JК
11 сентября 2001 :: Мейссан Тьерри
Страница: 254 из 286Is this because the perimeter wall was not required to carry much of the weight of the building, but was mainly a feature designed to give the WTC its required rigidity (against wind loading)? Notice the three parallel light-colored lines (about 4 inches wide) in the concrete. One also wonders why the pile of steel in the foreground was hoisted up the building, unless it was to be incorporated in the structure.
These figures illustrate this construction, and Figure 2-9 shows a cross-section through typical floor framing. Floors were designed for a uniform live load of 100 pounds per square foot (psf) over any 200-square-foor area with allowable live load reductions taken over larger areas. At building corners, this amounted to a uniform live load (unreduced) of 55 psf.
At approximately 10,000 locations in each building, viscoelastic dampers extended between the lower chords of the joists and gusset plates mounted on the exterior columns beneath the stiffened seats (Detail A in Figure 2-6). I find it really strange that dampers are attached to only one end of each truss. It doesn't make much sense to dampen vibration at one end while letting the other end «blow in the breeze». These dampers were the first application of this technology in a high-rise building, and were provided to reduce occupant perception of wind-induced building motion.
Figure 2-2 Representative structural framing plan, upper floors.
You may wish to compare the above floor plan with this one taken from Godfrey, GB (Editor); Multi-Storey Buildings in Steel, Second Edition; Collins, London, England,1985, ISBN 0 00 383031 4. The differences are quite telling.
Figure 2-2-A Structural system for typical floor.
The numbers in the figure denote:
13 — Perimeter frame
14 — Bar joists 900 mm deep
15 — Secondary joists
16 — Horizontal floor bracing
17 — Core box columns
That the second floor plan is more accurate than the first, is plain from the above photo where the diagonal brace members (the V-shaped features in the diagrams) are clearly visible in the concrete along the north wall, but not along the west wall (as in the second diagram but not the first).
Pairs of flat bars extended diagonally from the exterior wall to the top chord of adjacent trusses (this is puzzling, as the top chords of the trusses are set in the concrete slab, yet one can clearly see these bars on the concrete surface in the above mentioned photo) . These diagonal flat bars, which were typically provided with shear studs, provided horizontal shear transfer between the floor slab and exterior wall, as well as out-of-plane bracing for perimeter columns not directly supporting floor trusses (Figure 2-2).
These figures illustrate this construction, and Figure 2-9 shows a cross-section through typical floor framing. Floors were designed for a uniform live load of 100 pounds per square foot (psf) over any 200-square-foor area with allowable live load reductions taken over larger areas. At building corners, this amounted to a uniform live load (unreduced) of 55 psf.
At approximately 10,000 locations in each building, viscoelastic dampers extended between the lower chords of the joists and gusset plates mounted on the exterior columns beneath the stiffened seats (Detail A in Figure 2-6). I find it really strange that dampers are attached to only one end of each truss. It doesn't make much sense to dampen vibration at one end while letting the other end «blow in the breeze». These dampers were the first application of this technology in a high-rise building, and were provided to reduce occupant perception of wind-induced building motion.
Figure 2-2 Representative structural framing plan, upper floors.
You may wish to compare the above floor plan with this one taken from Godfrey, GB (Editor); Multi-Storey Buildings in Steel, Second Edition; Collins, London, England,1985, ISBN 0 00 383031 4. The differences are quite telling.
Figure 2-2-A Structural system for typical floor.
The numbers in the figure denote:
13 — Perimeter frame
14 — Bar joists 900 mm deep
15 — Secondary joists
16 — Horizontal floor bracing
17 — Core box columns
That the second floor plan is more accurate than the first, is plain from the above photo where the diagonal brace members (the V-shaped features in the diagrams) are clearly visible in the concrete along the north wall, but not along the west wall (as in the second diagram but not the first).
Pairs of flat bars extended diagonally from the exterior wall to the top chord of adjacent trusses (this is puzzling, as the top chords of the trusses are set in the concrete slab, yet one can clearly see these bars on the concrete surface in the above mentioned photo) . These diagonal flat bars, which were typically provided with shear studs, provided horizontal shear transfer between the floor slab and exterior wall, as well as out-of-plane bracing for perimeter columns not directly supporting floor trusses (Figure 2-2).