14" Deep Open Web Floor Truss deflecting
14" Deep Open Web Floor Truss deflecting
(OP)
I am investigating a floor truss that has a permanent deflection of 1/2" to 5/8". The trusses are residential floor trusses. 2x4 construction turned flat. I will be getting the web configuration and what ever info that is stamped on the truss concerning grade/species.
The owner is requesting a repair to level the trusses. He is okay with a beam at mid span of the trusses [single span truss becomes two span truss]. I also mentioned that the bottom flange could be reinforced.
The new two span truss may require a web member from the new support up to a panel point and possibly chord reinforcing as well.
There is no unusual load on the trusses from information that I have been told.
Does anyone have a feel for what may be the economical and practical solution to this?
The owner is requesting a repair to level the trusses. He is okay with a beam at mid span of the trusses [single span truss becomes two span truss]. I also mentioned that the bottom flange could be reinforced.
The new two span truss may require a web member from the new support up to a panel point and possibly chord reinforcing as well.
There is no unusual load on the trusses from information that I have been told.
Does anyone have a feel for what may be the economical and practical solution to this?





RE: 14" Deep Open Web Floor Truss deflecting
Mike McCann, PE, SE (WA)
RE: 14" Deep Open Web Floor Truss deflecting
RE: 14" Deep Open Web Floor Truss deflecting
chord reinforcements,
midspan support as you mentioned,
box beam..
RE: 14" Deep Open Web Floor Truss deflecting
The deflection could very well be within the building code limits. Cutting the bottom chord and jacking up the truss would reduce the static deflection. Creep would still occur over time.
But for the dynamic deflection/bounce an interior bearing would be the only way to limit it. IMHO
Garth Dreger PE - AZ Phoenix area
As EOR's we should take the responsibility to design our structures to support the components we allow in our design per that industry standards.
RE: 14" Deep Open Web Floor Truss deflecting
The trusses are deflected 1/2" min and over 5/8".
There are no unusual loads above.
Not sure at this time what is causing the deflection.
The floor above noticeably slopes.
If the owner signs off on a new support (beam, columns, footings), it will be below a vertical web member.
Model and run truss design. Provide chord reinforcement as needed.
If owner would prefer reinforcing the BC of truss.
Model and run truss design. Provide chord reinforcement as needed.
RE: 14" Deep Open Web Floor Truss deflecting
RE: 14" Deep Open Web Floor Truss deflecting
The stresses in your web members near a new central support will increase drastically however.
RE: 14" Deep Open Web Floor Truss deflecting
Any idea on what could be causing the deflection?
No noticeable damage to the trusses.
RE: 14" Deep Open Web Floor Truss deflecting
What is truss span and spacing?
You mention the floor "noticeably slopes"...are you meaning that one end of the truss is higher than the other or are you describing the deflection?
Is their any signs or expectation of excessive moisture in the building?
RE: 14" Deep Open Web Floor Truss deflecting
Mike McCann, PE, SE (WA)
RE: 14" Deep Open Web Floor Truss deflecting
If you get to the site, I'd be interested in seeing pics and details of the existing truss.
RE: 14" Deep Open Web Floor Truss deflecting
19.2" o.c. spacing
19'-9" span
The floor noticeably slopes due to the deflection.
Trusses were not installed upside down
no evidence of moisture
no unusual loads on the trusses
RE: 14" Deep Open Web Floor Truss deflecting
Cheep could add to the deflection if the trusses were loaded to a 40 psf/ 15psf maximum loading for 7 plus/minus days during construction.
What was the loading on the trusses at the time you saw them?
Garth Dreger PE - AZ Phoenix area
As EOR's we should take the responsibility to design our structures to support the components we allow in our design per that industry standards.
RE: 14" Deep Open Web Floor Truss deflecting
The dead load is plywood, hardwood flooring, kitchen island that is perpendicular to the trusses
I would like to jack up the trusses and add a 2x6 flat below the bottom chord
or some other way to stiffen the trusses.
RE: 14" Deep Open Web Floor Truss deflecting
One way to do that is (after supporting the truss) cutting a 1" section out of the bottom chord, at the center of the span. Jack up the truss to remove the deflection needed. Then attaching the add-on to transfer the forces.
Garth Dreger PE - AZ Phoenix area
As EOR's we should take the responsibility to design our structures to support the components we allow in our design per that industry standards.
RE: 14" Deep Open Web Floor Truss deflecting
RE: 14" Deep Open Web Floor Truss deflecting
The connection would need to be designed to transfer the bending from the bottom chord to the new 2x through shear.
Am I missing something?
RE: 14" Deep Open Web Floor Truss deflecting
How do you see just adding an 2x6 add-on is going to remove deflection from the truss?
For me you would have to shorten the bottom chord, or lengthen the top chord, or stiffen the top to bottom chords at the bearings (boxing the truss).
Garth Dreger PE - AZ Phoenix area
As EOR's we should take the responsibility to design our structures to support the components we allow in our design per that industry standards.
RE: 14" Deep Open Web Floor Truss deflecting
An elastic material that has permanently stretched on the tension face.
My thought is increasing the bottom chord 'I'.
By jacking up the member, you have removed the load.
Adding material increases the 'I'
Lower the member the load redistributes to the new bottom chord.
I could see that this might not remove all of the deflection, but should stiffen the truss.
RE: 14" Deep Open Web Floor Truss deflecting
Garth Dreger PE - AZ Phoenix area
As EOR's we should take the responsibility to design our structures to support the components we allow in our design per that industry standards.
RE: 14" Deep Open Web Floor Truss deflecting
That is one place that I did not look at very close.
If that is playing a part then the added member may not do much at all at the bottom chord, without re-doing the joints with plywood.
RE: 14" Deep Open Web Floor Truss deflecting
The question in my mind is - What caused the deflection? I have seen lumberyards deliver lumber packages and set it in the middle of a floor. If someone set a bunch of wall framing material in the middle of the floor after the sub-floor was on, that might have caused permanent deflection.
Of course there's not way to know if that happened or prove that it did. Everyone will just deny it.
I agree with those that say adding lumber to the BC will do nothing. It might make it a bit stiffer, but will not take the deflection out.
I think your only option is to add bearing under it to bring it back in line.
RE: 14" Deep Open Web Floor Truss deflecting
But I also wonder how much the island weighs?
How much flex is there from live load (walking)?
I learned the hard way years ago that sag and flex that meets code is very unacceptable in a large room. I recall a 30' wide living room that no one would walk across because you could feel the flex. We measured with and without loads and it was well within code. We still ended up stiffening it.
= = = = = = = = = = = = = = = = = = = =
Plymouth Tube
RE: 14" Deep Open Web Floor Truss deflecting
Mid-span support is going in with appropriate web member modifications.
Thanks for all the information
RE: 14" Deep Open Web Floor Truss deflecting
A confused student is a good student.
Nathaniel P. Wilkerson, PE
www.medeek.com
RE: 14" Deep Open Web Floor Truss deflecting
RE: 14" Deep Open Web Floor Truss deflecting
Mike McCann, PE, SE (WA)