Cut Bottom Cord of Engineered Truss

[Contraflexure74]

I've been asked to look at the following issue.

A domestic roof engineered truss has had the bottom cord cut to install an attic access hatch door. It has been cut adjacent to a node point. One truss cut only at the third point. The node gang nail at the cut point looks in distress and has partially pulled out. The adjoining trusses appear slightly out of plain. Load sharing maybe happening as plasterboard slabs are also Cracking in various locations upstairs under the truss. Cracks are less than 1mm. The upstairs internal stud walls may now be taking some roof load.

It has been like this for several years now (15 years). Should I be concerned? Any comments welcome. Anyone come across a quick economical fix option here in the past?

Engineered truss is triangular and spanning 11m at 450mm centres. 1 truss has been cut only.

 

[oldestguy]

What do you mean a "node pint" for likely a simple span truss? That's probably relieving plenty of tension that was cut. Consider jacking up the truss beyond it's working condition and add tension ability, as with bolted on steel straps. Be sure they take tension when jacks are removed.

 

[hokie66]

It's a chord, not a cord. And yes, you should be concerned. Wood trusses don't have excess capacity. You are doubtless correct that load sharing has occurred. That is a good thing about structures, they are often smarter than their designers, so seek alternate load paths. But you can't depend on it, no matter how long the defect has existed.

 

[MIStructE_IRE]

Its likely that since this 1 No. truss is no longer taking tension, the horizontal force is now applied to the wallplate which now laterally spans 900mm between trusses taking a horizontal pointload at the location of the cut truss.. Could this be justified to work? If not, simple jacking/strapping would work.

 

[Contraflexure74]

When I say node point there are diagonal timbers running to the roof ridge and rafter half span just beyond where the cut has been made.

Yes chord not cord. Typo.

How does one jack in a live house?

Not sure what you mean by strap...Strap what to what?

Do I jack and then splice on the cut cord missing piece and tell the client she has to sacrifice access to the attic?

 

[Ron]

Cf74....get a local structural engineer to look at it. He should be able to explain a repair procedure.

 

[Ron247]

Include a pic or sketch about what is going on. To my thinking, a truss with a cut bottom chord is useless.

 

[BridgeSmith]

You'll need to restore the load path for the tension to be carried by the chord. There are ways to route it around the access hatch, but as Ron said, you'll need a local structural engineer to design a solution.

 

[oldestguy]

For what it is worth. Your questions show that this is beyond your abilities and that this forum cannot be expected to give complete advice on the required repairs. However, the advice given to involve an experienced engineer is a valuable answer. Please go with it.

 

[Contraflexure74]

Not sure why you think this is beyond my abilities oldestguy. It's just I haven't come across this issue before. I thought that was the idea of this forum, that is to share experiences. All I was hoping for was that someone on the forum had encountered this scenario before and was willing to share their experiences/solutions. I don't expect details and calculations or complete advice as you put it.

 

[oldestguy]

OK I do see you have "been around" per your past posts. In that case how about proposing a solution? In that case provide a sketch and show what you would do. Ask for comments. The more you show of the situation, the better. I'd ask an experienced contractor for his plan also. Can a person do any welding in that attic? This may take 20 or so answers or suggestions (history here of similar things).

 

[BridgeSmith]

Not trying to be insulting, Contraflexure, but I think your initial question, "Should I be concerned?" made several of us question your grasp of the structural mechanics involved. It seemed obvious from your description of situation that you should be very concerned, and you should know that. (The improper terminology didn't help either - "cord" instead of "chord"; "plain" instead of "plane").

The suggestion to strap the bottom chord to restore the tension capacity is a good one. It doesn't have to be a single strap - split the load path around the access hatch (straps on each side of the bottom chord that go around each side of the hatch and connect back to the chord on the other side).

 

[oldestguy]

OG here with his first thought. Bolt onto the lower chord two steel plates or structural sections each attached to the remaing truss lower chord sections. Extend these to the opening. Construct a box made of steel (of sufficient strength)surrounding the opening, likely just behind the "frame" of that opening. Attach a cable and "come-along" to the new ends of the cut truss lower chord new steel pieces. Tighten that "come-along" so as to exceed the original tension stress in the lower chord and its orignal support positions. Weld the "box" to the steel extensions of the lower chord. Take away the "come along". That's a term for a portable winch. A more cumbersome and ugly job would be to use bolts instead of welding. Some diagonals might help get the tension into the long sections beside the opening. Edit-1: This might go better with cables, since the bottom chords of adjacent trusses might have to be penetrated or "gotten around".

 

[Ron247]

Contra,

A sketch or some pictures would help. Some of the comments you are getting are because of your questions. For example:

How does one jack in a live house?

I do not know what you mean by that question. What is a "live house".

As far as remedies, to repair the damaged truss would be to lose the attic access unless the truss was designed clearspan but does in fact have some interior walls under it. Without more information, none of us can advise although I imagine most responders have already faced this.

 

[Contraflexure74]

Ok now I understand the strap detail. It wasn't clear before. Thanks guys. This sorts out any further loss of tension. But how does one jack the roof in the correct sequence when the load due to cut truss has already been redistributed into adjoining trusses and are possibly loading non load bearing stud partitions directly below the trusses in question.

 

[BridgeSmith]

As OG said, you'll need to pull the ends of the bottom chord together, back to the original position and maybe a little extra to account for some give in the fasteners. This will likely temporarily block the access hatch. Once you have it back where it's supposed to be, attach the permanent strapping or connect to the frame, and release the temporary restraint.

 

[Contraflexure74]

Not sure this will bring the adjoining trusses back into their original position and unload the non load bearing stud walls. It's a good starting point though.

 

[Ron247]

Jacking the cut joist and the one on each side at the same time should put you close enough. The area above needs to be unloaded of any stored items before you do. If it only has dead load, it most likely did not load share very far beyond the adjacent joists.

 

[Contraflexure74]

Thanks all. I'm now clear on a proposed solution.

 

[XR250]

If it is only on or two trusses, I have yet to see this be a problem in practice (seen this hundreds of times in the field). The roof diaphragm will take up the thrust and transfer it to other areas. That being said, i usually design "strongbacks" to directly transfer the load to the adjacent trusses.