ASCE 7 Wind Loads on Rooftop Structures

[StructuralEddie]

Hey,

Thanks in advance for the help. Is uplift insignificant for rooftop structures greater than 60 ft? ASCE 7-10 requires uplift forces of 1.5x the lateral wind pressure applied as an uplift force on rooftop structures for buildings less than 60 ft, but does not require the uplift for rooftop structures on buildings greater than 60 ft. This seems like an arbitrary cutoff. I'm assuming structures up to 60 ft had test data and greater than 60 ft did not. So as an engineer, am I suppose to use my judgment in assigning uplift forces for these structures?

 

[charliealphabravo]

What do you mean by "rooftop structures"? Do you mean the roof itself? A flat or pitched roof with or without parapets? Or do you mean roof mounted structures like signs and penthouses?

The newest copy of ASCE-7 I have is 05 but can you say where the 1.5X requirement appears? I don't remember anything like this. Normally the MWFRS pressures come from Fig 6-6.

 

[RFreund]

Per section 29.6 of ASCE7-10.
The 1.5 can be reduced to 1 as a linear function of the equipment area. I'm not sure why it is limited to H<60ft though. They do say that the research is limited, but reference a couple of studies in the commentary.

EIT

http://www.HowToEngineer.com

 

[steellion]

Eddie, you need to consider the effects of your rooftop structure, screen wall, rooftop equipment, etc. on the roof framing regardless of the height. Follow the load path. ASCE requires an additional multiplier on the wind load for structures less than 60 ft. My understanding is that is in place because of several cases of rooftop units blowing off the roofs of buildings in hurricane-prone regions like Florida.

C.A.B., I don't believe the 1.5x uplift multiplier is in ASCE 7-05. It is in ASCE 7-10. However, both ASCE 7-05 and 7-10 require a multiplier of up to 1.9x the wind load on the face of rooftop structures for h < 60 feet. This has a huge impact on the design of these structures!

 

[StructuralEddie]

Thank you for the replies. Im talking about structures such as HVAC equipment, penthouses, and the such that require their own structural analysis for wind effects- not their effect on the supporting structure. For tall buildings in ASCE 7-10 (nothing in 05), these structures have specific design requirements for short buildings, but not tall buildings (>60 ft). If my building is 65', then do I just ignore all of these nasty uplift forces on the penthouses and HVAC equipment?

I know I'm speaking in extremes and the typical design of structures for wind loads consider uplift on the net roof and its components, but I'm trying to dig deeper into the bones of the wind provisions. I design manufacturing and industrial structures, so I operate in this gray area of structural engineering where I have to really understand the codes and their intent in order to apply them.

In the end, maybe I'll just figure out how to do wind tunnel tests in my garage and figure it out. You don't suppose there are any starter kits out there? ~

 

[RFreund]

That or maybe some computational fluid dynamics...

EIT

http://www.HowToEngineer.com

 

[msquared48]

I analyze and design rooftop structures for telecom regularly, and as steellion mentioned, the 1.9 factor, frequently ignored in the code, drives the design. As for the 1.5 factor, it mostly works out to 1.0 for me.

Mike McCann
MMC Engineering

 

[Triangled]

And, I believe, the 1.9 is 3.1 in Florida.

 

[msquared48]

I'll have to remember that 1.9 = 3.1 in Florida. Does Ron know this?

Must be the gator factor.

Mike McCann
MMC Engineering

 

[Triangled]

Yup. Unless I'm mistaken, gators were deposited rooftop after hurricane Wally, and they started tugging on the RTUs... And now, Gc = 3.1.

 

[Triangled]

Florida code
Section 1609.8

http://www.ecodes.biz/ecodes_suppor...lding/PDFs/Chapter 16 - Structural Design.pdf

 

[a2mfk]

My problem with Gator math has always been regardless of the numbers on the scoreboard at the end of the game, they still talk the same amount of trash. And also have the memory of an actual alligator except when it comes to the times they beat you. Sorry, football season is upon us, and I digress.

Thanks for pointing out that section Triangled in the FBC. I don't do a lot of rooftop wind design anymore and was not aware of this.

To the original post, wouldn't wind pressures increase with height on rooftop components the same way they do with exposure factors?

There is a table here (FWIW) with values up to 400ft:

http://www.academia.edu/2451278/WIND_LOAD_ANALYSIS_FOR_TALL_BUILDINGS

 

[UcfSE]

The 60-ft limit is based on the limits within the research. Some experts recommend applying the extra factors anyway for h > 60ft.

 

[paddingtongreen]

Consider a building, all the deflected air flows over the roof and sides. Consider a 300' wide, 30' high building, the wind has to pass a periphery of 360'. Consider the same shape but upright, here the the same amount of air passes over a periphery of 630'; it does not have to accelerate as much. Since the pressure/suction is proportional to velocity squared, you can expect higher suctions on the low building.

Greater pressures with increasing height are covered in the tables and formulae.

Michael.
"Science adjusts its views based on what's observed. Faith is the denial of observation so that belief can be preserved." ~ Tim Minchin