Civil Engineering Reference
In-Depth Information
Figure A.19
Trussed tube systems: optimal confi guration with inclined perimeter columns (
left
) and confi guration
with mega - bracings (
right
)
frames and the spacing between columns can be very much larger than in framed tubes, as shown in
Figure A.19. The function of diagonal members in trussed tubes is twofold. They withstand shear
actions generated by horizontal seismic forces and transfer gravity loads to the ground acting like
inclined columns. Diagonal members are tied together with spandrel beams along the perimeter of the
system, thus exhibiting a pure fl exural mode under horizontal forces. Optimal confi gurations for trussed
tube systems require closely spaced diagonal braces in both directions of the tube. Nevertheless, this
layout is impractical due to the interaction with architectural elements in the facade, e.g. claddings and
openings for windows. Multi-storey diagonal braces (also known as ' mega - braces ' ) are often utilized
for high-rise structures: in this confi guration, diagonals intersect peripheral columns at tube corners
(Figure A.19). Mega-brace members can also help to reduce shear lag effects either in webs and fl anges
of tube walls.
Trussed tube systems may exhibit large inelastic deformations and energy dissipation, provided that
buckling of diagonal braces is prevented and base columns employ seismic details with high
ductility.
Tube-in-tube and bundled tube systems possess higher lateral stiffness and strength than both framed
and trussed TSs. Tube- in - tube structures resist earthquake -induced horizontal forces through an interior
and an exterior framed tube (Figure A.20 ).
Floor slabs, acting as horizontal rigid diaphragms, tie exterior and interior tubes together so that they
interact under horizontal loads. The structural interaction between perimeter and interior tubes is similar
to that discussed above for HFs. The exterior tube resists most lateral loads in the upper fl oors, while
the interior tube carries most lateral loads at lower storeys. The lateral strength of tube- in - tube systems
is superior to that of HFs (Balendra, 1993). Similarly, the ductility and energy dissipation capacity of
high - rise with tube -in-tube structures are higher than those of HFs. The former possess, in fact, higher
redundancy and can give rise to more uniform action redistributions.
