Steel Question Moment capacity Please help

[Junior1234]

Hi Everyone,

This question may seem stupid but i hope someone is willing to help

Pin end cantilever column with a fixed connection on top (portal frame column, results in moment at the top. Is the moment capacity at the top connection Ms and therefore
i dont need to consider the effective length (Mb)

Same Token, if the bottom is fixed, cantilever column free top, the moment capacity is at the bottom

and therefore if we have two support ends max moment is in the middle therefore i look at Mb?

If not can i please get an explanation of where my understanding is correct

thank you eng family

 

[Tomfh]

Pin end cantilever column with a fixed connection on top (portal frame column, results in moment at the top. Is the moment capacity at the top connection Ms and therefore i dont need to consider the effective length (Mb)

Your moment capacity is not Ms. Ms is merely a limiting strength condition where the steel starts to yield. In reality your column will likely buckle prior to this (unless sufficiently restrained that it will yield prior to buckling). Your moment capacity will be determined by the column height (length), and the other relevant parameters, e.g. lateral restraints, bending moment diagram within the critical segment, etc.

 

[Agent666]

and therefore if we have two support ends max moment is in the middle therefore i look at Mb?

Agree with Tomfh. You'll only achieve Ms, or more correctly phi x Ms if you have full lateral restraint (FLR).

Not quite sure what you mean here either. Max moment will never be in the middle of a column that has no additional lateral loads applied midheight. Can you draw a picture of what you're meaning as I fear you're potentially misinterpreting something else fundamental here, but hard to know as that explanation doesn't make a whole lot of immediate sense to me?

https://engineervsheep.com

 

[Tomfh]

Not quite sure what you mean here either.

They are thinking that you check the capacity at the location the peak moment occurs, and that if this occurs at a point of restraint (e.g. top of column) then Ms applies (because it can't buckle there), and that if it occurs away from a restraint (e.g. middle of simple span beam) then Mb applies, because it could buckle there.

So there is some confusion about buckling, and about what Mb covers, etc.

 

[Junior1234]

Hi Everyone really appreciate the responses,
I should have been more clear though, the column is only experiencing lateral force

Case 1
Portal Frame, Column is pin on the ground, fixed connection to beam, lateral force, moment is max at the top where the connection to the beam is, do i use Mb or Ms for moment capacity and if i use Mb what is the effective length i consider.

 

[human909]

Hey Junior 1234. I while your question wasn't entirely clear, Tomfh I believe correctly interpreted you original question and answered it.

To reiterate and answer your question. You always need to consider Mb, though in many case Mb=Ms. As per AS4100 5.1 (which from previous question might be the code you follow) it is required that M*<Ms and M*<Mb.

The effective length you need to consider depends on the twist restraints on your column.

 

[Agent666]

Note the moment capacity of a column will also potentially be reduced by the axial load it is carrying once you consider if combined actions need consideration in accordance with chapter 8 of AS4100. This might further reduce the section capacity.

https://engineervsheep.com