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Span of TJI joists w/ no center support

toycept | Posted in Construction Techniques on March 3, 2008 06:34am

For new construction, the plan includes an outside foundation wall that is 18 feet x 18 feet (9 foot foundation wall for basement). The joists that span that 18 foot wide length (to create the 1st floor) are specified as 9′-6″ TJi members. They run the entire 18 foot length without any center beam with lolly collumn. My concern is when walking around on the first floor, it will deflect and not seem solid. Can anyone suggest whether it would be necessary to have a center beam the TJi members rest on (so the actual span is 9 feet and not 18 feet)…. or do the TJi members offer enough strength so a center beams is not necessary. Thank you for your insights.

I should mention the TJi’s are spaced 16 inches on center with 3/4″ T&G CDX subfloor and 3/4″ finish floor .


Edited 3/3/2008 10:41 am ET by toycept

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  1. davidmeiland | Mar 03, 2008 06:45pm | #1

    You can go to the Trus-Joist / I-level site and download the I-Joist Specifier's Guide. There are span tables in there. 9-1/2" I joists are undersized for 18' span at 16" OC. I would probably go with 11-7/8" 230 series.

    1. toycept | Mar 03, 2008 06:55pm | #2

      Thanks for the reply, David. I'm thinking I'd rather not go with a wider joist... i'm thinking probably better to go with a center beam with vertical column support. That way the span of the 9.5" joist is only 9 feet (not 18"). I'm just concerned about the deflection.To go with a wider TJi, i'd have to start altering the foundation wall heights.. whereas some TJi's are 9.5" and this section they would be 11-7/8")thanks.

      Edited 3/3/2008 10:55 am ET by toycept

      1. davidmeiland | Mar 03, 2008 07:00pm | #3

        A center girder will make them rock solid. Is there enough room for ductwork and drains? Any stone tile going on the floor above?

        1. toycept | Mar 03, 2008 07:08pm | #4

          yes... we're planning on the drains be situated under the slab running out to the septic... and the 1st floor side of the TJi's are fine for whatever we plan on there... just finished flooring. There's a vent from a washroom in the basement that will run between the TJi's to the outside. The center girder with the 9.5" TJi's should work fine. Thanks again.

          1. User avater
            BossHog | Mar 03, 2008 07:12pm | #5

            There is absoluntely no way in hell I would ever accept 9 1/2" I-joists for an 18' clear span. There's a thread on floor vibration here that will explain why I feel that way:http://forums.taunton.com/n/mb/message.asp?webtag=tp-breaktime&msg=21010.1With a center beam they should be more than adequate, though.
            We can stand here like the French, or we can do something about it [Marge Simpson]

          2. frammer52 | Mar 03, 2008 07:47pm | #6

            again min. specs. are not adequate.  should have at the very least 12" on center if have to go 9.5.  I don't understand why people don't go to 12" more.  the cost difference is not that big

          3. User avater
            BossHog | Mar 03, 2008 07:55pm | #7

            Going 12" O.C. wouldn't help at all. DEPTH gives stiffness - Not reduced spacing.
            Middle age is when you choose your cereal for the fiber, not the toy.

          4. frammer52 | Mar 03, 2008 10:11pm | #10

            I am going to respectfully disagree.  A room that was built with 2x10 joists 16 on center has vissably more deflrction than one built with 12 on center.  I will be glad to demonstate this in the real world

          5. User avater
            BossHog | Mar 03, 2008 10:31pm | #11

            Deflection and vibration are completely different things, and are only loosely related.
            Experience is what you got by not having it when you need it.

          6. Riversong | Mar 04, 2008 03:21am | #14

            DEPTH gives stiffness - Not reduced spacing.

            Depth helps more than spacing, but they both reduce deflection and vibration.

            For instance, a 2x10 SPF 16" oc first floor spanning 12' would have deflection of 0.28" and a frequency of vibration of 14.72 hertz (greater than 15 is considered acceptable).

            Reducing the spacing to 12" oc would reduce the deflection to 0.21" and increase the frequency of vibration to 17 Hz.

            Increasing the joist depth to 2x12 16"oc would reduce the deflection to 0.16" and increase the frequency to 19.75 Hz. 

            Riversong HouseWright

            Design *  * Build *  * Renovate *  * ConsultSolar & Super-Insulated Healthy Homes

    2. User avater
      CapnMac | Mar 03, 2008 07:59pm | #8

      I would probably go with 11-7/8"

      Ditto.

      But, I'm biased towards always using a 12" dimensioned floor, even if the tables will let you go smaller.  Mostly as you get more room for mechanicals in parallel in that depth, and you wind up with fewer problems when the mechanicals run perpendicular.  Much easier to make that 4.5" hole for DWV pipe if you are two sizes larger than need

      Plumbers sometimes get grumpy if you make them make precision-drilled holes in exactly-located areas of joists.  Grumpy plumbers are not very flexible.  And for their hourly rates, you really want happy plumbers <g>.Occupational hazard of my occupation not being around (sorry Bubba)

      1. RobWes | Mar 03, 2008 08:27pm | #9

        I was told by the yard that they always go taller vs. tighter spacing.
        Nothing worse than a bouncy floor. I'd add the LVL's

      2. davidmeiland | Mar 03, 2008 11:06pm | #12

        Fully agree. When I play a plumber on TV I am much happier installing drains and traps in a nominal 12" floor space. With a nominal 10" space it can be hard to get drilled holes appropriately centered in the joists and still maintain slope.

  2. IdahoDon | Mar 04, 2008 02:08am | #13

    are specified as 9'-6" TJi members.

    Is that a typo meaning 9-1/2" or is the specification implying a beam so the lengths aren't more than 9' 6"?

    You can span that with 9-1/2" and no projection under the floor only if you use a steel beam to cut the distance in half.  Probably $500 for a beam sutible for that span and $500 in labor and hangers to make it work.

    Having used a few steel beams I'd error to the side of deeper/stronger I-joists. 18' is a long way and you might even be better off with floor trusses cost wise.

     

    Beer was created so carpenters wouldn't rule the world.

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