Moment Curvature Analysis - curvature ductility calculations

[Anonymous]

Hi,

I would like to know the criteria at which the first yield & ultimate moment capacity is assessed?

Normally, I would specify Mander model for confined & unconfined concrete & Park's strain-hardening model for reinforcing steel. With the method prescribed, the first yield is assessed when one of the longitudinal rebar's yield strain exceeds and the ultimate capacity is assessed when the concrete compression strain or steel tension strain is reached first before the transverse reinforcement yields. But because Autodesk SAM software doesn't requests any of the previously stated information,  is the ultimate capacity assessed using a propriety bi-linear model?

Thanks,

Siva 

[dave_geeves]

Hi,

This is a very good question which I can't find the answer documented anywhere.  I believe that the first yield criteria is when the first reinforcing bar reaches the yield strain (ie when the stress strain relationship is no longer elastic)  .  I checked this by applying the moment at first yield to the section and noticing that the reinforcement strain was just at the yield point

The stress strain relationship can be any of the standard type (liner, bi-linear, parabolic rectangular etc...) but can also be defined as "user defined"  where specific curves can be manually input which would represent more realistic stress/strain relationships.

The ultimate moment is where either the concrete has reached the max allowable strain or the reinforcement has reached the max allowable strain and strain compatibility is attained.

I hope this helps.

Kind regards

Dave Geeves

[dave_geeves]

Hi Siva,

My apologies for being a bit previous with my last reply.  I thought I would demonstrate the procedure by creating a short video showing the moment curvature approach with a user defined material being defined for the reinforcement, but when I tried it, it didn't work.  I only got it to work if I extended my max strain to 0.1 and adjusted my stress/strain curve accordingly - but this is not what I wanted to show.

It appears that all but Eurocodes reinforcement properties do not allow a minimum negative strain to be set and this causes a problem with the strain compatibility solution.

So, it appears that a realistic stress strain relationship defined for a material does not allow a realistic moment curvature solution to be obtained.

Hopefully the developers of the software will take note of this, as this limitation not only reduces the capability of the moment curvature analysis but also many of the other strain compatibility solution, including the interaction surface solutions.

My apologies again

Kind regards

Dave Geeves