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AS 1684 SECTION 8 - RACKING AND SHEAR FORCES 1 SECTION 8 - RACKING (BRACING) AND SHEAR FORCES WEEK 13.

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Presentation on theme: "AS 1684 SECTION 8 - RACKING AND SHEAR FORCES 1 SECTION 8 - RACKING (BRACING) AND SHEAR FORCES WEEK 13."— Presentation transcript:

1 AS 1684 SECTION 8 - RACKING AND SHEAR FORCES 1 SECTION 8 - RACKING (BRACING) AND SHEAR FORCES WEEK 13

2 AS 1684 SECTION 8 - RACKING AND SHEAR FORCES 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 building’s foundation.

3 AS 1684 SECTION 8 - RACKING AND SHEAR FORCES 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 AS 1684 SECTION 8 - RACKING AND SHEAR FORCES 4 FIGURE 8.1 VARIOUS BRACING SYSTEMS CONNECTING HORIZONTAL DIAPHRAGMS

5 AS 1684 SECTION 8 - RACKING AND SHEAR FORCES 5 NOTES to Figure The wind force on unclad frames may be equal to or greater than those on a completed clad or veneered house.

6 AS 1684 SECTION 8 - RACKING AND SHEAR FORCES 6 NOTES to Figure 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 AS 1684 SECTION 8 - RACKING AND SHEAR FORCES 7 NOTES to Figure 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 AS 1684 SECTION 8 - RACKING AND SHEAR FORCES 8 Wind produces horizontal loads on buildings that must be transmitted through the structure to the foundation.

9 AS 1684 SECTION 8 - RACKING AND SHEAR FORCES 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 AS 1684 SECTION 8 - RACKING AND SHEAR FORCES 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 AS 1684 SECTION 8 - RACKING AND SHEAR FORCES 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. Without ceiling diaphragm With ceiling diaphragm

12 AS 1684 SECTION 8 - RACKING AND SHEAR FORCES 12

13 AS 1684 SECTION 8 - RACKING AND SHEAR FORCES 13

14 AS 1684 SECTION 8 - RACKING AND SHEAR FORCES 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 AS 1684 SECTION 8 - RACKING AND SHEAR FORCES 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 AS 1684 SECTION 8 - RACKING AND SHEAR FORCES 16 Determine the wind classification Determine the wind pressure Determine area of elevation Calculate racking force

17 AS 1684 SECTION 8 - RACKING AND SHEAR FORCES 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 AS 1684 SECTION 8 - RACKING AND SHEAR FORCES 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 AS 1684 SECTION 8 - RACKING AND SHEAR FORCES 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 AS 1684 SECTION 8 - RACKING AND SHEAR FORCES General (f)Check even distribution and spacing (g)Check connection of bracing to roof/ceilings and floors

21 AS 1684 SECTION 8 - RACKING AND SHEAR FORCES 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 AS 1684 SECTION 8 - RACKING AND SHEAR FORCES 22 When wind flows over a building it applies different pressures (forces) on a flat vertical wall to that on the sloping roof surface. * These values are indicative only and will vary with roof pitch, building height to depth ratio etc. Pressure on roof kPa* Pressure on wall kPa* 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 AS 1684 SECTION 8 - RACKING AND SHEAR FORCES Wind pressure on the building Pressures are given for single storey and upper storey of two storeys for both long wind at 90 O 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 90 O to the ridge and short wind parallel to the ridge sides of the building.

24 AS 1684 SECTION 8 - RACKING AND SHEAR FORCES 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 AS 1684 SECTION 8 - RACKING AND SHEAR FORCES 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. vertical wall Sloping roof surface All vertical surface  this is the worst wind direction +

26 AS 1684 SECTION 8 - RACKING AND SHEAR FORCES 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 1684 SECTION 8 - RACKING AND SHEAR FORCES 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). Ceiling diaphragm Floor Slab This is the area used to calculate single or upper storey bracing

28 AS 1684 SECTION 8 - RACKING AND SHEAR FORCES 28 This is the area used to calculate lower storey bracing Ceiling diaphragm Floor diaphragm 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)

