ASCE 7-05: Rooftop Structure 1.9x Wind Multiplier question
ASCE 7-05: Rooftop Structure 1.9x Wind Multiplier question
(OP)
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.
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.






RE: ASCE 7-05: Rooftop Structure 1.9x Wind Multiplier question
RE: ASCE 7-05: Rooftop Structure 1.9x Wind Multiplier question
RE: ASCE 7-05: Rooftop Structure 1.9x Wind Multiplier question
RE: ASCE 7-05: Rooftop Structure 1.9x Wind Multiplier question
RE: ASCE 7-05: Rooftop Structure 1.9x Wind Multiplier question
Here is an example of the problems we are having with this provision. Greenheck came to our office a few months back to show off their Miami-Dade NOA approved, 150mph tested rooftop fans and equipment doghouses. So I asked them if their NOA's were to the 2004 Florida building Code or the 2007 FBC. Their response "HUH? I don't know." I looked on their literature, no reference of Code edition. I looked online at the FBC website approvals page. Found their approval for 2007 FBC, but it listed a maximum allowable wind pressure (for one very typical model) that would be less than any wind pressure calculated using the 1.9x multiplier for any building in Miami-Dade county over 15 feet with an importance factor of 1.0 or higher. So how can it be Miami-Dade approved if it does not meet the wind pressures required by the 2007 Code??? I asked this of both the FBC code official and the Miami-Dade NOA head, and both were stumped. But did say should not be approved if it doesn't meet the new wind Code. We has since found that there are, in some cases, no products made that would meet the wind pressures by the new Code in Miami-Dade or Broward counties.
RE: ASCE 7-05: Rooftop Structure 1.9x Wind Multiplier question
RE: ASCE 7-05: Rooftop Structure 1.9x Wind Multiplier question
RE: ASCE 7-05: Rooftop Structure 1.9x Wind Multiplier question
I was down in Ft Lauderdale after the 2004 hurricanes to assess damage at a few facilities. The roof top equipment on every facility I looked at was utterly destroyed, even if the roof itself was still in reasonably good condition (though some roofs were partially or completely destroyed too *cough* pre-engineered metal buildings *cough*) It looked as if a bomb was set off in some of these rooftop air handlers. All the panels were completely gone, leaving the insides exposed and a big hole in the waterproofing through the roof. Water poured into the building through the destroyed units, ruining everything inside. So I think by making the wind pressures so much higher, they hope to either force the manufacturers to make units that actually hold up to the wind pressures, or force designers to put the units on the ground, where if they fail, the building interior is not compromised.
RE: ASCE 7-05: Rooftop Structure 1.9x Wind Multiplier question
Yes, I use the 1.9X factor!!! A lot of HVAC specs require sealed calculations and designs for seismic and wind forces.
RE: ASCE 7-05: Rooftop Structure 1.9x Wind Multiplier question
I sure would like to know where this factor came from and why? If one is using standard ASCE methodology the wind pressures would be adjusted for the height of the building, the area or project area of the unit, etc... Why would wind pressure be higher on roof top equipment then anything else? Perhaps it is just a factor of safety issue. I wish they would just come out and say it. Design for twice the wind load because this stuff keeps blowing off the roof.
John Southard, M.S., P.E.
http://www.pdhlibrary.com
RE: ASCE 7-05: Rooftop Structure 1.9x Wind Multiplier question
The wind is obviously powerful and those coefficients are probably not far off necessity.
RE: ASCE 7-05: Rooftop Structure 1.9x Wind Multiplier question
RE: ASCE 7-05: Rooftop Structure 1.9x Wind Multiplier question
RE: ASCE 7-05: Rooftop Structure 1.9x Wind Multiplier question
RE: ASCE 7-05: Rooftop Structure 1.9x Wind Multiplier question
The proposed eqn uses a GC factor of 1.9 while ASCE 7-05 uses a factor of 1.9 X the GC from fig 6-21 to 6-23. Plus, the proposed revision gives the 1.5 factor for uplift which was missing from 7-05. This looks reasonable and easy to use. I hope they don't gum it up and make it any more complicated.
Thanks for sharing this link.
I happen to be working on a building with a roof height of 59', which is covered by these equations. What happens if the building grows 2 feet taller? What equation should I use then?
RE: ASCE 7-05: Rooftop Structure 1.9x Wind Multiplier question
After reviewing the commentary, I come to agree with the seminar, and I now use the higher h value. I am curious how others have interpretated the value of h in Fig 6-21. I hope that clarify the definition of h in the new ASCE 7.
RE: ASCE 7-05: Rooftop Structure 1.9x Wind Multiplier question
qz = velocity pressure evaluated at height z of the centroid of area Af. (Eq 6-28)
The h in fig 6-21 should be the same as the z in Eq 6-28. This is the same elevation above grade discussed in your seminar.
RE: ASCE 7-05: Rooftop Structure 1.9x Wind Multiplier question
So my question is, in this case, do you think the 1.9x multiplier still applies? The wording of the paragraph which lists the 1.9x requirement really is not clear.
RE: ASCE 7-05: Rooftop Structure 1.9x Wind Multiplier question
I have a similar situation now. The roof height is 58' and the screen walls are 18'.
RE: ASCE 7-05: Rooftop Structure 1.9x Wind Multiplier question
RE: ASCE 7-05: Rooftop Structure 1.9x Wind Multiplier question
RE: ASCE 7-05: Rooftop Structure 1.9x Wind Multiplier question
RE: ASCE 7-05: Rooftop Structure 1.9x Wind Multiplier question
RE: ASCE 7-05: Rooftop Structure 1.9x Wind Multiplier question