Why does heating reinforcement penalise the allowable stress, while welding does not? 2

[bugbus]

I understand that many codes penalise heating of reinforcement over 450ºC (~850F) by reducing the allowable stress to around half of the original yield strength, e.g. from 500 MPa to 250 MPa.

But why is it that welding does not have the same rule when it exposes the steel to temperatures of 1000-1500ºC (1800~2700F)?

 

[WARose]

But why is it that welding does not have the same rule when it exposes the steel to temperatures of 1000-1500ºC (1800~2700F)?

I think because it is understood that welding shouldn't take place on loaded members. We have had a lot of conversations here about welding loaded members, and most of them is how much of the member to disregard as resisting force during welding.

Welding does effect allowable stresses when it comes to cyclic loads. Also, a engineer has to understand that you can't weld the same piece of steel over and over because of the residual stresses introduced (regardless if the loads are cyclic or not).

 

[bugbus]

@WARose - thanks for your comment, but I'm a bit unclear what you mean. Do you mean that welding should not take place on members that are currently loaded or those intended to be loaded in the future? I can think of plenty of examples of the latter, where we use welded splices for example. I'm not aware of any reduction in strength that the code would enforce in that situation.

I agree with your comment on fatigue / cyclic loading.

 

[WARose]

Do you mean that welding should not take place on members that are currently loaded or those intended to be loaded in the future?

I mean the former. (I.e. that have load on them during welding....which is a big no-no.)

 

[BridgeSmith]

Only a small portion of a reinforcing bar, both cross section and length is affected by the extreme temperatures of welding, whhereas heating the bar would presumably expose the entire cross section and a significant length of the bar to the detrimental temperatures.

 

[dik]

If the material is not heat treated, I'm not sure what the effect of heating it is. I suspect most rebar is not heat treated and it starts as a red hot billet of some kind.

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

-Dik

 

[BridgeSmith]

I'm not sure under what circumstances the penalty on the yield strength would be applicable. If it's applicable to rebar exposed to those temperatures while in service, then it's likely due to softening of the steel because of the heat. If the penalty applies to steel heated at some point prior to being cast into the concrete, I can only guess that it could cause some tempering to occur, which would make the steel more brittle and more likely to fracture a reduced strain, so to avoid a brittle failure mode of the reinforced concrete, the design stress limit is reduced.

 

[bugbus]

I mean the former. (I.e. that have load on them during welding....which is a big no-no.)

Yes, I agree

Only a small portion of a reinforcing bar, both cross section and length is affected by the extreme temperatures of welding

I agree generally, but there are some welds that are over the full cross section of the bar, some which probably require several passes to achieve, e.g. direct butt welds, butt welds onto anchor plates, etc.
Although I agree it is certainly more localised along the length of the bar, and with the weld material being deposited which I'm sure has some kind of effect by changing the structure of the metal locally where it has been exposed to those high temperatures.

I will do some more digging around and try to find more info

 

[LittleWheels]

Some parts of the world use thermo-mechanically controlled process reinforcement.

https://www.twi-global.com/technica...ermo-mechanically-controlled-processed-steels

https://www.twi-global.com/technica...-contolled-processing-steels-readily-weldable