Column load calculation in flat slab concrete building 1

[milkshakelake]

Which is the correct way to calculate axial loads on concrete columns: tributary area, or analysis using continuity of the slab? Taking tributary area would overestimate loads in outer columns and underestimate loads in interior columns.

However, ACI 315R-11 (which is limited to 5 story buildings) states: 9.9.1 Vertical reactions at columns and walls - Vertical reactions of the supporting members, Ru, should be determined as the total factored design load qu multiplied by the area bound by panel centerlines around the supporting member (Fig. 9.6.1). Maybe since it's limited to low rises, the "error" is within acceptable limits. But I have to design a 10 story building with lots of transfer slabs.

 

[Trenno]

If I'm not mistaken, one way to account for these effects is to design transfers to more stringent deflection criteria (ie span/500 - span/1000).

 

[Settingsun]

KootK, what is the axial force in column 1-2 on grid B? Is it 2.5 times the compression from normal unstaged analysis, or is it actually in tension (trying to hang from the upper floors)? Or something else?

 

[SteynvW]

Here is the the link of where that screenshot is taken from.

https://ottegroup.com/wp-content/uploads/Technical-Note-No.-TN-S03-Sequential-Const-Analysis.pdf

 

[Settingsun]

Thanks, SteynvW. Why does column 1-2 stretch by 13mm when it's in compression? And its compression is less than column 2-3? Need to think this through... but also happy to sit back and listen to a buildings person

 

[KootK]

In most cases, the antidote is the normal practice of designing the transfer slab in a 2D model independently of the carrying capacities and stiffnesses of the floors above. But yeah, you want a transfer stiff enough that you're not taxing upper level framing in significant, unexpected ways.

 

[rapt]

Kootk,

And then you get differential creep in the columns over the construction period that changes this further. Plus shrinkage, creep and cracking in the transfer member.

And then you stage prestress the transfer and the results are even closer to the independent 2D transfer beam/slab using tributary areas for the column loads!

So unless you can do the full building analysis allowing for stage construction, all non-linear effects and everything else, adopt the worst of full building elastic FEM and tributary areas for each column.

MIStructE_IRE
You either size the columns to be stressed more closely to the stress levels in the walls (architect trouble), or you artificially increase the axial stress in the walls by vertically prestressing them down to the foundations.

Or allow for a lot of adjustment in your lift setup and cladding.

Or build symmetrical buildings!

 

[KootK]

So unless you can do the full building analysis allowing for stage construction, all non-linear effects and everything else, adopt the worst of full building elastic FEM and tributary areas for each column.

Agreed. That's what I'd proposed in my previous post.