Wood Arch Design

[ehjelle]

I'm doing schematic design to determine approximate member size of a curved (140' radius) glu-lam wood arch. The base dimensions are approximately 185' width about a 35' rise at mid-span. These arches are spaced 10' to 12' o.c. to form a shelter roof for an outdoor stage.

I've turned up very little reference material for this situation, having searched through AITC (American Institute of Timber Construction) material, the Internet, as well as Eng-Tips.

I'm hoping someone out there can point me towards some sort of reference material that would help me with this.

Any help is much appreciated.

Erik Hjelle

 

[RWF7437]

Try the manufacturer's website.

Here's one to start with:

http://www.westernwoodstructures.com/

good luck

 

[mrengineer]

1) Do you have the AITC Timber Construction Manual? The 4th edition has a fair amount of info on arch design.
2) You might see if there is a local rep for one of the glu-lam manufacturers in your area. A lot of times if you give them a sketch with applied loads they can work with the manufacturer to get the info you need. We even defer the final design to the manufacturer in a lot of situations (especially the connections, such as in glu-lam trusses).

 

[KeySol]

ehjelle-

I recently designed a curved glu-lam. You need to get a hold of AITC 117-2004. STRUCTURAL GLUED LAMINATED TIMBER specifications. It should be available on the AITC website. There is another factor that is multiplied by the design values that is related to the radius of curvature. Hope it all works out.

 

[UcfSE]

Are you designing it as a curved beam or a true arch?

 

[dik]

I've designed a whole bunch of archrib structures in the last month or so. I get the geometry via AutoCAD using an estimated number of laminations. I then calculate the coordinates of the various joints, breaking the curved portion into 10 equal segments. Some of the archribs have sidewalls for a lower storey and some have a straight segment to the ridge. Also, some have collar ties.

I then calculate the snow loading and wind loading for the structure. I've done up a spreadsheet to calculate these. In Canada, we have a snow and rain load component. Under loading for curved roofs, there are 3 loading conditions and assuming just a regular sloped roof, there are 2 loading conditions. Condition 1 being the same for both cases... that's 4 snow load conditions... and two different wind load conditions.

I enter the geometry into Risa and the loads generating a combined solution. Risa also does an excellent job of p-delta calcs. On a recent 50' span, increasing the depth by approx 4" reduced the design moment by nearly 50%... Taking the output from Risa, I then check combined stresses at a few of the more significant points. For the Risa solution, I often use the graphical output for Axial force, Flexure and shear.

Almost got it down to an artform... Dik

 

[ehjelle]

We just got the 5th edition of the Timber Construction Manual - which points back to 4th edition for & some AITC tech-notes for analysis. Seems my original search wasn't as comprehensive as I thought... ;-)

As mrengineer mentioned, I'll most likely be deferring the final design to the glu-lam manufacturer. However, not having done this sort of design before, I'd like to have some understanding of the analysis, forces, etc. involved as I get started on the project.

UcfSE - After doing a bit of reading this morning, I believe this will be treated as a true arch. Given the overall dimensions and what can practically be shipped, a three-hinged arch analysis seems to make sense.

Thanks for all the advice/info - I've got enough to do some reading/learning this weekend.

Erik

 

[RARWOOD]

You definitely should contact a glulam supplier for this project. Your biggest problem probably is going to be the supplier's ability to manufacture and ship the archs.

Shipping restrictions likely will require the use of a three hinged arch with two half arches or the use of a radial arch broken into three sections with full moment splices at the joints.

 

[FloridaPE]

Erik,

These types of arches are used in the fertilizer industry to store raw phosphate. Several years ago, a CSX Rockport phosphate terminal in Tampa was struck by lightening and we did the analysis and repair design for the wood arches. The arches were 190' clear span with a rise of 85' at the center, and the section was 12" x 60".

These were continuous arches, with no hinges except for the large steel shoe and pin at the buttress support. Since we only had to repair the arch, we never found out how it was fabricated, shipped, and erected.

At the time (1985), the computer programs were very basic so we divided the arch into 4 foot segments and then check each segment along the arch. The best reference at the time was AITC. Although forces in a true arch are axial in nature; the wind loading in Florida (110 mph) created some odd loading patterns.

Good luck,

John Carter