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TitleEngineering Basis of NZS 3604 Updated4
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Document Text Contents
Page 1

BASIS OF
ENGINEERING

NZS 3604

Page 57

56

BRANZ Engineering Basis of NZS 3604

The parallel support system factor for load sharing in bending (k4) was taken as
1.1, recognising the load sharing provided by dwangs, lining and cladding.

Under dead load only and dead plus live load cases, the bearing strength
perpendicular to the grain on the bottom plate was also considered. This may be
critical for short studs.

The bearing area factor (k3) was taken as 1.0.

6.2.3. Design for serviceability (SLS)

General
The deflection of the stud under wind face loading was the critical load case for
serviceability.

Loads
Wind (face loading on studs):

The differential pressure coefficients across external walls (∑(Cpe, Cpi)) was taken
as 1.1. A face load of 0.40 kPa was considered appropriate for internal wall studs
to give a minimum level of general robustness against uneven wind loads and
domestic scale impacts.

Live and earthquake loads were not considered for serviceability.

Structural model used for serviceability
The model used for serviceability is shown in Figure 8 of this book.


Figure 8. Structural model used for serviceability of studs

The systems effect of the linings and claddings resulting in an increased stiffness
of the studs in the wall was allowed for by effectively increasing the stud E
value by a factor of 1.69. This factor was derived from full-size face loading
tests conducted on lined and clad 2.4 m high walls in the 1970s and has been
applied to the Elb value from NZS 3603. An SG8 stud thus has an effective E for
serviceability of 5.4 x 1.69 = 9.13 GPa.

Zone Wind speed (V
des

) Design wind
pressure (p)
(kPa)

Low (also internal walls) 26 0.40

Medium 30 0.53

High 37 0.82

Very high 42 1.06

Extra high 45 1.22

st
ud

h
ei

gh
t

W

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BRANZ Engineering Basis of NZS 3604

Deflection criteria
Maximum deflection = stud height/180, with an upper limit of 15 mm.

6.3 Stud spacing adjustment factor (clause 8.5.5)

6.3.1 General description
The stud spacing adjustment factor allows studs of the required cross-section
to be substituted for by studs of a smaller cross-section spaced more closely
together. This is particularly relevant to the situation of studs in raking walls,
which are desirably all the same cross-section width although they are different
heights.

The basis of the recommendations is as follows:

1. The stud size is determined by bending stiffness requirements only.
2. The reduced stiffness of a smaller stud size may be compensated for

by placing these studs closer together in proportion to the ratio of the
smaller cross-section moment of inertia to the larger (using equation 6.3
below).

To ensure that the first condition is met, only studs 3 m and higher may be
substituted for, as the sizes of these studs are determined by bending stiffness
considerations. However, this condition was omitted in the published standard.

Hence, the following formula applies:

(6.1)

or:
(6.2)

where:

S = stud spacing

I = moment of inertia

rearranging:

S
2
= S

1
x (I

2
/I

1
) (6.3)

to get the stud spacing factor:

(I
2
/I

1
)

Example: Table 8.1 requires a 150 x 50 mm stud at 600 mm centres.

What spacing must be used with a 100 x 75 mm stud?

From the ‘Original larger stud size’ row labelled 150 x 50, run along to

the ‘Desired smaller stud size’ column headed 100 x 75. The spacing

adjustment factor is 0.38. The maximum spacing of the 100 x 75 mm

stud is 0.38 x 600 = 230 mm.

Alternatively, a 100 x 100 mm stud may be used at 0.53 x 600 = 320

mm spacing.

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BRANZ Engineering Basis of NZS 3604

Fixing Q 2 HDG 'tee' straps (see Figure 9.3(A)) Veranda beam/post

Similarly to Fixing O:


ɸQ =ɸ x n x k
1
x k

12
x Q

sk


For perpendicular to grain direction:

b
e

= 90 mm

Q
skp

= 6.97 for M12 bolt in J5 timber (dry)

Q
sk

= 2 x 6.97 = 13.8


For parallel to grain direction:

b
e
= 90 mm

Q
skp

= 10.4 for M12 bolt in J5 timber (dry)

Q
sk

= 1.25 x 2 x 10.4 = 26


using:

ɸ = 0.7 for other fasteners
n = 2 for parallel and 4 for perpendicular

k
1
= 1.0 under wind uplift

k
12

= 0.7 for green timber

ɸQ = 0.7 x 2 x 1.0 x 0.7 x 26 (parallel) or 0.7 x 4 x 1.0 x 0.7 x 13.8 (perpendicular)
= 25.5 k

Fixing R 1 / 90 x 3.15 mm nails Purlin or batten/rafter

Tests on this configuration were carried out at BRANZ and AHI Roofing (report SR0949/1). AS/NZS
1170.0 and EM1 were used to derive a capacity of 0.55 kN.

Fixing S 2 / 90 x 9.15 mm nails Purlin or batten/rafter

Tests on this configuration were carried out at BRANZ and AHI Roofing (report SR0949/1). AS/NZS
1170.0 and EM1 were used to derive a capacity of 0.8 kN.

Fixing T 1 / 10 g self-drilling screw, 80 mm long Purlin or batten/rafter

Tests on this configuration were carried out at BRANZ and AHI Roofing (report SR0949/1). AS/NZS
1170.0 and EM1 were used to derive a capacity of 2.4 kN.

Fixing U 1 / 14 g self-drilling Type 17 screw, 100 mm long Purlin or batten/rafter

Tests on this configuration were carried out at BRANZ and AHI Roofing (report SR0949/1). AS/NZS
1170.0 and EM1 were used to derive a capacity of 5.5 kN.

Page 114

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B
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3
6

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4

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Private Bag 50908, Porirua 5240, New Zealand
T +64 4 237 1170 F +64 4 237 1171
E [email protected]
www.branz.co.nz

ISBN 978-1-877330-90-2 (pbk)
ISBN 978-1-877330-91-9 (epub)

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