Fire Damaged Steel Trusses 5

[Mark_J]

Hi,

I was hoping to get some recommendations on how to fix one steel roof truss that has buckled top chord in 3 bays and fire likely diminished the adjacent truss and brace truss at same location. Trusses spanning about 75'. It seems that the owner may be OK with putting a beam under the buckled truss but I'd like to offer a solution that may be around the same cost and no beam with columns. It's a beer warehouse, I wouldn't want columns in a warehouse.

Good ways to shore. All the roof in that area is coming off and getting replaced as well.

 

[Mark_J]

Cross brace and adjacent truss.

 

[Mark_J]

Main truss damaged

 

[Mark_J]

This is what I was thinking:

1) Weld all members to existing gussets where possible
2) Knock out rivets and place 1/4" gusset plates each side. Cut existing leg angles as required.
3) Replace all bolt holes
4) New web angles to match, weld ends to new gusset. Continuous top and bottom angles length of damage.
5) Buckled top chord interference use heat to return to somewhat straight to avoid interference.

Both sides on severely damaged truss. One side on adjacent truss.

Do same with the cross brace truss.

Anyone see anything wrong with this approach? Anything that the contractor should be made aware of that may not be so obvious.

 

[BAretired]

Members which have undergone obvious fire damage should be removed and replaced; the steel has lost some of its original strength. Yield strength, modulus of elasticity and stiffness are likely to be reduced where existing steel has been exposed to high temperatures. If I were the owner, I would not accept your recommended procedure.

 

[Mark_J]

BA: Little confused. All new steel, just not removing existing. Reinforce actual damage, not all the way to original splice points.

Anyway, was wondering about use of heat to straighten the top chord so splice angles can be added without interference. I read about that on this forum, from a 2011 thread.

 

[BAretired]

It may be okay, but it looks clunky. I would replace all joists which have been bent out of shape. Just my opinion.

 

[Mark_J]

BA, I don't like clunky either. I'm going to recommend replacing the top chord only, replace rivets with bolts and done. Contractor can shore it up as needed. The fire started from a battery charging, a little tiny battery. Sparks caught boxes on fire. I originally thought it was a diesel fire from generator, it wasn't. So as far as hiw hot the fire was, I am assuming on the lower end of fires.

Top chord is bent, replace top chord.

 

[EdStainless]

The only property not changed is the modulus (stiffness).
Everything else is a gamble.
All heat affected steel needs to be replaced.
Unless you can get this design to pass using the lowest yield strength possible, say 15ksi.
Unless you want to spot grind places and take hardness readings to see how bad it actually is.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, consulting work welcomed

 

[dik]

I'd likely be taking a few coupons to see what the extent of the damage is.

-----*****-----
So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik

 

[Retrograde]

I'd likely be taking a few coupons to see what the extent of the damage is.

I agree. I have done this before to justify not replacing rafter beams that were exposed to fire.

 

[HTURKAK]

I saw this thread two days ago but preferred to keep silence to heard about others taughts and the practice at the other side of the ocean. I am not in a position to encourage which could be risky for the integrity of structure for future use.
My opinion is , and assessment study should be conducted to see the most suitable reinstatement method.
Probably the simplest answer would be demolishing and new construction of the trusses . My biggest concern would be hardening of the steel due to rapid cooling (e.g. extinguishing the fire water spray ) .
The straightening the top chord still can be an option.

I will suggest you to look
- ''The Effects of Fire on Structural Steel ''
-

https://www.aisc.org/steel-solutions-center/engineering-faqs/11.2.-steel-exposed-to-fire/#9366

My opinion..


According to the grace of God which is given
unto me, as a wise masterbuilder, I have laid the foundation, and another buildeth thereon. . . .
I Corinthians 3:10

 

[Aesur]

I'm with HRURKAK here, I would require complete replacement of the steel trusses and the wood planks unless the fee was sufficiently high enough to warrant checking the steel properties and modeling the steel trusses (rarely is, but maybe yours is). Additionally, it may be easier to construct unless there is another story above. I assume insurance is involved here which may be part of the decision, but I believe consideration should be made for repair versus replacement costs. I recently was involved in the review of a similar framed school that burned and it was cheaper to tear down and rebuilt versus try to retrofit and fix, based on the value they managed to even get more use and rooms out of the rebuilt costs versus the retrofit costs.

 

[Mark_J]

Aesur, I actually read his reply completely different. I read it as, the easy way out is to just replace anything fire touched and call it a day. In reality, the steel strength is still adequate if a member can be returned to its original shape. In this case I believe it can.

Not sure if the steel contractor believes he can, will discuss it with him.

 

[Aesur]

In reality, the steel strength is still adequate if a member can be returned to its original shape.

I disagree with that statement, it may still be similar strength, but unless you test it, you don't know for sure if the fire changed the properties or not.

 

[SWComposites]

plus "straightening" the bent member is likely to introduce yielding which will also reduce the capacity of the member.
why not just remove and replace the heat exposed trusses?

 

[jerseyshore]

I don't know why'd you mess around with something like this. The amount of investigation, testing, and calculations needed, plus the actual physical labor involved, seem so much more than just providing new members.

I try to look at most projects, especially ones like this, as if I was critiquing myself or getting it peer reviewed by a real PITA engineer. If there was ever a problem in the future a thorough engineer would nail you for not performing the necessary calculations and testing that would be required. So that means you have to do that now, up front.

Unless there is absolutely no way to get new members in there I don't see how anyone has enough time to perform a comprehensive analysis on something that probably has a good chance of not even calcing out on paper in the end.

 

[Mark_J]

The reason to not just replace trusses that were exposed to fire is:

1) I don't think they lost their strength. Fire from cardboard boxes is not considered a hot fire.
2) Money, insurance doesn't want to pay. To repair the roof above the trusses is 150K, insurance offers 150K only. What about all the smoke damage throughout the entire building that needs to be updated?

I'll get some of the rivets that get knocked out tested. They would have undergone the same heating and cooling cycle as the rest, I would consider that due diligence. I think replace anything touched by fire to be the most expensive option possible, and unfortunately, no budget for it.

I'm not sure if you all read the article posted above but I'll copy it below:

"Dill1 concludes that, while exposure to fire will almost certainly cause warping and twisting of members, it does not inevitably follow that the strength of the steel is reduced. It is almost certain that any steel that has been heated hot enough to undergo damaging grain coarsening or that has been cooled rapidly enough to harden it will be so badly distorted that it would have no consideration for re-use anyway. This leads to the general statement that steel that has been through a fire but that can be made dimensionally re-usable by straightening with the methods that are available may be continued in use with full expectation of performance in accordance with its specified mechanical properties. Essentially then, the question is one of economics: if the steel can be straightened for less money than fabricating and installing a new piece, then that should be done.

Two possible exceptions to the above include quenched and tempered structural steels and high-strength fasteners. The mechanical properties of such heat-treated items may be affected by prolonged fire exposure and should be tested to determine the effects of the fire, if any."

 

[jerseyshore]

Insurance is never going to want to pay for anything. That's the point of hiring an engineer to provide a report on the required steps necessary to test and analyze the structure. If the insurance company finds someone else to say no it doesn't need anything just slam it back into place then let them sign and seal the repair drawing.

 

[SWComposites]

Yeah, since when did insurance companies want to pay for anything? How do they think they can get away with only replacing the roof?

How does testing a rivet cover the structural members? they are likely to have very different alloys and material properties. The quoted document clearly states that structural steels should be tested to determine the effect of fire exposure.