Civil Engineering Reference
In-Depth Information
Each diaphragm may be built up from one or more separate
panels provided the vertical joint between panels is structur-
ally connected. It is also good practice to tie the heads of pan-
els together with a horizontal wall plate or to use the floor or
roof construction bearing on the wall to provide this tie.
Each panel is built up from a timber frame of studs and hori-
zontal rails lined on one or both faces with an acceptable struc-
tural material such as OSB/3 or plasterboard. It is allowable to
have two sheathing layers on one face of the timber frame but
then no allowance can be made for any sheathing on the other
face. The sheathing may be applied as vertical or horizontal
sheets fastened to the frame with a specified fastener config-
uration around the perimeter of a sheet, with usually the same
fastener at twice the spacing for fixings to the frame within the
perimeter of the sheet. There are restrictions on the minimum
size of sheet that may be used as a racking sheathing.
The horizontal shear applied to the diaphragm is transmitted
by the sheathings and their fastening to the bottom horizon-
tal member of the timber frame. A horizontal shear connec-
tion has then to be made to the supporting structure by either
mechanical fasteners or by friction between the timber and the
structure it bears on or a combination thereof. PD 6693-1 gives
a value of 0.4 for the coefficient of sliding friction between
timber and wood-based products and other materials such as
concrete, masonry, steel, etc. If reliance is placed solely on
friction to take the horizontal shear then mechanical fixings
should be provide to locate the diaphragm and also to resist
forces normal the plane of the diaphragm.
Any vertical holding down forces have to be connected to
the structure below whether a timber construction or the foun-
dation and this substrate has to be capable of sustaining the
uplift force.
i.e. f
w,d
=
u
f
p,d
particularly where the vertical connection, f
w,d
,
between the bottom rail of the diaphragm and the substrate is
in withdrawal.
The factor K
i,w
is the minimum of
1.0 or [1.0 + (H/
μ
L)
2
+ (2M
d,stab
/
μ
f
p,d,t
L
2
)]
0.5
+ H/
μ
L
where M
d,stab
is the net overturning destabilising moment aris-
ing from V
H
countered by the vertical actions.
Where masonry cladding of the timber frame occurs, some
reduction of the wind force on the timber frame is allowed.
This is described in Annex D of PD 6693-1 but this shield-
ing effect will be found to be much less than in BS 5268-6.1
and 6.2.
The design racking shear strength of a diaphragm wall, F
v,Rd
,
is given by
F
v,Rd
= K
opening
K
i,w
f
p,d,t
L
where K
opening
= 1 - 1.9 A / HL is the factor to allow for the reduc-
tion in racking strength due to the aggregate area A of openings
in the diaphragm
f
p,d
= F
f,Rd
(1.15 + s) / s and f
p,d,t
= ∑f
p,
d (see 9.7.4.4)
where
f
p,d
is the design shear capacity of the perimeter sheathing fasten-
ers in kN/m
f,Rd
is the design lateral capacity of an individual fastener in kN
s is the fastener spacing in m.
Where more than one sheathing is used on a diaphragm
wall, the total design shear capacity of fasteners is given by
f
p,d,t
= f
p,d,1
+ K
comb
f
p,d,2
where f
p,d,1
is the design shear capacity of the perimeter fasteners
of the primary sheathing layer and f
p,d,2
is the design shear cap-
acity of the perimeter fasteners of the secondary sheathing layer
where f
p,d,1
≥ f
p,d,2
comb
is the sheathing combination factor, for example
0.75 for secondary sheathing on the opposite side of the framing
with the sheathing identical to the primary sheathing with
regard to material, thickness, fasteners and fastener spacing
0.50 for secondary sheathing on the opposite side of the fram-
ing with the sheathing identical to the primary sheathing
with regard to material, thickness, fasteners and fastener
spacing
0.50 for secondary sheathing on the opposite side of the fram-
ing with the sheathing different from the primary sheath-
ing with regard to material, thickness, fasteners and fas-
tener spacing.
The value of f
p,d
for wood-based materials may be obtained
by calculation in accordance with 8.3 whilst values for vari-
ous plasterboard thicknesses and combination of thicknesses
is given in PD 6693-1. For example, a 12.5 mm plasterboard
19.8.7.2 Design of vertical diaphragms
The accumulated applied horizontal shear forces, F
H
, acting on
a diaphragm will create a destabilising moment acting about
the leeward bottom corner of the diaphragm, countered by the
permanent actions and any specific holding down, V
HD
, pro-
vided acting about this point.
Figure 19.22
shows the action
on a diaphragm that is held down only at the windward corner.
The action V
R
can come from the return wall and any forces
acting on this wall. Any actions on the return wall within a 45°
line drawn from the base of the wall can be considered con-
tributing to V
R
. For stability, due account has to be taken of
whether actions are favourable or unfavourable and the appro-
priate partial load factor used.
The development of
Figure 19.22
is shown in
Figure 19.23
where the fasteners between the sheathings and the bottom rail
are assumed resist uplift as well as horizontal shear. It may be
necessary to supplement this holding down with the tie down
force, V
HD
, at the windward corner in order to have sufficient
shear capacity in the remaining fasteners to resist the hori-
zontal shear. The strength of the fastenings in uplift may be
a proportion of the shear strength of the bottom rail fasteners,
