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RE: Manufactured floor trusses

Posted by TomR on May 31st, 1998 05:20 AM
In reply to Floor trusses by Ron Verbos on May 29th, 1998 10:08 PM [Go to top of thread]


There are several types of manufactured beams and trusses. The ones that come to mind are wood I-beams, paralams, bridged trusses. Iím sure there are more, but youíll get the idea with these descriptions:

Wood I-beams look the shape of an "I" at cross-section. The center part of the "I" is usually plywood of different thickness and widths, depending on its load rating. The top and bottom portions are about the size of a 2X3, with a groove cut in one of its long sides. The groove is filled with an adhesive, and the long edges of the plywood are fitted into the grooves. The finished product is essentially a wooden version of a steel I-beam, if you know what that is.

Paralams, or gluelams, are laminated layers of wood. They can be made like plywood, only much thicker, or from little pieces of wood strands, mixed with glue and pressed together.

Bridged trusses look like two 2X3ís or 2X4ís on their sides parallel to each other, with steel or wood braces/webbing between them. The resulting product looks much like open steel girders on many bridges, where two steel beams have a pattern of X-braces between them.

Being manufactured, things like straightness and strength can be predicted and controlled. They are generally regarded as stronger than dimensional lumber of equal size, and can span longer distances between support points. Paralams can be sized for 40+ foot spans. They are solid and thick, and would loose their integrity if holes were made through them for plumbing etc.. Therefore they are usually fitted in main support beam applications.

The other two examples are usually found in floor joist applications. Wood I-beams could also be engineered for 40-foot distances, but the end product would be so thick as to be impractical, so they usually donít fill that role. Same goes for Bridged trusses. The I-beams, depending on brand, often have hole knockouts. The bridged trusses ore somewhat wide open, and make the easiest plumbing routing you could have of either manufactured trusses or dimensional lumber. I think the manufactured stuff costs more, depending on area. I say "I think," mostly because the price comes down all the time as demand causes more companies to manufacture them. I think ultimately they will save money in the building of the house, not to mention providing a more complete and efficient use for wood byproducts. Some areas (maybe yours) may be more into using them than mine.

Once 200 years ago, the building material of choice depended on what was growing/lying about where you wanted to build. If that was wood, you probably had to cut the trees yourself. There was probably not much available in the way of information on calculating loads then, and if there were, you probably did not know how to read anyway. So you used as much wood as you could possibly get, any size, any shape.

With the advent of load charts and dimensional lumber, the quantity of lumber needed could be calculated, with predicted and consistent results, and wood was less wasted in the building process. Your "New" type of floor trusses is another step to improve on that plan. As far as how long they will last, well; there has been questions on the adhesives/laminating material used, and whether it will break down over time. On the other hand, I know builders who would rather use laminated beams as the main beam instead of steel because in a fire the steel is quick to warp, and the wood beams have a better fire rating. Also, truss floors have the advantage (unless installed poorly) of being very quiet. Read: No Squeaks.

I have worked on several 200+ homes...solid as the day they were built. I suspect these engineered beams may not be holding up 200 years from now, but I canít say that modern houses of any material combination would be much better. Not to worry, though. Either one will probably outlast you and me.

Happy house hunting - TomR

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