ECN Forum Forums General Topics of the Trade General Discussion Area prefab floor joists? how too?

"middle of a joist, top to bottom, is not doing any work to support the load above." Not true, all holes are reduction, but in the middle they have far less effect than a hole towards the top or bottom. Holes toward the top start to effect compression, toward the bottom start to effect tension. As holes drift up or down they have more effect by a certain factor, the closer to the edge the higher the factor, what that factor is I don't know. On these "I beam type" the majority of the tension and compression value is consentrated in the 2X4 at the top and bottom, they feel that within the web the effect is marginal, cut that 2X and you're in trouble...


Generaly holes in the middle have the least effect, until they start to get into the shear zones. The closer they get to the ends, the more effect on shear forces at the hole. They also increase in value by a certain factor the closer you get. Note the size of holes allowed toward the ends...




[This message has been edited by e57 (edited 01-07-2006).]