shear in columns in infilled

[Samra]

Dear all i am modelling an infilled rc frame using a truss for infill... im doing pushover analysis....when i compare my local forces for columns the sum of total shear force in the column base does not come equal to total base shear.......but if sum shear forces at the base of columns for bare frame at a given timestep they come exactly equal to the total reactions at the support....can somebody explain me why is that so and how do i get story shears for my frames?

[selimgunay]

Because, in the case of the RC frame infill, horizontal components of the diagonal truss axial forces contribute to the total base shear. If you consider those forces in your calculations, you should achieve equilibrium.

[Samra]

thanks for your response, but this explanation is acceptable for bottom story as also verified by my calculations where the total force at col base plus the horizontal component of the axial force in strut adds up to total base shear... but again what about the upper stories where the forces from strut are eventually transferred to the columns, shouldn't the sum of column shears there be equivalent to the applied nodal force.. if yes it isnt coming out to be this way... and that is what make me perplexed....

[selimgunay]

From equilibrium, some of the external horizontal forces above a story should be equal to the sum of the column shear forces and the horizontal components of the truss forces. Isn't this what you are getting?

[Samra]

no it is not.....

[selimgunay]

You can first make everything elastic and check if your equilibrium calculations are correct.

[Samra]

one more question please, if i apply monotonically increasing lateral loads on my infilled frame and at every time step extract the result for shear in cols, what i am getting is a slight negative values on the first time step ( which is the elastic shears) and later as the load increase they become positive and start to increase until the infill breaks....why is there a negative shear profile initially?

[selimgunay]

Although the negative forces sound a bit strange, one possible explanation could be the difference in deformation patterns, where the frame deforms in
a flexural mode and the infill wall (diagonal strut) deformation is governed by shear, which may result in the different direction of column and infill forces in the elastic range. Is the first step the only elastic step? Does inelastic response start immediately at the second step?

[selimgunay]

Or you may be getting these slight negative forces on the columns as a result of gravity analysis.

[Samra]

Thankyou for your kind response, i actually start to get positive values on the 3rd time step and i think that's elastic too because cracking doesn't start that early stage....so the possibility seems to be the gravity analysis as you have pointed out.... but why would there be negative shear due to gravity analysis?

[selimgunay]

The total base shear or story shear should be zero in gravity analysis, however this does not restrict having slightly negative forces on the columns and slightly positive horizontal forces on the infill struts, the sum of which would be zero.

[Samra]

Thank you so much for your kind response