Civil Engineering Reference
In-Depth Information
gable wind girder. Generally braced bays should be provided
at least every eight bays (
Figure 15.7
).
Common forms of bracing for single storey steel frames
are shown in
Figure 15.8
. The bracing members are typically
rolled hollow sections, angles or flats. For economy it is usual
to incline the bracing members at approximately 45°. A single
brace is designed to take compression and tension whereas
cross bracing members are usually designed for tension only.
Where a bracing member cannot be permitted in a particular
bay, if, for example, the bracing member would encroach on
a door opening, the bracing member can either be transferred
to another bay or it can be replaced with a portal frame brace
(
Figure 15.8
). If a portal framed brace is used then the service‑
ability limitation should be checked since this form of bracing
is significantly less stiff than a braced bay.
warehouses, retail premises and sports centres. With such
buildings the cladding can be utilised to brace the steel frame
and assist in providing lateral stability. The cladding can also
be used to provide restraint to the compression flange of the
steel frame members. In this way economies can be made in
design of the steel frame.
Wall and roof cladding will stiffen and strengthen the struc‑
ture and will reduce the deflections of the bare steel frame.
Stressed‑skin design is the evaluation of the shear diaphragm
resistance that the roof and side sheeting panels can provide or
contribute to the lateral stability of the building. The sheeting
panels must be securely fixed to the secondary members (purl‑
ins) by mechanical fasteners through the trough of the sheeting.
In the case of low pitch portal frame buildings, wind loading
from the gable posts is transmitted to the side braced bays by
the diaphragm action of the roof sheeting (
Figure 15.9
).
Careful specification and detailing of the fixings for the roof
sheeting is required in order to ensure proper diaphragm action
occurs. The following must also be considered in the design:
15.3.1.2 Stressed-skin design
Single storey steel framed buildings are often clad with pro‑
filed metal sheeting. Examples of such buildings are factories,
Side walls must be adequately braced to transmit horizontal loads
■
to the foundations.
Sheeting panels must be adequately fixed to the secondary mem‑
■
bers (purlins).
Braced bay
Braced bay
Max 8 Bays
(Generally)
Side Bracing
('K' bracing)
Figure 15.5 Portal frame bracing
Figure 15.7 Side elevation arrangement of braced bays
Vertical Bracing
Vertical Bracing
Gable Posts
Gable Posts
Eaves Wind Girder
Eaves Wind Girder
Vertical Bracing
Vertical Bracing
Lattice Griders
Ties at bottom chord level
Figure 15.6 Plan bracing layout at bottom chord level of lattice girder
