The 9/11 Forum

Just a note about normal demolitions. The largest CD was a 25 story structure which I believe had much more masonry that any of the towers.

Why does a CD have to be one massive explosive event is you are intending to say destroy hundreds of joints over a couple of second? I am not proposing that hundreds of column splices destroyed over this time would leave no audible signs, but I am suggesting that hundreds of small events could be blending into background noise and hard to pick out from the mic location.

Since the facade doesn't seem to crumple up, it possibly implies that the upper facade slipped past the lower on... probably outside it. Does this mean that the core went first and it pulled the perimeter columns in and with it the lower attached facade so that the upper facade and columns above slide over the those below? This would be a kind of inward pulling verinage. The inward bowing of the facade seems to suggest it was pulled in and it is not dropping as perhaps many read the kink in the roofline.

Since there may have been a consideration not to destroy the adjacent structures so close on the east and west the destruction had to be an implosion and this means that the center must be somewhat "hollowed out" and the area around it pulled in. Could the dropping of the top penthouse through the core be the mechanism which accomplished this?

Hudson Dept Store was basically a 25 story block with a smaller high rise for services and several basements.

CD's don't usually destroy every single beam to column connection id the building is tall enough and has enough PE for gravity to bust the rest. So a typical one would implode the building by taking out a series of main columns in the center, the mass above it drops down and the beams connecting to the peripheral columns pull those columns and floors and attached facade with it.

CDs are done for economy and time. Dismantling a building by "hand" piece by piece is expensive and time consuming and in the case of a long span bridge a tactical nightmare.

The collapse needs to break the structure up or else the result remains a difficult clean up. So preparing a lot of connections by weakening insures that the gravity forces will break it completely apart and pile it in the foot print. Several failed CDs fail to release enough gravitational dynamic load to break apart the structure. Reinforced concrete with rebar presents more challenges than steel frames with shear and moment connections.

The hull and core design is a less complicated CD with the long span column free interior giving fewer but larger columns, girders, connections and so forth... and facilitating an implosion by pulling down the core which then drags the outer region down after it. In addition with fewer structural members mean they are heavier per foot and more "lethal" in their destructive effect when falling. This means you have beams framing into girders and then into columns.

Perimeter columns would be "attacked" at the spandrel height and not readily visible through the windows as the spandrels cover the floor structure.

Steel columns don't have to be blasted apart.. they are usually resting on atop the other with a bearing plate and relatively thin splice places welded on. The girders framing into them provide the lateral support so destroying the splices and removing / destroying the shear or moment connections at the girder to column connections will allow the columns to drop like pick up sticks.

All high rise have what's called a core. This is where the elevators and egress stairs are located, various services, bathrooms and risers for wiring, plumbing and ventilation.

Many high rises are more typically a complete lattice type frame with a bay system of 20' x 20' or so. They have deeper stronger girders framing into the columns with beams framing into the girders. The floor spans are repetitively short. These designs, like the Empire State Building are less susceptible to the type of collapse where the core is taken down and it pulls the rest of the building with it. The certainly would not experience a runaway floor collapse.

All three WTC towers were versions of the long span column free interior concept. The difference between the twins and 7 was that 7 had spaced columns and long structural spandrels and a non structural curtain wall attached (hung) to the spandrels which the twins had a structural facade - no curtain wall. The facade was part of the wind shear strategy and was vierendeel truss. Lattice frames usually have large diagonal members to brace the frame like the diagonals in a truss and they may be connected at the out columns as well as in the core to increase its overall stiffness.

Do you know what curtain wall is? Here is the wiki definition.

"A curtain wall is an outer covering of a building in which the outer walls are non-structural, but merely keep out the weather. As the curtain wall is non-structural it can be made of a lightweight material reducing construction costs. When glass is used as the curtain wall, a great advantage is that natural light can penetrate deeper within the building. The curtain wall façade does not carry any dead load weight from the building other than its own dead load weight. The wall transfers horizontal wind loads that are incident upon it to the main building structure through connections at floors or columns of the building. A curtain wall is designed to resist air and water infiltration, sway induced by wind and seismic forces acting on the building, and its own dead load weight forces.

Curtain walls are typically designed with extruded aluminum members, although the first curtain walls were made of steel. The aluminium frame is typically infilled with glass, which provides an architecturally pleasing building, as well as benefits such as daylighting. However, parameters related to solar gain control such as thermal comfort and visual comfort are more difficult to control when using highly-glazed curtain walls. Other common infills include: stone veneer, metal panels, louvers, and operable windows or vents.

Curtain walls differ from store-front systems in that they are designed to span multiple floors, and take into consideration design requirements such as: thermal expansion and contraction; building sway and movement; water diversion; and thermal efficiency for cost-effective heating, cooling, and lighting in the building."

And this is precisely what WTC 7 had.

Where were the "exterior" columns"?