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

e57,

I still have a lot to learn about the manufactured products, we had a call from a GC this past spring, can you please meet me at the job site, i have a problem with my electrician, when we get to the job site, the GC meets us, and takes us to the basement, look at what this idiot did, at this point i really din't understand the amount of damage done, and as he was talking to us, he went to his knees, and had trouble breathing, we left just as the medics were taking him away, i think this is a great topic, and would sure like to get more information on how others, deal with these products.

Here is another link to TJI installation instructions. http://www.trusjoist.com/PDFFiles/2035.pdf . This file also shows where holes can be drilled in LVL microlams.

Chris

[This message has been edited by raider1 (edited 01-06-2006).]

It sounds like some are talking about microlam and others are TGI's here.

Tom

I'm taliking about engineered lumber, technicaly all spanning structures. Most of the engineered stuff acronims are really brand names by the manufacturer.

LK was it your guy who did the work? If the guy had a bad ticker, it was only a matter of time, the straw that broke the camels back may have actually saved his life. He could have been home in bed for that eventual heart attack...

Other glue lam stories:

Security guy drills a 1/4" hole on a 12X8" lam, engineer refuses to sign off until replaced at the tune of $10+K BC.

Plumber took a notch on the side of one to get toilet to go where shown, same results...

I don't let new guys near them! And everytime I see one I brow beat it to death. Lams are an item unto themselves...

The original one we were discussing with the web between 2 2X's can be just as much hassle depending on how much the engineer is tightening the belt.

Over the years I have had several conversations with engineers about how and why these are used, sometimes with varying result. And I have a good friend who is an engineer for the Golden Gate Bridge who summed the deal out best... "Some people like to use them as a replacement for steel." Esentially you can span wider than with dimentional lumber, and its lighter than steel would be, so you dont have to over build to support the wiehgt of the span. He said most often people who do that will use them at full or close to full strength values. Holes in any lumber reduce the bearing values, and how big the hole is, and where the hole is reduce the value by so much... He also mentioned some good clues to question weather or not the Engineer was of that frame of mind, "replacement for steel"...

• Combinations of lams and wood beams used together.
• Multiple wood beams set in patterns close together.
• Or exessively wide spans.

The way all spanning structure works, is that the top portion must have a compression value, and lower portion a tention value. Each area near the supports on the ends has a shear value. And any portion within that rectangle is what he reffered to as "webbing". And any hole in the web reduces the load bearing value in multiple places at varying degrees along the span. Combinations lead to higher reduction values.

Doh, I need to draw this... See drawing (Improved, but loads slow.)



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

Not our job, the GC wanted another electrician to do the job right, so he called us , it ended up i recommended another EC, that did a lot of work with manufactured beams, and yes it was a good thing, he got help early on, i met him at post office last week, and i could not belive it was him, many pounds lighter.

Nice drawing

[This message has been edited by LK (edited 01-06-2006).]

Thanks about the drawing... I figured I need to print this up, as I seem to draw it everytime a get a new "new guy", as they seem to butcher regular lumber too.

Oh and, yes, equally spaced holes add a certain factor to reductions in the web as they can double up.

For ease in stapling I use one of the Desa PowerFast staple guns.

I like the drawing, but would ask why drilling in the neutral plane of a TJI would have any more affect than drilling a hole in the same spot of dimensional lumber. I was told early on that the middle of a joist, top to bottom, is not doing any work to support the load above. However, if you drilled the same size hole near the top or bottom chord that you had effectively diminished the depth of the joist and its corresponding strength.

Here is another link to one of these discussions. https://www.electrical-contractor.net/ubb/Forum18/HTML/000022.html

"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).]

Oh yes, as for cable supports, the best method I know so far is a stacker or strap with a screw... Like those zip ties with the hole for a screw. Even if you get a staple in they have a tendancy to not hold well.

Cable supports, are another concern, how to support along the beam, screw, and strap seems to hold better then a staple, what other ways have been used?