Design of L’Ambiance Plaza
The L’Ambiance Plaza was designed as a 16-storey building in the centre of
Bridgeport, Connecticut. It consisted of 2 offset rectangular buildings (19.2 x 34.1m)
joined by an intermediate Lift Shaft which was to service both towers.
The structural frame of the buildings was a steel column grid, while posttensioned concrete slabs were used to span the floor levels throughout. Designers of
the building used the method of post-tensioning the slabs to overcome the concrete’s
tensile weakness – thus inserting high strength tensile steel along the length and width
of the slabs prior to pouring. These specific cables were then tensioned with hydraulic
jacks to the required stress level thus placing the concrete in a compressive state and
capable of withstanding greater tensile forces.
Each concrete slab was constructed on site at ground level, one-on-top of the
other separated by specified bond breakers. The Lift-Slab technique of construction
used to mount the concrete floor slabs on the frame was patented by Youtz & Slick
1948:
2/3 slabs were lifted at a time into position by a hydraulic lifting apparatus
located on top of each column. The extended lifting rods hoisted the slabs up into
position by the lifting collars cast in the slabs. Steel wedges were used to hold them in
place and then each slab was then permanently fixed in place by welding the steel
wedges to each column.
Two shear walls were designed in each building to resist lateral loading for the
completed structure except for the top two floors – these floors depended on the rigid
joints between the steel columns and the concrete slabs alone to supply sufficient
lateral restraint. These shear walls were essential to maintain lateral rigidity and
designers specified that they should be within 3 floors of the lifted slabs throughout
the construction.
Failure of the Structure
To this day there is no conclusive evidence to prove exactly why the
L’Ambiance Plaza collapsed. A full investigation was never carried out in an effort to
compensate those affected by the disaster as quickly as possible. A number of credible
theories do exist on what caused the failure.
Theory 1: “An overloaded steel angle welded to a shearhead arm channel deformed,
causing the jack rod and lifting nut to slip out and the collapse to begin”. (National
Bureau of Standards)
According to this theory, the failure occurred at column E4.8 or the adjacent
E3.8 – the most heavily loaded column in the building. Shortly before the collapse,
the construction workers had lifted the 9th, 10th and 11th floor package into its final
position and began tack-welding the steel wedges into place. A jack, positioned on top
of column E4.8 or E3.8 was used to slightly adjust the position of the slab. When the
sheerheads and lifting angles had lifted the three 320-tonne slabs into place they were
dangerously close to maximum capacity. Following sample tests on these sheerheads
and lifting angles by NBS they were found to deform under loads of less than 80tonnes. This meant the jack rod and lifting nut (holding these two pieces together) had
room to slip out of the lifting angle. The floor packages then fell from the jack and
this impact load on the column caused it to fail. This resulted in the floors below
collapsing.
Theory 2: “The instability of the wedges holding the twelfth floor and roof package
caused the collapse”. (Thornton-Tomasetti Engineers)
Thornton-Tomasetti Engineers (T-T) concluded at the end of their
investigation that the instability of the wedges at column 3E caused the 12th floor/
roof package to fall initiating the collapse. Contradiction to the NBS report, T-T found
that the collapse did not occur during the mounting process. In fact, their belief is that
the wedges were already fixed into place prior to failure and this theory is backed up
by the examination of columns E3.8 and E4.8 which found no evidence of any impact.
However, they did find abnormal tack welds on the wedges which supported the 12th
floor package. Indentations were found on the underside of the level 9 sheerhead. The
shallowness of the indentations indicated that, while both lifting nuts slipped out, they
were not heavily loaded at the time.
Their investigation also found that the shearhead gaps on columns 3E and 3.8E
(16 mm) were much larger than the gaps on the rest of the building (6 mm - 8 mm)
and other buildings built with the lift-slab technique (6.35 mm - 9.5 mm). In addition
to these abnormally large gaps, the shearheads used on these two columns did not
have cut outs in their lifting angles to restrict relative shifting, and were installed
eccentrically.
