Why do corner columns at the exterior take more axial load than interior columns?

[Tygra_1983]

Hi there,

I'm a bit confused and I am hoping someone can help. I thought due to the larger tributary area that an interior column has from the slab above it, therefore it would take more axial load. However, working in SAP2000 the corner columns at the exterior are more heavily axially loaded than the interior columns. Could someone explain to me why this is the case from a structural perspective?

Many thanks.

 

[Agent666]

Might help if you tell us the load case, structural system, etc? The answer to that might explain why, or maybe you made a mistake modelling loads.

For a portal frame system you could under seismic have an axial load larger than gravity. Simply because on internal columns the beam shears cancel at each level either side of the columns, but exterior corner columns the beam shears add up over the height of the building.

https://engineervsheep.com

 

[Tygra_1983]

Hi Agent666

It's a rigid frame tall building system simply under the dead load for the load case.

Here, see the axial force diagram of the left window.

Many thanks

 

[Agent666]

You sure you have supports provided under the internal columns and not just at the four corners?

You sure the model is even solving and not spitting out ill condition errors in the analysis log?

https://engineervsheep.com

 

[dik]

They may be carrying cladding loading...

-----*****-----
So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik

 

[Tygra_1983]

@Agent666 - Yes, I have assigned supports under the interior columns as well. I am not quite sure how to check the analysis log.

@ dik - I haven't modeled any cladding. It's just the structural skeleton.

 

[Tygra_1983]

I think this might be the analysis log:

Seems okay.

 

[dik]

If there is no cladding weight, then likely, something is not correct.

-----*****-----
So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik

 

[Agent666]

Review deflections? Usually that will give you an idea of what doesn't appear right.

Check anything that might affect the axial stiffness, is one of you E values orders of magnitude higher/lower than it should be. Is one of your column sections size defined in the wrong units and much larger than it should be.

What you're seeing appears wrong. Try modelling something similar from scratch, do you observe the same effect or have you inadvertently not made the same mistake somewhere along the way?

https://engineervsheep.com

 

[Tygra_1983]

From watching a Youtube video I modelled it in the same way as the video and I am now getting correct results - axial force is greater now in the interior columns. Problem solved. Thanks for all your help guys.