add panels on roof, does it add wind load to roof structure members?

[johndeng]

MWFRS wind load for flat/monoslope roof is only uplift load, no downward load. so we don't add wind load for joist/girder design.
But when we add tilted panels on half of the roof, panels will take uplift and downward wind load, most time downward wind load is over 20psf and uplift is only 15psf. do we need to add panel downward wind load to roof stuctural members, when we check the roof joist/girders?
Or we don't need to, since joists/girders are not roof c/c.

Thanks

 

[msquared48]

Where I am, snow controls at 25psf minimum over the wind anyway, so I rarely worry about it as a downward force. Uplift is quite another matter though.

Mike McCann
MMC Engineering

 

[johndeng]

we also have snow 25psf, but the basic load combination is
Dead+0.75Wind+0.75Live+0.75Snow both snow and wind count

 

[EngineeringEric]

I might have misunderstood... but are you asking that you are configuring a roof that will now apply a downward load (additive to gravity) but you are not sure if you should design the structural system to resist that load? IF so, it is my opinion that a structure should be engineered to adequately resist the known loading combinations.

 

[johndeng]

The job is to put solar panels on roof. What I saying is that beside gravity loads and snow load, the new panels on roof will draw some downward wind load while roof original design never care about.
Do we need to use the load combination and include wind load on panels? Or we should ignore since joist/girders are main force resisting over areas bigger than 700 square feet?

 

[EngineeringEric]

I would include it, you can utilize load reduction factors for the live loads... but really one can argue that solar panels will not be walked on nor have material stored on therefore will have a reduction of live load since the space is not accessible. But you will have a lot of snow drift to consider.

 

[johndeng]

That's 20psf!
Just wondering I can get away with it since building members won't design like roof components and cladding.

 

[wannabeSE]

I don't know what code you are using. Typically, joist and girders are considered components and cladding. For most (all?) cases in ASCE 7, there is a minimum C&C wind load acting toward or away from the surface (10psf min for ASCE 7-05).

 

[johndeng]

I have been working many years, only see Dead and Live load used for roof members, wind we only consider uplift.
there are so many roof are adding panels, nobody even think they would add downward wind due to panels.

 

[JAE]

When we add roof top members that are fairly tall on a roof we do consider the lateral overturning on the unit which causes downward and upward forces on the roof members.

I don't think it is proper to ignore the vertical components of the wind load for solar panels.

 

[johndeng]

JAE, I always feel you are the one that I can trust. Thanks!
Just want to confirm that for any enclosed building roof (flat/monoslope/gable) framing, net wind load is uplift, so regular design we only have D (say 20psf)+L/S (30psf). For roof c/c, it may have smaller downward load, but uplift is bigger. Joist/girder cover bigger than 700psf area are not designed as component/cladding.

When we add panels on half of the roof, panels are at 10 degrees, having about 14psf uplift and 19psf downward load. we need add weights to hold the uplift and horizontal load, that's about 17psf dead load added on roof. Usually we only consider the added roof dead load, and existing roof framing may still figure since 17psf is only added to half of the roof area. Very strange in my area, all roof panels never get any roof framing reinforcement from all projects.

But if I need to consider the wind downward load, I believe no existing roof can hold it:
without considering wind downward load: Dead+0.75Live/0.75Snow= 20psf+17psf/2+0.75*25psf=47.25psf <50psf design
consider wind downward load: Dead+0.75Wind+0.75Live/0.75Snow= 20psf+17psf/2+0.75*19psf/2+0.75*25psf=54.375psf >50psf design

So I am thinking the panels may be only treated as component/cladding for all connections, but for roof joist/girder, the panel induced wind load won't apply. Am I right?

 

[racookpe1978]

I would include it, you can utilize load reduction factors for the live loads... but really one can argue that solar panels will not be walked on nor have material stored on therefore will have a reduction of live load since the space is not accessible.

On the other hand, live loads do need to consider that ALL loading (and maintenance and solar panel installation weights (stacks of panels and frames and motors and people and tools) are going to be concentrated in the walkways between the rows of solar panels. Assuming "live loads" are nicely spread out all over a "generic roof" is no longer correct.

 

[davefitz]

It sounds like a good problem to be tested at a wind tunnel or evaluated using CFD, so that generalized curves can be used for all practical situations.

"Whom the gods would destroy, they first make mad "

 

[JAE]

jondeng, I think I'm catching a drift of your question - it appears that you are suggesting that since MWFRS wind doesn't include downward roof pressures then because of that you can ignore wind for larger girders in the roof because their tributary wind area is large? Is that a correct interpretation of your question?

I might suggest that with a roof design the joist/girder designs do indeed include C&C wind. The perspective I take is that MWFRS wind pressures and C&C wind pressures aren't mutually exclusive for a member design. The wind itself doesn't "know" that it is MWFRS or C&C - it's just wind. The fact that girders and joists may have large areas (>700 s.f.) doesn't mean you can ignore C&C wind (which might include downward pressure). It only means that the larger area allows a reduction in the C&C wind pressures to a level more closely matched to the MWFRS wind.

The ASCE 7, which we use, may have language that specifically says - If > 700 s.f. then USE MWFRS vs. reducing C&C (I'll have to check). But the premise that you simply ignore a downward pressure may not be right.

 

[EngineeringEric]

ASCE 7 6.5.15.1 Rooftop Structures and Equipt. for Buidlings. These forces will be horizontal but will result in an overturning moment, this moment will be turned into a force couple which in my opinion needs to be accounted for in a realistic loading situation.

 

[EngineeringEric]

Now this may not be the only wind force to consider but i had to use it this morning for a stack so i thought i'd share. But it is difficult to dispute the requirement of those loads in your situation.

 

[johndeng]

EngineeringEric: Thanks, these are BALLASTED solar arrays, just sit, not attached to any roof framing.So each has very small horizonal load, larger up/down wind due to the 10 degree tilt. no moment force couple will occur.

JAE: Can I have a silly question? for regular building roof design, do you have min. 10psf downward wind applied to joist/girder beside the dead and live/snow load? Thank you!

 

[JAE]

I haven't used that 10 psf minimum for downward loads - only as a check on total lateral MWFRS loads.

 

[haynewp]

It's in 7-05 section 6.1.4.2 for C&C. MWFRS has the 10 psf applied only to the vertical plane projection. I would check the joist/girder for the 10psf wind in either direction based on 6.1.4.2.

 

[a2mfk]

I think your main problem would be snow drifts due to these new obstructions you are placing on the roof, no?