Fig. 3.56 Belted truss
system.
(a) The diagonally braced core
is here carrying all of the
horizontal load.
(b) The introduction of the
rigid girder at the top of the
building allows the perimeter
columns to act compositely
with the core to resist the
horizontal load.Steel structures(a) (^) (b)
diaphragm to conduct wind forces to the verti-
cal-plane bracing. In the case of high-rise
structures more attention is given to the latter
function but the floor grids which are adopted
are nevertheless similar to those which are
used in lower structures.^13
For frames with more than 30 storeys,
neither rigid-frame action nor diagonal or
diaphragm core-bracing are capable of provid-
ing sufficient lateral strength in the vertical
plane to resist wind load effectively. A combin-
ation of core diaphragm bracing and rigid
joints provides sufficient lateral strength for
buildings of up to 60 storeys in height; the
belted truss system (Fig. 3.56), which allows
the core to act compositely with the perimeter
columns of the building and which reduces the
bending moment on the core due to the lateral
loads, is another arrangement which is
suitable for buildings in this height range. For
higher buildings the lateral strength is
increased by making use of the full cross-
sectional width of the building, so that it
behaves like a single vertical cantilever.
Various techniques have been used to achieve
this. In the framed-tube system (Fig. 3.57),
which was used in the World Trade Centre
building in New York, the closely-spaced
perimeter columns form a cantilever tube;
interior columns play no part in the resistance
of lateral load in this system and the whole of
the wind load is resisted by the external walls.
The walls which are parallel to the direction of
the wind are rigid frames and form a shear
connection between the walls which are
normal (at right angles) to the wind direction,
which act as flanges. The building is therefore
a box girder which behaves as a vertical
cantilever in response to lateral load. The
action of the trussed tube (Fig. 3.58) is similar.
In the case of the tube-in-tube structure (Fig.
3.59) the stiffness of the tube, which is formed
by the perimeter walls, is augmented by
composite action with a core tube. This
requires that floor structures be stiff enough to
provide a shear connection between the two
tubes. Bundled-tube structures (Fig. 3.60),
which were developed to reduce the shear-lag
effect which occurs in single-tube construction,
produce the stiffest buildings for a given size of
cross-section.
95
13 See, Schueller, High Rise Building Structures, John Wiley,
London, 1977.