14-08-2013, 01:55 AM
Jeffrey Orling Wrote:Tony Szamboti Wrote:Why would you challenge something like that?
Not all the facade columns had the same FOS... the higher FOS would have been where they used the thinnest steel to reduce OAL weight so that the columns at the lower part were not solid and could be bolted together. They used higher grade steel because A36 was not stiff enough... There is no load reason to use the higher grade steel.
I don't trust everything that NIST writes either and therefore everything they say is taken with a grain of salt. The FOS at the base was nothing like 5:1. I find the pull in makes little sense. But I wasn't there.
Neither were you.
There was a load reason to use higher grade steel (higher yield strength) at different locations on the exterior. That was because the engineers wanted to keep the unit stress between the core and perimeter the same at each story to eliminate differential deflection between them and the floor warpage it would cause and for wind load resistance. Heavier wall columns would increase the load and the unit stress at different locations.
See the Engineering News Record article on this here http://911research.wtc7.net/mirrors/guar...record.htm. I quoted the part on the exterior columns below.
HOW COLUMNS WILL BE DESIGNED FOR 110-STORY BUILDINGS
For record-height towers of New York's World Trade Center, engineers proportion columns to avoid floor warpage when high-strength steels are used for exterior columns and A36 steel for interior columns.
A design procedure that will be used for structural framing of the 1,350-ft high twin towers of the World Trade Center in New York City gives the exterior columns tremendous reserve strength. Live loads on these columns can be increased more than 2,000% before failure occurs.
The procedure calls for proportioning of columns in each story for the same unit stress under gravity loads, regardless of the grade of steel in the columns. Thus, all columns will shorten the same amount, and differential shortening will be eliminated as a possible cause of floor warpage. The reserve strength of high strength steel members will then be available to resist wind stresses.
The structural engineers adopted this particular design because of the great length of the columns, use of different grades of steel and their plan to take wind stresses in the exterior columns only.
The concept was explained to the New York Architectural League by John Skilling, a partner in Worthington, Skilling, Helle and Jackson, of Seattle, consulting structural engineers on the World Trade Center (see p. 124).
Record-height towers. The. Port of New York Authority's World Trade Center will provide offices and exhibit areas for government agencies, trade services and private business concerned with exports and imports. The project will occupy a 16-acre site along the Hudson River in downtown Manhattan. Its twin towers, 110 stories high, will be 100 ft taller than the Empire State Building (excluding its TV antennas on --top), currently the world's tallest building (ENR Jan. 23, p. 33). Rising the full 1,3 50-ft height without a setback, each tower will be 208 ft square. It will be designed to resist a 45-psf wind, with both low sway and low acceleration.
Exterior columns will be spaced 39 inches c-c. Made of various high-strength steels, they will be 14-inch square hollow-box sections, for high torsional and bending resistance, and windows will be set between them. Spandrels welded to the columns at each floor will convert the exterior walls into giant Vierendeel trusses.
Your FoS study is not correct.