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Title: SECTION 8 - RACKING (BRACING) AND SHEAR FORCES WEEK 13


1
SECTION 8 -RACKING (BRACING)AND SHEAR
FORCESWEEK 13
2
8.1 GENERAL
Permanent bracing shall be provided to enable the
roof, wall and floor framework to resist
horizontal forces applied to the building
(racking forces). Appropriate connection shall
also be provided to transfer these forces through
the framework and subfloor structure to the
buildings foundation.
3
8.1 GENERAL
Where required, bracing within the building,
which normally occurs in vertical planes, shall
be constructed into walls or subfloor supports
and distributed evenly throughout. Where
buildings are more than one storey in height,
wall bracing shall be designed for each storey.
4
FIGURE  8.1   VARIOUS BRACING SYSTEMS CONNECTING
HORIZONTAL DIAPHRAGMS
5
NOTES to Figure 8.1
  • 1. The wind force on unclad frames may be equal
    to or greater than those on a completed clad or
    veneered house.

6
NOTES to Figure 8.1
  • 2.Horizontal wind (racking) forces are applied to
    external surfaces that are supported by
    horizontal or near horizontal diaphragms.
    Diaphragms include roofs, ceilings and floor
    surfaces including their associated framing.

7
NOTES to Figure 8.1
  • 3.Each horizontal diaphragm transfers racking
    forces to lower level diaphragms by connections
    and bracing. This continues down to the subfloor
    supports or concrete slab on the ground, where
    the forces are then resisted by the foundations.

8
Wind produces horizontal loads on buildings that
must be transmitted through the structure to the
foundation.
9
  • In a conventionally constructed house these loads
    are transmitted to the ground by a complex
    interaction between the walls, ceiling/roof
    structure and floor structure.

10
  • The ceiling and floor form large horizontal
    diaphragms and normally play an important part in
    this action as most walls rely on support from
    this ceiling or floor diaphragm to prevent them
    blowing over.

11
The wind forces are transmitted to the ceiling
diaphragm from the walls and also the roof. They
are then transferred through the ceiling
diaphragm to the bracing walls that transmit them
to the floor structure, foundations and then into
the ground.
With ceiling diaphragm
Without ceiling diaphragm
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8.2 TEMPORARY BRACING
Temporary bracing shall be equivalent to at least
60 of permanent bracing required. Temporary
bracing may form part of the installed permanent
bracing.
15
8.3.1 General
Bracing shall be designed and provided for each
storey of the house and for the subfloor, where
required, in accordance with the following
procedure
16
  • Determine the wind classification
  • Determine the wind pressure
  • Determine area of elevation
  • Calculate racking force

17
NOTE To calculate the number of braces required
for wall bracing, the required racking force (kN)
is divided by the capacity of each brace.
18
  • The total capacity of each brace is equal to the
    length of the brace multiplied by its unit
    capacity (kN/m) as given in Table 8.18 (pg 141).

19
  • For example
  • a diagonal brace Type (c)
  • (as per Table 8.18) has a total capacity
    of 1.5 kN/m
  • Multiplied x length of bracing wall
  • 1.5kN/m x 2.4m 3.6 kN
  • for a 2.4 m long section of braced wall.

20
8.3.1 General
(f) Check even distribution and
spacing (g) Check connection of bracing to
roof/ceilings and floors
21
8.3.2 Wind pressure on the building
Wind pressures on the surfaces of the building
depend on the wind classification, width of
building and roof pitch. Tables 8.1 to 8.5 give
pressures depending on these variables.
22
When wind flows over a building it applies
different pressures (forces) on a flat vertical
wall to that on the sloping roof surface.
Pressure on roof - 0.77 kPa
Pressure on wall - 1.10 kPa
These values are indicative only and will vary
with roof pitch, building height to depth ratio
etc.
The tables need to know the ratio between how
much roof area the wind sees as opposed to how
much wall area the wind sees. The building
width and roof pitch will establish this ratio.
23
8.3.2 Wind pressure on the building
Pressures are given for single storey and upper
storey of two storeys for both long wind at 90O
to the ridge and short wind parallel to the ridge
sides of the building, and lower storey of two
storeys or subfloor for both long wind at 90O to
the ridge and short wind parallel to the ridge
sides of the building.
24
8.3.3 Area of elevation
The wind direction used shall be that resulting
in the greatest load for the length and width of
the building, respectively. As wind can blow
from any direction, the elevation used shall be
that for the worst direction. For example
...........
25
8.3.3 Area of elevation
In the case of a single-storey house having a
gable at one end and a hip at the other, the
gable end facing the wind will result in a
greater amount of load at right angles to the
width of the house than the hip end facing the
wind.
Sloping roof surface
All vertical surface \ this is the worst wind
direction

