Civil Engineering Reference
In-Depth Information
SYMMETRIC
ASYMMETRIC
SYMMETRIC
ASYMMETRIC
Outrigger
Outrigger
Outrigger
Outrigger
Figure A.15
Typical layouts of outrigger braced frames with central and offset cores
Table A.4
Behaviour of walls for different height - to - width ratios (
see
also Figure 2.12).
Height - to - width ratios
Contribution to response
Shear
Bending
1
✓
3
✓
4
✓
✓
6
✓
overturning moments). Lateral load resistance of OBFs is primarily provided by core, outrigger and
exterior columns. The dynamic behaviour of OBFs is signifi cantly affected by the location and the
number of outriggers along the structure height. Relative location of outriggers considerably infl uences
storey drift response (Stafford-Smith and Salim, 1981; Hoenderkamp and Snijder, 2003 ). Their optimum
location along the building height is a trade-off between the (fl exural) stiffness of the deep truss beam
and the rotation of the core section where it is connected to the outrigger. Parametric analyses have
shown that systems with outriggers at intermediate frame heights are very effective for earthquake
response (Taranath, 1998). Multi-truss systems are preferable to single- outrigger confi gurations because
they increase structural redundancy and enhance global strength, overstrength and ductility.
(iii) Structural Walls
Structural walls (SWs) are vertical systems which are frequently combined with RC, steel and com-
posite framed structures to control lateral defl ections. These systems are often classifi ed according to
their height- to - width (
H
/
L
) ratio (also known as vertical aspect ratio) in ' squat ' and ' slender ' (or ' can-
tilever') walls. Squat walls have low slenderness: their
H
/
L
ratios vary between 1 and 3. Slender or
cantilever walls are those with
H
/
L
6. Under horizontal loads, the ratio of bending-to-shear defl ections
of structural walls increases with the system aspect ratio
H/L
. Consequently, squat and slender walls
are governed by shear and fl exural modes, respectively. Relationships between horizontal forces and
corresponding deformations are provided in Section 2.3.1.1. Rough estimates of the structural behaviour
of SWs can be obtained from Table A.4 .
Squat and cantilever walls have high in-plane stiffness and strength (also known as ' membrane
action'). Bending is resisted in wall systems through chord effects at the edges. This load mechanism
is similar to diaphragm actions, which have been discussed for horizontal systems in Section A.2.1 .
Lateral stiffness and strength of structural walls are increased by using cross sections with I- shape rather
than narrow rectangular shapes, as discussed in Section 2.3.2.2. The former layout is a viable solution
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