17-05-2011, 01:48 AM
(This post was last modified: 22-05-2011, 03:37 AM by Jeffrey Orling.)
1. Buildings rarely collapse. They are designed to stand and carry the normal expected loads. And typical office floors, hotel floors and apartment floors are rarely loaded anywhere near the limit and over the entire floor area.
2. It's rather difficult TO impose an excessive load any floor in a uniform manner in any typical application. For example, if could do this if you turned a WTC tenant floor into a warehouse floor and loaded it up to the ceiling, with for example steel hardware. Of course what would happen as the shelves were filled with inventory the floor would begin to sag and someone would say... We're over loading this floor and we'll have to store the inventory on a stronger floor or in a warehouse designed for such loads.
3. The towers floors collapse because they experience an extremely rare totally unanticipated over load when the floors above were destroy and became the imposed load of 1000's of pounds per square foot on a floor designed to support 58#/SF. You could drop a wrecking ball or a tug boat or a tank on the floors and destroy them.. or a stack of safes or a extremely heaving equipment, motors, transformers, steel beams and so forth.
Side note: Recently I designed a roof terrace which was supported on steel beams which spanned over the existing roof into the parapet walls which were strong enough to take the load. The problem the contractor had was where to store all the steel beams on the roof BEFORE they were installed in beam pockets in the parapet wall. If he stacked them up in a neat pile ON the roof, it would likely collapse! So he had to rest them on top of two walls which formed the corner of the building on top of the parapet so the weight was not on the ROOF, but on the load bearing walls.
3. The twin towers were a very unique design with very lightweight construction for the very long span floor system. Rather than have a field of columns distributed throughout the floor, the design called for pre fab assemblies with would rest between the facade and a beam attached to the perimeter of the core. Cheap, fast to build, lightweight.. column free for flexible interiors... but vulnerable to the failure they experience. The Empire State Building... where I worked for a time on the 74th floor had a completely different structural system with columns in a 25' (or so) grid with masonry exterior walls supported by the frame. The ESB could not collapse as the twin towers... but perhaps one bay in the grid might if a huge load was imposed within THAT bay.
4. Because the floor system was a composite with the embedded trusses it also acted as bracing for the facade and for one side of the perimeter core columns. The facade most definitely required the floors for bracing. Without the floors the facade could not stand long.... too tall and too thin. The core columns could stand much like a steel grid frame of a typical high rise structure without the floors outside the core for bracing.
5. But even the core columns required bracing to stand 1362 feet tall. Absent the bracing the core columns were unstable and would collapse from their own weight. Surely you don't think a column which is 22"x52" made from 36' sections welded together and stacked up AND tapering down to as small as 12x12 at the top could stand with bracing or guys 1,363 feet tall.... or 700 feet tall. Or could remain stable with tons of debris collapsing and crashing around them?
So first the floors collapsed... then the facade fell away and finally the core columns buckled when the bracing WITHIN the core was destroyed by falling debris... inside the core.
2. It's rather difficult TO impose an excessive load any floor in a uniform manner in any typical application. For example, if could do this if you turned a WTC tenant floor into a warehouse floor and loaded it up to the ceiling, with for example steel hardware. Of course what would happen as the shelves were filled with inventory the floor would begin to sag and someone would say... We're over loading this floor and we'll have to store the inventory on a stronger floor or in a warehouse designed for such loads.
3. The towers floors collapse because they experience an extremely rare totally unanticipated over load when the floors above were destroy and became the imposed load of 1000's of pounds per square foot on a floor designed to support 58#/SF. You could drop a wrecking ball or a tug boat or a tank on the floors and destroy them.. or a stack of safes or a extremely heaving equipment, motors, transformers, steel beams and so forth.
Side note: Recently I designed a roof terrace which was supported on steel beams which spanned over the existing roof into the parapet walls which were strong enough to take the load. The problem the contractor had was where to store all the steel beams on the roof BEFORE they were installed in beam pockets in the parapet wall. If he stacked them up in a neat pile ON the roof, it would likely collapse! So he had to rest them on top of two walls which formed the corner of the building on top of the parapet so the weight was not on the ROOF, but on the load bearing walls.
3. The twin towers were a very unique design with very lightweight construction for the very long span floor system. Rather than have a field of columns distributed throughout the floor, the design called for pre fab assemblies with would rest between the facade and a beam attached to the perimeter of the core. Cheap, fast to build, lightweight.. column free for flexible interiors... but vulnerable to the failure they experience. The Empire State Building... where I worked for a time on the 74th floor had a completely different structural system with columns in a 25' (or so) grid with masonry exterior walls supported by the frame. The ESB could not collapse as the twin towers... but perhaps one bay in the grid might if a huge load was imposed within THAT bay.
4. Because the floor system was a composite with the embedded trusses it also acted as bracing for the facade and for one side of the perimeter core columns. The facade most definitely required the floors for bracing. Without the floors the facade could not stand long.... too tall and too thin. The core columns could stand much like a steel grid frame of a typical high rise structure without the floors outside the core for bracing.
5. But even the core columns required bracing to stand 1362 feet tall. Absent the bracing the core columns were unstable and would collapse from their own weight. Surely you don't think a column which is 22"x52" made from 36' sections welded together and stacked up AND tapering down to as small as 12x12 at the top could stand with bracing or guys 1,363 feet tall.... or 700 feet tall. Or could remain stable with tons of debris collapsing and crashing around them?
So first the floors collapsed... then the facade fell away and finally the core columns buckled when the bracing WITHIN the core was destroyed by falling debris... inside the core.