Historic barn 1

[Sawbux]

I am involved with re-rooofing of an historic barn built in 1948. The roof is in the form of a gothic arch, framed with nail laminated arches made up of 4 -1x8's nailed together with staggered joints - the roof face sawn to a radius. It was reinforced in the 90's by installing glulam arches and 4x12 purlins to take the sag out of it. So now the arch structure only needs to span the 12 ft. between the supporting beams. Still, using an average member depth for the arch members and analyzing them as a beam spanning 12 ft, the bending stresses end up being about 2400 psi. The new roof requires a layer of plywood or OSB over the existing skip sheathing, which should stiffen the roof- but how to figure? The dead load of the roofing will increase from 1.5 psf (cedar shingles) to 5.5 psf (rubber shakes + plywood). Need to justify to building department that new roof will not overload the structure.

Anyone out there done something along similar lines? How did you analyze segmental built up arches like this?

SAWBUX SE, PE - WA

 

[msquared48]

I am having a problem visualizing this. Can you post a picture or a sketch?

Mike McCann, PE, SE (WA)

 

[Sawbux]

Here is a photo, near the peak, showing the rafters & retrofitted glulam arches.

 

[Sawbux]

Here is a USDA plan, showing gothic rooof barn with same rafter construction detail - circa 1941!

 

[SlideRuleEra]

Sawbux, that's quite a project. Even though the barn was built in 1948, using 1941 plans, the design is a first generation gothic roof (circa 1910) with sawed, vertical rafter segments laminated using nails. This design wasted lumber and was labor intensive to fabricate. While it remained popular, the design was quickly superseded with a second generation (c. 1920) with bent horizontal rafter segments laminated using nails. Both types tended to sag and suffer damage from moderate wind. To my knowledge, the first generation, like the one you are working with, never received much engineering analysis. The second generation was analyzed and lead to a third generation (c. 1935) using glued laminated rafters. These were reasonably successful. With the dramatic increase in use of mechanized farm equipment in the 1940's the need for barns with huge hay lofts important for storing draft animal feed went away... therefore, so did barns with gothic roofs.

The 1939 USDA Forest Products Lab report "The Glued Laminated Wooden Arch" does briefly address structural tests performed on the rafter type used in your project. See info beginning on page 77 at this link:

https://books.google.com/books?id=m...after arch forest products laboratory&f=false

Within limits, gothic arches can be analyzed as a three-hinged arch. The hinges are at the weak connections of the arch to two supporting wall plates and at the pointed top. The above report addresses this subject.

From looking at the photo, I believe you will find that the 1990's frame is to hold the roof in proper position, not to take over structural support.

The 1x8 and 1x4 rafter segments may be full sized, rough cut... therefore stronger than dressed lumber due to dimensions alone. Rough cut was preferred to maximize friction between the nail laminated rafter segments.

If considering the rafters as beams spanning the 1990's frame, it should be reasonable to consider them as continuous with M = wl^2 / 10 instead a simple beam's M= wl^2 / 8.

The proposed plywood / OSB roof deck will actually be a big help. In the 1930's there was some investigation of installing the skip sheathing on a diagonal. Supposedly, it helped improve performance. Plywood / OSB would be even better.

Hope these thoughts help. With today's software tools, you may be the first to truly get an understanding of how a sawed-rafter gothic roof actually works.

http://www.SlideRuleEra.net

http://www.VacuumTubeEra.net

 

[BAretired]

The 2x8@24" ties under the ridge seem to be missing on the first photo.

BA

 

[Sawbux]

Thanks, SlideRule for the long and informative post!
Over my long career, I often run into similar problems, structures that are difficult to rationalize why they stand up! Meeting modern codes and load criteria is most difficult!

Luckily, this barn is located in Western WA where snow isn't too much of an issue!

The second barn on the site was built in 1933 and uses the nailed horizontally laminated arch system you describe. This one I can get to figure somewhat close to the design loads- assuming the 12ft. support spacing of the new internal glulam arch frame. This barn is continuing in use as an educational farm with animals, so city kids can get a little appreciation of what rural life is about.
I'm trying to work up a computer model with RISA. I'll post if I can get it to work out!

SAWBUX

 

[Ron]

Is there any reason you can't remove the skip sheathing and replace with thicker, tongue and groove plywood? Fastening of any option is going to be critical to achieve a diaphragm.

 

[Sawbux]

Diaphram action is not needed. The retrofitted glulam framing has rod bracing in the long direction and arches are sufficient in the cross direction.

Major reason not to removed the skip sheeting is cost. Also it would be difficult to bend the thicker plywood to the radius = 28 ft.

We need 1/2" PW or 7/16 OSB to provide nailing for the new roofing, per the manufacturer.

 

[Sawbux]

I spent 1/2 my Sunday trying to get a RISA model to work. Along the way I learned some things, like RISA doesn't have properties for 1x members, as they are not stress rated under NDS. You have to enter Custom Wood Properties under an obscure menu location. Also the node locations for the segmented members don't line up, so you either need to "kink" the members or put in solid links to simulate the side to side nailing of the laminations.

So this morning I hand calced them as 3 hinge arches. The axial stresses come out pretty low - 120 psi with 20 psf snow & 9 psf DL.

At this point, I think I'll call it good. Rationalizing that the arches figure for the load and the new retrofit framing acts as extra insurance!