Reducing Core Foundation Size

[LucasAU]

I have a two storey building consisting a concrete slab at first floor and a steel roof.
There are 2 core walls in the middle and 12 band beam running in Y direction with 8m spacing supporting 260mm concrete slab. Each band beam is 5 x 8m span = 40m long. So the dimension of the building is quite large (120m x 40m).
Now I have worked out the overturning moment from wind in X direction is 3000kNm while 8000kNm in Y direction. The axial force in each core is quite small (2000kN) because they are locating at the edge. Considering this, if I design the core wall footing to carry the whole overturning moment, the size of the pad foundation will be quite large. (founding material is basalt rock with 800kPa allowable bearing). So I am wondering if I can reduce the core pad size and consider the beam-column frame to take the overturning too?

Simply put, if the building has no core wall then the lateral load will become axial force in the columns of the frames (still pin the column-pad footing). In another word, can I reduce the footing size of the core and let it rotate under lateral load so then lateral load will go to the frame and result in axial force in each column? What about the X direction (considering there is not beam running in X direction)?

 

[hokie66]

Not sure why you would describe the load on columns in a moment frame to be axial. If the load is horizontal, the columns would be resisting the forces by bending. The floor structure would also have to resist this bending.

Another way of reducing the size of core pads is to employ rock anchors. These should be quite simple to install in the basalt.

 

[LucasAU]

Sorry for misleading. I was trying to say I was gonna design the pad footing to be pinned so the footing will be subject to axial force only without any moment (with the beam-column connection to be rigid it still gives you lateral resistance). In short, can I let the foundation for core wall to 'fail' in overturning and what happens if it rotate? Will the loading transfer to the beam-column frame and then rely the footing under column to take the overturning?

 

[KootK]

In short, can I let the foundation for core wall to 'fail' in overturning and what happens if it rotate? Will the loading transfer to the beam-column frame and then rely the footing under column to take the overturning?

Yes and yes. Search the term "foundation rocking" and you'll encounter a similar principle used often in seismic design. It'll be prudent to still design the core foundation for the loading case associated with the rocking of course. So not just pure axial on the core foundation.

I like Hokie's rock anchor solution as well. Given that you don't have frames in the other direction, you may wind up using rock anchors at the core for that reason anyhow. And, if that's the case, I'd be inclined to just use them in both directions. Making use of the stiffest systems available for your lateral loads is intuitively appealing even if other options are possible.

 

[LucasAU]

Yes, the other direction concerns me the most. Can I rely column-flat slab system to take lateral load in that direction if the core foundation rotate? The builder is inexperience in anchor and wants it to be easy and cheap for them, that's why I have to reduce the core foundation size to make it look reasonable for them (they have been questioning why a two storey building requires such a big pad like a high rise building footing...). If I let the core foundation rotate and let the column-beam and column-slab to take the lateral forces, the increased axial force in column wont change much considering the numbers and the spans of the frames.

 

[hokie66]

Good luck with your inexperienced builder. Sounds like a recipe for disaster. Don’t make it your problem.

A word to the wise...your building is 120 metres long, and has restraining cores both ends, therefore you have shrinkage cracking issues.

 

[KootK]

Can I rely column-flat slab system to take lateral load in that direction if the core foundation rotate?

Yes, so long as:

1) You can demonstrate that lateral system is viable and;

2) Your local codes don't prohibit that system as is sometimes the case in high seismic situations.