timber arch bow string (tied arch) 1

[rittz]

Does anyone know of a design example for a tied arch (glulam upper circular arch .... and glulam lower tie). Want to analyze an existing 50 ft span tied arch …2 ft oc
Rise 9 ft. Upper arch glulam 1 1 ¾” x 7 ½”.. lower tie 2 x 6. … rods support lower tie

 

[IsaacStructural]

I usually analyze these using risa 2d or 3d. The two biggest issues are to make sure to check for unbalanced snow/live load and to verify all connections. These are often controlled by tension failure in the bottom chord. In my area, we reinforce these by adding two continuouse steel rods that run the length of the bottom chord and connect to truss shoe at each end.

M.S. Structural Engineering
Licensed Structural Engineer and Licensed Professional Engineer (Illinois)

 

[msquared48]

Bowstring trusses are notorious for connection failures in the lower chord. I had to deal with two recently with the same problems. - built sometime between 1930 and 1955. Yours sounds like it might be newer.

Mike McCann
MMC Engineering

http://mmcengineering.tripod.com

 

[shobroco]

50' is not a very big span for a bowstring & most timber design guides walk you through this; the Canadian Wood Council's design manual does a good job & I'd guess the American one is the same. A 1956 American wood design manual that I have also does it, & it's not too complicated for a simple bowstring tied arch (which you seem to describe, as opposed to a bowstring truss). However, I'm pretty well baffled by your description. The span is 50', the arch is an 11 3/4" X 7 1/2" glulam, which sounds reasonable, but the spacing is 2'? And the tie is a 2X6? This has to be unique in bowstring construction. I've built 60' span arched structures with glulam ribs, at 16" & 24" spacings, but they were 1 3/4" X 7 1/2" ribs (2x8s), not 8X12s.

 

[rittz]

shobroco ..... there is a mis-print in my post. THe upper chord is 1-3/4" wide and
7-1/2" deep. Yes the tie is an ordinary 2 x 6 ??? with a plywood gusset the arch at the springline OMG. The spacing of the arches is indeed 24" the Canadian Wood Council's design manual 2005 does not have a design guide of a tied arch.

 

[msquared48]

Diagonals?

Mike McCann
MMC Engineering

http://mmcengineering.tripod.com

 

[shobroco]

I don't think you'll get it to work by analyzing it. The ones like that that I did 25 years ago are no longer made but they were produced for agricultural buildings & the engineering came from the manufacturer. The bottom of the arches were on the foundations, so they didn't need a tie on each one; and the roofs were amazingly flexible. Jump on one side & the other side bowed out, but somehow they stayed up. As I say, though, I think analysis will show you it can't be done (just like a lot of PEMBs).

 

[ThePioneer]

Does not sound like this is a bowstring truss. Sounds like a glued laminated tied-segment. These were very common in agricultural and light industrial and commercial buildings in the 1940s - mid 1960s. Are there any vertical and diagonal webs that connect curved upper chord to the horizontal bottom chord?

Andreas

 

[msquared48]

Sounds more like a tied arch if there are no diagonals here.

Mike McCann
MMC Engineering

http://mmcengineering.tripod.com

 

[shobroco]

rittz says rods support the tie, which means it's not a truss, it's a tied arch, and the ones I'm familiar with are also not trusses, they have no diagonals or rod supports because they sit at floor level instead of on top of above-grade walls, and the height of the building comes entirely from their rise. They could have matching halves of larger radius & be gussetted at the centre or be actual semi-circles but either way we are essentially talking about 50-60' span buildings built of single 2x8s. I'm sure everyone will say it can't be done, which is why I said you won't get analysis to show that it works, but I know for certain of a least a few that have stood for decades in locations of very significant snow load. It was long before the days of digital photos, but I think I could dig out some old shots & scan them if pressed.

 

[rittz]

Hello shobroco
I would not like to see you go to much trouble... but world like to see
the shots if you had a spare minute. Thanks a bunch

 

[shobroco]

Sorry, I thought I knew where to put my hands on some photos, but no luck yet. I'll look around a bit more & post some if I find them. They are from the days of Kodak Instamatics & developing film.

 

[ThePioneer]

Our factory buildings in Peshtigo, WI are full of tied-segment glued laminated timber arches. Some date back to 1940. All doing just fine aftr all these years. The old Thompson Boat factory across the alley has tied segment arches dating to 1949 that are still holding. I am aware of dozens and dozens and dozens of buildings in Wisconsin with these type of glued laminated timber roof framing members that are doing fine after 60 years or more.

Andreas

 

[rittz]

To The Pioneer

Do you have print-out which would give me a simple (or approx) method of analysis of a tied arch? (I forgot how to do a 2-hinged arch 40 years ago +. Besides I have never got into finite analysis (born too soon). The one I am looking at has an ordinary 2x6 tie (not a tension rod). There are no web members. The lower chord is attached to the laminated arch member with plywood gussets.(Span 50 ft spacing 2 ft). Perhaps it was home made??

Ritz

 

[ThePioneer]

Ritz,

I do not have much information on design of such members. I did find a 1950s competitors brochure on 24" o/c glued laminated tied-segment arches. Glued laminated timber curved top chord and solid lumber gusseted horizontal bottom chord.

Andreas