Civil Engineering Reference
In-Depth Information
19.8.4 Mechanically jointed components
Attention must be paid to the influence of joint slip in any
member that relies upon mechanically fasteners for its struc-
tural integrity.
Where the shear force varies along the length of a mem-
ber, as in a beam, resulting in maximum and minimum spacing
dimensions of fasteners, s
max
and s
min
, then provided 4s
min
≥
s
max
, a uniform effective spacing of s
ef
= 0.75s
min
+ 0.25s
max
may be used.
Note that the spacing of fasteners may be determined by the
requirements of fixing one material to another e.g. the need to
prevent a board material distorting.
Annexes B and C of BS EN1995-1-1 give methods of calcu-
lating mechanically jointed beams and columns.
The shear forces in the width of the diaphragm can be
assumed to be uniformly distributed across the width. Where
the sheets forming the diaphragm are laid in a block bonded
pattern the spacing of fasteners in the width direction can be
increased by a factor of 1.5 up to a maximum of 150 mm.
19.8.7.1 Vertical diaphragms - general
The first drafts of BS EN1995-1-1 contained what is now
described as Method A for the design of vertical diaphragm
panels to resist horizontal forces in the plane of the wall. This
relied on the design strength of the fasteners connecting the
wall sheathings to the bottom rail of the timber frame wall to
take the horizontal force and required holding down connec-
tions at the ends of walls or where discontinuities occurred
within the length of the wall due to door or window openings.
Despite being allowed on a statistical basis to increase the
design strength of fasteners connecting the sheathing to the
timber frame by a factor of 1.2, it was found that the racking
shear strength with Method A was lower than that achieved
using BS 5268-6.1 or 6.2 when account was taken of the dif-
ferences between ultimate limit states design and permissible
stress design. The holding down requirements of Method A are
not normally required in a BS 5268 design.
The UK therefore submitted an alternative design proced-
ure which is included in BS EN1995-1-1 as Method B. This is
effectively the BS 5268 method converted to limit states.
Method B has problems with regard to acceptability in that
the BS 5268 design method is based on the analysis of many
hundreds of racking tests on various configurations of timber
frame walls. The results are therefore not verifiable by calcu-
lation alone. Nevertheless it was included in BS EN 1995-1-
1:2004.
A compromise was sought (named Method C!) and it was
soon recognised that the omission in Method A of the contri-
bution of the fasteners between the wall studs and the bottom
rail of the timber frame panel was the 10% to 15% difference
in racking strength between Methods A and B. A calculation
procedure that allowed the elimination of the holding down
requirements of Method A was also developed.
The design procedure for vertical diaphragms given in PD
6693-1 is Method C. It is relevant to storey height wall panels
and where necessary utilises a uniformly distributed verti-
cal connection force at the bottom edge of the diaphragm to
eliminate requirement for specific holding down connections,
provided that the fasteners in the residual length of the bot-
tom edge of the diaphragm can resist the horizontal shear
force.
A racking wall is built up from one or more wall diaphragms.
The wall may be broken down into separate diaphragms by
discontinuities created, for example, by doors and similar
openings. Fully framed window openings may occur within a
diaphragm albeit with a reduction in racking strength. Smaller
openings with a dimension not more than 300 mm may occur
in a diaphragm without reduction in strength.
19.8.5 Multi plane trusses
Bolted and connectored trusses have compression, tension and
web members lying in different planes. The important consid-
eration is to have a 'balanced' construction with an odd number
of planes otherwise the truss will bow out plane due to eccen-
tricity. For example, in the simplest triangular truss the rafters
could lie in the outer planes (nos 1 and 3) and the tie in the
centre (plane 2). This concept can be extended to 3, 5, 7 or even
9 planes.
BS EN1995-1-1 9.2.1 gives guidance on assessing effective
lengths of members and the distribution of moments in, for
example, continuous rafters. The principles of frame analysis
for these components are given in BS EN1995-1-1 5.4.2.
19.8.6 Single plane trusses
Better known as trussed rafters, a simplified analytical pro-
cedure is given in BS EN1995-1-1 5.4.3 and design consider-
ations in 9.2.2.
19.8.7 Diaphragms
It is difficult to produce rigid moment resistant joints in a tim-
ber frame so most structures rely on some form of horizontal
bracing usually in the form of horizontal diaphragms to trans-
mit wind forces and the like to vertical diaphragms and thence
to the foundations. Horizontal diaphragms occur in many
buildings either as roof or floor decking, roof sarking or ceil-
ings and vertical diaphragms are created by wall sheathings
such as OSB/3 and plasterboard.
Horizontal diaphragms can be created by careful arrange-
ment of floor and roof decking sheets together with calculation
and specification of the connection of the decking to the sup-
porting members. Where reliance is placed solely on the deck-
ing to form the diaphragm the ratio of length to breadth of the
diaphragm should not exceed 2.
Diaphragms with a length to breadth ratio of up to 6 can be
designed with perimeter timber members creating the flanges
of a horizontal web beam as described in BS EN1995-1-1
9.2.3.2(2).
