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.

 

[slickdeals]

Structureguy: I work in Miami, where are you located?

 

[Ron]

Structuresguy...I believe the intent in ASCE 7-05 was for rooftop structures to be those that are added to the structure such as equipment, equipment support frames, screen walls,etc., not those jutting up through the roof and being part of the main wind frame.

 

[ARLORD]

For an explanation of their intent see the commentary on page 300, section C6.5.11, Fig. 6-8, 6-18, 6-19, 6-21, and 6-22.

 

[SteveGregory]

I agree with Ron and Arlord

 

[structuresguy]

Yes, I have read that section before. It is clear that it applies to equipment on the roof, but not necessarily clear what the intent of "rooftop structures" may be. Obviously, I place an enclosure on the roof, say to enclose a piece of equipment on all sides and top, this would apply. Well, how would that be different than if the roof popped up, with say a large skylight with vertical sides? I don't think it would be any different in actual behavior as far as the wind is concerned. But now the glazing of the skylight would need to be designed for 1.9x multiplier. These pressures get very high very quick.

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.

 

[SteveGregory]

No more rooftop units! It sounds like the code just made your job easier. You can tell them to put their units on the ground or in the building.

 

[structuresguy]

I DO!!! But the architects never want that great big air powered chiller right by the front entrance. I can't imagine why not.

 

[structuresguy]

Actually, it's funny that we joke about it. But I seriously think that that was one of the reasons for adding the 1.9x multiplier. If not a primary reason, then certainly it is a cause-effect type solution.

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.

 

[SteveGregory]

I also do consulting work for a major AHU manufacturer and one of my jobs is to design the anchorage for these units. Many of the units are the size of a mobile home trailer.

Yes, I use the 1.9X factor!!! A lot of HVAC specs require sealed calculations and designs for seismic and wind forces.

 

[southard2]

I hate the new factor. I'm thinking that perhaps the new 1.9 factor is so that you can just apply a lateral force to the roof top unit. Say versus applying both lateral load and an uplift load. I think the end results are about the same now.

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

 

[Ron]

In 1992, after hurricane Andrew, I was asked to evaluate a variety of structures that had been damaged. In particular, one client had 6 "big box" structures with HVAC units on the roof, of course. Two of those, one in Naranja and one in Coral Gables, had 3000 lb units that were literally rolled across the roof and fell onto the ground on the opposite side of the building. One of them fell onto a lower roof before hitting the ground, thus bring the roof structure down as well.

The wind is obviously powerful and those coefficients are probably not far off necessity.

 

[WillisV]

This link for a proposed revision to this provision in ASCE 7-10 (to include uplift as well) has some interesting discussion.

http://www.dca.state.fl.us/FBC/comm...lRevisionstoWL14-4RR-RooftopEquipment-fyi.htm

 

[steellion]

The 1.9 factor is based on testing, and it seems to be empirically appropriate based on the stories in this thread.

 

[structuresguy]

Ron's experience after Andrew in 1992 is exactly the same as mine in 2004 after the hurricanes in Ft Lauderdale. One facility at the airport, a large cargo shipping hanger, had very large exhaust fans on the roof, about 10-15 ft (dont remember exactly now) fan blade diameter. There were 4 or 5 of these along the length of the ridge of the building. The housing total was about the size of a small car. Every single one of them had blown completely off the roof, tumbling across the roof and tearing it to shreds as it went. They landed in the parking lot, and came to rest about 100 feet from the building. These fans weighed well over 1000 lbs. And the real kicker to it all, they had just been installed a few months previous. So its not like they failed because they were old and rusty and designed to some old wind code.

 

[SteveGregory]

Willis,
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?

 

[ARLORD]

Interesting side note to this conversation. For roof top units, in figure 6-21 of ASCE7-05, I have always used h = height of the roof top unit above the top of roof. However, after attending a wind seminar, we were told that h = the height of the building + 1/2 the height of the roof top unit. Big difference.

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.

 

[SteveGregory]

In Section 6.5.15 "Design Wind Loads on Other Structures",
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.

 

[structuresguy]

OK, question time. I am working on a project right now where we are adding a new rooftop air handler to a building where the mean roof height is about 56 feet. So according to the ASCE code, we would need to use the 1.9x multiplier, since it says for buildings with h<=60ft. But the unit itself is 8 feet tall, and sitting on a curb of about 18 inches. So the top of the unit would be at about 65.5 feet. In eq. 6-28, it says to evaluate q at the centroid of the equipment. So in our case the centroid would be at 61.5 feet. This is above the 60 feet requirement.

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.

 

[SteveGregory]

Use 1.9X, since your roof height is 56'.

I have a similar situation now. The roof height is 58' and the screen walls are 18'.