vertical wall
26
For example, the relatively simple building shape
shown in Figure 8.2(A) must be broken into two
parts (shapes) in Wind Direction 2 because gable
ends are calculated using a different table.
After calculating the separate bracing
requirements for each part the bracing elements
used must also be distributed accordingly.

27
As indicated by Figures 8.2 (A) and Note 1, the
area of an elevation includes only the top half
of the wall. Note 1 - h half the height of the
wall (half of the floor to ceiling height).
This is the area used to calculate single or
upper storey bracing
Ceiling diaphragm
Floor Slab
28
As indicated by Figures 8.2 (B) and Note 1, the
area of an elevation For lower storey of two
storey section h half the height of the lower
storey (i.e. lower storey floor to lower storey
ceiling)
This is the area used to calculate lower storey
bracing
29
Note 3 of Figures 8.2 (A, B C) pg 113
states The area of elevation of the triangular
portion of eaves overhang up to 1000 mm wide may
be ignored in the determination of area of
elevation.
Area of Elevation
30
Include the area of enclosed verandah in the
total area. Also include any roof area over an
open verandah
Calculate area of enclosed verandah separately
using its width and pitch and distribute bracing
accordingly.
Do not include areas of open verandahs
Open Verandah
Open Verandah
Enclosed Verandah
Enclosed Verandah
Building with open and enclosed verandahs, with
main roof pitched separately from verandahs.
Building with open and enclosed verandahs, with
main roof pitched from verandah beams.
31
8.3.4 Racking force (pg 116)
The total racking force, in kN, shall be
calculated as follows
Projected area of elevation (m2) Lateral wind
pressure (kPa) Total racking force
x

32
TABLE 8.1 (pg 116)
  • Gable ends and flat, vertical surfaces only

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Table 8.2 is used for determining the pressure on
single or upper storey elevations where the wind
direction is at 90O to the ridge and for wind
speeds N1, N2, N3 N4.
35
continued
N2
36
Table 8.3 is used for determining the pressure on
lower storey elevations where the wind direction
is at 90O to a ridge and for wind speeds N1, N2,
N3 N4.
37
continued
N2
38
Table 8.4 is used for determining the pressure on
single or upper storey elevations where the wind
direction is parallel to a ridge and for wind
speeds N1, N2, N3 N4.
39
N2
40
Table 8.5 is used for determining the pressure on
lower storey elevations where the wind direction
is parallel to a ridge and for wind speeds N1,
N2, N3 N4.
41
N2
42
  • 8.3.6.2  Nominal wall bracing (pg 140)

Nominal wall bracing is wall framing lined with
sheet materials such as plywood, plasterboard,
fibre cement or hardboard, or the like, with the
wall frames nominally fixed to the floor and the
roof or ceiling frame. (table 9.4 pg 167)
43
The most common nominal bracing material used in
houses is plasterboard wall linings.
Plasterboard, fixed to the wall frame
appropriately (to manufacturers specification) is
given structural bracing status with a
reasonable strength rating. Fixed to the wall
frame with nominal fixings, however, its bracing
strength is much lower.
44
  • 8.3.6.2  Nominal wall bracing

The maximum amount that can be resisted by
nominal wall bracing is 50 of the total racking
forces determined from Clause 8.3.4 . Nominal
wall bracing shall be evenly distributed
throughout the building. If this is not the case,
the contribution of nominal bracing shall be
ignored. The minimum length of nominal bracing
walls shall be 450 mm.
45
  • 8.3.6.2  Nominal wall bracing

The minimum length of nominal bracing walls shall
be 450 mm. The bracing capacity of nominal
bracing is scheduled in Table 8.17.
46
Where sheet wall lining is placed over the top of
a structural brace, the value of the sheet wall
lining can not be given its nominal value for the
section that overlaps the structural brace.
47
  • 8.3.6.3  Structural wall bracing

