Design of Cupola and Steeple for Wind 1

[keen_mentee]

Hi,

I am trying to design a cupola/steeple. The cupola/steeple is made up of steel and aluminium trusses. These trusses will be covered externally with architectural cladding. I am using ASCE 7-16, Chapter 29 to design the steel/aluminum frame. I am confused as to if my structure will fall under Other Structure(chimneys, tanks, open signs, single-plane open frames, and trussed towers) and I should use Equation 29.4-1 or, since my structure (cupola/steeple) is mounted on top of a roof, it would be considered as a Rooftop Structures and Equipment for Buildings and I should use equation 29.4-2 and 29.4-3? The calculation of lateral wind force using 29.4-1 and 29.4-2 gives me a big difference in wind load. This is mainly because in equation 29.4-1, GCf=0.85*1.4=1.19 while using equation 29.4-2, Gcr=1.9. How would you suggest me to design the cupola/steeple metal frame?

Thank you,

 

[JStephen]

I hadn't really noticed the wording in there.
29.4- "The design wind force for other structures (chimneys, tanks, open signs, single-plane open frames, and trussed towers), whether ground or roof mounted, shall be determined by..(29.4-1)
29.4.1 "The lateral force and vertical force for rooftop structures and equipment...shall be determined as (29.4-2)"
So appears to be a bit of discrepancy for anything that could reasonably fall under both.
That said, I'd vote for the Rooftop Structures simply because it's higher.
It occurs to me that a steeple blowing off would be a lot more noticeable failure than an AC unit blowing off.

 

[r13]

Word by word, I don't think the cupola/steeple's structural response resembles any of the stated structure - chimneys, tanks, open signs, single-plane open frames, and trussed towers. I'll use "The lateral force and vertical force for rooftop structures and equipment..."

 

[phamENG]

A cupola and a steeple are two different things, though I agree they could be called "similar." Which one you use will probably depend on how big it is.

I've designed both - a cupola that sat on top of a gable roof with a couple of vents and weather vane on top - maybe 11 feet tall from the base intersection with the roof (only about 8 feet tall from the ridge). I designed this one based on component and cladding wall pressures.

A steeple that sat on top of a masonry tower. The tower was about 55 feet tall, and steeple was about 90 feet tall, so the finial sat about 145ft above the street. It was a steel frame with wood plank cladding. I designed it using the chimney provisions. After all, the thing was shaped like a giant chimney, which is what that provision is there for.

So do you have a cupola, or a steeple? How big is it?

 

[keen_mentee]

JStephen and retired13: Mostly I have been designing as Rooftop structure since it is higher, as you said. Thank you for your reply.

phamENG: I was asking as a general question for cupola and steeple, as I have been designing them with similar approach. For your cupola design, did you use ASCE 7-16 Section 30.3.2? What if the height for the building is greater than 60 ft?
For the steeple, If I design it as chimney provisions,I would get GCf=1.19 (approx.) which reduces the force as compared to Section 29.4.1 (Rooftop Structure and Equipment). I could do that as well to reduce the loads on my metal skeleton. But I am concerned if this is allowed by most jurisdiction or not. What do you think? Currently I am designing a steeple with roof height of 150 ft and the steeple height of 50 ft. I appreciate your time to address my question.

Thank you,
Ruchin Khadka

 

[phamENG]

My jurisdiction still uses the 2015 IBC, so we're in ASCE 7-10 (and probably will be for a few more years). Yes, I used chapter 30. Though of course I used the additional factors required for the various calculation methods, not just velocity pressure. If the building is greater than 60 feet, you'll have to find accepted literature addressing the topic as ASCE 7 doesn't cover it (except to say that you can use accepted practices).

Using the chimney provisions doesn't reduce the force, it more accurately estimates it. Read the commentary on rooftop equipment. It's about size relative to the building and how close it is to the roof top. As wind breaks around a building, it has to speed up (basic Bernoulli). This is part of what creates areas of higher pressure around the edges of the building. A 5' tall RTU on a 40' tall building is going to get caught in that "wind speed up" zone and be exposed to higher loads. For a 150' steeple on a 50' building, a little bit of the steeple may be effected, but overall it'll have a minimal effect. If the footprint of the steeple matches the footprint of the building (like at the entrance to a lot of churches), you don't get any wind separation between the two and the rooftop structures absolutely doesn't apply.

 

[keen_mentee]

phamENG: Thank you for the detail reply. It clears up many of my confusions.

Thank you,
Ruchin Khadka