Ultimate Limit State Design

[patelam]

I have designed a beam for bending which is laterally supported over its length. This means that its governing failure in bending will be local inelastic buckling.

Does it means that we are allowing the beam to undergo inelastic zone when loaded to design load? Or the beam will still remain in elastic zone as there is enough margin created by the factor applied to the load?

 

[Agent666]

Depends on what load case you're considering. For seismic quite possibly forming a ductile plastic hinge and undergoing some significant inelastic rotation depending on your arrangement.

For a gravity case, depends on your ULS combination and the increased ratio of this over and beyond an unfactored load case and the corresponding ratio between your elastic and plastic section moduli (assuming only bending is relevant). You also have to consider that the real yield strength isn't always going to be the lower characteristic 5% value usually utilised by codes for design purposes. You also typically have a strength reduction factor less than 1.0,further increasing the divide.

Do the calcs and see. But I'd imagine for a gravity case you may still have a significant margin between the 'yield point' and the actual stress distribution.

This is a great video to understand the intricacies of the overall safety indices that might apply to our structures

https://youtu.be/itie93Adetc

https://engineervsheep.com

 

[human909]

I have designed a beam for bending which is laterally supported over its length. This means that its governing failure in bending will be local inelastic buckling.

Unless I'm misunderstanding you. If you section is compact then your governing failure won't be inelastic buckling it will be plastic yielding. (This is essentially what Agent666 has said.)

Does it means that we are allowing the beam to undergo inelastic zone when loaded to design loadCAPACITY?

In most cases a design load won't push your members into inelastic zones. But yes the design capacity of your beam normally allows for some inelastic yielding. Hence the plastic section modulus.

Or the beam will still remain in elastic zone as there is enough margin created by the factor applied to the load?

Have a look at the diffence between the section modulus capacity and the PLASTIC section modulus capacity. You can see that plastic yielding is likely if you push your member to the design capacity.

But like Agent666 said normally you keep your design loads well away from the design capacity as often other (defelection) requirements govern.

 

[dik]

Attached is a little write up on plastic design resulting from an earlier thread on this topic. Any corrections or comments will be greatly appreciated since this is 'first kick at the cat' (I like dogs, actually).

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik