Responsibility for RTU Rooftop Curb Design 1

[IsaacStructural]

Just wondering, in your experience who is responsible for the design of rooftop curbs for RTUs?

I've generally provided a reinforced opening in my concrete slab/metal deck and left the anchorage and detailing of the actual curb to the mechanical engineer. A mechanical engineer in my office thinks this is my responsibility. To me it doesn't really seem like a component of the building's structural system, and what do I know about how tall and wide to design and detail these curbs.

I'm in Chicago, with low wind loads and no seismic requirements, so I'm not really overly concerned, I think whatever reasonable solution they provide will suffice, but just wondering, how do others handle this? am I in the minority?

Licensed Structural Engineer and Licensed Professional Engineer (Illinois)

 

[AELLC]

It is the Mech Eng.

Doesn't make sense for the Struct - what, does the Mech expect us to provide details for all his stuff hung from our bar joists, etc?

 

[jayrod12]

It's always on the RTU manufacturer for us unless we are specifically asked to design it. If we are retained to do the design then we require much more information to be provided such as the things you list above.

but 99% of our RTU projects the curb and anchorage are by the supplier, we just provide the support system in the building.

 

[Archie264]

Isaac,

Chicago has low wind loads and no seismic requirements? That's a surprise to me. I thought it's nickname was the "Windy City"? Also, I know Illinois licensure regulations are very stringent so I assumed there was considerable seismic activity but that was just that: an assumption.

Interesting.

 

[IsaacStructural]

Archie264, Chicago has their own Code, the Chicago Building Code, which specifically exempts seismic requirements and the wind loading a simplified approach, 20psf main wind force for buildings up to 200feet tall. Cladding and Components have different requirements, but nothing extreme. Other than a freak tornado, the winds in Chicago are comparable to the rest of the state.

There is some seismic concern as you head to southern Illinois and the New Madrid fault line, but in Chicago, the computed seismic values are quite low, and you generally end up in Category A if you go through the IBC/ASCE methods.

Outside the city (suburbs) various IBC 20XX codes have been adopted.

Licensed Structural Engineer and Licensed Professional Engineer (Illinois)

 

[AELLC]

20 psf? I hate to think what happens if a 200 ft bldg. is in a wind alley downtown.

 

[IsaacStructural]

After 200ft the values start to climb for every 100ft, so by the time you get to 1000ft building, the value is 42psf.

If you compute the wind loads in ASCE 7-05 you often times get loads of less than 20psf, at least for shorter buildings in the area. the velocity pressure coefficients in ASCE don't climb as much as you think. For Exposure A at 0-15ft you've got .70 and all the way up to 200ft you've got 1.20, so a less than double. On the other extreme, for Exposure D, you get 1.03 up to 1.61.

I don't think it is a big concern, global collapse from straight line winds seems unlikely to me, it doesn't seem the wind occurs in such a uniform fashion. Anyhow, we haven't had any 200ft buildings blow over, so the real world testing seems to concur (subject to future revision as needed!)

Licensed Structural Engineer and Licensed Professional Engineer (Illinois)

 

[steellion]

-The curb should be in the mechanical engineer/unit manufacturer's scope. Almost always. We detail the support steel for the curbs, but we don't dimension this steel, as the mechanical unit dimensions almost always change from the design basis by the time the actual units are submitted.

-I just did a quick napkin calc and came up with approximately 23 PSF for combined windward+leeward pressure at 200ft, Exposure B. 20 PSF for the entire structure seems to be a reasonable and conservative analysis method. However, I think our clients would want us to use the "long" ASCE method to make the lateral design as tight as possible.

 

[AELLC]

Well, the reasons things don't fall down on a regular basis here in the USA (except decks), is you are just eating in the FoS.

You can get a nasty uniform wind force if the wind direction is parallel to your boulevard, and all the velocity gets amplified when there are tall buildings each side. Then, if the boulevard dead ends with a cross street, a small building right there is going to see a lot of wind pressure.

 

[IsaacStructural]

Steelion, I've done the same calcs in the past, and come to the same conclusion. I think if you plotted out the two methods and the resulting overturning moment, there would be an overlap phase, Chicago would be more conservative for some heights/exposures and slightly less conservative for other heights/exposures.

I have worked on taller structures (tallest was over 650ft), and in those cases we did have a wind tunnel analysis done, and the final result was lower design values, which was anticipated.

AELLC,

I've felt the sort of winds you are talking about, but in general, I believe those are really more of a concern for components and cladding more than global collapse, we don't really have cases of skyscrapers adjacent to stud framed single family houses, or something of that sort. The buildings mostly increase and decrease in height gradually. In general, from what I've seen, the more accurate wind tunnel tests yield lower design values than either Chicago Building Code or ASCE.

Like Steelion provided, you can crunch the numbers for yourself and see what design value you'd get using ASCE 7.



Licensed Structural Engineer and Licensed Professional Engineer (Illinois)

 

[dhengr]

IsaacStructural:
It seems that the way design and construction is going these days, we all spend more time shirking the responsibility for designing something, and trying to lay it off on someone else, than we do actually getting the bldg. properly designed and built. And, I’m not pointing the finger primarily at the structural engineer here. But, where have the master builders and the master designers and project coordinators all gone, and who pays them for their efforts. Everyone always has money to fight about this crap being done wrong, after the fact.

I think those curbs are primarily the responsibility of the mech. people, but if you want them done right you should probably stay involved at least on the periphery, shop drawings, early inspections, willingness to help and advise, etc. They can never, or at least rarely do, give us sufficient info. on the RTU’s when we are actually involved in our design process. If you provide the opening and structure it adequately, and cleanly, to give them the most options, that should be enough. Many times they are still farting around with size, wt. and connection points when the roofing is going on the bldg. and we’re barely involved any longer. When this guy is actually in your own office, I’d kick is butt, he’s just trying to get you to do his work for him so he can sit back and relax. Fact is, if he can’t pick it out of a chart or table, he probably doesn’t know how, or what a good detail looks like, so help teach him. He should always be able to come to you with structural questions, which is the best way, the best detail, but his department should do the work and cover the cost.

 

[AELLC]

Burke's Structural Law #12: RTU's are always at least 50% heaver than specified on the Mech. drawings.