Dave Gaines has already given you a very good response. In addition, you might want to reference the following:
Evaluation, Maintenance and Upgrading of Wood Structures, ASCE.
NAVFAC MO 111.1, Inspection of Wood Beams and Trusses
(They have a lot of good photos)
Historical Considerations in Evaluating Timber Structures, Forest Products Lab
I had to evaluate an old bow string truss many years ago. The split rings were corroded, and loose. But the truss held while the connectors were re-tightened.
Subject: RE: Failure Modes for Split Ring Connections in Heavy Timber Trusses
Date: Tue, 22 Jan 2008 11:45:03 -0600
Where is the heavy timber truss project located? It sounds like you're seeing rust on the bolts so you should expect the same corrosion on the split rings.
I don't see how anyone can inspect for rust or dry-rot on the inside of a multi-layered wood connection without taking it apart. Fungus dry-rot will also result from exposure to moisture. Sudden collapse can occur if the split ring connections have deteriorated due to either rust or dry-rot. We've seen many examples of this here on the west coast. Often times it occurs when the loading changes due to roof construction, remodeling, during heavy snow loads if you have that in your area or during rainfall if the roof slope or drainage has been compromised.
I've seen lots of truss failures from my work on roof collapse projects. A few years ago I worked on a commercial building with split ring connections in a folded plate roof that collapsed here in Southern California. The bottom chords failed during a heavy rain storm. The loads increased because long term deflection caused ponding on the roof. When water collected on the roof the loads increased and the bottom chords failed suddenly, collapsing large parts of the roof. The owner nearly lost some of the concrete tilt-up walls that the roof was attached to. There was no warning that it was going to collapse but there were other signs the owner should have been checking that could have saved his building from collapse. The roof had been leaking around skylights over a long period of time, causing water to run over the split ring connections. The long term roof deflection was visible from below which should have driven an inspection of the roof slope and drainage. Although the roof was over 20 feet above the floor, a closer inspection of the wood should have been performed. Still, the dry-rot we found on top of the roof and inside the 4-layer split-ring connections was not visible until the roof was removed. When the split rings were eventually disassembled, rust and dry-rot was evident in many of the connections where the roof had failed.
With your project I would suggest going to the expense of shoring the roof and opening up some of the tension chord connections. It's a lot less expensive than repairing the trusses after a roof collapse. Some investigation could be done to find likely candidates for corrosion or failure. Inspection of the wood timbers and connections for water stains and signs of water intrusion should be performed. You can look for outward direct or secondary indications of any slippage or displacement of the connections. The roof slope and deflection should be checked for signs of excessive deflection. Diagonal and compression chords of the trusses should be inspected for signs of slippage, cracking, crushing or other failures at the connections. Tension chords should be checked for signs of splitting longitudinally or cracking across the plane of the bolts.
Another Structural Engineer here in Southern California who is highly qualified at inspections of heavy wood timber trusses is Steve Provenghi of Mackintosh and Mackintosh. Steve's office specializes in wood truss repair, fire damage repair and roof collapse repair. Steve performs forensic engineering and his office also designs structural and architectural repair work on commercial buildings. His number is (323) 662-1184.
Good luck with your project.
Dave Gaines, P.E.
Structural Project Engineer
HDR ONE COMPANY | Many Solutions
251 S. Lake Ave, Suite 1000
Pasadena, CA 91101
From: Felker, Brian P CIV NAVFAC Lant, CISE [mailto:brian.felker(--nospam--at)navy.mil]
Sent: Tuesday, January 22, 2008 6:56 AM
Subject: Failure Modes for Split Ring Connections in Heavy Timber Trusses
Anybody know of a good source of information about failure modes of split ring connections in heavy timber trusses? I am dealing with a WWII-vintage historical wood structure, consisting of enormous wood trusses, with split ring connections. We have reason to suspect corrosion of the split rings may be occurring, since the environment is humid, and the wood was salt-treated for fire protection, and since we have detected severe corrosion of the bolts within the split ring connections. Unfortunately, short of spending thousands of dollars in shoring, there is no way to get a direct visual inspection of the split ring connectors. A firm I have been in contact with claims they are able to perform visual inspection of the joints (without disassembly), and detect whether split ring failure is impending. Being unfamiliar with the modes of failure of this type of connection, I am not sure how to evaluate their claim. Can a ductile failure be anticipated? With plenty of warning signs? Or could failure be sudden, without apparent warning? Is there some useful body of information on the subject?
Brian P. Felker, P.E.
NAVFAC Atlantic CISE
Ph: (757) 322-4436 Fax: (757) 322-4416
Helping your favorite cause is as easy as instant messaging. You IM, we give. Learn more.