1950's Bowstring Truss Problems 2

[msquared48]

I have a bowstring truss project from a year ago (circa 1955) that is resurrecting itself and ran across a good technical paper on the web regarding the inspection of, inherent problems, and the repair of these trusses at ...

http://ascelibrary.org/doi/pdf/10.1061/(ASCE)1084-0680(1996)1:1(25)

Since we all like to share here, and sharing is good in analyzing problem children, does anyone have any direct personal experience on any special designs or fixes that may not have been addressed in the above, and any similar articles?

And they are problem children.

Mike McCann
MMC Engineering

 

[MikeHalloran]

Sorry, no ASCE library card.

On a distantly related note, just this summer I drove my Mountaineer across the Hadley (NY) Parabolic Bridge, only to find an informational placard on the other side noting that it was closed to all traffic.

Of course, the placard is seriously out of date; the Wikipedia article is less so.

http://en.wikipedia.org/wiki/Hadley_Parabolic_Bridge

Mike Halloran
Pembroke Pines, FL, USA

 

[XR250]

I worked on a building that had all the common problems - moisture damage and connection and member loads significantly greater than reasonable.
They had all sagged and had been shored a number of times.
With the owner's budget, further shoring was the only repair option.

 

[ornerynorsk]

Mike H, the lucky stars shine upon you!

From the article "In 2006 it was reconstructed and restored with federal and state grants, and reopened without any load restrictions."

It is better to have enough ideas for some of them to be wrong, than to be always right by having no ideas at all.

 

[Sawbux]

I have repaired a number of these trusses. Common problem is failure of bottom chord. As mentioned in summary of the publication, the tension properties of wood were over-estimated until recently. Also there were problems with mis-graded wood. 1:12 slope of grain was a common limit and also max knot sizes. Also the strength of connections with large diameter bolts (1-1/2") was over estimated. Connections with long, stiff gusset plates also fail due to un-anticipated rotation in the joint - leading to prying and splitting cross-grain. Seasoning checks and splits are also a problem. Many buildings were thrown up (especially in WWII) using green lumber.

Many of my repair designs involved installing cables and pretensioning to remove dead load. Ends were attached to existing heel joint straps or saddled around the ends of the truss bottom chords. Damage to bottom chord and other members were repaired with epoxy and/or sistering reinforcements to the sides.

 

[MikeHalloran]

Re Hadly Parbolic Bridge, I found some photos of the "reconstruction" showing some serious rolled beams under the new deck, concealed by the truss, which must now be largely decorative.




Mike Halloran
Pembroke Pines, FL, USA

 

[msquared48]

I found this too, and thought it was a good summary...

http://www.westernwoodstructures.com/index.php/white-papers/bowstring-trusses/

Mike McCann
MMC Engineering

 

[msquared48]

And this one:

http://www.google.com/url?sa=t&rct=...dGKmTmP8Ee1EvQSjXlrjdFw&bvm=bv.51773540,d.cGE

Mike McCann
MMC Engineering