Reducing Core Size 1

[StructuralAddict]

Hi!


In my design, the core around the elevators (working as shear walls) has to be reduced as the number of elevators will reduce from 3 (GF to Roof) to 2 (underground parking).
Is it possible to change the cross-section of the core in the lower levels?
Thanks

 

[jayrod12]

Sure it is. Anything is possible with enough space for structure and money. How tall is the building above GF?

 

[StructuralAddict]

It's 320 ft above GF and 36 ft below GF

 

[jayrod12]

Again, anything is possible with enough structure depth. You'd end up requiring some fairly substantial transfer beams in the ground floor framing. And considering you'd still require the pit for the 3rd elevator to extend below the GF level, you should indicate to the owner that they likely aren't gaining that space on the first parking level. At best, you'd gain the additional floor space on the second parking level.

Is there really that much savings to be had by eliminating an elevator for the bottom 3 floors. Seems like saving less than 10% of one elevator to gain 100 square feet of floor space total is a wasted effort.

 

[StructuralAddict]

The thing is that they need to have a driveway in place of the third elevator. They will gain many parking spaces by doing so.

 

[jayrod12]

Ok. But you still need a pit for that elevator. On an elevator of that size, it's likely a substantial depth, 6 or 8 feet. You likely have 12 ft floor to floor heights, you won't have enough clearance below the pit on the first parking level to use the space. They need to find another solution.

 

[StructuralAddict]

True, that's a very good point. The arch. dwgs. are showing the third elevator going from the 2nd floor all the way up. I think the architect wanted the third elevator pit to be located in the first floor. The arch. design is still in progress though.

 

[jayrod12]

Ok, then you would still need some significant beams in the main floor framing to transfer the loads to nearby columns and the remaining core. But I don't see why you can't make it work.

 

[StructuralAddict]

Thanks very much jayrod12.. we'll give it some thought..

 

[KootK]

One issue to watch out for is that, when you make substantial changes to your core, you tend to get very large shear forces at the levels near the discontinuity. It's hard to explain but it presents much like the backstay effect at podiums. This has caught me completely off guard in the past.

 

[ggcdn]

If this is a seismic area, you are venturing into irregularities that may be prohibited in your local building code, and for very good reason. But it's definitely possible. Your critical section will probably be the smaller core so I would make sure it can take the total flexure, shear x flexural overstrength, and confine it well. Especially the lower parts of any walls that step back. Those areas have traditionally been points of failure due to crushing.