Affects of Improper Heat Cambering

[NBEEBE]

I'm looking for some insight on the affects of improper heat cambering of beams. I have two possible situations that I'm wondering about:

1. Beams are heated beyond acceptable temperatures (1200deg) and quenched (water cooled via garden hose) once in "desired" position.

2. Beams are NOT heated beyond acceptable but are still rapidly water cooled.

The members that this pertains to are already erected and are supporting a concrete slab. the beams are showing deflection much greater than anticipated and the finger is being pointed at the company who performed the cambering. Reports show that camber was correct and within tolerance (-0" +1/2") prior to leaving their shop.
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I'm NOT an engineer or a chemist by any means but my thoughts (besides both methods being completely unacceptable) are that:

For #1: The steel is being heated to a point of the "atomic structure" becoming non-uniform (non homogeneous), and then rapidly cooled which might be resulting in this non uniform structure being the final "weaker" configuration.... Could this cause the steel to deflect "easier"?

For#2: The steel in the locally heated and rapidly cooled areas is less ductile and more prone to cracking rather than bending/deflecting. There are currently no signs of fracture that we know of.


I may be way off base here so any insight or thoughts on this would be great.

 

[paddingtongreen]

As long as the steel hasn't cracked, I think the cambering operation is not to blame. In heat straightening steel it is heated to about 1100[sup]o[/sup], but I've not seen them quenching it afterwards.
Without more information, I can't offer any ideas as to why the excessive deflection.

Michael.
Timing has a lot to do with the outcome of a rain dance.

 

[rowingengineer]

my investigation would run something like this:

1. check the beams where installed with the camber in the right direction.

2. Check the thickness of the slab and make sure there was no over pour.

3. Steel tokens to be taken, use these to steel strength and most importantly E (had s situation where some bolts where supplied from out of country, these bolts has some contaminates in the mix which reduced the E significantly, These were very long bolts > 1000m long so extension was an issue when tightening)

4. Investigate assumptions of design v's construction (assuming a design check shows everything to be ok, I would concentrate on connection behavior at the supports where the steel is in compression to start with)

starting with the assumption that the cambering caused the deflection would seem a little hasty.

http://www.nceng.com.au/

"Programming today is a race between software engineers striving to build bigger and better idiot-proof programs, and the Universe trying to produce bigger and better idiots. So far, the Universe is winning."

 

[gtaw]

Steel has to be heated above the lower T1 temperature to begin the transformation into austenite. If the steel is heated above 1335 degrees F, depending on the carbon content and the other alloying constituents, i.e., carbon equivalency, quenching (rapid cooling) can result in a microstructure that is harder and stronger than it was to begin with.

The potential problem from heating above the T1 and quenching is that any increase in strength and hardness is accompanied by a reduction in ductility. However, most steels used for steel building construction usually formulated with a low carbon equivalency so that excessive hardness is not a major concern.

One must consider how steel beams and other shapes are made. They are rolled at temperatures above T1 (1800 – 2200 degrees) and either air cooled or water cooled. The exception is plate and shapes that are finished by rolling at lower temperatures to gain the advantage of additional mechanical work to strengthen the steel (Thermo- Mechanically Controlled Processing. However, structural steel shapes used for building construction are not usually rolled using TMCP due to their high cost.

Deflections are not a function of the strength of the steel, but rather the Modulus of Elasticity. If deflection is the governing factor in a steel design, there is little advantage in using high cost TMCP steels.

Back to the issue at hand, whether the technique employed to camber the beams is the root of the problem. I do not believe cambering is the problem. Heating and quenching the steel would make it stronger, less ductile, but would have little if any effect on the defl


Best regards - Al

 

[NBEEBE]

Just wanted to say thanks for everyones replies. Much appreciated. I'll report back once there is a conclusion of who/what the actual issue is deemed to be. Thanks again