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I'm watching the progress of a new office building being built over the road (essentially a manual timelapse).

The way the building progressed was by the pouring of a concrete core in the centre of the building to the full height (~8 storeys), before a steel beam frame was constructed around it to the full width/length of the building, obviously making a frame for the outer walls and floors.

Although the core is substantial (not just a pillar or something), it wouldn't seem to have any structural effect on the outer parts of the floors of the building (near the outer walls).

My lay-person's guess on the purpose of the core is as a kind of protected fire space (dry-risers? wet-risers?) containing the stairs and lifts/elevators, but if I knew the answer I wouldn't be asking here.

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  • $\begingroup$ Using the outer walls to carry structural loads is problematic, because the outer walls are usually full of "holes" for windows, access to balconies in residential buildings, etc. Thick outer walls (for strength) also limit light natural access, since daylight isn't "aimed" horizontally through the windows into the building. $\endgroup$ – alephzero Nov 7 '18 at 11:41
  • $\begingroup$ @alephzero In this case though, wouldn't it be the structural steel holding the loads, not the walls? Does the core in that case usually support the structural steel as well? I'm just curious because the building addition I'm working at now seems to be all steel structure with a lot of windows still and no concrete core that I'm aware of. I know very little about structural so I'm curious about all the different ways to design a structure. $\endgroup$ – JMac Nov 7 '18 at 12:29
  • $\begingroup$ It depends on the size of the building, but a steel frame with no structural panels between the columns that carry shear force doesn't have much lateral stiffness, so the building will tend to sway in the wind. The OP's design works pretty much like a big tree-house - you have a central "trunk" carrying the main structural loads, "branches" cantilevered out from the trunk supporting each floor, and something around the outside to mainly to keep the wind and rain out, not to support the structure. And just like an old tree, the trunk can be "hollow" without compromising how the design works. $\endgroup$ – alephzero Nov 7 '18 at 13:10
  • $\begingroup$ @alephzero - True that the concrete core takes lateral loads, but the steel frame does usually take vertical loads; the cost and thickness of cantilevering slabs would not be favourable. $\endgroup$ – AndyT Nov 8 '18 at 11:40
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The concrete core at the center of the building serves in several ways.

A structural hard center that is fire resistant and shields emergency utilities and elevators and emergency egress means like fire and smoke resistant corridors and stairs.

It is the main lateral load bearing component of the structure, designed to resist earthquakes and wind loads. It is basically a big shear wall cage.

The fact that it is the stiffest and heaviest part of the structure gives balance and symmetry to the massing distribution of building which is highly effective for limiting lateral torsional wobbling of the building at a big earthquake. At the same time being at the center of the building puts emergency exit corridors and stairs equally near to all occupants at each floor.

The reason they built the core completed fully first could be they used post-stressed vertical reinforcement and high strength concrete. Post-stress bars are long continuous vertical bars tightened at the very top of the building with special jacks and make the concrete core very strong and ductile for performing under lateral and vertical loads.

One of the most effective ways of designing an earthquake resistant building is to reduce the overall weigh of the floors and by using of thin post-stressed slabs as floors that have their tendons anchored at the central concrete core this is achieved.

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