50' span trusses (broken bottom chords)

[PSUengineer1]

Hi,

I have pre-engineered wood trusses, mfr. unknown. 50' spans. After a fire, approximately 6 feet of bottom chords of three trusses are gone, and vertical and diagonal web members above the missing bottom chords are heavily charred.

Considering that I do not know the mfr., I am airing on the side of caution and calling for trusses to be replaced. The damage occurred near the trusses mid-spans. Is this even a candidate for repair? If repairable, what are repair recommendations?

Thank you.

 

[dik]

Can you reconstruct the damaged chord by sistering a dimensioned lumber member on each side (extending the member to develop the tension capacity) and replace the web diagonal and maybe sister members where they join up with solid wood?

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

-Dik

 

[Agent666]

I'd imagine if the roofs needing to be repaired /replaced then it's fairly easy to just drop some new trusses to the same dimensions in there. By the time you consider the design fees and time to repair the damage its a no brainer to replace as trusses are a comparatively cheap and mass produced item.

The other thing is once you get into analysing the existing trusses, you might find they don't work for some reason or another. Best not to open that can of worms in my opinion. Replace old compromised structure with new compliant structure.

 

[skeletron]

@Agent666: If you find that the truss doesn't work and replace the 1 damaged truss with a 2020 compliant truss, is that really doing anything for a redundant system such as a roof? By that logic, if you find that 1 truss does not work, you should actually be replacing more structure and (effectively) upgrading the structure.

 

[Agent666]

Usually in a repair scenario you fix the broken thing to current standards. If there was no fire you would be doing nothing after all. You're always going to come across this type of thing because of the ever changing nature of codes and building regulations. Best you can do is inform owner, and if its something deemed critical you might advise acting, I believe at that point you've done your duty.

Like for like replacement, but new work should comply with current rules is the usual unwritten rule around these parts. Obviously local jurisdictions may vary in what is acceptable or deemed appropriate.

 

[r13]

Below is the CALIFORNIA BUILDING CODE requirements regarding alterations and repairs of the existing structures. I remember either ASCE or IBC has the similar requirements.

3405A.1 General

Buildings and structures, and parts thereof, shall be repaired in compliance with Section 3405A and 3401A.2. Work on nondamaged components that is necessary for the required repair of damaged components shall be considered part of the repair and shall not be subject to the requirements for alterations in this chapter. Routine maintenance required by Section 3401A.2, ordinary repairs exempt from permit in accordance with Section 105.2, and abatement of wear due to normal service conditions shall not be subject to the requirements for repairs in this section.

3405A.3 Substantial Structural Damage to Gravity Load- Carrying Components

Gravity load-carrying components that have sustained substantial structural damage shall be rehabilitated to comply with the applicable provisions of this code for dead and live loads. Snow loads shall be considered if the substantial structural damage was caused by or related to snow load effects. Existing gravity load-carrying structural elements shall be permitted to be designed for live loads approved prior to the damage. If the approved live load is less than that required by Section 1607A, the area designed for the nonconforming live load shall be posted with placards of approved design, indicating the approved live load. Nondamaged gravity load-carrying components that receive dead, live or snow loads from rehabilitated components shall also be rehabilitated or shown to have the capacity to carry the design loads of the rehabilitation design. New structural members and connections required by this rehabilitation design shall comply with the detailing provisions of this code for new buildings of similar structure, purpose and location.

3405A.4 Less Than Substantial Structural Damage

For damage less than substantial structural damage, repairs shall be allowed that restore the building to its pre-damage state. New structural members and connections used for this repair shall comply with the detailing provisions of this code for new buildings of similar structure, purpose and location.

 

[KootK]

If you don't have to replace the roof deck, something like this might make for a cost effective repair.