The Structural Systems oF Tall Buildings - Tall Buildings: Structural Systems and Aerodynamic Form - page 25

Rigid frames resist lateral loads by dissipating energy through their ductility and
may undergo excessive lateral deformations. Under lateral loads, the slope of the
deformed shape, in other words, the inter-storey drift between adjacent floors is
higher in the lower storeys. Shear trusses and walls, despite being less ductile than the
frame, dissipate energy while staying within elastic limits because of the larger size of
the shear area subjected to the shear force caused by lateral loads, and exhibit smaller
lateral deformations. Thus, ductility is not as important as it is with rigid frames. The
slope of the deformed shape, in other words, the inter-storey drift between adjacent
floors, is higher in the upper storeys, and is greatest at the top. The disadvantages
of the frame compared with the shear truss or wall, and of the shear truss or wall
compared with the frame, are compensated by one another in a system where they
are used together, in which the frame contributes to the shear truss or wall in the upper
storeys, while the shear truss or wall contributes to the frame in the lower storeys. In
this way, the shear-frame system exhibits very effective behaviour against lateral loads
by giving the structure a greater stiffness than a system of “shear truss / shear wall” or
“rigid frame” acting alone ( Figure 3.12 ).
When shear trusses and shear walls are designed as cores that surround elevator
shafts and stairwells, they form partially closed cores since the cross-section of the
core is not completely but partially closed by beams and/or floor slabs ( Figure 3.13b ).
The partially closed cores in general are arranged in rectangular or circular shapes.
The effort is made to approximate the behaviour of a closed core by strengthening
beams and/or floor slabs in the open part of the core, providing sufficient stiffness
against shear and bending.
The location and shape of the shear trusses and shear walls affect their performance
under lateral loads to an important degree. By arranging them in such a way that the
resultant lateral force acts through the centre of rigidity of the building, shear-frame

Rigid connection

Shear shape
(frame behaviour)

Flexural shape
(shear wall / shear truss
behaviour)

Frame
behaviour

Shear wall / shear truss
behaviour

Shear-frame
behaviour

Figure 3.12 The behaviour of the shear-frame system under lateral loads