Roof top structure with flat roof

[jeffhed]

I am designing an awning that is going on top of a 2nd story roof. I used section 29.5.1 to determine my roof top structure horizontal MWFRS and uplift loads. Section 29.6 Parapets refers you to section 27.4.5 for the effect of parapets on the MWFRS of structures. I don't have an actual parapet on the awning, but this is an open structure and section 27.4.3 states that if the roof angle is less than 5 degrees and there are fascia panels present that the fascia panels must be considered an inverted parapet. Typically when I do these awnings the only portion of the awning that receives any wind pressures is the fascia panel so normally I would calculate the MWFRS wind pressure on the fascia panels according to 27.4.5 and my positive and negative pressures on the roof according to to section 27.4.3 for open buildings. In this case the fact that section 29.6 in the roof top structure section refers me to section 27.4.5 for parapets, it makes it seem like I should calculate my MWFRS loads with section 29.5.1 but then do I have to add my parapet loads from 27.4.5? Once again, I don't have an actual parapet, only the requirement in 27.4.3 that for open buildings with roof slopes less than 5 degrees that the fascia panel be treated as an inverted parapet. My mean roof height of the existing building is 34'-8" and the transverse direction building width is 115'-0". The awning is 13'-0" tall and the fascia is 8" tall so its total height is 13'-8". Vertical area, Af = 15 psf. So Af < 0.1Bh so GCr = 1.9. qh = 30 psf. Rooftop structure Fh = 55 psf. If I have to parapet pressures according to section 27.4.5 (44 psf) for the "effect of the parapet on MWFRS" then my MWFRS pressures are 99 psf. Is this correct? I am unsure on this one as it is an open, rooftop structure. Chapter 29 doesn't discern between open or enclosed structures, just roof top structures but does refer me to 27.4.5 for parapets. Chapter 27 doesn't discern between structures on grade and structures on a roof. On one hand since chapter 29 does not discuss open structures, the requirement to treat the fascia panels as an inverted parapet does not apply. But on the other hand this IS an open structure it is just also a rooftop structure. Do I:
1. Add the forces together = 99 psf or
2. Use the MWFRS loads determined by chapter 29, 55 psf?

 

[JAE]

I'm away from ASCE 7 right now but I don't think your option 1 is correct. You don't add parapet pressures and MWFRS pressures to the same area.

Not sure about the 55 psf though - would have to calculate it out.



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[jeffhed]

JAE,
I don't think option is is correct either. Chapter 29 doesn't differentiate between open and enclosed structures so I am leaning towards using the worst case of chapter 27 and 29 which is controlled by chapter 29.