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


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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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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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


Thermal Factor

Unbalanced Load




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


Thermal Factor

Unbalanced Load


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 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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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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Vertical line - ½ on 12 limit

Horizontal line - roof too small to care?

Transition – curve fit ?

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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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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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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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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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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


Thermal Factor

Unbalanced Load




Ponding

FAQ’s25


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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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)28


Windward drift if s < 20’

Truncated drift

hd windwarddrift height based on fetch for lower roof

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Outline


Thermal Factor

Unbalanced Load




Ponding

FAQ’s30


In ASCE 7-05 upwind fetch for parapet wall clear

In ASCE 7-05 upwind fetch for RTU unclear

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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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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


Thermal Factor

Unbalanced Load




Ponding

FAQ’s34


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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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


Thermal Factor

Unbalanced Load




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


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 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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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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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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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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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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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

asce 7-10 snow load provision -...

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