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shoring an open web floor truss

shoring an open web floor truss

shoring an open web floor truss

(OP)
What's a biomed guy doing in here? (hopefully not getting into trouble)...  I have a 2 story home with open web floor trusses spanning 40ft bearing on front and rear walls; 24"oc.  Trusses have settled (26yrs old) onto an interior nonloadbearing wall.  Deflection of 2.25in now exists midspan on second floor over 4 trusses with drywall cracks and separation along 2nd floor ceiling and buckling/pop-up of 3/4"TG subfloor.  I glued&nailed 1/2" plywood gussets over front 26ft last summer from below(remaining 14ft inaccessible)but problem has progressed since.  Can I hang a beam under the bottom cord of the gusset section and jack-up sagging members or will I need to put additional vertical bracing between top and bottom cords at the site of the new beam.  Section is over a garage so posts/footers not a problem.  It's been a while since eng.mech. course but I do know bottom cord isn't designed for vertical forces.

RE: shoring an open web floor truss

My gut feeling is that these trusses were designed with the intent to bear somewhere along their 40 ft length.  It's just not plausible to me that an "engineered" product would be so grossly underdesigned. What's the depth of the truss?  Is the 2.25" deflection occuring on the trusses bearing on the wall?  ...or on trusses that don't have this wall below them (you say only 4 trusses)?  

Most likely you'll need a vertical member at the concentrated load of the support....

I'd talk to a local truss fabricator and see if they have someone who can come out and take a look.

RE: shoring an open web floor truss

Often what happens is truss designers assume all walls are load-bearing, or that they can bear wherever they want to make their trusses cheaper.  Perhaps that is what happened here; the truss desiger assumed the non-load-bearing wall was load-bearing.  Whatever the cause, it sounds like you definitely have a problem that need fixing.  That may involve adding a beam somewhere or stiffening the truss itself, but you will have to follow a complete load path to the foundation, whatever is done.

I would hire a structural engineer to provide recommended fixes and then adhere to them.  You wouldn't want to have that liability on your head should something go wrong, and if you want to sell your house down the road you will need a professional fix anyway.

RE: shoring an open web floor truss

You need more help than can bee given here.  Consider having a structures person come and evaluate your problem.  Your span (40')is very long for a residential structure.  I suspect there was a load bearing wall than was removed.  The deflection is too high and your flooring system is showing distress, structural failure is possible.  

Typical max truss delfection I use is limited to L/480 or one inch under live and dead load, you indicate 2.25 inches under dead load.

RE: shoring an open web floor truss

(OP)
Thanks for the thoughts.  My plan is to cut into the 4" garage slab and pour footers to support posts up to a new beam.  I will jack up the affected trusses (slowly) and place the new beam midspan.  My biggest concern is transmitting the vertical load to the top cord.  Are 1/2" plywood gussets on either side adequate or do I need to scab in a 2by between TC and BC?  To answer the posted questions... the maximum deflection is midspan and not positioned over the existing downstairs wall.  Truss depth 16".  Lots of construction going on in SW FL; after making many calls over several months I have not been able to get a structural engr to come out and look.  

RE: shoring an open web floor truss

I'd be careful about putting a new beam at midspan. There might be a better location for the interior support and/or the web reinforcing might need to be carefully considered. Conceptually the idea might be fine but the devil is in the details. Get a structural engineer who is familiar with wood truss design to evaluate the web conditions and to find the best location for interior support.
If, as was previously mentioned, the original designer assumed interior walls were bearing, then those wall locations would seem to be the best locations for introducing support now.

RE: shoring an open web floor truss

Agree with Houseguy.  The truss should have a bearing interior point.  Their typically is a vertical leg compomonent (double blocked and adjacent 45 degree web components.  Often the interior is both designed for bearing (down load), and wind loading ( up-lift load).   There are many components in the design to consider:  Foundation, truss bearing points, jacking points, uplift connection, connection, beam size, beam support.  

Hope this is helpfull  
Boo1

RE: shoring an open web floor truss

This is probably too late, but never hurts to add more information.

I would contact a truss plant and talk to them as if you are building a new home. Tell them the span you have (40') and where you are proposing the beam. I am still surprised that 40' floor trusses are 24" o.c. Any good designer would have noticed the span and NOT just design with ALL walls as being bearing (per UcsfSe). I designed roof and floor trusses for 10 years. The main detail I was taught was to insure there was a beam or post or concrete wall under any bearing point. Now maybe the orginal plans called for a beam and posts, but the builder decided to eliminate them to make the space more open, and failed to inform the truss designer.

Anyway, I would definitely contact a truss plant. Talk to them and find out your options. Most plants might even put you in touch with an engineer.

RE: shoring an open web floor truss

The best advice given above is to hire a structural engineer experienced in wood design.  

If you contact a local truss company after describing your problem you might ask them to reccomend a structural engineer.

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