Solar Panels effect on PEB Building's Roof. 2

[TekEngr]

I am desiging a pre engineered building which is 44m in width , 120m in length and 8.5m eave height (Metal roof slope is 4 degree).
I made the calculations for wind forces by using the code ASCE 7-05 in result all the wind load forces are upward (suction) on roof steel rafters , but now client wants to place solar panels on roof , so my question is that does the wind load calculation will be effect due to addition of solar panel? Is there will be any downward wind force will be devolpe due to addition of solar panels on my steel frams and roof purlins ???

 

[malikasal]

I do not see how the presence of the solar panel will affect your wind calculation. usually the panels are directly screwed to to the roof single skin or on purlin.

so other than the additional weight i see no difference what so ever.



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[IRstuff]

Wind force is a consequence of pressure differential between the interior of the structure and the windward side

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[Aesur]

I recommend reading ASCE7-16, the wind chapters deal with solar panels on a roof. I understand that you are using ASCE7-05, however you can convert your loading back to ASD levels from 7-16 for solar, additionally SEAOC PV Documents have similar information which is what was used to add the provisions to ASCE. The loading applied to the roof varies based on the configuration of the panels which is determined by a solar installation company typically. For instance if the panels are intended to be raised and tilted toward the sun (usually in more northern locations from the equator) then the applied loads can be significant.

Also it is worth noting that how the panels are attached can make a huge difference in member sizes, typically panels are attached in 4 locations per panel this can create larger tributary area point loads on certain members. Another common attachment is using rails on the roof that can attach at even further spacing.

If the owner hasn't selected a specific solar system I recommend making it clear on your plans what assumptions and limits are on the roof regarding solar. I have seen many buildings "designed for solar" that cannot actually support an economical/efficient solar system because the assumptions used were not sufficient for the economical/efficient solar system selected based on location or building, direction or sun, etc.. that required higher tilt panels or similar changes.

Question - even for 4 degree roof slope buildings for purlin designs, C&C loading would have a small pressure, how are you coming up with suction only? Reference Figure 6-11B in ASCE7-05.

 

[TekEngr]

@ Aesur , thank you for your explanation in detail.
you are correct main frame rafter is carrying only uplift wind force but roof purlins carrying uplift and downward forces both.
currently I am considering that the solar panels are directly screwing on the roof sheet with the same roof slope (there will be no tilt) so in this case there will be no additional wind force effect , is my consideration is correct ??