ASCE 7-05: Rooftop Structure 1.9x Wind Multiplier question 3

[structuresguy]

They introduced a new provision in ASCE 7-05 that adds a 1.9x wind pressure multiplier for "Rooftop Structures and Equipment", section 6.5.15.1. My question is "what is the definition of a roof top structure?". If I have a small pop-up roof area, say elevator overtravel or something similar, that is in the field of the roof, I would say that it applies. But what if I have a portion of the building, say an atrium, that pops up on the leading edge of the building. Does it apply to this higher roof area? Obviously, applying a 1.9x multiplier will result in much higher wind pressures than simply using a larger h value to calculate wind pressure for the higher roof. What if the same condition applies to the backside of the atrium, so that the wind has to travel across the field of the main roof before it hits the higher roof?

Second part is about roof equipment. I am in Florida. Since this code has been in effect, we have seen several cases where rooftop equipment that has Miami-Dade NOA approval does not have adequate rated wind pressure resistance to meet this new 1.9x criteria. Most equipment manufacturers still have not heard of this criteria. In many cases, no one even makes a piece of equipment rated high enough to meet the requirements. I contacted both the FBC and the Miami-Dade NOA office, and neither director had been aware of this criteria before. When questioned about approved products not meeting the ASCE7-05 criteria, I was told that NOA approval does not supercede wind pressure design criteria, and to reject such products. What are the rest of you doing in this case? This can't be limited just to Florida, since the gulf and east coasts have similar wind speeds.

 

[structuresguy]

my plan was to use the 1.9x multiplier. Just wanted to see if anyone thought otherwise. In this case, the pressure on the roof top unit will be 107psf. But the unit manufacturer that we want to specify (I think it was Trane) can only handle up to 85 psf. so my mechanical engineer was all in a lather when I told him 107.

 

[smartin]

I have found this discussion quite useful, but I'm an engineer in Florida working on designing the anchorage for roof top units on a pair of buildings that are 165' in height. Do any of you have any experience with what to do in this case. Prior to ASCE 7-05, the buildings less than 60' limit did not apply. What are your thoughts in applying the 1.9x multiplier in this case?

 

[SteveGregory]

I would think that the increased wind load at that height would be enough without any extra 1.9X effects.

 

[structuresguy]

My understanding is that the code only requires the 1.9x multiplier for units on building less than 60 feet. So, I agree with Steve that the higher Kh would take care of the pressure on the tall building.

 

[smartin]

Thanks all for your input. I was concerned that there would still be increased turbulence above and beyond the standard wind pressure at that height. Does anyone know why they cut-off the requirement at 60 feet? Is this the height to which they have testing?