Plastic design is another method of designing continuous steel beams. It's beneficial if they are the same or similar spans. It provides additional strength by relying on the redistribution of moments after initial failure. The classic example is a beam with two fixed ends, and a uniform load. Elastically this can be loaded to wl^2/12 when the end moment fails and the mid span moment is wl^2/24. Loading can further be applied until the mid span moment equals the end span moment at wl^2/16, where with three hinges the fixed end beam continues to fail without increasing load. Continuity is achieved by designing the cantilevered beam connections for both shear and moment. Some jurisdictions stipulate the minimum design moment, usually about 25% of the section capacity. It is excellent for large warehouse type construction.
The plastic design approach takes advantage of this redistribution. I've generally found it to be more economical than elastic design. There are a couple of reasons for this. There is generally a reduction in weight of steel and the number of connections; and they are generally more simple. Alternate loading on adjacent spans is not normally an issue. This usually only increases the amount of the cantilevered moment design. Gerber beams take a huge 'hit' on this. Price is reduced because there are fewer sections to handle. For long continuous beams with multiple supports, only the end spans increase in size due to the reduced number of 'plastic hinges'. Beam lengths can generally be about 60', or whatever the maximum transportable length is. Statically indeterminate structures are not normally an issue... beams are elastic with fixed hinge strengths.
Because it relies on redistribution of moments, there are requirements that the sections be 'stockier'; this is why most jurisdictions require Class 1 sections. There are occasional times that a lighter Class 2 section has the moment resistance, but cannot be used. Bracing and lateral support are only slightly more restrictive, and columns are generally 'pin ended' with real cross bracing. Plastic design can be used for rigid frames, too.
On the Vista Cargo project, outside Toronto, I didn't do an estimate for the original design(I didn't know what was estimated), but the final design weight was had a savings of slightly more than 2 psf of steel. The engineering manager had to seal the drawings (a company policy) and I had to teach him the design methodology... It was the first plastic design he had done. It was about 400,000 sq.ft, and the first project I had worked on for that firm. He was aware of my design skills from the Cornwall Centre in Regina.
At university, our class was the first one to use limit states (not plastic design) for both concrete and steel. Working stress design was only briefly mentioned, for historical reasons... back in 1965.
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So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates
-Dik
