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I -joists or open web floor trusses

wallyo | Posted in Construction Techniques on March 14, 2003 08:55am

We are closing on a building lot today, and are starting to thing of home design.  More and more I am noticing that houses here are using open web floor trusses in their construction insdead of I-joists.  Now I can see advantages and disadvantages to both:

Open Web Floor Trusses allow you the ability to run duct work, plumbing, wiring with ease (great for multi story home which is what we will be doing) and have a wide flange, but have more joints, but they can only be trimed on site with in a few feet.  In the summer we can here our roof trusses making noise from expansion (it can be 60 degrees at night and 105 during the day).

While I-joists can be trimed to any lenght, have less joints only two not counting web splices, no order lag time, and also can be had withwide flanges, but have limits on duct work and plumbing cut outs.

The main qustion is which is quiter and offers a stiffer floor?

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  1. DaveRicheson | Mar 14, 2003 11:39pm | #1

    Both should be equaly quite. Theses are floor structural member, not subject to the temperature variances that you see in a roof. They are inside the conditioned envelope of the home. Proper specification and engineering of of I-joist or open web floor trusses has more to do whith the "stiffeness" .  Quietness of the floor memebrain has a lot to do with installation.

    Dave

  2. woodroe | Mar 15, 2003 05:18am | #2

    You seem to know some advantaes of open web joists, but there are heating limitations too. Most think it places the heating trunk lines in the floor joists.

    This can be done, but the joists need to be designed to accomodate the duct size required.

    Rim joists need to be left open to allow installation of the trunks.

    Return and supply trunks in the joists make it difficult to pass each other with individual supplies.

    The return air ducting to the trunk line must be run in pipe in the space as opposed to panning the bottom of the joists because of the open webs.

    In short, it will cost you more to install a forced air heating/cooling system, and you will probably not get both the supply and return turnks up in the joists. Pre-planning with your heating contractor is important.

    Should you ever have a fire in your home, it can spread more rapidly throughout your floor (assuming the ceiling is finished) with the open webs. In short, I would use the I-joists, and perhaps add more height to the floor below if maximising the headroom below is the goal.

    Steve

    1. UncleDunc | Mar 15, 2003 05:39am | #3

      >> Should you ever have a fire in your home, it can spread more rapidly ... with the open webs.

      FHB did a story on open web trusses, and one of the people they interviewed for the story made the same point. I don't remember the exact wording, but it was something to the effect that if he were an inspector, he would insist on sheetrock on at least one side of at least every other truss.

      1. DaveRicheson | Mar 15, 2003 03:31pm | #4

        ICC addresses this issue in R502.12 and R502.12.1. This section reguires Draft Stopping for every 1000sf of flooring that has usable space above or below the concealed structural members.

        This has been enforced in commercial construction here for many years. It may be new to residential construction, but now that it is there, maybe it will be enforced, and reduce fire spread through these open web floor trusses.

        Dave

  3. User avater
    BossHog | Mar 15, 2003 03:37pm | #5

    Lots of room for opinion on different floor systems. I see advantages to both.

    For I-joists - Lowest material cost, and they are readily available. You can generally get them from a stocking dealer in a day or 2. (This may vary in your area) They're nice and straight, which is hard to beat. Some say they don't last long in a fire, as in a basement with no ceiling.

    Open webbed floor trusses offer advantages too. They are the most expensive floor system to buy, but the least expensive to install. Sure is nice to just drop 'em in place and nail 'em down. Running plumbing and electrical through them is great. One of the downsides is longer lead time - It varies, but is probably minimum a week, and can be several. Ask around and check this out before deciding.

    As for the fire issue - It doesn't bother me a whole lot. For floor trusses to be a hazard, consider this: What are the odds that 1. your house will ever burn down. 2. anyone will be home when it does. and 3. the fire will get into the floor system and burn through it before everyone gets out. Seems pretty remote to me.

    Probably the biggest advantage of floor trusses is the one that's least taken advantage of - Eliminating posts and beams in basements. Think about how many threads have been posted here about removing posts and/or altering beams. Get rid of 'em from the start, and you'll never be sorry. This requires more depth, but is well worth it, IMHO. And it leaves more room for mechanicals.

    A sort of humorour side note - I have an Uncle who used deep floor trusses to clear span his 28' wide basement. The trusses have a nice wide duct chase in the center of the span. He finished the basement, but wanted to have access to wiring and such at a later date. So he put an access hatch in the garage, and put a plywood "floor" in the duct chase. So now he can use a mechanic's creeper to buzz down the middle of his trusses to work on stuff. (Wood heat, so no ductwork)

    One more point I want to make about wood webbed floor trusses and HVAC. If you buy the trusses and install 'em without planning ahead, your HVAC guy is likely to be pissed. Get the truss company and the HVAC guy together BEFORE anything is ordered, and plan out the duct runs. Truss companies don't automatically align webs and/or duct chases unless asked to. This can be a nasty surprise for a homeowner when their HVAC guy says he can't put the ductwork in the trusses.

    Well, I've droned on enough about this subject. Would be happy to discuss it further if you like. But one more thing - Check out the thread about Floor Vibration before you order anything:

    Best of luck with your house.

    Remember men, you are fighting for the lady's honor, which is probably more than she ever did.

    1. wallyo | Apr 17, 2003 09:47pm | #6

      Boss

      If I got things right you are in the truss business.  If this is correct I have a question for you. 

      I was checking a house out last night that is in the framing stages for the upper floor they were using open web floor trusses ( engineered I-joists on the first fl and open webs on the second are becoming more common here).  Now I don't know if your company handles floor trusses  but this is my question.  I noticed that the cord on these trusses had splices in them, Now I always thought,never actually seeing them that the cords were continious.  If there are splices how come they don't flex where the splices are?   these trusses where about 14" with flat 2x4 on the top and bottom cord.  Also the Bonus room garage trusses  had there bottom cords spliced too.

      Now if any one did this with a 2x12 floor joist it would not be allowed.  Hence the ridge beam cut short story.  Can you request when ordering them that the cords be a continious piece of lumber?

      I would think it has to do with the whole tension and compression thing but how?

      Thanks wally

      1. User avater
        BossHog | Apr 17, 2003 10:44pm | #7

        You got it right - I'm in the truss business. For 18 excruciatingly long years.

        Floor trusses are different than 2X joists. With a 2X joist, you only have one piece of lumber. You could theoretically splice it, but the splice would have a lot of bending stress in it.

        Floor trusses are typically spliced if they're over 16' or 20' long, depending on what lengths of lumber the truss supplier stocks. I've never heard of 2X4 in lengths longer than 20'. Even if they are available, 2X4s over 20' would be terribly expensive. Also - Truss plants typically aren't set up to handle lumber over 20' long.

        With a floor truss, remember that the different pieces handle loads in different ways. The top chord is always in compression. (In a simple span truss)The bottom chord is always in tension. The webs can be either tension or compression members.

        Since the chords are in tension or compression, splicing the isn't any big deal. The force in that panel is calculated, and a plate is sized to handle the force. You might notice that the splice typically isn't in the middle of the truss, but more out towards the ends.

        Attached you should see a drawing of a typical 24' truss. The tension menebers are colored in as red, and the compression members are blue. The splice locations are noted.(It's kinda crude - I'm no artist) Maybe that will help demonstrate the forces involved.

        Hopefully that explains it a bit. If not, let me know and I'll take another crack at it.Everywhere is walking distance if you have the time.

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