See TABLE 8.18 pg 141 For sheet-braced walls, the
sheeting shall be continuous from the top plate
to the bottom plate Unless otherwise specified,
sheet-bracing walls shall be a minimum of 900 mm
wide to satisfy the requirements of their
nominated ratings.
48
A2
A4
49
TABLE   8.18   (continued)
A3 A4
50
TABLE   8.18   (continued)
A3 A4
51
TABLE   8.18   (continued)
A3 A4
52
TABLE   8.18   (continued)
A4
53
A3
A4
54
A3 A4
55
A2 A4
56
A2 A4
57
EXAMPLE Required Racking force 22kN less
provision for 50 nominal bracing 11kN. The
proposed method of bracing is 2100mm long cut-in
timber or metal angle braces. Type c Each brace
is rated at 3.15kN (2.1 m long x 1.5kN/m). 11kN
/ 3.15 3.5 therefore 4 x 2.1m (12.6kN total)
long braces are required plus 9.4kN of nominal
bracing. (Check that 9.4kN of nominal bracing
is achievable and also that the cut-in braces are
not spaced more than required by 8.3.6.7)
58
EXAMPLE contd Of course there are other
combinations for the above situation 4 x 0.9
long ply braces rated at 3.4kN/m 12.24kN plus
9.76kN of nominal bracing (type g) or 2 x 0.9
long hardboard braces rated at 3.4kN/m 6.12kN
plus 2 x 2.1 long metal angle 6.3kN plus 9.58kN
of nominal bracing. (type l)
59
  • 8.3.6.4  Wall capacity and height modification
    pg 147

The capacity of bracing walls given in Table 8.18
is appropriate to wall heights up to and
including 2700 mm. For wall heights greater than
2700 mm the capacity shall be multiplied by the
values given in Table 8.19.
60
  • 8.3.6.5 Length and capacity for plywood bracing
    walls

Where the same structural plywood bracing system
is fixed to both sides of the wall, the capacity
of the wall will equal the combined capacity of
the bracing system on each side.
61
  • 8.3.6.6 Location and distribution of
    bracing

Bracing shall be approximately evenly distributed
and shall be provided in both directions (see
Figure 8.5).
Bracing shall initially be placed in external
walls and where possible at the corners of the
building.
62
FIGURE  8.5   LOCATION OF BRACING
63
  • 8.3.6.7 Spacing of bracing walls in single
    storey or upper storey of two storey construction

A3
For single or upper-storey construction, the
maximum distance between braced walls at right
angles to the building length or width shall not
exceed 9000 mm for wind classifications up to N2
(see Figure 8.6).
64
  • 8.3.6.7 Spacing of bracing walls in single
    storey or upper storey of two storey construction

A3
For wind classifications greater than N2, spacing
shall be in accordance with Table 8.20 (pg 150)
(N3) and Table 8.21 (N4) for the relevant wind
classification, ceiling depth and roof
pitch. NOTE Ceiling depth is measured parallel
to the wind direction being considered.
65
N3
NOTE A ceiling depth of 16 m is to be used for
all ceiling depths greater than 16 m.
66
  • 8.3.6.7 Spacing of bracing walls in single
    storey or upper storey of two

A3
Where bracing cannot be placed in external walls
because of openings or the like, a structural
diaphragm ceiling can be used to transfer racking
forces to bracing walls that can support the
loads. Alternatively, wall frames may be
designed for portal action. (This requires
engineering advice)
67
FIGURE  8.6   SPACING OF BRACING
68
The ceiling and floor diaphragms play important
roles in the transfer of wind loads from the
walls and roof to the braces. The ability of a
ceiling or floor diaphragm to effectively
transfer the wind load depends on the depth of
the diaphragm.
69
  • Narrow or long diaphragms will not transfer the
    wind loads as effectively as a deeper diaphragm.
    The smaller the length to depth ratio the more
    effective the diaphragm.
  • For this reason the spacing of bracing walls in
    limited as per Clause 8.3.6.7.

