Civil Engineering Reference
In-Depth Information
a
other loadings
NNN
NNN
a
C
B
L
a
A
a
a
N
N
(7)
C
C
C
NNN
NNN
NNN
Figure 19.25 Single member braced by lateral supports (BSI, 2004).
Permission to reproduce extracts from BS EN 1995-1-1 is granted by BSI
Figure 19.26 Lateral support for a system of members (BSI, 2004).
Permission to reproduce extracts from BS EN 1995-1-1 is granted by BSI
The value of
Φ
should be at least 0.005 radians for h ≤ 5.0 m
and 0.005 5/
h
for h > 5.0m where h is the height in m. There
should be an initial sinusoidal curvature between nodes corre-
sponding to an eccentricity of
e
where
e
is at least 0.0025l.
The elastic constants to be used in the analysis are
The maximum horizontal defl ection of the bracing system
under q
d
and any other actions should not exceed L/500.
19.8.10 Detailing and control
Section 10 of BS EN1995-1-1 describes structural detailing
and control of materials, glued joints (but not design), connec-
tions and mechanical fastenings. It gives details on nails (e.g.
pre-boring), bolts and washers, dowels and screws.
The requirements for transportation and erection are given
together with the requirements for quality assurance. Note that
a further part of EN 1995 covering the construction of timber
buildings is in the course of preparation.
Special rules for diaphragms are given:
(a)
for fl oor and roof diaphragms the maximum spacing of
fasteners, nails and screws, around the perimeter of a
sheathing sheet should be 150 mm;
(b)
for vertical diaphragms the maximum perimeter spacing
of fasteners should be 150 mm for nails and 200 mm for
screws.
Note that for horizontal diaphragms smooth nails should not
be used and whilst not specifi cally stated with regard to vertical
diaphragms it must be assumed that this condition also applies.
Special rules for the dimensional tolerances on fabrication
and erection of trussed rafters are given.
E
G
E
(see
EC5-1-1 2.2.2 and
mean
mean
and G
=
and G
and G
d
=
d
d
d
2.4.1
(2)P
)
γ
d
γ
γ
M
M
γ
M
M
M
19.8.9 Bracing of members either singly or in a system
EC5-1-1 9.2.5 gives methods of calculating bracing forces for
single members and a series of members to prevent instability
and excessive defl ection.
For single members the forces in the bracing system shown
in
Figure 19.24
should be calculated assuming the spring stiff-
ness of the support Ck
N
a
d
Ck where k
s
is a factor given in
EC5-1-1
Table 19.2
and has the value 4 in the UK National
Annex. The force at the support F
d
= N
d
/ k
f,1
for a solid timber
member and N
d
/ k
f,2
for glulam and LVL. The values of k
f,1
and
k
f,2
are given in EC5-1-1
Table 19.2
with the values of 60 and
100 respectively specifi ed in the UK National Annex. Note that
the difference between solid timber and glulam/LVL is due to
the maximum initial bow allowed in the member
Ck
s
.
The value of N
d
for a rectangular beam should be taken as
)
M
cri
( )
d
[9.36] where k
crit
is determined for the unbraced
beam, M
d
is the maximum design moment and h is the beam
(
k
)
−
h
crit
19.9 Conclusions
Eurocode 5-1-1 introduces signifi cant changes in the execu-
tion of timber design from BS 5268 and its predecessor CP112.
Surprisingly much of the technology in EC5-1-1 was known at
the time of drafting the fi rst edition of CP112 in 1952 but at
that time it was considered slide rules could not cope with the
complexity of the consequent calculations. Hence, the simpli-
fi cations in CP112 many of which were carried forward to BS
5268.
To fully comprehend the effects of time and moisture on the
design of timber an understanding of the principles set out else-
where is required. It may help the reader to solve some those
problems that were not described in either CP112 or BS 5268!
depth.
For a series of parallel members which require lateral sup-
ports within their span, a bracing system capable of sustaining a
uniformly distributed load, q
d
in addition to any other horizon-
tal actions such as wind is required as shown in
Figure 19.25
.
nN
d
The value of q
d
is given by k
kL
[9.37] where k
L
is the
L
kL
f
kL
f
kL
kL
15
minimum of 1.0 and
and k
f,3
is given in the UK National
L
Annex as 60 where the spacing of members is not greater than
600 mm and 40 otherwise.
