contributed to the diaphragm shear capacity of the slabs. Reinforced-concrete gravity beams that are 700 mm deep span 10. 6 m between
the core and the perimeter frame. The perimeter columns vary in
cross section from 1,200 mm square to 700 mm square. Composite
columns are used from the mat foundation level to level 29, along
with embedded W 360 steel column sections of varying weights.
This made it possible to use 1,100 mm square columns in all typical office floors from level 40 down to level 5. Below level 5 columns
1,200 mm square are required because of the increased story heights
within mechanical floors and the double height of the podium levels. The reinforced concrete in the perimeter frame columns ranges
in cube compressive strength from 50 to 80 MPa, and the beam and
slab floor framing is all constructed using concrete with a cube compressive strength of 40 MPa.
The main lobby of the office tower, a 24 m high space that extends from the building core to the perimeter frame, is on the north
side of the building. To increase the area of the lobby, the north columns of the tower, which are vertical from level 12 to the tower
roof, slope away from the building core, following a circular arch.
The result of this movement is that the main tower columns passing through the lobby are 24 m tall and curved. Thus large bending
moments are developed in the columns.
A complete three-dimensional finite-element analysis model of the lobby lamella structure was constructed to study the
effectiveness of the bracing scheme that
had been developed and to guide the architectural design of these elements. A series
of nonlinear buckling analyses were carried out on the lamella scheme, each model
adding the next layer of bracing elements.
© SKIDMORE, OWINGS & MERRILL, LLP, TOP; © SKIDMORE, OWINGS & MERRILL, LLP/© PAWEL SULIMA, BOTTOM
The models analyzed included A elements
alone, A and B elements, all elements except D elements, and, finally,
all lobby elements. The buckling mode of the first model was weak-axis buckling of the A elements. The addition of the B elements reduced
the weak-axis buckling length of the A elements so that strong-axis
buckling controlled and reduced the proportion of the applied load
that was carried by the A elements. This reduction resulted from the
B elements sharing the load. The product of these two factors was that
the buckling load increased by a factor of 2. The addition of the C and
E elements reduced the strong-axis buckling length of the A elements,
nominally increasing the buckling load. However, up to this point the
buckling load of the A elements was still slightly lower than their load
demand. The addition of the D elements had the single biggest effect
on the buckling load of the system. By tying together the A, B, C, and
E elements, buckling failure of any of these elements is effectively prevented, and the critical buckling mode became the buckling of the A
elements at the first conventional story above the lobby. This study confirmed the concept of the lamella bracing scheme and demonstrated
the structural importance of all of the elements in the lamella.
To prevent the construction of the lobby lamella from adversely
affecting the overall construction schedule, time for observing the incrementally beneficial effects obtained by adding bracing elements
was incorporated into the design schedule. While the A and B elements had to be in place prior to the construction of the floor slab
above, construction of the typical floors above that could proceed as
work continued on the lobby lamella. There was a limit to the number
of floors that could be poured prior to the installation of the C and E
The main lobby of the
tower is a 24 m high space
located at the north side
of the building. The main
tower columns passing
through the lobby are
24 m tall and curved,
developing large bending
moments in the columns.
SEPTEMBER 2012 Civil Engineering