assigning effect of cracking in concrete beams

[mohamad reza]

hello seismosoft

in concrete beams there is initial crack due to service loads such as crack due to deflection under Dead loads and Live loads.for considering reduction of moment of inertia which caused by cracking,it is customary for assigning a modifier coefficient it is commonly o.5 for moment of inertia in beams.

how we can assign this reduction coefficient for beams in program? in fiber model can not consider this initial reduction . this reduction in moment of inertia obviously has effect on period of structure and it is important to consider it in model ,

regard

[huffte]

Mohamad,

One way to accomplish this would be to use a material with E-effective set equal to 1/2 of the actual E-value. OF course, this will reduce not only EI, but EA - axial stiffness - as well.

This would only be applicable to elastic frame elements I believe. For nonlinear analyses with fiber elements, this should not be necessary, I would think. The fiber-element approach takes account of the nonlinearity explicitly. The reduced mement of inertia method has typically been used when the analyst has access only to linear analysis engines in an effort to account for nonlinear effects.

[seismosoft]

Dear Mohamed, huffte is indeed correct.

If one is running elastic analysis (such as eigenvalue analysis), then concrete cracking effects may be accounted for through the reduction of the Young Modulus (E) assigned to the concrete.

If, however, one is running inelastic analysis (such as pushover or nonlinear dynamic analysis) using fibre elements, then concrete cracking is typically taken into account already by the concrete stress-strain relationship.

Seismosoft Support

[zthaer]

dear all;
i ' m concerned about how to introduce these values in Eigenvalue analysis, since if i modified, for instance, Ec (i.e. for columns) to 0.7Ec i reduced the shear and axial rigidity by 30%
how to separate these quantities in seismostruct ??

Component Flexural Rigidity Shear Rigidity Axial Rigidity
Beams 0.5EcIg 0.4EcAw -
Columns 0.7 EcIg 0.4 EcAw EcAg
Walls 0.5 EcIg 0.4 EcAw EcAg

best;
Thaer;

[seismosoft]

On way to avoid this problem would be to employ the elastic frame element elfrm with reduced EA and EI values.

Seismosoft Support

[zthaer]

dear seismostruct team;
is there any more simplified method to do since i built the model already, and i see that the program tends to adopt the Eurocode in which the effective stiffness could be 50% of the gross one for all actions.

best;
thaer

[seismosoft]

Hi Zthaer,
Unfortunately it is not possible to automatically change the inelastic frame elements with elastic ones.
Regards,
Seismosoft Support