Mornin! I need to price a job that has been stick built yet the are using the prefab floor joists. the ones with a piece of plywood on edge and two sticks glued to each edge. just trying to figure out my hours for this job. how to wire it. i'm pretty sure you cand put holes in them. any help would be great

There have been many discussions on this, here is one https://www.electrical-contractor.net/ubb/Forum18/HTML/000053.html

A friend of mine broad back a scrap of one form a construction site. There were these knockoutse scroed in them you just knock out with your hammer.

I don't even conceder if it has enginered joists or standard 2X when bidding. So many varebles in a job I don't think it makes too much difference.

Myself I think it's a plus when they have I-joists cause they drill fast. They don't seem to use cross braces with them here too which is on less thing that might be in your way.

Tom

Tom

At this time last year, I was wiring my first building with these things.....

I was very fortunate in that I had an absolutely free hand in deciding what receptacles went where, light placement, etc. I also, unfortunately had absolutely no information as to room layout, equipment location, etc.

This was particularily vexing, as with this type of construction- and unlike regular feaming- there is absolutely NO going back and making changes or additions later on; you pretty much only have one chance to 'get it right.' Once the drywall is up, any changes call for major, and I mean major, work.

I was lucky; my guesses were 98% on the money. The remaining two were- luck again- possible with minor work. (I was able to remove the cove base, rout a slot for the wires, and re-cover. The MC used was also protected by a length of steel channel).

Yes, some have pre-punched holes for utilities. I have found them to be tough to knock out. They do drill fast and where you drill is not as much of an issue as dimensional lumber.

One thing to watch for is the spacing. I have had to drill some that were on 12" centers. Make sure you have a right angle drill and a very short spur bit.

Silly carpenters almost NEVER put those holes anywhere close to lining up. Rare if they even think of it. Most can be drilled*, per manufacturers +/or engineers instructions. Always check with GC, and engineer if they can or can not be drilled, and for what size holes.

*Add a set of bits into the bid for the job, the glue in them trash bits like no one biz... 2 sets for a large job.

Here's a horror story about these. On a job 6-7 years ago, (When these were kinda new) the GC had a meeting about them, and said under no curcumstances should the ones on that job be drilled in any way. Everything in the cieling had to enter from the sides. (We ran a gutter down both sides and that was it for us, a pipe down each wall and a whip for each set of lights between the joists.) The building was open spanned from property line to property line, short dimention was 50', with several tons of AC equipment on the roof. The cieling had several sections with these engineered beams, and glue lams stacked next to eachother.

While I'm up in the lift at the gutters, I notice the Sprinkler guy drilling the mess right in the middle, so I yell over to him that he wasn't supposed to drill them, and we argue from two different lifts. "Just following the plans..." None of my biz, etc. The GC is no where to be found all morning. My boss shows up, and I tell him. So my boss calls him, and the GC is back in a flash. He then throws everyone off the job site, and he's hopping mad, throwing stuff. We don't go back for 3 weeks or so. When we do go back, there is a giant steel beam down the center of the building. And a new sprinkler sub... Apparently these all started to split at the 4+" holes that were drilled, but that probhably had more to do with the glue lams.

Get manufacturer specs before drilling any holes in them. Get the GC to sign off on anmy holes in the micro lam beams as well.

NJI -

Go to:
http://www.aecinfo.com/1/resourcefile/00/22/89/bcwspe22.htm
This will give information about hole location and sizing.

You can also call Boise Engineered Wood Products at 800-232-0788. They are very helpfull.

Steve

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

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?

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.