Civil Engineering Reference
In-Depth Information
The edge beam must be continuous.
b)
from various timber grades and the triangulated frameworks
are assembled together using punched metal plate connectors.
For trussed rafters to function properly as designed and to
ensure overall stability of the complete roof structure they
require the addition of stability bracing. These bracing mem‑
bers serve two functions: they prevent lateral instability of the
compression members within the trussed rafter by increasing
their buckling strength; and they also provide overall stiffness
to the roof structure and assist in transferring the lateral loads
on the roof structure to the side or gable walls.
Figure 15.21
shows a typical trussed rafter roof with stability bracing.
Shear strength of 'web'
F
bt
≤ f
v,d
2
Where t
- thickness of panel (mm)
f
v,d
- design panel shear strength of decking (N/mm
2
)
-
kf
m
kf
m
k
dv
k
dv
kf
,k
γ
m
f
v,k
- characteristic panel shear strength
Whilst the truss rafter supplier is responsible for the design
of the truss based on the loads provided, it is the building
designer who is responsible for the stability of the roof struc‑
ture overall. The building designer should make sure that the
bracing is adequate to ensure the overall stability of the whole
roof structure and supporting walls and that the roof is properly
fi xed to the vertical wall components to resist uplift forces.
c)
Shear strength of panel support fi xings (along XX)
≤
2b R
s
F
d
d
Where R
d
- design shear resistance of fastener (N)
s
- spacing of fasteners (mm)
15.3.3.3 Stiff vertical diaphragms
The stiff vertical wall diaphragms used in timber framed struc‑
tures are constructed from wood‑based sheet material mech‑
anically fi xed (using nails and screws) to a timber frame. The
racking resistance provided by the panel is developed primarily
by the perimeter panel fi xings (
Figure 15.20
). The panel is
fi xed to the sole plate by bolting or other suitable anchorages
to prevent sliding. The design must also ensure that an ad‑
equate factor of safety exists to prevent overturning. Eurocode
5, 9.2.4.1 provides detailed design guidance.
15.4 Multi‑storey buildings
15.4.1 Lateral stability of multi‑storey steel and
concrete buildings
Lateral stability of multi‑storey buildings, be they steel or con‑
crete construction, depends upon the provision of either braced
bays or moment frames within the building. In the UK, by far
the majority of buildings depend on braced bays for lateral sta‑
bility. That is, they depend on either vertical steel bracing or
reinforced concrete shear walls to provide stability and to pre‑
vent sway of the building frame. The stability requirements
of multi‑storey buildings are similar in principal to low‑rise
structures as follows:
15.3.3.4 Trussed rafter roofs and stability bracing
Trussed rafters are invariably part of a timber framed house
or residential development and are usually designed and
manufactured by specialist suppliers. The trussed rafters are
designed to support the weight of the roof covering, ceiling
loads including imposed loads and water tanks, etc. They must
of course also be designed to resist wind loading. Spans of
up to 25 m can be achieved. Trussed rafters are manufactured
Floors and roof members are required to act as stiff diaphragms.
■
Braced bays (vertical bracing or shear walls) are required in two
■
directions at right angles.
Vertical Load
Lateral Force
Racking Resistance
Nail or
Screw Fixings
Wall Panel
Sole Plate
Figure 15.20 Typical wall panel resisting lateral and vertical load
