Church Steeple Project

[Skinnattittar]

First; thank you everyone for your help! I've browsed these forums for a while now, but have never posted or asked a question myself.

I did some searching and did not find anything addressing this issue posted so far, but if anyone knows of something similar, feel free to let me know!

Project: Renovations, Repairs, and Replacements on a church built circa 1850.

Conditions: We are on the final stages of the planned projects - replacement of the church steeple and spire. The supporting structure below the steeple has seen many renovations over the century-and-half, including steel beams and heavy framing support. We have decided to use the existing structure and support the replacement steeple in the same location as the existing steeple. The site is in a 110mph Zone, Exposure B, with many buildings and tall trees around.

Steeple: The steeple will stand approximately 28'6", with an 18'6" Spire, atop 18'6" of the main building. There is a clock and bell on two platforms, we estimate their combined weight of 2,000lb. There will be a perforation of 3'6"x6'8" on four sides (steeple is 8'0" square) approximately 20'0" up from base of steeple.

Construction: The steeple will use 8"x8"x28'4" cedar posts (we're trying to convince the builder to switch to modern PT columns) at all four corners with 6x6/4x6 lateral girts and for all framing. The exterior will be 1/2" plywood with traditional wood cladding.

Problem: Wind Design.

Details:

The structural design is pretty straight forward. I calculated the wind load to be 28.2 psf (using ASCE 7-05). The issue now is figuring out how to actually analyze the steeple to withstand wind loading and the best methods of making connections to the existing building. Thus far I have proposed a structural analysis using MultiFrame and using standard shear wall design, both of which resulted in 6/12 nailing and less than 10kips uplift at the post bases.

Question: Basically my boss isn't comfortable with using these methods for such a tall structure (we typically just deal with standard residential construction). Using MultiFrame and its panel tools doesn't actually give results of shear/tension as each connection, I've just interpreted its results to be similar to what I've found with traditional shear wall designs put through MultiFrame. So now I'm reaching out to see what other engineers think or might know about designing such structures.

 

[JAE]

It sounds like you have the wrong tool to properly analyze the framing for this steeple. If you don't know how to do the analysis by hand, and you don't have the proper software to assist you in the analysis, then I would suggest you need either to 1) get the proper software (assuming you know how to do the analysis by hand anyway) or 2) hire someone who has the tools and knowledge.

Most good structural engineers would know how to approach, analyze, and check/design something like this with no problem.


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[Skinnattittar]

JAE: I was hoping for a little more guidance than "ask someone else to do it." I do not think that will teach me much. I am a structural engineer with about 500 projects under by belt, almost half of which have required wind design (as opposed to just WFCM Checklist).

 

[JAE]

OK - some suggestions:
I would use a standard 3D analysis program (RISA 3D, RAM Advance, etc.) and model the structure with applied ASCE 7 wind loads.
Using a shear wall analysis seems a little fishy to me but maybe it might work. As you suggested there are no outputs from it that would give you connection values.
I would look at wind directions other than orthogonal.
I would be concerned with the shear wall aspect ratio of the tower (20' to 8' seems a little high and outside normal wood sheathed shear wall analyses.
Your wind pressure seems appropriate - I would use an importance factor higher than 1.0 for a church and use Figure 6-21 of ASCE 7-05 (or equivalent) based on your h/D ratio.
Look at second order effects due to probable high flexibility of the tower.
Be sure the open steeple "windows" don't allow moisture in to rot your wood.
I'd use steel framing myself and have the wood be infill/surface material.

I'm not trying to be difficult here - but I'm not sure exactly what you are asking.
You gave very nice detail on the problem but your question seems very general and vague.
Perhaps some more specific questions?





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[boo1]

check

http://www.structuremag.org/wp-content/uploads/2014/08/F-Steeple-Proper-Apr-081.pdf

 

[JAE]

Perhaps some more specific questions?

Helloooo - anyone home?

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[Tmoose]

RE: Anyone home?

Skinnattittar (Civil
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[Skinnattittar]

RE: Anyone home?

Sorry for the silence, this project is still going on and changing.

I created a rather heavy plan involving thick sheathing and 10d nails with cross bracing. The sheathing would act as gussets (with appropriate nailing). Hardware would strengthen connections. It would be a rather involved build but that was the best I could manage building a 30ft tower atop a 20ft building with an almost 20ft spire.

All that done and agreed to by the client, he came back several months later claiming to have "slightly modified" this design, but had completely gutted it. No cross bracing, no complete sheathing, all the nailing and hardware gone and just doing standard nailing. Gone engineered lumber replaced with ungraded cedar (he was obsessed with going out and cutting down trees himself to make some kind of collage thing about the church renovations to put on display... somewhere). There were many other changes but there's no need to go into detail.

Ultimately, I was able to, for a little while, convince him that if he wanted his new requirements to be met (which he insists are not new) we really need to go to steel construction. So I did a traditional steel design, but now he wants to try doing engineered material again, but using the steel layout. /table-flip