Distribution of base shear forces

[gmenichini]

Dear SeismoSoft Support
I have to perform some time history dynamic analysis in a prefabricated single-storey building realized with 3 row of columns and rigid roof that i modelled with a rigid diaphragm, how you can see in the figure below:

The distributed beam load and mass on central row are higher than the two lateral rows, so the axial forces in the central row columns are higher than the other lateral colums.

When run the dynamic analysis, i have noticed that, despite the presence of the rigid diaphragm, the cumulative shear forces on central pillars are larger than the lateral ones.

Continue in the next post

[gmenichini]

In order to study this issue i realized a simple structure with two columns (Heigth 10m) fully restrained at the base and connected at the top with a rigid rod, the loads applied are shown in figure:

The two columns are identical and composed with a inelastic FB elements and with a RC section whose materials are C55/67 con_hs and steel B450C stl_bl

If we perform an elastic hand calculation we found:
RA=FA, RB=FB and VA=VB=H/2

Instead, with the values of H=10kN FB=50kN I run a linear static analysis changing the FA value for each analysis. I obtain (table of results):



It seems that the distribution of shear forces at the columns’ base depend on the axial force present.
It is this thing possible?? Maybe I did something wrong in modelling?

[huffte]

For reinforced concrete M - P interaction, it is common to observe an increase in moment capacity with increased axial load. It is also common to observe an increase increase effective, cracked moment of inertia with increased axial load. So it appears, in your case, that this is happening - the columns with higher axial load are both stiffer and stronger. So the phenomenon is not considered unusual.

Best of luck gmenichini.

[gmenichini]

Thank you for your timely and detailed answer.
Can you suggest me some litrature references where i can better study the theme??

Best regards

Giovanni

[huffte]

I would simply recommend a reinforced concrete textbook with a good discussion of column moment - axial load interaction. And if you have a good section analysis program, just play with some moment - curvature diagrams at various levels of axial load to observe the change in moment capacity and stiffness (stiffness is proportional to the slope of the moment - curvature plot). Typically, you will find that for axial loads up to the balanced point, moment capacity increases with increasing axial load. Once you get past the balanced point, the reverse happens. So, in summary gmenichini, a good RC text and a section analysis program will enable you to convince yourself of the validity, or lack thereof, of your results. Best of luck with all your work, gmenichini.

[gmenichini]

Thank you huffte for your answer and your precious help.

Best luck you too