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RE: Pf - Snow Load on Flat Roofs Subjected to Solar PV Array

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This message was also sent by MStuart(--nospam--at)Pennoni.com to: Harold.Sprague(--nospam--at)parsons.com

Thanks for the input

D. Matthew Stuart, P.E., S.E., F.ASCE, F.SEI, SECB, MgtEng
Structural Division Manager
Pennoni Associates Inc.
Office 215-222-3000 x7895
Mobile 908-309-8657
http://www.pennoni.com | mstuart(--nospam--at)pennoni.com

-----Original Message-----
From: Sprague, Harold [mailto:Harold.Sprague(--nospam--at)parsons.com]
Sent: Tuesday, November 18, 2014 6:27 PM
To: seaint-seaosc(--nospam--at)mail-list.com
Cc: Matthew Stuart
Subject: RE: [SEAINT-SEAOSC] Pf - Snow Load on Flat Roofs Subjected to Solar 
 PV Array

Matt,

That is a very good question.  I would contact Dr. O'Rourke directly for any 
 specific studies.

According to Michael O’Rourke in his snow load guide:

3. A client wants to install a new RTU on an existing large roof.
The existing roof structure can marginally support the proposed RTU, but has 
 no additional capacity for drift loads. Is there a means to install the RTU 
 so as to avoid formation of large windward drifts? Commentary Section C7.8 
 states in relation to arrays of rooftop solar collectors: “By elevating 
collectors several feet (a meter or more) above the roof . . . the potential 
 for drifting will be diminished significantly.” It is the author’s 
opinion that conventional RTUs should be elevated such that there is a 2 ft 
gap between the bottom of the RTU dunnage/framework and the top of the 
balanced snow surface. Thus, for the RTU in Example 8-2, the RTU should be 
elevated a minimum of 3.17 ft (balanced + minimum gap; 1.17 ft + 2.0 ft = 
3.17 ft) above the roof surface. If the projection is much taller than an 
RTU, such as a billboard or cooling tower, then the gap probably should be 
greater than 2 ft.

ASCE 7-10 Commentary:
C7.8 ROOF PROJECTIONS AND PARAPETS
Drifts around penthouses, roof obstructions, and parapet walls are also of 
the “windward step” type because the length of the upper roof is small 
or no upper roof exists. Solar panels, mechanical equipment, parapet walls, 
 and penthouses are examples of roof projections that may cause “windward” 
 drifts on the roof around them. The drift-load provisions in Sections 7.7 
and 7.8 cover most of these situations adequately, but flat-plate solar 
collectors may warrant some additional attention. Roofs equipped with several 
 rows of them are subjected to additional snow loads. Before the collectors 
 were installed, these roofs may have sustained minimal snow loads, especially 
 if they were windswept. First, because a roof with collectors is apt to be 
 somewhat “sheltered” by the collectors, it seems appropriate to assume 
 the roof is partially exposed and calculate a uniform snow load for the 
entire area as though the collectors did not exist. Second, the extra snow 
that might fall on the collectors and then slide onto the roof should be 
computed using the “cold roofs-all other surfaces” curve in Fig. 7-2b. 
This value should be applied as a uniform load on the roof at the base of 
each collector over an area about 2 ft (0.6 m) wide along the length of the 
 collector. The uniform load combined with the load at the base of each 
collector probably represents a reasonable design load for such situations, 
 except in very windy areas where extensive snow drifting is to be expected 
 among the collectors. By elevating collectors several feet (a meter or more) 
 above the roof on an open system of structural supports, the potential for 
 drifting will be diminished signifi cantly. Finally, the collectors should 
 be designed to sustain a load calculated by using the “unobstructed 
slippery surfaces” curve in Fig. 7-2a. This last load should not be used 
in the design of the roof because the heavier load of sliding snow from the 
collectors has already been considered. The influence of solar collectors 
on snow accumulation is discussed in Corotis et al. (1979) and O’Rourke 
(1979).

Regards,
Harold O. Sprague, F ASCE, PE
Principal Technical Consultant
PARSONS
2800 NE 76th Terr. ♦ Gladstone, MO 64119 Office/Mobile – (202) 697-0662 
 ♦ Cell - (816) 213-9587 harold.sprague(--nospam--at)parsons.com ♦ www.parsons.com

-----Original Message-----
From: seaint-seaosc(--nospam--at)mail-list.com [mailto:seaint-seaosc(--nospam--at)mail-list.com] On 
Behalf Of Matthew Stuart Sent: Friday, November 14, 2014 5:43 AM
To: seaint-seaosc(--nospam--at)mail-list.com
Subject: [SEAINT-SEAOSC] Pf - Snow Load on Flat Roofs Subjected to Solar PV 
 Array

Harold,

I sent you an inquiry via email to your Hotmail address but didn't hear back 
 from you so I'll try the SEAINT list.

Do you know of any studies that show that a roof mounted solar array can 
contribute to the accumulation of snow on a flat roof (i.e. less likely to 
get blown) such that the reduction allowed on the ground snow load for the 
flat roof snow load is unconservative?

D. Matthew Stuart, P.E., S.E., F.ASCE, F.SEI, SECB, MgtEng Structural Division 
 Manager Pennoni Associates Inc. Office 215-222-3000 x7895
Mobile 908-309-8657
http://www.pennoni.com | mstuart(--nospam--at)pennoni.com

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