Civil Engineering Reference
In-Depth Information
19.7.4.2 Staples
Staples are not widely used in the UK. Staples are effectively
two separate nails with the following specifi c requirements:
lg
d1
d
■
The equivalent diameter of rectangular section staples is
bh
bbh
where b and h are the staple dimensions.
The width of the staple crown should be a minimum 6d and the
l
■
Figure 19.19 Geometry of a raised thread screw
pointside penetration 14d.
There should be at least 2 staples in a connection.
■
(ii)
the threaded part being produced by rolling or for-
ging such that the thread diameter, d, is greater than
the smooth shank diameter (these screws are often
referred to as 'raised thread screws' (
Figure 19.19
).
( c )
The length of thread created may be either the full length
of the screw or part length (raised thread screws with d
greater than 6 mm are usually part threaded).
(d)
The effective diameter d
ef
of a screw should be taken as
either
A staple may be calculated as 2 nails provided the angle between
■
the line of the staple crown and the grain is more than 30° other-
wise the shear capacity has to be down rated to 70% of the full
capacity.
The characteristic yield moment of each leg is given by 240d
2.6
provided the wire has a minimum tensile strength of 800 N/mm
2
.
There is a reduction in strength for a series of staples in a line.
■
■
■
Staple spacings differ from nails (EC5-1-1
Table 19.3
).
19.7.4.3 Bolts
( (ii) )
the smooth shank diameter (ds in
Figure 19.19
) where
the smooth shank extends a distance of 4d
ef
either side
of the shear plane under consideration or
(ii)
1.1 × root thread diameter where the condition
described in (i) (above) does not exist.
Note that for the screws defi ned in (ii) (above) it may
be convenient to take d
ef
as the smaller of the smooth
shank diameter and 1.1 × root thread diameter (d1 in
Figure 19.19
) so the location of the shear plane relative to
the shank and the threaded part is immaterial.
( e )
The lateral load carrying capacity of screws should be cal-
culated in accordance with Section 8.2 of BS EN1995-1-1
with the effective diameter d
ef
being used instead of d in
the expressions in 8.2.
( (ii) )
For screws with an effective diameter, d
ef
, of 6 mm or less
the lateral load carrying capacity should be calculated in
accordance with the rules of 8.3.1 of BS EN1995-1-1 for
nails otherwise the lateral load carrying capacity should be
in accordance with the rules for bolts given in 8.5.1.
The characteristic tensile strength for bolts is 400 N/mm
2
for grade
■
4.8 bolts and 800 N/mm
2
for grade 8.8.
Bolt spacings and edge distances are given in EC5-1-1
■
Table 19.4
.
There is a reduction for the number of bolts lying in line paral-
■
lel to the grain, for example, the capacity of 8 bolts in line at the
standard spacing of 4d with the load parallel to grain is reduced
to 4.8 bolts.
To achieve the design strength, nuts and bolt heads must have the
■
correct washers (EC5-1-1 cl. 10.4.3). The washer diameter should
be 3 × bolt diameter and washer thickness 0.3d.
For bolts the characteristic axial strength value F
■
ax,Rk
is net washer
area × 3 × characteristic compression perpendicular to grain
strength of the material under the washer.
For external steel plates, an equivalent diameter of bearing is given
■
by the smaller of 12 × plate thickness or 4 × fastener diameter.
19.7.4.4 Dowels
Dowels are designed in a similar manner to bolts but with lower
spacings and edge distances as given in EC5-1-1
Table 19.5
.
σ
m,c
σ
m,c
19.7.4.5 Screws
Section 8.7.1 of EC5-1-1 is not easy to follow. The follow-
ing sets out the intentions of 8.7.1 in a more comprehensive
manner.
(a)
Screws should conform with the requirements of BS
EN14592 (
Timber Structures: Fasteners - Requirements
).
(b)
Screws may be formed by either
( (ii) )
the threaded part being turned down from the original
rod diameter such that the thread diameter, d, is equal
to the smooth shank diameter (as given in BS 1210
Specifi cation for Wood Screws
) or
σσσ
bw
bw
σσσ
b
b
σ
m,t
σ
m,t
Stress distribution
I beam
Box beam
Figure 19.20 Thin webbed beams
