Moment redistrubution

[OliverLonergan]

After carrying out a number of tests on a reinforced concrete propped cantiliver to ascertain the amount of moment redistribution occuring under a point load placed at the centre. the results appear to be the opposite of that expected, (simple support reaction decreases as plastic hinge forms at fixed end). I would like to hear anyones view and opinions on how they would feel the results should be analised in the above situation to ascertain the amount of moment redistribution occuring (if any??). thanks for your ideas.

 

[ThatEngineerDude]

OliverLonergan,
Building codes such as the 1997 UBC permits an increase or decrease of negative moments calculated by elastic theory at the supports of continuous flexural members by not more than 20 percent base on Ro balanced, see 1908.4
As your simple support reaction decreases as plastic hinge forms at fixed end perhaps you are experiencing the sumation of moments from propped end.
A note of caution, you may be approaching limits of elastic theory or shear failure or if over balanced (too much steel)near sudden brittle failure.
Try to calculate the applicable amount of moment redistribution expected by elastic theory. With only one negative moment, may not be applicable in your instance.

 

[dik]

Both the right and left reactions should be 1/2 of the point load with the propped end being reduced by the plastic moment / span and the reaction at the cantilever should be increased by this same amount. Since the reactions from the moment are 'ideally' fixed, then the propped reaction should increase linearly with the increased load (almost, anyway)... not sure why you are getting the opposite results...

The bottom reinforcing at the cantilever support is likely acting as compression steel, increasing the moment capacity. How have you determined the negative plastic moment (one at the cantilever)? The 'true' plastic moment capacity of the cantilever support should occur at the point where the propped support reaction starts to increase at 1/2 the increase in the applied point load.

My understanding is that plastic redistribution works better for doubly reinforced sections as well as lower concrete strengths, that is, lower strength concrete redistributes better (not as brittle) and the confining nature of doubly reinforced also helps...