Re: [design] Big Boxing

Yes, the WTC floor structure had bar-joists spanning the large
open floors from the exterior column-beam grid to the interior
core.

Bar joists hardly resist moment forces at their points of bearing,
and this is one of their fundamental weaknesses although very
cost-beneficial compared to the bolted or welded connections
of rolled members.

At WTC we photographed among the debris the bearing angles
for the bar joists at the core and at the exterior wall. None showed
remnants of the bar joist connectors, which easily break away under
relatively light lateral movement -- they are customarily secured
with 2-4 bolts which provide almost no resistance to lateral
movement.

Some studies of WTC show the bar joists sagging as heat weakens
them and then pulling away from their supports at each end, then
as one falls atop that below, a cascading pancaking occurs,
pretty much accounting for the directly downward imploding of
the structures.

It is also suggested that the initial impacts and subsequent
extreme swaying of the towers fractured the bar joist connections
and thus assisted their collapse as they sagged from heat.

Had those connections been stronger, the much stronger exterior
shell and core could have remained connected and likely would
have withstood the attack if the tube had not been eviscerated
by the collapsing floor plates.

Among WTC debris the exterior sheel and the interior core
columns are the large remnants. Few bar joists could be seen,
well except for a few of their end connectors which remain attached
to the more durable structural elements.

Weak bar joist connections are known to be the cause of
blow-downs and collapses of open space structures in earthquakes,
hurricanes and tornados. It is odd that Robertson did not make
stronger connectors to resist lateral movement. Except that
bar joists have relatively low compressive strength along their
horizontal axis, so Robertson may have been unable to design
for that exceptional horizontal loading. Reason: bar joists were
invented to reduce the cost of structural steel, and were never
intended to resist the varieties of loading rolled members can
handle.

Recall that the Port Authority elected to increase the towers'
height by 20, then 40, stories because of economies Robertson
introduced with his daring design.

There are photos of models at 70, 90 and 110 stories, with the
floor plate remaining constant. Which means Robertson was
squeezing more potential out of earlier design assumptions.

This squeeze is not uncommon in high design, and it is always
a late decision in the design process to set the number of floors,
with the number riding on how deft the structural designers
are. To be sure, it is done with some ground, air, space and
marine craft to design right against the limits of failure, push
to those limits by financial and legal design.



Folow-ups
  • Re: [design] Big Boxing
    • From: Michael Kaplan
  • Re: [design] Big Boxing
    • From: brian carroll
  • Replies
    Re: [design] Big Boxing, John Young
    [design] Big Boxing, brian carroll
    Re: [design] Big Boxing, Michael Kaplan
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