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  1. #1
    Join Date
    Jul 2005
    Location
    Cumming, Georgia
    Posts
    112

    Default Modifying 2 x 4 Manufactured Floor Trusses

    I'm an engineer looking for practical advice on making modifications to existing 2x4 open web floor trusses. The trusses that are spaning the entire 25 ft. width of the basement garage in an existing home are sagging and need reinforcing. The floor system is filled with mechanicals and there's no easy way to modify or replace the trusses. The obvious solution is to put a support beam and posts down the middle of the garage, but this changes the stresses in the truss members and the plate connections. I can’t turn to the manufacturer because the home was built in the 80's, and there's no to find out who made the trusses. I’d love to hear from anyone that has found a creative way to solve this kind of problem.
    - Rich

  2. #2
    Join Date
    Jan 2007
    Location
    mass.
    Posts
    401

    Default Re: Modifying 2 x 4 Manufactured Floor Trusses

    I usually have an engineer tell me how, but seeing how an engineer is asking either plywood the sides after you take the sag out (very slowly jacking )
    Or sister with LVL's
    12 Hours is only 1/2 a day

  3. #3
    Join Date
    Jan 2009
    Location
    Richmond, Va
    Posts
    54

    Default Re: Modifying 2 x 4 Manufactured Floor Trusses

    I've seen an ad for a product that attaches to the bottom of each joist (assuming they're accessible) and allows you to apply tension along the bottom of the joist using either a wedge or a turnbuckle.
    They are steel and hang below the joist. Unfortunately, I've forgotten the name and I can't find anything with a search. I think the ad was in JLC.

    Someone may be familiar with them and know the name. I'll look in some back issues.

  4. #4
    Join Date
    Jul 2005
    Location
    Cumming, Georgia
    Posts
    112

    Default Re: Modifying 2 x 4 Manufactured Floor Trusses

    Quote Originally Posted by djawns View Post
    I've seen an ad for a product that attaches to the bottom of each joist (assuming they're accessible) and allows you to apply tension along the bottom of the joist using either a wedge or a turnbuckle.. . . . . .. I'll look in some back issues.
    Please do. That sounds *very* promising!
    - Rich

  5. #5
    Join Date
    Jan 2009
    Location
    Richmond, Va
    Posts
    54

    Default Re: Modifying 2 x 4 Manufactured Floor Trusses


  6. #6
    Join Date
    Jul 2005
    Location
    Cumming, Georgia
    Posts
    112

    Default Re: Modifying 2 x 4 Manufactured Floor Trusses

    Thanks, djawns. I've been through a year of back issues and have been Googling for hours, all with no real results. Thanks for helping an old dog learn a new trick!
    - Rich

  7. #7
    Join Date
    Jun 2004
    Posts
    581

    Default Re: Modifying 2 x 4 Manufactured Floor Trusses

    Those links are for products used on solid lumber not trusses.

    I think adding posts and modifying the trusses near the posts would be a reasonable solution.

  8. #8
    Join Date
    Aug 2009
    Location
    St. Paul, MN & Northern WI
    Posts
    267

    Default Re: Modifying 2 x 4 Manufactured Floor Trusses

    Rich:
    Given the meager info. you have provided about the real truss configurations, details, loading, etc. etc., about the best that can be offered in the way of a fix, is just add some truss de-sagger. It would probably be wise to get a local structural engineer involved, who would know what he was looking at, what to look for, and what details to analyze as part of the fix.

    You are quite right that a beam down the center changes the truss span length, but it also drastically changes the truss member and joint loadings and stresses; it can sometimes be done but requires much attention to details. The two links that djawns offered are really just variations on the same theme; with different bottom (chord) tension members, different number and length of vertical compression members (struts) btwn. existing truss bot. chord and new tension strap. They both prove that anything can be patented and sold without being particularly novel and without much real engineering to truly cover their usage. They might work in some conditions, but would be wholly inadequate, or even dangerous, in others. This scheme might be made to work in your situation, but we need to know much more about the details. 25' is not a particularly long span for 2x4 floor trusses, but at what spacing, truss depth, exact truss configuration? What are the real loads, are there bearing walls on the truss from above, any other concentrated loads, what are the design L.L’s? What are the real D.L’s, given your comment about mech. equip. Is there any evidence of truss member, joint or truss plate distress. These are all part of a proper structural inspection in prep. for developing a solution.

