Posted by ngon nguyen on September 25, 2001 at 12:04:43:In Reply to: Re: WTC Collapse posted by John A. Giambalvo on September 24, 2001 at 06:51:40:
I do not know how to enlarge my response window to see what I actually post. Here is my theory about WTC collapse:
WTC has two towers of 1368 & 1362 foot tall with 110 floors each. In plan, each tower has a square shape of 208 foot each side. Its perimeter is a framed tube built from W14 (beam-column frame on the perimeter). Frame tubes are among the most widely used structural systems for providing lateral stability and lateral loading in tall buildings. This system has been used successfully in many of the world tallest structures. However, the efficiency of the framed tube is limited by two factors:
1. Shear lag in the flanges of the tube reduces the effectiveness of columns away from the corners;
2. The axial stiffness of columns in the webs of the tube, especially the columns well away from the corners, is not used effectively for resisting lateral load.To overcome this and to increase the stiffness, the viscoelastic damper was conceived and developed as a part of the structural design for the twin towers of WTC. They are an integral and an essential part of the structural system, being designed to assist the tubular steel frame in limiting wind-induced building oscillation to levels below human perception.
Each of the two towers employs approximately 10,000 viscoelastic dampers. The dampers are distributed evenly throughout the buildings (about 100 on each floor- from the 10th to the 100th floor). They are located between the lower chords of the horizontal trusses and the columns of the outside wall. Upon oscillation of the building there is a relative motion between the lower chord of the truss and a column on the building perimeter.( Note this also means the top chord will rotate relatively to the perimeter column). this motion generates shear deformation in the viscoelastic part of the damper thereby dissipating a part of the energy of oscillation.
For vertical loads, besides the perimeter columns there are rows of columns at the center core surrounding all the elevators. Steel Joists are employed to carry lightweight concrete deck spanning between the perimeter columns and the center core columns. These Steel Joists are trusses made out of angle irons (L-shaped rolled steel). Their top chords are connected or the whole truss is supported by a small shear plate welded to the columns. (The truss has an inverted trapezoidal shape). Theoretically, These plates are to carry shear only however because of the welding, there is some restraint against rotation. As mentioned above the perimeter column rotates relatively with the top chord, thereby inducing bending in the shear plate. The shear plate is under combined shear and bending , is subject to cyclic wind loading all the time.
As the airplane strikes the tower and fire is started. The elevator shafts acted as a flue spreading the fire to all floors. The viscoelastic system is destroyed first because they are made out of rubber. Thereby more rotation between the top chords and the perimeter columns combined with heat leading to the shear plate failure. One floor fails leads to another and so on. This is a progressive failure and a DESIGN FLAW!!! Had the designer paid attention to his detail and the possible failure mechanism, progressive collapse could have been avoided. Had in place of the shear plate a true hinge (such as a door hinge) been designed the progressive failure would not happen. Another point well worth mentioned is that above 400 degree C lightweight concrete would become powder. At this temperature, the calcium silicates commence to decompose into quicklime and silica. This process is irreversible and there is a progressive loss of strength with time.
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