Old Wood Trusses Inspection - Evalutation

[RFreund]

I recently looked at a small two story apartment/residence. The building is a fairly simple rectangle with 2nd floor trusses spanning between exterior walls. It was built around 1980. There is a 2nd floor wall at mid-span of the trusses which supports attic joists. There was a crack in the 2nd floor ceiling. This happens where there is a flush header in the 2nd floor ceiling and likely overstressing the floor truss below the end of it.
However, the remainder of the second floor also slopes toward the center wall (i.e. the 2nd floor wood trusses are deflecting). The trusses are 12" deep spanning about 22.5' and deflecting about 1.0"-1.75" at the center.
There are a couple locations that I can see the 2nd floor trusses. The trusses are in fairly good condition and I don't see any issues with the plate connectors. My question:
While a repair is needed in the one location I mention above, it seems reasonable that the original trusses might be ok despite the excessive deflection.
A quick M/d = C = T gives a fairly reasonable value for compression even when carrying the interior wall. Could the deflection just be related to creep? Is the deflection reason enough to recommend reinforcing all trusses or putting an additional support below?

EIT

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[KootK]

1) What do you estimate the elastic deflection for the trusses would be based on an effective depth of 10.5" to be including a 15% bump for shear deflection? They're pretty high L/d for a floor truss even without the minor transfer load so I'd anticipate a fair bit of displacement even before creep.

2) Did you get a good enough look at the joints to be able to comment on whether or not they were tightly fabricated or full of small gaps? Depending on the fabrication method, I feel that 16" ought to be the practical limit on truss depth. When these things get highly compressed spatially, you tend to get a lot of minor gaps between members that were assumed to be in contact in the analysis such that all of the joint loads are carried by the plates until the gaps close. In this sense, creep could definitely make a meaningful contribution to the deflection that you're seeing.

 

[XR250]

Ahhhh, the 80's - worst decade for floor trusses IMHO. 22 ft. is a lot for a 12" truss. Probably meets minimum manufacturer's requirements (which suck, BTW). My experience is that a 12" truss at this span will sag considerably with very little sustained load due to nail plate creep. The crack is definitely due to the truss sag and possibly also caused by the ceiling joists pulling apart at the header owing to their likely job of resisting the rafter thrust.
Hard to fix these things once they have sagged this far.
Is adding a girder underneath near the midspan an option?

 

[RFreund]

Meant to respond to this-
Thanks for the replies. A few responses:

@Kootk:
1.) I get about 1.4375" including 15%, so a fair bit. I also found some deflection to be about 2".
2.) There are some gaps at web to chord locations. Generally the gap is between the two web members, but tight to the chord.

@XR250

Is adding a girder underneath near the midspan an option?

Yes it is. It's just odd (to me) to tell some one that they need to add new columns and a girder when the building has been around for 40 years. However, they can see the slope to the floor so they understand. I'm trying to justify it to myself.

I modelled it as a truss and found that the chords are close to their capacity (as expected). I'm not sure that I can say confidently either way. I suppose I could ask them to test a sample of the wood. Then run the analysis. Seems like a lot of effort and it might be justified, but that's what I'm struggling with.



EIT

http://www.HowToEngineer.com

 

[dhengr]

RFreund:
You are expected to be a smart Structural Engineer, using good engineering judgement, not a magician or a crystal ball type. You need to approach these types of problems in a stepwise fashion. There isn’t always a simple solution to every problem, and that’s not all your fault, but it does take some explaining. And, at each step, if there is not a clearcut indication of direction or action path you need more info. and study to narrow the direction toward a reasonable solution. These steps do not come automatically and instantaneously, and others must be made aware of this.

 

[Ron247]

Does the 2nd floor wall also support the 2nd floor rafters in addition to the joists or are there stifflegs from the ridge board to the wall?

 

[XR250]

Does the 2nd floor wall also support the 2nd floor rafters in addition to the joists or are there stifflegs from the ridge board to the wall?

Ron,

Why are those the only two options? Seems like the likelier scenario is the wall is only supporting ceiling joists and rafters are bearing on the exterior walls.

 

[Ron247]

They are not the only 2 options, just the ones I was curious about. I do not know what his roof live or snow is. I have seen rafter braces and temporary ridge board stifflegs left in place with a 20 psf RLL on a building as narrow as what RFreund is noting. Even if they are not needed, if someone installs them, they load the wall.

 

[XR250]

@Ron247,

LOL, Yes, you could read your statement both ways.

 

[RFreund]

Thanks for the replies. Wise words DH - thank you.

Here is a section:

EIT

http://www.HowToEngineer.com