Plastic Steel Design 1

[abusementpark]

I hear a lot about ways to design steel beyond the elastic limit with moment redistribution and plastic hinges. Is this something that is easy implemented into general practice? In other words, for those understand these concepts and how to implement them, how much does this knowledge help you a regular basis?

 

[youngstructural]

I am in a very significant seismic zone for nearly all of my structures... If I did not use moment redistribution and plastic design, I would find my sizes increasing significantly (say in the order of 30% by weight, or more).

Both in reinforced concrete and steel design I design to at least nominally plastic (ductility level of 1.25) and more commonly for larger structures a ductility level of 2.

Just as a quick reminder, the ductility level is defined as the ratio of displacement experienced by the structure at ULS to the deflection experienced at onset of inelastic responce.

I would highly recommend learning about these tools no matter where you are, since, even in non-seismic regions moment redistribution can save you from having to go up a size.

Cheers,

YS

B.Eng (Carleton)
Working in New Zealand, thinking of my snow covered home...

 

[dik]

I've used plastic design for nearly 40 years and it generally results in minimal weight and cost (I've not used it for seismic areas and shakedown and reversals may mess with the economy)

The general provision for designing the moment connections for 25% of the strength usually accommodates moment generated from unbalanced loading. Unbalanced loading, other than the connection has no effect on the member size, hence a general reduction in weight. Also the use of the plastic modulus instead of the section modulus gives approximately a 15% increase in member strength. The continuity also minimises deflections. There are generally fewer pieces to handle further reducing costs. The beams can often span the long cantilever distance.

Dik

 

[haynewp]

Plastic analysis and design can be both easy and potentially very difficult at the same time.

The 2 best sources I can immediately give you is an introduction to shakedown by Galambos (free from AISC) and a link to Mastan which is a free program with plastic hinging analysis ability.

http://www.aisc.org/Content/Navigat...s/Introduction_to_the_Shakedown_Behavior_.htm

http://www.mastan2.com/

 

[abusementpark]

haynewp,

Thanks for the links. That will give me a good start.

 

[PersonalProfile]

Isnt AS4100 based on plastic design? (background is mechanical - not structural).

Regards,
Lyle

 

[hokie66]

No, AS4100 is a design standard or code, not a design method or philosophy. For those reading this who wonder what AS4100 is, it is the Australian Standard for steel structures.

 

[youngstructural]

Parallel to what Hokie said, it actually depends on how you define plastic design. If you think of plastic design as simply elastic methods using the plastic section modulus, then yes, AS4100 permits/shows plastic design. However I believe the OP was asking the much more complicated question of plastic analysis (involving the calculation of hinges and rotation limits). In this case Hokie's comment applies; As in it is a method of analysis, not an issue of code clauses, though most codes will have guidance on permitted sway and rotations which will be the boundaries to your plastic design problem.

Reinforced concrete structures can also be designed plastically by providing sufficient confining steel to permit the beams (or, VERY rarely, columns) to reliably sustain plastic hinge formation. My comment on column plastic hinge formation is because in nearly all structural designs, you want to have weak beams and strong columns to avoid soft-storey effects which can easily lead to premature structural collapse. In very rare circumstances (some bridges, DRE systems, etc) high-precision analysis is used to use hinge formations in columns or infill walls. I do not believe I would be brave enough to design this way, but am aware that it is possible.

Cheers,

YS

B.Eng (Carleton)
Working in New Zealand, thinking of my snow covered home...

 

[dik]

Youngstructural...

Once the system is initially overloaded and unloaded and the first hinges have formed... it is then an elastic system <G>.

Dik

 

[youngstructural]

Dik... Not always. And this again depends again on what you mean by plastic design...

Regards,

YS

B.Eng (Carleton)
Working in New Zealand, thinking of my snow covered home...

 

[dik]

Sorry... generally speaking...

Dik