Snow Load Provision -...
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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 7-10 Snow 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- roof load right after heavy snow w/o wind
No time for thermal, 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 the maximum size of a ‘single’ heavy snow storm
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Minimum Roof Snow Load
After the single heavy snow storm -eventually the wind blows , thermal effects have time to act, and we then get Ps on the roof.
“ This minimum roof load is a separate uniform load case. It need not be used in determining or in combination with drift, sliding, unbalanced or partial 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 drifting is less than the ground snow load , but with special circumstances pr>pg
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Thermal Factor
In the SEAW report, biggest differences were for Freezer buildings – going from 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 Ct
factor
Unheated and open air Ct = 1.2
Structures intentionally kept below freezing 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
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 upper limit slope based on Cs chart
Unbalance load for roof slope up to 70º
Angle of repose for drift same as fresh fallen snow?
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Unbalanced-Upper Limit Slope
Observations by TTEA- unbalance for 6 on 12 & less
Consistent with max slope of roof step drifts 1V:2H
Seems drifted snow has smaller angle 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 slope exceeding 7 on 12 (30.2º)…unbalanced snow loads are not required 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-Lower Limit Slope
In 7-05 lower limit was complicated -slopes less than larger of 70/W +0.5 and 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 to be a physical limit
Venturi tube has angle < 4º avoids separation
½ on 12 has angle > 4º separation , wind shadow & drift
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Unbalanced-Lower Limit Slope
In ASCE 7-10 lower limit relation simplified
“ For hip and gable roofs … with a slope less than 2.38º (1/2 on 12) unbalanced snow loads are not required 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 originally for roof steps
lu restriction not a issue for steps
Fig 7-9 now also used for gables
lu=25 ft seemed arbitrary
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Unbalanced-Small Width
The question of whether the lu=25 ft should apply to gable roof drifts is complicated by the following issues
Theoretical issue- Fig 7-9 is empirical relation based on case histories with a mean value of lu=172 ft
Practical issue-relation gives negative values 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 method was used to simulate max annual drifts for a # of locations & winters
Big differences between upper Midwest & Pacific NW
However results suggest that …“For W less 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 a truncated drift required if lower adjacent roofwithin 20 ft. of higher level roof
In ASCE 7-05 roofs A,B & C all get drifts
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Drfit Load on Adjacent Roof
In reality drift only if lower roof in wind shadow of upper roof
In ASCE 7-10 we assume a 1(V) to 6(H) wind shadow after Tabler’s work on snow fences
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Drift Load on Adjacent Roof
Leeward drift if s < 20’ & s < 6h (in wind shadow)
Drift height smaller of hd and(6h-s)/6
Drift length smaller of 6hd
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Drift Load on Adjacent Roof
Windward drift if s < 20’
Truncated drift
hd windwarddrift height based on fetch for 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
Parapet Wall & RTU Drift
In ASCE 7-05 upwind fetch for parapet wall clear
In ASCE 7-05 upwind fetch for RTU unclear
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Parapet Wall & RTU Drift
In reality for North wind –Drift North of RTU is windward drift w/ fetch = LN
Drift South of RTU is leeward drift w/ effective fetch < LN
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Parapet Wall & RTU Drift
ASCE 7-10 clarifies and simplifies the RTU case by specifying windward drift for both sides
“ For roof projections ,lu shall be taken equal to the greater of the length of the roof upwind or downwind of the projection”
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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 on lower roof in 7-05
Surcharge taken as 0.4pfW
Applies to slopes greater than ¼on 12(slippery) or 2 on 12(non-slip)
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Sliding Load on Adjacent Roof
New provision
Sliding load on adjacent if s<15’and h>s (45ºsliding shadow)
Load pro-rated 0.4pfW(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 a ponding analysis was required only for roof slopes less than ¼ on 12
Envisions a free draining eave
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Ponding
New provision in 7-10 account for impounded water in susceptible bays w/ anyslope
Problems arise w/o SE/ME/Arch interaction
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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 snow drift loads adjacent to the unit?
For normal sized RTU’s( not billboards) specify a 2 foot gap between the bottom of the dunnage/framework and the top of the balanced snow
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FAQ # 2 Combined Loads
For the step sketched below should sliding and drifting loads be combined?
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FAQ # 2 Combined Loads
No – in ASCE 7 both the drift load and the sliding load are intended to be 50 year MRI events . While it is possible to have some drift and some sliding simultaneously on a roof , that load combination is not envisioned in ASCE 7. The return period for the simultaneous occurrence of the 50 year drift and the 50 year sliding load would be much 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 completely independent , 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 snow come from the same source area- the upper level roof.
For a step , ASCE 7 is clear in that the larger 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 a doorway in the end wall of a tall gable seems excessive . Should it be designed as a roof step ?
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FAQ # 3 Awing Drift
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FAQ # 3 Awing Drift
Yes – failures have been observed for cases where “ h”is moderate ( ~5 to 15 ft.) .
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FAQ # 3 Awing Drift
If ‘h’ is large , one expects smaller drifts due to lower trapping efficiency
If the horizontal extent of the awing is small – drift limited by angle of repose of driftedsnow.1:4 shown typical – but not conservative
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FAQ # 4 Odd Drift Geometries
ASCE 7 specifies drifts for simple geometries based on the size of the upwind snow source area ( fetch and ground snow )
For more complex geometries, a reasonable approach is to match the cross-sectional area of the odd shaped drift to that for a roof step with the same fetch and ground load
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FAQ # 4 Odd Drift Geometries
Approach used for gable roof drifts in 7-05
Area for roof step drift is A = 2(hd)²
For a triangular gable drift with a 1:S slope , areas match
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FAQ # 4 Odd Drift Geometries
Roof step with a sloping lower level roof
hd is drift height for windward or leeward roof step with same fetch and ground load
Applies for slopes lessthan 3:12 for typical angle of repose
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FAQ # 4 Odd Drift Geometries
Step with non-vertical separation wall
hd is height for windward or leeward drift at step with same fetch and ground load
Applies for slopes steeper than 3:12 for typical angle of repose
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FAQ # 5 Drift at Addition
For an addition adjacent to an existing lower lever roof , how do I avoid 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 of addition "
From a structural engineering standpoint – 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 the wall is trapped by the wall
Shielding- all of the snow for a distance downwind of the wall, shielded by the wall
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FAQ # 5 Addition- Trapping
Wall or step traps a portion of upwind snow , even w/ excess drift space available
Trapping efficiency typically about 50% at leeward step, less at windward
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FAQ # 5 Addition-Shielding
Wall will shield allsnow within a certain distance downwind of wall
10 ho rule is conservative for Ce factor
Measurementssuggests 5 ho isconservative for shielding
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FAQ # 5 Wall Implementation
One proposed fixinvolved a number of shielding walls
Still have windward drift
Due to cost , owner decided to reinforce existing roofs
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FAQ # 5 Exotic Measures
Reduce space for drift accumulation with light geofoam blocks or a false roof
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FAQ # 5 Exotic Measures
Set of baffles which redirect wind and minimize leeward drift. May have difficulty convincing the local code official since approach 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 the Snow Load Provisions of ASCE 7–10 ”ASCE Press