WTC 1 floor load distribution

Yellow areas floor load is carried by the outer corner core columns.

Green areas floor load is carried by the outer core columns.

Pink area floor load is carried by the inner core columns.

The gray area floor load is carried by the red perimeter wall columns.

The other perimeter columns carry the load outside the yellow/green areas. Note that the perimeter columns carry different floor loads as the width of the floor varies.

In case the red perimeter columns fail at one wall side (buckling due to heat and the bolted floor connections to the perimeter columns shear off), the local load on the gray floor area is only transmitted to the opposite violet outer core columns

As only a few floors are involved, the violet outer (opposite) core columns will not be overloaded (they carry already plenty load from above).

Structure above the locally failed red perimeter columns may drop down to the ground or is carried by the intact structure above (redundancy due to spandrels - already seen at the initial damage - the hole in the wall - and further failures are arrested).

There is no way that local failures of red perimeter columns in one wall will expand to other sides perimeter wall columns (and cause local collapse around the whole area and later global collapse of the core and other structure below - due to free fall of upper block and impact of upper block on columns below causing a shock wave in the structure below - and the upper block, intact, pushing down the structure below). One reason is that the floors are only bolted to the wall columns and the bolts are the weakest link and will shear off before any load is transmitted from the perimeter to the core. Therefore local collapse of perimeter columns will not really affect core structure. The hat trusses will shear off if trying to transmit loads from perimeter to core. Read also about collapse arrest.

Anders Björkman 5 June 2008