Civil Engineering Reference
In-Depth Information
Shear Walls
Lift Shaft
Figure 15.30 RC framed building with lift shaft and shear walls providing stability
15.5.1 Stability of precast concrete framed buildings
The stability of precast concrete framed buildings is achieved
in the same way as the other forms of construction previously
described, notably floor diaphragms and shear walls (refer to
Sections 15.4.1 and 15.4.2 above).
Floor types (a) and (b) are suitable to act as horizontal dia‑
phragms to transit the lateral loads to the vertical bracing. The
design should be sufficiently flexible to allow for additional
floor penetrations which may be added at a late stage. The PC
slabs will typically require a structural topping 75-100 mm
thick comprising small aggregate structural concrete with a
continuous layer of fabric.
Floors types (c) and (d) are usually chosen for economy
and where future access to plant, pipework or services is
likely to be required. This flooring has only nominal shear
stiffness and additionally it is often required to be remov‑
able hence this floor type is unsuitable to act as a horizontal
diaphragm.
Where large openings or plant penetrations occur in a slab
which is acting as a floor diaphragm, additional plan bracing
must be incorporated in the floor structure to transfer the shear
and bending across the opening. If design of the floor structure
permits, it is preferable to separate the lateral load resisting
system from the plant supports since this is likely to reduce the
effect of late alterations.
15.5.2 Design of precast concrete shear walls
The shear walls are normally designed as vertical cantilevers with
the in‑plane stiffness resisting overturning and sliding (refer to
Section 15.4.4 above). These walls of course carry a proportion of
the vertical load from the floors, roof and walls above. The shear
walls can be readily constructed using precast concrete panels
(
Figure 15.32
). It can be seen that continuity reinforcement and
in situ
strips of concrete are required to complete the shear wall.
It is noted that masonry infill panels should not generally be
used to provide building stability since there is always the pos‑
sibility that such walls could be removed at some later stage.
15.6 Further stability requirements
Some further stability requirements for various types of struc‑
ture are discussed below.
15.6.1.2 Vertical bracing
Vertical bracing for industrial structures is usually tension
only X bracing. The bracing members can be back‑to‑back
angles, channels, hollow sections or UC sections depending
on the loads involved and length of the bracing spar. Where
X bracing cannot be accommodated then N or K bracing
can be used depending upon restrictions imposed by the
plant.
The vertical bracing bays are usually arranged at the extrem‑
ities of the steel framing as previously described. However,
the restraint effect of the bracing can cause problems due to
expansion or contraction of the steel frame. With industrial
structures the bracing can in some instances be extremely
substantial imposing high restraint forces on the vertical bra‑
cing. In these situations it can be preferable to locate bracing
bays near the centre of the structural frame and thereby avoid
the thermal effects on the bracing. Erection of the steel frame
would therefore commence at the centre of the framing and
work outwards (
Figure 15.33
).
15.6.1 Heavy industrial steelwork
Stability design for heavy industrial structures often requires
special consideration due to three factors:
Additional lateral or dynamic loads from items of mechanical
■
plant.
Sometimes extremely heavy mass at high level.
■
■
Large floor penetrations or discontinuous floor to accommodate
plant.
15.6.1.1 Floor construction
Floor construction in an industrial plant typically consists of
one or more of the following types:
(a)
In situ
reinforced concrete
(b) Pre‑cast concrete slabs with a structural topping
(c) Durbar steel plate/chequer plate
(d) Open grid steel flooring
