It’s difficult to explain without drawing a picture, and I don’t have the software to create an accurate diagram, possibly one of our resident engineers can assist with that. Moisture conditions, however, have no real bearing on how a truss will perform under load.
In an engineered truss, all members of the system (top & bottom chords and internal web members) are under compression or tension when loaded. The relatively short distances between supporting members allows the truss to span a great distance without “sagging,” but when overloaded, the compressed members will twist between supports (connecting points) until the point of failure.
Truss top chords are generally in compression. When subjected to compressive stress, wood members can buckle. The longer and more slender the member is, the less compressive force it takes to buckle.
This truss illustrates the action of the various stresses occuring along the wood members. The applied loads induce stresses and movement in the truss members. A stable truss will resist these stresses. The wood members are designed to resist the stress according to the allowable design values published in the National Design Specification For Wood Construction (NDS). NDS is published by the National Forest Products Association. Forces at the member joints are resisted by metal connector plates that are held in place by “teeth” punched out of the base metal at right angles. The plates are rated for lateral resistance (tooth holding), shear, and tension and require review and approval by each of the model codes.
Yes, ditto what Robert said. I appreciate it as well. Very good information and illustrations.
As a side note, I asked the buyers realtor yesterday what came of the roof issue. She told me that the buyer had an independant engineer take a look and it does appear to be from excess weight. The seller now has said, they will be contacting their own engineer to take a look.
I asked the realtor today to please keep me posted on the outcome. What seems a bit perplexing to me is the roof was put on with a permit. Aren’t those things taken into consideration during the City’s inspection process? I’ll post when I hear anything new. Thanks for the excellent feedback here.
No Bert, they are not taken into consideration. I did an inspection on a two story, wood frame second story home in Coral Springs about 10 years ago. All of the exterior stucco siding had buckled and was broken all around the home. The trusses in the attic looked far worse than those pictured. I told the homeowner, a Realtor I was doing a lot of work for at the time, to get an engineer and to see what went wrong. Last I heard…it was headed to court as the engineer said that the second story had to be rebuilt.
