ASCE 7 Mean Roof Height

[joder]

I have a question that requires engineering judgement because I don't believe the Code nor the Guide to the Wind Load Provisions addresses this question. I have made my judgement, but would like input from others.

I am working on a structure that has a large sprawing low roof surrounding a gymnasium that pops up higher and has a separate higher roof. The average roof height of the low roof is 25 ft. The average roof height of the gymnasium is 31 ft. The area of the low roof is about 3 times the area of the high roof.

The building is right at 1500 ft. from an open body of water. The wind speed per Fig 6-1, interpolated, is about 134 mph.

In my opinion, and as Structural Engineer of Record, I am insisting that this building is in Exposure C. because the mean roof height of the Gymnasium is greater than 30 ft. and Exposure C conditions exist less than 1500 ft. away. Note the Exception under the definition of Exposure B (6.5.6.3)

The architect and general contractor want to save a lot of money on this Pre-Engineered Metal Building by calling it Exposure B because the "mean" roof height is less than 30 ft and its greater than 1500 ft. from Exposure C conditions. If one were to average the low roofs into the high roof, the average roof height would indeed be less than 30 ft. My argument for making it Exposure C is that the wind hitting the gymnasium doesn't know that the lower roofs exist and will not be reduced accoringly. Furthermore, knowing that PEMBs are usually designed based on the tributary width of the frame spacing, the frames of the gymnasium are not helped by the frames of the low roofs.

I believe a reasonable, yet highly unorthodox, compromise would be to allow the low rise structures to be designed for Exposure B and the gymnasium for Exposure C.

What do you think?

 

[DaveVikingPE]

Questions I have:

1. Is the building truly "regular" in geometry? Will your MWFRS reflect that the lower roofs really matter concerning wind load? The commentary, section C6.2, page 282, talks about the issue of regular-shaped buildings. That might be of some help. Now, if you're near a body of water, how can your surface roughness get you a "B"?


2. The "EXCEPTION" in section 6.5.6.3 says that the upwind distance may be reduced to 1500 ft. If you're right at 1500 ft, then it's 1500 ft.

 

[joder]

David: The building is not a box, so I'd say it is not regular. It's somewhat irregular in plan and it's not regular in elevation since the middle pops up. I believe engineering judgement is involved. A wind tunnel test would be too expensive.

The exception basically says that if I'm 1500 ft. from a body of water and less than 30 ft. tall, I can use Exposure B.

 

[UcfSE]

The wind hitting the higher are will know the low area exists because it has to, in part, flow over the low area to get to the high area. Because of this, and my reasoning below, I would probably pick MRH of the higher roof.

While I don't have an answer per se, I would try to find examples of high and low roofs elsewhere in the code, such as figure 6-12 in 7-02, "Stepped Roofs". The variable h is the MRH and is shown to the high roof; the relative heights of the low and high roof are defined separately. While this isn't strictly your case, it may help define what the code intends for MRH in your case. If they would take the MRH to the high roof in this case, it seems reasonable to do so in your case as well. This is also the more conservative choice.

If you do what the contractor wants and something happens in a few years, who do you think they will blame? It's your liability therefore your decision, period.

Another option is to look in the wind code of another country. The Australian and British codes are pretty well defined compared to ASCE 7.

 

[JKW05]

Joder, I wouldn't hesitate to use Exposure C.

Sure the architect and contractor want to use the lower exposure, but in the end their butt isn't on the line. I'm guessing from the description that this may be a school. And from your wind velocity, I'm guessing this is in a hurricane region. So I find it a little disappointing that a "design professional" (i.e. architect) would quible about a couple of feet in the technical determination of wind loads on such an occupancy, which would surely be utilized in the event of a natural disaster. It doesn't surprise me coming from the contractor: he looks good to the owner for trying to save money with NO liability since he just has to build what you are responsible for designing.

For what it's worth, I see in the ASCE Commentary C6.5.6 that "interpolation between exposure categories [is] permitted. One acceptable method . . . is provided in Section C6.5.6.4." (This is apparently a typo, should be C.6.5.6.6). I've never tried this; Personally I think the code requirements for wind loads are trying to be to precise for the accuracy that we can really predict.

 

[DaveVikingPE]

OK, so the building's not regular, so... First iteration, I wouldn't include the lower roofs in determining the mean roof height.

The way I interpret the exception is that you can go with Exposure B where surface roughness B "prevails in the upwind direction for a distance of at least [1500] ft..." In your case, the nearness to an open body of water indicates surface roughness C and thus Exposure C is appropriate. Your arguing with people, it sounds to me, who are trying to change the surface roughness from C to B and based on your description of what's really there, it's surface roughness C, obviously.

 

[UcfSE]

Note that not every surface has to be the same exposure. You might have C on one side and B on the other three.

 

[DaveVikingPE]

That's a good point, but from joder's description, it sounds like it's all surface roughness C.

 

[joder]

UcfSe thanks for pointing out that different surfaces can have different Exposures. I have never considered that.

DaveViking: Exposure B conditions exist for 1500 ft. between the the body of water and my structure, so if less than 30 ft. tall, Exposure B should be okay.

This is a church family life center. It is in a hurricane zone. The 1.15 Importance factor has been applied.

 

[UcfSE]

That note is in ASCE 7-02 6.5.6.1. I think it's the same section number in 7-05 as well.

 

[DaveVikingPE]

? Then why insist on exposure C in the first place?

 

[joder]

Dave:

I am insisting on Exposure C because the higher roof IS greater than 30 ft. The architect wants to average the high and low roofs to say that the "mean" roof height is less than 30 ft. That would be a wrong interpretation of "mean roof height". I believe that UcfSE has given me the ammunition to clarify the correct interpretation, by utilizing Fig. 6-12 Stepped Roofs. That Figure identifies the mean roof height as that of the highest of the roofs.

Post-script: The architect is going to lower the high roof so that it is less than 30 ft. Then, I will accept Exposure B. They really are way over budget doing all they can to cut costs.

 

[bylar]

the person responsible should make the determination, definitely not the contractor.