hello
if concrete models con_ma and con_me are employed for eigenvalue analysis, the compressive strength (fc) values must be reduced by a factor that is equal to the square of the stiffness.what dose mean'stiffness'?
Eigenvalue analysis
Re: Eigenvalue analysis
bakhtiyar, the real f'c value must be reduced by a factor that is equal to the square of the "stiffness reduction", not the square of the "stiffness".
For example, one frequent assumption is that concrete cracking effects reduce the effective stiffness to 1/2, or 1/3, or 1/4 of the stifffness computed based on gross section dimensions and naterial properties.
So, the account for cracking, if you wished to model an effective stiffness of 1/2 of the gross stifness, then you could specify an artificial f'c which would equal (.5)^2 = 0.25 times the actual f'c.
And so on. Sometimes users will simply wish to model some arbitrary fraction as mentioned above. To obtain a more realistic estimate, a moment - curvature analysis of the cross-section with the appropriate axial load level can be performed to find a truer value of effective stiffness - to - gross stiffness ratio.
For example, one frequent assumption is that concrete cracking effects reduce the effective stiffness to 1/2, or 1/3, or 1/4 of the stifffness computed based on gross section dimensions and naterial properties.
So, the account for cracking, if you wished to model an effective stiffness of 1/2 of the gross stifness, then you could specify an artificial f'c which would equal (.5)^2 = 0.25 times the actual f'c.
And so on. Sometimes users will simply wish to model some arbitrary fraction as mentioned above. To obtain a more realistic estimate, a moment - curvature analysis of the cross-section with the appropriate axial load level can be performed to find a truer value of effective stiffness - to - gross stiffness ratio.
Tim Huff