Truss Stability

[tehparadox86]

I have a friend who decided to put holes thru the sides of his truss to run cabling thru and now he does not want to remove the cabling since it's installed. He's had a structural engineer look at it and the engineer told him to remove the cabling and scab the sides of the bottom chords with 4x4. I'm looking at scabbing the tops and bottoms of the bottom chord to allow the cables to remain in place. Thoughts or any other recommendations that I can give to the engineer to see if his calcs will work?

 

[jayrod12]

How large are the holes?

You shouldn't necessarily go against the engineer, but more ask him to work with you for an alternate solution. Scabbing above and below could work, but you'll run into issues where the web members come down.

And if scabbing above and below is acceptable, why couldn't your friend have just run his cabling above the bottom chord instead of drilling through.

 

[tehparadox86]

The holes are 1.25" in diameter. He didn't think it through when he made the holes and just installed the cables thinking it was not a big deal. I was upset that he didn't just build a platform on top of the bottom chords, but now its too late.

 

[tehparadox86]

Also, I'm not an expert in structural design, but if I used a large enough member to scab the top and bottom chords, would my main design concern be to make sure the section modulus is large enough to compensate for the loss of area from the hole?

 

[JAE]

Not a section modulus issue (i.e. not a bending issue really).
This is a bottom chord tension member and your member area has been reduced by almost 36%.

So the trusses are very compromised.

You could sister on a steel strapping on the bottom of the chord - the concern is the nailing attachment to the wood at the ends of the strapping to take the tensile load and detour it around the compromised sections of chord. (strapping direct to the wood and NOT through any ceiling sheathing.)



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[tehparadox86]

JAE, can you show me an example of these steel strapping to be used?

 

[jayrod12]

Simpson strong-tie makes many straps that would be useful in this scenario. Likely the coiled strap as it comes in whatever length you need. As JAE alluded to, the amount of tensile resistance might be tricky to provide. And the engineer would still need to ensure any bending capacity left in the bottom chord is still adequate for the ceiling loads it supports. It is mostly a tension member, however there is some bending to be resisted as well.

 

[JAE]

Here's one Simpson product - many others like it:

https://www.strongtie.com/coiledstraps_specialtystraps/cs-cmst_productgroup_wcc/p/cs.cmst

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[BAretired]

It's not clear to me how the truss was intended to function. It is not a standard truss; in fact it is not a truss at all by the usual definition as it is not composed entirely of triangles. This means that some of the joints are required to carry moment if the load is unsymmetrical as it would be with snow on one side of the roof and not on the other.

The scale of the drawing suggests that there are three sizes of member in each truss which I am guessing are 2x6, 2x4 and 2x2. The analysis of this truss is not a simple matter. If the original supplier is known and is still in business, it might be a good idea to contact them for a recommendation on remedial measures. Alternatively, the truss should be re-analyzed to ensure that remedial measures are appropriate.

BA

 

[JAE]

BAretired....now that you mention it ...and I actually look at the truss...it is quite strangely configured so I would totally agree that this is no simple repair and my earlier comment about section modulus not applying to this could be wrong as the bottom chord, with the large open space in the middle, could indeed be serving as a floor or attic joist of some kind and bending as well as working axially.

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[Canuck65]

The truss configuration appears to be for an 'attic truss'. They are designed for usable space, usually at 40psf residential loading in the central area. Therefore, I see the reluctance to take the cabling out and put it on top of the bottom chord as there will be floor sheathing that is required to be installed on the top of the bottom chord. Those holes should never have been drilled through the bottom chord of the truss. The cables should be removed and then the bottom chord be sistered. I have never heard of 4x4's being used to sister what likely is 2x material. I would much rather have 2x material sistered onto either side of the bottom chord than a 4x4 on one side. The cables should be moved to the side, outside of the useable attic space and strung along the top of the bottom chord. This is a very easy, straightforward repair if the cables are moved.

 

[dhengr]

Mklee1986:
The truss guys call that a ‘bonus room truss’ (or other names, in different regions). It is for an extra room under the roof, in the attic. It will have a 7’ long by 3’ high gable type truss on top, to form the ridge. I agree with BA and JAE, they are not a simple truss and the middle 16’ of the bot. chord will have a fl. load on it. The top chords and bot. chord are more likely 2x6’s or x8’s and 2x10’s or 2x12’s. That drwg. is right out of the truss guys shop drawing package for the job. It will have design loads, stresses and manuf’ers. title block, if you know how to read their wild drafting notations, a wealth of info. to help starting to fix this. The bot. chord is definitely a bending member too, not just a tension member, and the trick will be that the holes are all at its mid depth, so that he can sister 2x4’s or 2x6’s on both sides above and below the holes. What a damn dumb stunt. Structural Engineers always put big members where they are not needed, just to make mechs. and H/O’s drill big holes. As BA suggested, I’d go back to the truss guys for a fix, the drwg. package should show who to talk to.

 

[Ron247]

I agree with Canuck and dhengr, it appears to be a bonus room or attic room truss. The bottom chord is treated as both tension and bending I think. As far as scabbing the sides, I would consider one long scab with holes in it to match the cabling. Split the scab long ways and install in 2 pieces. Make the scab the same width as the bottom chord/beam.

 

[XR250]

I’d go back to the truss guys for a fix, the drwg. package should show who to talk to.

Around here, the truss guys won't provide any fixes. That let local PE's take care of it.

 

[HouseBoy]

Agree with the others - this is a standard "attic truss" (at least that's what we call them around here). The "piggy back" on top is a common feature that we see for taller trusses to accommodate shipping.

Since the bottom chord of the truss has both tension and bending, I would not expect that adding separate pieces above and below the holes would be effective unless you can produce shear-flow between the original piece and the added piece. That seems difficult to do.

What I have done in similar circumstances is to request the output from the truss designer showing the member forces (shear, bending and tension). Then the reduced member properties can be calculated/estimated. I usually do not like holes in truss members but in some cases I like to add a plywood piece on each side of the affected area as a way to improve the shear transfer around the openings. (That's more difficult if the affected length is more than 8 ft. long.)

Some engineering judgement involved but first I want to determine the stress levels of the individual members in the existing configuration in order to decide which direction to go.

 

[BAretired]

The attached sketch which I obtained from the internet indicates a 28'-0" attic truss with piggyback. With the possible exception of the bottom chord splice, that truss appears to be stable. The bottom chord truss plate would need to be checked to ensure it can carry the midspan moment from the attic floor load in addition to tension from the truss.

HouseBoy considers the sketch shown in the original post to be a "standard attic truss". This gives me some concern as it does not appear to me to be standard. In particular, the truss is missing bracing members between top chord 7-8 and horizontal member 6-9. And it is missing a diagonal brace between joints 4 and 17 and also between joints 11 (not marked) and 15. If truss plate joints are considered pin joints, the OP's truss becomes a mechanism which is unacceptable.

If truss plates are capable of resisting moments, then the truss may be adequate as detailed, but I would not consider this to be the case even though they evidently do resist some moment.

If the original truss is adequate as built and if the 1.25" dia. holes are centered in the bottom chord, I would be surprised if the capacity of the truss is significantly affected.




BA