13 Apr 01 23:25
I had a peculier observation in analysing a simple single bay single storyed symmetrical frame,with fixed supports and with a uniformly distributed load on the beam.
A very simple problem to solve indeed. The moment at the column base shall be 50% of the moment at the top of the column. Since, we deal with only relative stiffnesses between the column and the beam at the junctions, the answer shall be same for the given load,irrespective of the actual stiffness of the members as long as the stiffness ratio between the beam and the column is unchanged.
I observe that the result is not so. For values of lower stiffnesses (lower I/L), the results are accurate. But the support moment at the column base reduces drastically (as low as 10% of the top moment for higher stiffnesses)if the stiffness of the individual members are increased keeping their ratios same. However, the other moments remain unchanged.
I am not able to find any logical reason for this.
I fear that this may give rise to a gross underestimation of column base moments leading to underdesign of foundations, if the results from the output are taken as correct.
Can anyone offer any reason for this?
Has anyone noticed this before?
Can anyone check this in any other proven software and inform the result?