Glulam Arch Question

[Engrman]

Designing a swimming pool enclosure that has three-hinged glulam arches. In the parallel direction the arches handle the wind load. Since the roof structure will be purlins framing to the arches and not bearing on the perimeter wall I say there needs to be a wind girt at the top of the wall that will span between the arches, similar to pre-engrd mtl bldgs. A colleague has convinced the contractor that the walls's double top pl does the job and no wind girt is needed. What do you guys say?

 

[ChipB]

LMAO!! Thanks! I needed a good joke!

You're using a glulam top plate too, right.

(Did I answer your question?)

 

[RARWOOD]

Are you talking about wind in the direction parallel to the arches?

With a stud wall you need to get the wind load into the roof diaphragm. The diaphragm then will transfer the wind load into the arches. The double top plate may be required to serve as a diaphragm chord.

 

[Lion06]

Depending on the length of the top plate and the sizes, it is possible. How do you plan on getting the wind loads into the foundation. Just getting it into the diaphragm with your double top plate (or whatever method you come up with is one thing). Once in the diaphragm, you have to get it into the foundation.
Do you have shearwalls or any kind of moment connections between the purlins and the arches for the perpindicular direction?
RARSWC-
He said that the wind parallel to the arches would just be handled by the arches, which makes sense. His question is for wind perpindicular to teh arches.

 

[ChipB]

Whoops. Sorry. Duh. Must have had a bad brain day yesterday (as compared to other days.)

I second StructuralEIT and his questions.

I'm actually kind of shocked someone is doing this over a pool. I'd think the humidity combined with the chlorine would not be very good for either the wood or the glue. I've seen concrete T beams significantly deteriorate under the same use.

 

[hokie66]

The way I read the original post, he IS talking about wind parallel to the arches, and he has a wall perpendicular to the arches which needs its top reaction transferred to the arches. He did say that the member in question would span BETWEEN the arches.

If that is the question, Engrman, you do need either diaphragm action or a horizontally spanning member (call it a girt, plate, or whatever) at the top of the wall, connecting to the arches. Since you didn't mention a diaphragm, I assumed you were not using one.

 

[hokie66]

ChipB,
There are lots of pool roofs done in glulam all over the world. Needs attention to detail, specification of appropriate grade, coatings, condensation control, etc. Glulam is material of choice in many areas.

 

[Engrman]

hokie66, You are right I was talking about wind parallel to the trusses. There is a diaphragm. It just seemed to me that with an 8/12 pitch the connection at the wall gets a little flaky and a wind girt would do a better job of transferring the wall lateral load. Maybe the blkg btwn the wall and first purlin does the job.

ChipB, Timberweld, the glulam fabricator, is aware of the enviromental concerns and the specifications cover it.

Everybody else, thanks for the replies.

 

[ChipB]

Well, you learn something new everyday. The church I grew up in had glulam arches. Since then I've always been partial to glulam arches.

This is going to depend on your connection detail at this eave. Either way, you're going to have to transfer tension and shear at this location. You can do it with a double top plate, just make sure you get all the forces transfered. Personally, I'd probably tension strap the plates across the outside face of the arches, and transfer the shear directly to the arches.

 

[hokie66]

In the direction we are talking about, I don't see where diaphragm axial chord forces come from, as each arch takes its part of the load. Unless there is a need for load sharing.

 

[ChipB]

hokie66
You are still getting axial chord forces between the arches, and the shear is transferred to the arch.

 

[hokie66]

ChipB,
I would think the shear would be transferred from the diaphragm sheeting (I assume it is plywood) into the purlins, then directly into the arches all along the length of the arches rather than concentrated at the eaves. Thus the purlins would themselves be the "chords". Am I missing something?

 

[Engrman]

ChipB, Maybe I'm missing something but it seems to me that somehow the wall wind load has to make it into the diaphragm and has to do so at the eaves. The roof wind load acts as you described with the sheathing acting as sub-diaphragms btwn the arches.

 

[hokie66]

Engrman,
Not sure if you were querying me or ChipB. But anyway, I believe if you are connecting the top of your wall to the diaphragm, then both the wall wind load and the roof wind load distribute along the arch and there is no special requirement for tensile connection through the arch at the eave. Only talking about wind parallel to the arch, there may be eave strut/tie requirements for wind perpendicular or diagonal.

 

[ChipB]

Given 30'x150' building with frames at 30'
Eave Ht: 20'
Roof Pitch: 4:12
Wind pressure: 25 psf
Projected Trib. Area acting on Diaphragm: .5(20)+(4/12)(30/2)=15'
You have a series of sub-diaphrams as Engrman has stated.
SHEAR
V=25psf(15sf)(30/2)=5625 lbs
Assuming the diaphragm is directly connected to the frame, part of this shear (roof portion) will be disbursed to the frame through the connections from the diaphragm to the frame. The remaining shear is disbursed to the frame through the connection of the top plates to the frame.

CHORD FORCES
M=25psf(15sf)(30)^2/8=42188ft-lbs
Tension/Compression=M/D=(42188)/30=1410 lbs

I'd tension strap it across the frames to balance out.

If you use your purlins as chords, I still think you'll end up with a bunch of tension and compression at the plates. Reason being, which is the stiffer element(s): a 30'x30' diaphragm or a series of 30'x6' diaphragms? If they all have to deflect the same, (30)^4 >>>>>(5)(6)^4