Holes drilled in bottom chord of truss 1

[CANeng11]

An electrician drilled 3/4" holes through a 4 ply 2x10 bottom chord of a wood truss and the builder is asking for a repair detail. See the attached pictures. I believe the truss manufacturer is telling them they will need to replace the truss, but they are asking for alternative solutions since this would be very difficult to replace. Any input would be appreciated. I have requested the truss designs from the manufacturer.

 

[phamENG]

Doesn't seem too bad. Hopefully they didn't design it to 1.0001 utilization.

If you do need to reinforce it, maybe some steel straps on the bottom? Simpson coil straps come in 50' lengths, I believe.

 

[KootK]

Former truss designer here. Late 90's before and during college. This is silly if all of the holes are in the middle third of the cross section. Any attempt to "repair" it is probably just going to make it worse. I wouldn't love being sort of liable for the whole girder on account of ok'ing this one silly issue but I'd probably be willing to do it if it was my project as a whole and there was no other way to keep things moving. These things almost never fall down unless they're churches. If this is your only involvement in the project, I'd turn it away or charge a nice healthy fee for the risk you assume.

Consider playing chess with me on the Social Chess app at iTunes. Same handle. Fear not, I suck.

 

[msquared48]

Have the electrician stick his fingers in the holes while you repair them. Should impress him...

Mike McCann, PE, SE (WA, HI)

 

[HouseBoy]

Several things come to mind:

It's a 4 ply truss so that means a 3 ply truss was over 1.00 unity check. (right?)
Hard to believe the 4 ply was designed for CSI over 1.00 (as phamENG wrote above) and I would not expect the bottom chord to be the highest CSI out of all the members (although it's possible I guess).

3/4" hole causes about 8% loss of area in a 2x10. I don't remember seeing a 4 ply girder truss that was designed with bottom chord CSI greater than 92%

I wonder if there is some other reason the truss maker is saying remove and replace. Maybe they just don't like electricians drilling holes in their trusses. Can't really blame them for that....

Looks like reinforcement would be very difficult to do effectively.

If I HAD to take a position, I'd ask the truss maker for the truss design output and see where the high CSI is really occurring and then (assuming I could rightfully say) write an opinion indicating that the location of the holes is not in the stress critical zone.

Yes, there's a lot of "I hate to stick my neck out for this" here and that is perfectly natural but if I had to take a position, I THINK that's how I'd go about it.

 

[struct_eeyore]

Do you have software with which you could model the truss and a good idea of the loads that it is carrying? I always like a sanity check before approving something like this. It might be that the 4-ply is not even necessary in the panel that you are dealing with. I've used Visual Analysis with, what I assume, are good results for truss mods like this.

 

[RPMG]

Inject it with adhesive with the wires still in it. "Repaired"

 

[msquared48]

To repair the truss is to void any factory warranty and assume the liability yourself. Just remember that.

My question is, if this is an open web truss, why did he have to drill any holes at all?

Mike McCann, PE, SE (WA, HI)

 

[HouseBoy]

Make sure it's STRUCTURAL adhesive. Then it'll be fine.

 

[r13]

Are those live wires, or low voltage cables? Is bare wire allowed in wood structures?

 

[KootK]

Some things to keep in mind:

1) A wood member's tension capacity will be governed by flaws/imperfection that create stress concentrations. It's not really a % cross section remaining kind of thing.

2) Because of #1, in a 2x10, a 3/4" diameter hole far from an imperfection can be expected to lower tension capacity by 0%.

3) Because of #1, in a 2x10, a 3/4" diameter hole beside an imperfection probably also lowers tension capacity by 0%. Heck, it might actually relive stress concentration.

Consider playing chess with me on the Social Chess app at iTunes. Same handle. Fear not, I suck.

 

[JLNJ]

If it were an existing structure (and now it is), the code will allow 5% over as altered under gravity load. You are almost there.

 

[KootK]

2/3 of the supported hanger nails probably didn't get installed so you might be able to claw some back there. Insert emoticon for "facetious" here.

Consider playing chess with me on the Social Chess app at iTunes. Same handle. Fear not, I suck.

 

[LittleInch]

Is that really a truss?

Looks like a beam to me.

Needs a drawing for sure, but given there are three holes in it already to presumably tie the 4 individual beams together I can't see a hole that small in the centre of the beam making any difference.

There's so much else going on with this beam - vertical loads, floor loads from those beam, stair loads? that it won't be easy to so any calcs.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[oldrunner]

Review the submittal drawings and see if there is a detail for allowing holes and where the holes should be and their maximum diameter.

 

[r13]

oldrunner,

I guess this is a pre-engineered truss, since the manufacturer has suggested "replacement" as the solution (see OP statement).

 

[oldrunner]

A pre-engineered truss should have notes concerning holes that can be drilled or not. If it's pre-engineered, then there should be information on the trusses capacity. Did I miss something in the picture? Is it only one hole in the middle of the bottom chord?

 

[SteelPE]

I agree that drilling a little hole in that truss shouldn't cause an issue. If you want to prove it to yourself, that calculation is going to be tough without getting the calculations from the manufacturer. Having the manufacturer recommend replacement would bother me though.

I noticed that there is a wall under the truss.... is it possible to add a few posts in the wall (and in the basement if there is one) to support the loads from above? I understand stress reversal may be an issue, but it appears as if the chords are well braced, so maybe it's not a bad option.

 

[BAretired]

The third photo in the original post seems to indicate that holes were drilled in web members as well as the bottom chord. The adequacy of the entire truss should be reviewed, not just the bottom chord. It seems unlikely that the holes shown will significantly affect the capacity of the truss, but this should come from the truss fabricator in order to maintain his warranty. If he refuses to provide such assurance, the EOR must review and accept responsibility for the design of the truss as built.

EDIT: The dark circles are not holes; they are screw heads. I see no 3/4" dia. holes in the photo below.

BA

 

[phamENG]

Are those holes in the webs, or TimberLOK style screws to build up the multi-ply truss? One of the earlier photos shows holes with wires running through and what appear to be large TimberLOK screws - this picture just looks like the screws. I could certainly be mistaken, though.