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14" Deep Open Web Floor Truss deflecting

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?

RE: 14" Deep Open Web Floor Truss deflecting

I would strongly recommend a site visit here. I would not trust the judgment of a non professional

Mike McCann, PE, SE (WA)


RE: 14" Deep Open Web Floor Truss deflecting

(OP)
Site visit is planned. Getting information gathered before I get out to look at it.

RE: 14" Deep Open Web Floor Truss deflecting

when you have your data posted, i am sure you can get three or four workable solutions on this site including
chord reinforcements,
midspan support as you mentioned,
box beam..

RE: 14" Deep Open Web Floor Truss deflecting

What span of trusses? Does not sound bad for a 24' span. A 4' span would need to be repaired.

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

(OP)
I have been to the site and I have the truss design with grade and species.
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

I would expect a new support approximately midspan beam under a vertical web would create a situation where chord reinforcements were no longer necessary but web reinforcements may be necessary on either side of the new midspan bearing condition.

RE: 14" Deep Open Web Floor Truss deflecting

agree with Triangled. Unless there is damaged chord members or plates then the stresses will go down considerably in them.

The stresses in your web members near a new central support will increase drastically however.

RE: 14" Deep Open Web Floor Truss deflecting

(OP)
Thanks
Any idea on what could be causing the deflection?
No noticeable damage to the trusses.

RE: 14" Deep Open Web Floor Truss deflecting

What kind of trusses, ie. metal plate connected? Or trus joist style wood chords and metal webs, or some other variant?
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

Could the truss have been cambered, but installed upside down?

Mike McCann, PE, SE (WA)


RE: 14" Deep Open Web Floor Truss deflecting

In the OP you didn't mention the span of the truss.

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

(OP)
open web 4x2 metal plate connected trusses
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

For that span and spacing, assuming a 40 psf LL and 15 psf DL, I would expect a 0.25" dead load and 0.5" live load deflections.
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

(OP)
40/15 was the design loading
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

Just jacking up the truss and adding a 2x6 to the bottom chord will do nothing. You have to transfer forces to and from the add-on for the add-on to have any effect.
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

Wouldn't be surprised if the island is inducing the unexpected additional deflection

RE: 14" Deep Open Web Floor Truss deflecting

(OP)
Attaching the 2x to the bottom chord after jacking the truss up will not help.
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

Maybe I am missing something.

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

(OP)
I see where you are coming from now.
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

If these are metal connected wood trusses than this type of deflection typically occurs due to member slip at the truss metal plates connections.

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

(OP)
There are metal plate connected trusses.
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

A span of less than 20' with a 14" deep floor truss should be pretty stiff. I'm surprised that there are problems with it.

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

I would put money on construction supplies being stacked on the floor
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

(OP)
Analyzed the trusses with a mid-span support.
Mid-span support is going in with appropriate web member modifications.

Thanks for all the information

RE: 14" Deep Open Web Floor Truss deflecting

Any chance of getting some pictures of the trusses and fix, I would be interested in taking a look at these.

A confused student is a good student.
Nathaniel P. Wilkerson, PE
www.medeek.com

RE: 14" Deep Open Web Floor Truss deflecting

ya pictures always help and i would be interested in seeing the repair after it is installed. thanks in advance and good luck with everything!

RE: 14" Deep Open Web Floor Truss deflecting

Has any bridging recommended by the manufacturer been installed?

Mike McCann, PE, SE (WA)


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