rot damaged parallel chord web joists

[Gwylliam]

I recently took a new FM position and have been uncovering deficiencies in my facilities. I recently discovered a half dozen parallel cord trusses which appear to be in some stages of rotting due to wetting caused from standing water caused by modification of the roof with no provision for water to drain. These trusses span 40 feet of low pitch roof. I want to know if there is a method of repair or is my employer looking at having to replace these?

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[KootK]

Ughh... your client has rot in what is, with out question, the most hard working and fragile joint in a top chord bearing truss. What's worse, this particular truss typology is notoriously difficult to reinforce, particularly at the joints. I'm usually of the mind that anything can be reinforced but my money's on replacement or sistering with new, full span members unless it can be demonstrated that the trusses are okay as is somehow.

A partial, two sided sistering with LVL might have potential depending on the truss depth. Hang that from the side of the steel beam somehow maybe? The achilleas heel of this setup, in my mind, will be dreaming up a way to pass the truss shear from the steel webs of the existing truss into the LVL.

 

[XR250]

Those are some ugly trusses. I wonder how the webs are fastened to the chords?

 

[dik]

Terrible problem... you can have 10 to 20% loss of strength without losing any material weight. as noted... a bad spot. Is this representative of a large number of trusses? It's an ugly repair to try to sister something that doesn't have the strength to start with. Have you looked into penetrating epoxies; they use it for repairing boat hull damage caused by brown rot (aka dry-rot).

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[KootK]

I wonder how the webs are fastened to the chords?

Usually there's some kind of dowel forming the pin. On the one hand it's ingenious because:

1) Vertical shear transfer is web to web without involving the chord and;

2) Horizontal shear transfer mobilizes only chord bearing parallel to grain.

On the other hand, this setup makes reinforcing, shoring, and point load support a challenge because those things do often generate perpendicular to grain stresses.

I find it telling that modern versions of these trusses often hand the bearing web connection a bit differently.

 

[XR250]

Thanks KootK.

 

[KootK]

This is a conceptual repair that is used with steel joists sometimes. I'm not sure that I'd be willing to attempt this personally though.