Guide for floor truss repairs? 1

[kbsooner]

Got another one to bounce of you all.

Ran into a spot in an existing town home with 18" plated connection floor trusses spanning +/-24 feet. About 10 foot in, 2 of the trusses have web blockouts about 24" wide with no diagonal members, I'm guessing for mechanical, but there is nothing running through them. Right at the edge of this blockout the trusses have deflected about 1", causing a bow in the ceiling. See pic, red squares are the blockout, arrow at max deflection. I'm going to attempt to contact the truss mfr. in the morning, but I am already thinking of a fix if I don't get cooperation on that end. The owner is going to do minimal jacking, but is concerned about finishes on the floor above. So the goal is to push up as much as possible and reinforce the trusses from further deflection.

I can check the chords to make sure they are adequate, but I am sure we need to reinforce the panel without a web member as the current configuration is bending the chords. Is the easiest/fastest solution to use plywood each side, glue and screw to the chords? Any type of guide out there for this type of fix? Anyone else ran up against this and have opinion/thoughts on this?

 

[SlideRuleEra]

Kbsooner - That truss has significant deflection for a reason, and I doubt that it is totally because of a couple of missing diagonals.
An 18" deep truss spanning 24' has a reasonable span to depth ratio of 16. Assuming the trusses had no deflection when installed, deflection seems high at L / 288, with minimal load.

In the photo there appear to be vertical members left of the area of concern. Do these go the roof members, possibly applying an unplanned for point load to the truss?
If this is true, an educated guess could be that the builder may have (incorrectly) though that roof members would help "hold up" the trusses where diagonal members were removed. Instead, the roof is loading the truss... just speculation.

If, you have not already done so, suggest that you compare deflection of the two altered trusses to those that have not been altered to determine the extent of the problem. Also take a look at deflection at other point on the trusses, say at 1/4 span and 3/4 span to get a more complete idea of what is going on.

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[kbsooner]

Great points. I just got back from the site and can confirm there is a partition wall above this location that more than likely is tied to the floor trusses at the floor above that could be imparting additional loads as you noted. These loads would apply to all the trusses in this room, and the trusses with full web configurations either side of these two have minimal deflections. Additionally further investigation today I noted L/216 that the span of these 2 trusses is actually 18', so we are looking at L/216, pretty high for a 18' truss and I doubt meets the spec they designed for. I am guessing it is experiencing more load than anticipated, the original drawings show the truss was designed for 65psf total load. Just looking at the deflected shape in person it is obvious the max is at the start of the open web panel. I'm not well versed in TPI truss design, but these open panels give me pause.

 

[SlideRuleEra]

Kbsooner - You mention that the two trusses have an 18' span. Are they 18' trusses... or are they 24' trusses that have an intermediate bearing at 18 feet? It makes a big difference, and does not sound good either way.

A true 18' long truss (18" high) has a span to depth ratio of 12, and deflection should be truly minimal... which, at 1", it is not.

If the trusses are 24' long and are NOT designed for intermediate bearing at 18' that creates another problem. Point overload at the 18' location.
Even if they were designed for bearing at 18', the 1" deflection is troubling. This is because any two-span, continuous member is much "stiffer" that a simple span member and should have minimal deflection at any point.

You seem to be concerned that the deflection is localized to ONLY the area where the diagonals are missing. IMHO, that is not the way deflection "works". A structural member tends to deflect along it's entire length, not just where there are local defects.

I do believe you are on the correct path to a solution by reinforcing the trusses with plywood. However, if the loads on the trusses are excessive, this may be required for essentially the entire length of the trusses, not just the area where the diagonals are missing. Also, this could apply to all the trusses, not just the two in question - this is another reason to try to determine the cause of the loading.

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[kbsooner]

sorry my statements were written quickly. Truss is 18' simple span, no midspan support. Max deflection is actually measuring out at 3/4". The max deflection is occurring at the open truss panel, which also is at center span of the truss, so it is deflecting over the entire length, but it does have a noticeable inflection point right at the node. Will investigate further on the loading...

 

[woodman88]

Floor trusses with chases work great for uniform loads and not so good for concentrated loads.
Floor trusses deflect due to loading. As a floor the loads mainly come from above, I would look at the loads to the floor. The partition wall above should not effect the chased trusses unless it is only over those two trusses. But the possibility that a large load was placed over those trusses at the wall, such as one side of a water bed, an large aquarium, etc. Could cause the deflection over time.
What I think I see just to the left of the chase is a run of floor truss strong backs (e.i. the 2x members running across the trusses and should be attached to the verticals in the adjacent trusses and to the verticals attached to the side of the chased trusses). The dark one appears to be flush to the top of the bottom chords in the picture. But the lighter one seems to have a gap between the strong back and the top of the chased trusses bottom chord. Which means there was a failure of the strong back system for the floor trusses.
As for repairing the trusses 1) did they fail due to one or two panel joint(s) failures. Or 2) due to general plate/web deflection at each joint?
If 1) then you should be able to just repair those joints in the trusses. If 2) then you may be looking at boxing then trusses with plywood/OSB each face at the ends and center of the trusses.
In both cases when jacking up the trusses. I would have them jacked at least 1/4" higher than the adjacent trusses for the repair. But the actual amount of jacking is a experience vs guessing matter as how much the truss will deflect to engage the plywood/OSB and nails depends on many factors.

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.