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Originally Posted by Delphinus
I don't think an I-beam has more "rigidity" per se than a equivalent sized rectangular solid beam.. My understanding (which is very limited) is that an I-beam has similar load capacities (depending on the direction of force, which is either vertical or lateral) than the equivalent solid shape, but requires far less material. I.e., it's a more efficient structure in terms of function per unit mass, but that doesn't necessarily mean it's more rigid or has more load capacity overall. Plus, an I-beam is made out of "engineered lumber" whereas a 2x10 is just a piece of wood. You can make an I-beam based on byproducts of other products, but you need a tree for a 2x10, sort of thing.
(Maybe Carpentersreef, if he's out there, can elaborate.. Mitch??)
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As far(or short) as I have learned, Tony you are pretty much right on the money. The load carrying capacity ( or ressistance to deflection) of the member has a lot to due with the cross sectional shape. The taller the shape is the more strength it has against this type of load in this manner. The reason it's shaped like it is, is like you said, to save material cost. The web part doesn't do much other than hold the top and bottom flange together. In this case the top flange is under compression and the bottom flange is under tension. And blah blah.
In short form;
-in the olden days we used say 2x8s. Since they are a tree, lets apply a factor of safety of
2.5 when designing.
-wood-I's are a little more predictable. Using typical living space loading values and predictability we can save some money and use a safety factor of say
1.5. Floor deflection doesn't mean structural failure but mearly a little discomfort to an allowable level.
I do though agree that when these wood-I's were interduced they were spanned way too long. Presently, like Jon stated, I can feel the beasts walking around in my house and the when the spin cycle starts I don't have to add coins to my vibro-sonic mattress. Saves $ and backache.
Kari