    As an engineer would develop this scheme: what is the tension in the bot. strap, what is the compression in the various vert. struts, what compression is imparted into the existing bot. chord, how many nails are req’rd. in the strap end connections to the existing bot. chord? You have to take great care in where you locate all of these and how they effect the existing truss. Consider the following: tie a cable to a tree and taught to the bumper of your truck; and without much lateral force on the cable you can pull the truck up an incline. The statics dictate very high cable tension, with fairly small lateral load on cable, until cable deflection is quite large. How does this relate to your problem and the links above?
    Dick Hackbarth, PE
    RWH&AI, Consulting Engineers

  9. #9
    Join Date
    Jul 2005
    Location
    Cumming, Georgia
    Posts
    112

    Default Re: Modifying 2 x 4 Manufactured Floor Trusses

    Thanks for your comments on this, Dick. The trusses are spaced 24" OC and support first floor LL of 40 psf plus dead load of 10-20 psf. The second floor is supported by similar trusses that span the width of the house (25'), and the roof trusses also span the width of the house, so the only load on these trusses is from the first floor.

    The trusses are readily accessible from the garage where there is about 7'-8" of headroom, so it's easy to work on the bottom chord of each truss. It is NOT easy to work in between trusses or on the sides of a truss because of the electrical, HVAC, plumbing, and gas lines that are in the way. It's real crowded in there.

    A few of the trusses have deflected after a slow water leak went undetected for several months, soaking the top chord of the trusses and the OSB subfloor. The wood of the trusses is sound and is now quite dry - moisture content less than 9% in all the places that were measured.

    I don't see the need to replace the trusses, but feel they need to be strengthened, or stiffened, or both. The trusses can easily be jacked back into place before the remedial work is done. As a structural engineer, I'm looking for a practical alternative to putting a girder and posts in the middle of the garage, and I thought that perhaps the guys who do framing for a living would have a suggestion or two.

    I don't think the Joist Jack is appropriate in this case, but it's good to know that it's out there. Right now I like the concept of putting a 1/8 inch steel bar on the bottom chord and attaching it with screws, but I need to model it and see what that does to the member forces. And even if that checks out, well, I am still not convinced that it will work - I've never done it before. (And even if it does work, the owner may still want to add posts to the middle because that is what he is comfortable with.)

    Please let me know if you have other thoughts on this that you would like to share.
    - Rich

  10. #10
    Join Date
    Aug 2009
    Location
    St. Paul, MN & Northern WI
    Posts
    267

    Default Re: Modifying 2 x 4 Manufactured Floor Trusses

    Rich:
    I trust you mean a 3.5" wide fl. truss made with 2x4's. How deep did you say the trusses are? What about concentrated loads over these trusses, walls, bathtubs, kitchen island. Wet wood has a lower allowable fiber stress and a propensity to creep and a greater sensitivity to duration of loading. A concentrated creep shortening of the top chord over a couple panel lengths would cause some rotation (deflection) in that immediate area. See if you can get a sense of that shortening, by measuring from butt jnt. to butt jnt. over a couple panel points. I mean the vert. butt jnt. btwn. two diag. chord members at a panel point, up near the top chord, a distinct vert. line in the jnt. You said only a few trusses really show the sagging and how does this relate to the wet spot, and what do you think the deflection is? Do only a few trusses need repair? Stretch a string 3 or 4" below each brg. and measure up to the bot. chord and the top chord at each panel point, and plot the deflected shape. Check the deflection on a few good trusses too, for comparison. Draw an accurate elevation of the truss, and if you can run it on a simple structural program to get a handle of the forces in the members. As a refresher, take a look at design with cables and the difference in their loaded shape under a uniform load (parabolic) and under a number of point loads (harped at each point load). Cable or 1/8" bar, you’ll be dealing with that type of tension member. Do some calcs. and get a feel for some of the forces involved, I suspect the end connections must be much more elaborate than the invention sketches show, and 6 or 8 screws at each end won’t do the trick. Think in terms of post tensioning or you won’t have sufficiently pre-load your 1/8" thick bar. The beam and columns at mid span are not an easy fix for your problem either, so they shouldn’t add much comfort for you or the owner. And, you as his engineer, should show him his alternatives, and explain them. Reread my first post and answer all the questions, I’m sure there will be more. What’s your min. allowable head room under these trusses, you said the bot. chord is about 7' - 8" above the garage slab.
    Dick Hackbarth, PE
    RWH&AI, Consulting Engineers

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