How is permissible moment computed?

[Anonymous]

I used the load optimization feature to get multiple envelopes that I transferred to a design beam.

I thought of permissible moment as the flexure that a beam can carry at different points, computed through section properties. However, from the two screenshots, it can be seen that the permissible moment changes with different loadings. This is using the AASHTO LRFD 7 design code. Why is this so?

 

I ask because I plan to see how a design needs to be changed when designing the same structure for both AASHTO and Eurocodes + UK annexes. I originally planned to do just get differences from design moments and adjust the sections to completely account for that change in moment. However, I do not know anymore if that would work, as the loads themselves seemingly contribute to the beam's moment capacity.

Also, I was wondering if the resistance factor is part of the computations. Below is a screenshot of a beam with the same loading but with an extremely increased girder depth. (I also adjusted reinforcements but we can ignore that). Does this software account for the resistance factor of 0.9 for tension-controlled girders and decreasing to 0.75 as it becomes more compression-controlled?

Thank you!

[dave_geeves · Accepted solution]

Hi,

Without having access to your model it is quite difficult to say what is causing this.  If your model is a grillage with offsets (eccentricities applied to obtain the correct elevation of each element) than an axial force will be generated into the beams members.  The value of the applied axial force will be different for each load case applied.  When the moment capacity is calculated by a strain compatibility analysis then the axial force is taken into account so that the resistance to bending changes as the axial force changes.

Yes, the resistance factor is applied and this can be seen in the detailed calculations when the appropriate point of interest is selected and the results button is clicked.  The appropriate resistance factor is determined as defined in the AASHTO code.

I hope this helps and if it has answered your query than please accept my reply as a solution so that others may benefit.  Thanks.

Kind regards

Dave Geeves