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

e57...as you said to note the size of the holes allowed toward the ends...I'd ask that you look at the size of the holes allowed as you get closer to the middle. They increase usually by 1" diameter for every foot you go away from the ends. And as you correctly state: the top is in compression and the bottom is in tension...this means that somewhere between the compression and the tension there is zero stress. That is why you can drill larger holes toward the center. Having worked with engineered lumber for about 20 years, I'd say to check the manufacturers specs for where holes can be drilled and more importantly ask the GC if the design for your particular building has any specific engineering preventing you from following those manufacturers specs.

Be careful with generalised statements about the bending stresses being 'maxima at centre span' and shear force being 'minima at the supports'. That's only true for beams or joists simply supported at the ends. It certainly is not the case for cantilevers, or overhung joists, or where intermediate props (like a studwall) are introduced or where point loads are applied, or where the ends are fixed, or where the beam is considered continuous, [ie. with multiple supports].

Alan

"I'd ask that you look at the size of the holes allowed as you get closer to the middle. They increase usually by 1" diameter for every foot you go away from the ends."

Exactly - the amount of material you would be allowed to remove would increase toward the middle, and decrease toward the supports. So as not to dramaticaly compromise the shear value requirements that would be higher toward the ends. I have talked to alot of people about this... (In fact I ask every Structural Engineer I come in contact with the same line of questions, as I am facinated by the slight differences in opinion.) The idea that there is a zero area between the top and bottom areas that does little work is being dispelled by many people as "Olde School", all removal of material is a reduction to some degree. Imagine these I beam types as a trelessed bridge, with many cross members between the top and bottom. Imagine your hole is where two of them cross. Your hole is removing both of them. You are actually removing and reducing four areas of web tension from the top across to the bottom.

Some of the differences in engineering opinion that I find funny are with dimensional lumber and Glue/micro lams. As tension and compression are not consentrated in the top and bottom edges. Tension and compression values are more spread out in the body of the dimensions of the material. And both also have web tension values, and shear zone areas. The differences in engineering opinion that I find funny is how that web is formed, and the importance of holes in the inside 3rd as opposed to the outside 3rd's. That whole "3rd's rule" that has been drilled around for so long.

Here are the differences:
"Olde school", imagine the most important web tension from the top of one supported side to the bottom of the other side, so the web is one big X across the whole span, making large holes in the middle 3rd a tabboo.

"New skool" sees all spanning structers in a simular light, with many smallers X's from top to bottom, but with more importance toward the edges, top, sides (shear) and especially the bottom. (like that drawing I made) The reasoning for this is that they see wooden structure (Dimensional lumber and glue lam types) as being more dynamic, as the bonds of the grain of wood do not actually stretch from one corner to the other. But more closely together.

A compromise to this would be to split the span in fifths and avoid the two outside shear areas, and the center fifth. Like I said, I have an engineer friend, who likes to BBQ... Many drunken nights of talking shop.

Alan, very true, cantilevers and such reverse everything to some wildly varying degree. Tension and compression make wide sweeping arcs over the support and move shear area toward the center of spans. Several hundred years of engineering can not be expained simply... By me, (Not an Engineer) or any one else.



[This message has been edited by e57 (edited 01-08-2006).]

Another useful model explaining strength principles is your "Sunday Paper."

Stand it on edge, and it supports very little. Roll it up, and it supports a lot.

The difference is in the rigidity. Form it into some sort of shape, and the shape is much harder to distort. leave the sheets straight, and it doesn't take much to get that bending started. And, once the bend starts, there goes the strength!

With this model in mind, it seems pretty clear that the "important" area is the area where the bend is. The bend in a formed steel purlin, the "intersections" in the web of an I-Beam, the flanges of an I-Joist....you get the picture!

Right at the dawn of "craftsmanship" [c.2550BC], was erected the Great Pyramid at Giza. 1000 tons of cut white-limestone blocks were hauled up every day for 16 years during the construction. In the central chamber, intended for King Khufu's [2606BC - 2584BC] sarcophagus, the 'roof' [ie the top-half of the pyramid!] was supported on massive 50 ton slabs of granite hauled up from Aswan. 5 of these had been exactly inched into place, when one of them developed a small crack, due to excessive bending stress. To reduce the load imposed on this beam, Khufu's "engineer" laid further sets of similar slabs over the first, 4 layers in all, then over that an angled layer to direct force to outside of the slab-beam assembly. A 'laminated beam', built 4500 years ago. And, it's got holes in it!, in the form of 3 ft crawlspaces beween the layers. This whole remarkable structure was not exceeded in height until the Eiffel Tower was built in the 1880s, even though literally millions of 2-ton limestone outer casing blocks were robbed-out to build Cairo in the Middle Ages.

Alan

Alan, is this about my "hundreds of years of engineering" comment?

For a thousand years or so after the fall of the Romans the formula for concrete and mortar was lost.

Craftsmanship, and engineering science have died several times. And fortunatley so, I would hate know the punishment for failing to head the engineers warning on the drilling of "the stone lam".

bump

Thanks! Knew it was here somewhere.

-Hal

"They take longer to staple wires to if you are running the length of the thing. Lots of bounce back."

Mike, I got around this by stapling into the top of the bottom 2x, or on the bottom of tyhe top 2x. Since they're 1.5" thich, they're out of (legal) danger of being hit by fasteners.