Plastic Design % Strain

[scottee]

This is a question for my fellow structural engineers:-

I am conducting a brief study into what levels of strain are typically realised or designed for in plastic design of steel structures?

I would like this information to develop an understanding of how a rigid coating with low strain levels would react given high strain on the steel.

Many thanks in advance

Peter

 

[KootK]

Tough question.

Firstly, true plastic design, including significant moment distribution, is rare nowadays. It just never caught on as planned back in the 70's. My guess is that computers made it more expedient to stick with elastic analysis.

That being said, many flexural members are designed such that their cross sections are expected to fully plastify at ultimate loads. And, at full plastification, the extreme fiber strain is theoretically infinite. Of course, theses strain levels would only exist briefly at locations of peak moment.

So it seems that your coating would be in trouble at ultimate strength loads. Maybe your coating doesn't need to remain in tact at such extreme load levels however. In order to avoid permanent set deflections, most steel beams will be designed such that they are never actually strained beyond yield in service. As such, if your coating only needs to be viable at serviceability load levels, I would recommend considering the yield strain (Fy/E) a conservative upper limit.

I realize that my answer probably leaves a fair bit to be desired. So far no one else has chimed in, however, and we hate to leave a poster behind. At minimum, my response should give your post a bump.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[dhengr]

Scottee:
You can certainly get strains well in excess of .2% at load conditions near the max. serviceability conditions. That’s starting to yield on some of the steel surfaces, long before the whole member turning into a wet noddle. You can see some coatings cracking at discrete locations in some structures in normal service. Once plastification really starts happening in the member surface strains go pretty crazy. But, at this point you are primarily worried about preventing structural failure or collapse, not a little paint cracking. Have you talked to the coating manufacturer about this, do you even know what strain the coating will tolerate on a given base coat and sub-straight? Maybe this condition dictates what coating systems you can use on various structures. Again, talk to the coating manufacturer.