70
The above diaphragm, has a large length to depth
ratio, (the length being the distance between
braces) will not transfer the wind loads
effectively.
71
By adding an intermediate brace, the diaphragm is
broken into two. Individually they have a
smaller length to depth ratio and will transfer
the wind loads effectively
72
The same diaphragm, with the wind from the other
direction, will transfer loads very effectively
because its length to depth ratio is small.
73
  • 8.3.6.9 Fixing of top of bracing walls

All internal bracing walls shall be fixed to the
floor for lower storey bracing walls, the ceiling
or roof frame, and/or the external wall frame,
with structural connections of equivalent shear
capacity to the bracing capacity of that
particular bracing wall.
74
  • 8.3.6.9 Fixing of top of bracing walls

Nominal and other bracing walls with bracing
capacity up to 1.5 kN/m require nominal fixing
only, i.e. no additional fixing
requirements. For typical details and shear
capacities, see Table 8.22. pg 152
75
Fixing of top of bracing walls Wind loads,
transferred from the roof and walls to ceiling
and floor diaphragms are then transferred through
braces to the ground. These braces, however,
can only transfer these loads if the brace is
connected to the ceiling or floor above and the
floor below.
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The strength of these connections must be at
least equal to the load the brace can transfer
e.g. a cut-in timber or metal brace 2.4 m long
can transfer a total of 3.6kN (2.4 x 1.5kN/m) a
3.6kN connection to the diaphragm is required. or
alternatively the strength of the brace can be
reduced to equal the strength of the
connection(s) . e.g. if a 2.8kN connection is
used for the above brace, its bracing capacity
will be reduced to 2.8kN.
78
Connection used equals the total brace
capacity. Refer to table 8.22 pg 155
79
Connections used equals the total brace
capacity. Refer to table 8.22 pg153
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A3
A4
84
(b)
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A3
A4
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A4
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  • 8.3.6.10 Fixing of bottom of bracing walls pg 155

The bottom plate of timber-framed bracing walls
shall be fixed at the ends of the bracing panel
and, if required, intermediately to the floor
frame or concrete slab with connections
determined from Table 8.18. pg 141
92
  • 8.3.6.10 Fixing of bottom of bracing walls

Where bottom plate fixing information is not
given in Table 8.18, the bottom plates shall be
fixed at the ends of each bracing panel using
tie-down fixings determined from Table 8.23 and
Table 8.24.
93
  • 8.3.6.10 Fixing of bottom of bracing walls

For bracing wall systems of capacity 6 kN/m or
greater given in Table 8.18, which do not specify
intermediate bottom plate fixings, additional
intermediate bottom plate fixings of a minimum of
1/M10 bolt, or 2/No. 14 Type 17 screws, at
max.1200 mm centres shall be used.
94
NOTES 1 Some bracing wall systems require
fixings to be full-length anchor rods, that is
from the top plate to the floor frame or concrete
slab. 2 The maximum tension load of 8.5 kN given
in the Notes to Span Tables for studs in the
Supplements is not applicable when considering
the uplift force at the ends of bracing
walls. 3 Where provided, the bottom plate
tie-down details given in Table 8.18 may be used
in lieu of the details determined from Table 8.23
and 8.24.
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A4
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  • 8.3.7 Roof Bracing pg 158
  • 8.3.7.1 Pitched roofs (coupled and non-coupled
    roofs)

The following shall apply to the bracing of
pitched roofs (a) Hip roofs   Hip roofs shall
not require any specific bracing as they are
restrained against longitudinal movement by hips,
valleys and the like.
103
  • 8.3.7.1 Pitched roofs (coupled and non-coupled
    roofs)

(b) Gable roofs (including cathedral roofs)   For
wind classifications up to N2 gable roof
buildings with a roof pitch greater than 10 but
less than 25, shall be provided with roof
bracing in accordance with Clause .
Alternatively, for wind classifications up to N4
and roof pitches to 35 bracing shall be in
accordance with Table 8.25, Table 8.26, and the
following (i) Ridge to internal wall  minimum
of two timber braces in opposing directions at
approximately 45 (see Table 8.25 and
8.26). (ii) Diagonal metal bracing  single or
double diagonal bracing shall be designed and
installed in accordance with engineering
principles.
104
FIGURE  8.9   GABLE ROOF BRACING
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