ASCE 7-10 Snow Load Provision
Objectives
SEAoO Conference September 2011 Michael O’Rourke PE , Ph.D. Rensselaer
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
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Minimum Roof Snow Load
Minimum Roof Snow Load Thermal Factor Unbalanced Load Drift Load on Adjacent Roof Parapet Wall & RTU Drift Sliding Load on Adjacent Roof Ponding FAQ’s
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
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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”
The 20 psf value is our estimate of the maximum size of a ‘single’ heavy snow storm
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Outline
Thermal Factor
Minimum Roof Snow Load Thermal Factor Unbalanced Load Drift Load on Adjacent Roof Parapet Wall & RTU Drift Sliding Load on Adjacent Roof Ponding FAQ’s
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
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
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
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Unbalanced Loads-Gable Roof
Minimum Roof Snow Load Thermal Factor Unbalanced Load Drift Load on Adjacent Roof Parapet Wall & RTU drift Sliding Load on Adjacent Roof Ponding FAQ’s
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Upper Limit Roof Slope - change Lower Limit Roof Slope - simplify Small Eave to Ridge Distance -change
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Unbalanced- Upper Limit Slope
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?
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
Unbalanced Loads-Gable Roof
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”
Upper Limit Roof Slope Lower Limit Roof Slope Small Eave to Ridge Distance
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Unbalanced-Lower Limit Slope
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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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Vertical line - ½ on 12 limit Horizontal line - roof too small to care? Transition – curve fit ?
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Unbalanced-Lower Limit Slope
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
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
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Unbalanced-Small Width
Upper Limit Roof Slope Lower Limit Roof Slope Small Eave to Ridge Distance
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
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Unbalanced-Small Width
The question of whether the l u=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 l u=172 ft Practical issue-relation gives negative values for low Pg and small W=lu hence some limit needed 23
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
Drift Load on Adjacent Roof
Minimum Roof Snow Load Thermal Factor Unbalanced Load Drift Load on Adjacent Roof Parapet Wall & RTU Drift Sliding Load on Adjacent Roof Ponding FAQ’s
In ASCE 7-05 a truncated drift required if lower adjacent roof within 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
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Drift 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
Leeward drift if s < 20’ & s < 6h (in wind shadow) Drift height smaller of hd and (6h-s)/6 Drift length smaller of 6hd and (6h-s)
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Drift Load on Adjacent Roof
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Outline
Windward drift if s < 20’ Truncated drift hd windward drift height based on fetch for lower roof
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Minimum Roof Snow Load Thermal Factor Unbalanced Load Drift Load on Adjacent Roof Parapet Wall & RTU Drift Sliding Load on Adjacent Roof Ponding FAQ’s 30
Parapet Wall & RTU Drift
Parapet Wall & RTU Drift
In ASCE 7-05 upwind fetch for parapet wall clear In ASCE 7-05 upwind fetch for RTU unclear
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
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Outline
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”
Minimum Roof Snow Load Thermal Factor Unbalanced Load Drift Load on Adjacent Roof Parapet Wall & RTU Drift Sliding Load on Adjacent Roof Ponding FAQ’s
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Sliding Load on Adjacent Roof
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Sliding Load on Adjacent Roof
Sliding load on lower roof in 7-05 Surcharge taken as 0.4pf W Applies to slopes greater than ¼ on 12(slippery) or 2 on 12(non-slip)
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New provision Sliding load on adjacent if s<15’ and h>s (45º sliding shadow) Load pro-rated 0.4pf W(15-s)/15
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Outline
Ponding
Minimum Roof Snow Load Thermal Factor Unbalanced Load Drift Load on Adjacent Roof Parapet Wall & RTU Drift Sliding Load on Adjacent Roof Ponding FAQ’s
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
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Outline
New provision in 7-10 account for impounded water in susceptible bays w/ any slope Problems arise w/o SE/ME/Arch interaction
Minimum Roof Snow Load Thermal Factor Unbalanced Load Drift Load on Adjacent Roof Parapet Wall & RTU Drift Sliding Load on Adjacent Roof Ponding FAQ’s
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FAQ #1 Elevated RTU
FAQ # 2 Combined Loads
For a new heavy RTU on a large , existing roof , how can I avoid snow drift loads adjacent to the unit?
For the step sketched below should sliding and drifting loads be combined?
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 41
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FAQ # 2 Combined Loads
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.
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
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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
FAQ # 3 Awing Drift
Yes – failures have been observed for cases where “ h” is moderate ( ~5 to 15 ft.) .
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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 drifted snow.1:4 shown typical – but not conservative 48
FAQ # 4 Odd Drift Geometries
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
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
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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 less than 3:12 for typical angle of repose
Step with nonvertical 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
FAQ#5 Addition-Snow Bay
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. 53
Leeward & windward drift on new roof Lower head room for portion of addition From a structural engineering standpoint – this works 54
FAQ # 5 Addition- New Walls
FAQ # 5 Addition- Trapping
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
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
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FAQ # 5 Wall Implementation
Wall will shield all snow within a certain distance downwind of wall 10 ho rule is conservative for Ce factor Measurements suggests 5 ho is conservative for shielding
One proposed fix involved 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
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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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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
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