Finally, until a wedge is completely welded into place it depends on friction to
hold it. Normally, this is sufficient. The large shearhead gaps on columns 3E and 3.8E
and the presence of hydraulic fuel on these wedges, however, would have demanded
an extremely high friction coefficient to hold the wedges into place. On the day of
collapse, the lateral load from the hydraulic jack exerted on the heavily loaded wedges
caused the west wedge to roll. Then the local adjustments to slab elevations caused
the remaining wedge to roll out initiating the collapse of the 11th floor package and
the west tower.
Theory 3: “The improper design of the post-tensioning tendons caused the collapse”.
(Schupack Suarez Engineers)
General layout of post-tensioning tendons
* Each line represents 1-5 mono-strand tendons (magenta)
The vertical tendon layout used in the east tower distributes the weight of the
slab to the east-west column lines which in turn distribute the weight to the columns.
The west tower does not follow the same layout however. At column 4.8E the tendons
split in two, both diverging from the column line. In the east tower there are no
tendons present to distribute the weight of the slab to the column line. The design
details of the post-tensioned floor slabs do not show the location of the shear walls or
the openings for the walls at columns 11A, 8A, and 2H. The design did not take these
opening into account. Detailed finite element analysis showed that tensile stresses
along column line E, east of column 4.8E, exceeded the cracking strength of the
concrete. Therefore once a crack began, it would immediately spread to column 4.8E.
Theory 4: “Questionable weld details and substandard welds could have caused the
collapse”. (Occupational Safety and Health Administration)
OSHA found that the header bar-to-channel welds on one side of the 9th floor
shearhead at column E3.8 had failed. The technique of one-sided square-groove welds
for the header bar-to-channel connection was not approved by the American Welding
Society. This technique meant strength could not be determined as the penetration was
not known. A sample of 30 welds examined by the OSHA found only 13 to be
acceptable. The questionable weld details and the substandard welding coupled with
drawings that indicated that the welds would undoubtedly experience forces that they
could not resist all point to weld failure as the trigger of the collapse.
Theory 5: “The sensitivity of L’Ambiance Plaza to lateral displacement caused its
collapse”. (Failure Analysis Associates)
When the concrete slabs are temporarily resting on the wedges, the connection
is rotationally stiff, but as soon as the slab is lifted off one of the wedges into its final
position it can rotate freely from the column. Once the wedges are fully welded into
their final position the connection becomes rigid again. In the absence of lateral
loading, the tower is completely stable.
Lateral loading and displacement, however, can cause the slab to lift off one of
its wedges causing the structure to become laterally flexible. The FAA investigation
and analysis lead them to the conclusion that the towers’ sensitivity to lateral
displacement caused its collapse. While the FAA acknowledges that another
mechanism could have triggered the lateral displacement, they believe that lateral
jacking provided sufficient displacement to initiate the collapse
Preventative Measures
Keeping the 5 theories mentioned above at the forefront of the cause of the
disaster, the following preventative measures could be taken to ensure that the same
failure would not re-occur in lift-slab construction:
Temporary lateral bracing should be used throughout the construction of the
building. Theory 5 showed that the sensitivity of the structure type to lateral loading is
very high and the lift-slab method required lateral hydraulic jacking to plumb the
building.
Sway bracing cables should be used to keep the stacked floors from shifting
from side-to-side also. Temporary posts (Cribbing) should be used to support the
concrete slabs vertically while the temporary wedges are welded permanently in place.
These would ensure a non catastrophic failure even if the jack bolted connections
failed (theory 1) as the supports would prevent severe impact loading on the lower
floors.
Certain procedural deficiencies were evident from this failure:
Both Theories 2 & 4 highlight that a number of the welds used to secure the
slabs to the columns did not comply with the American Welding Society regulations.
This is mal-practice - regulations must be adhered to.
Similarly, Theory 3 specifies that the arrangement of the tendons used for
post-tensioning was quite abnormal (Grid Line 4.8-E). This arrangement did not
comply with the American Concrete Institute Building Code – an error which could
have been avoided at design stage.
Finally, the primary engineer is required to take responsibility for each part of
the design and construction of the building which should also be reviewed by a
second engineer to ensure all defects are removed and the design is safe to construct.