[Kelvin]

Lexy,

The concept of tube and bundled-tube both allow that a structural interior is not required - at least in the way that you are thinking. In both of these systems the function of interior core is moved to the outside edges, where sets of closely spaced perimeter columns and rigidly connected beams provide resistance to the major structural loads including horizontal shear. This is the force that pushes the building back when the wind blows, but there also needs to be a means of carrying that load to the ground so it has to have a vertical component as well. Even so, tube structures also often use interior columns but they are assigned as gravity-load columns (meaning pure vertical loads derived from floors) and do not otherwise contribute to the lateral load resisting system.

Bundled tubes were conceived after it was realized that the major drawback to "single" tube design was the large accumulation of shear forces at corners - so-called shear lag - which reduced some of the basic efficiency. To counter this, smaller tubes were "tied" together, breaking up the total shear-lag into lesser pieces and improve the overall efficiency.

In other framing systems, a structural core might be employed - the type that you are thinking - that is reinforced concrete. However, as buildings get taller, this system becomes less efficient and tube concepts are favoured. In these cases, the continuous rigid plane of the concrete or block wall induces horizontal rigidity.

Another alternative are braced frames (e.g. ESB) and even braced tubes (e.g. John Hancock). In these systems the diagonal elements of X- and/or K-bracing provide the necessary load path to the ground.

So while tubes are efficient structurally, the trade off is the loss of open area at the perimeter - meaning possibly compromised window area. Recall that the WTC had 39" column centres but only about 20" left for glass. At the time, they weren't sure if it would be accepted but obviously it was. Similarly, the large diagonals on the exterior of Hancock were initially a concern because they thought offices with the reduced window area would be hard to sell. Strangely enough - once the building became accepted and even an icon - those offices now sell at a premium.

Your final question - as to why there is an apparent lack of a reinforced core in the construction photos - is that there is none (in tubes). Again, the rigidity necessary for stability is provided by the closely spaced exterior columns and heavy beams. There are "cores" where the elevators and mechanical services are located, but mostly comprised of conventional construction. The core in the WTC was simply steel column (to carry interior "gravity" loads) and covered with gypsum board.

This is perhaps the great irony of the WTC design: the rigidity of the tube system employed allowed that a structural core was not required - one that may have also prevented spread of the fire (i.e. the core of the building was easily compromised as a result of the attack and allowed greater distribution of flammable fuels). By the same token, it was the rigidity of the tube system, and it's incredibly over-redundant frame, which allowed the building to even stand after loss of great pieces of the exterior frame.