29 AS 1684 SECTION 8 - RACKING AND SHEAR FORCES 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 AS 1684 SECTION 8 - RACKING AND SHEAR FORCES 30 Include the area of enclosed verandah in the total area. Also include any roof area over an open verandah Building with open and enclosed verandahs, with main roof pitched from verandah beams. Width Building with open and enclosed verandahs, with main roof pitched separately from verandahs. 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

31 AS 1684 SECTION 8 - RACKING AND SHEAR FORCES Racking force (pg 116) The total racking force, in kN, shall be calculated as follows: Projected area of elevation (m 2 ) Lateral wind pressure (kPa) Total racking force x =

32 AS 1684 SECTION 8 - RACKING AND SHEAR FORCES 32 TABLE 8.1 (pg 116) Gable ends and flat, vertical surfaces only

33 AS 1684 SECTION 8 - RACKING AND SHEAR FORCES 33

34 AS 1684 SECTION 8 - RACKING AND SHEAR FORCES 34 Table 8.2 is used for determining the pressure on single or upper storey elevations where the wind direction is at 90 O to the ridge and for wind speeds N1, N2, N3 & N4.

35 AS 1684 SECTION 8 - RACKING AND SHEAR FORCES 35 N2 continued WIND 90 O TO RIDGE A3

36 AS 1684 SECTION 8 - RACKING AND SHEAR FORCES 36 Table 8.3 is used for determining the pressure on lower storey elevations where the wind direction is at 90 O to a ridge and for wind speeds N1, N2, N3 & N4.

37 AS 1684 SECTION 8 - RACKING AND SHEAR FORCES 37 N2 continued TABLE 8.3 PRESSURE (kPa) ON PROJECTED AREA—LOWER STOREY OR SUBFLOOR OF SINGLE OR TWO STOREY—LONG LENGTH OF BUILDING—HIP OR GABLE ENDS WIND 90 O TO RIDGE A3

38 AS 1684 SECTION 8 - RACKING AND SHEAR FORCES 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 AS 1684 SECTION 8 - RACKING AND SHEAR FORCES 39 N2 WIND PARALLEL TO RIDGE A3

40 AS 1684 SECTION 8 - RACKING AND SHEAR FORCES 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 AS 1684 SECTION 8 - RACKING AND SHEAR FORCES 41 N2 WIND PARALLEL TO RIDGE A3

42 AS 1684 SECTION 8 - RACKING AND SHEAR FORCES 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 AS 1684 SECTION 8 - RACKING AND SHEAR FORCES 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 AS 1684 SECTION 8 - RACKING AND SHEAR FORCES 44 The maximum amount that can be resisted by nominal wall bracing is 50% of the total racking forces determined from Clause 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 Nominal wall bracing

45 AS 1684 SECTION 8 - RACKING AND SHEAR FORCES 45 The minimum length of nominal bracing walls shall be 450 mm. The bracing capacity of nominal bracing is scheduled in Table Nominal wall bracing

46 AS 1684 SECTION 8 - RACKING AND SHEAR FORCES 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 AS 1684 SECTION 8 - RACKING AND SHEAR FORCES 47 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 Structural wall bracing

48 AS 1684 SECTION 8 - RACKING AND SHEAR FORCES 48 A2 A4

49 AS 1684 SECTION 8 - RACKING AND SHEAR FORCES 49 TABLE 8.18 (continued) A3 A4

50 AS 1684 SECTION 8 - RACKING AND SHEAR FORCES 50 A3 A4 TABLE 8.18 (continued)

51 AS 1684 SECTION 8 - RACKING AND SHEAR FORCES 51 TABLE 8.18 (continued) A3 A4

52 AS 1684 SECTION 8 - RACKING AND SHEAR FORCES 52 TABLE 8.18 (continued) A4

53 AS 1684 SECTION 8 - RACKING AND SHEAR FORCES 53 TABLE 8.18 (continued) A4 A3

54 AS 1684 SECTION 8 - RACKING AND SHEAR FORCES 54 A3 A4 TABLE 8.18 (continued)

55 AS 1684 SECTION 8 - RACKING AND SHEAR FORCES 55 A2 A4 TABLE 8.18 (continued)

56 AS 1684 SECTION 8 - RACKING AND SHEAR FORCES 56 TABLE 8.18 (continued) A2 A4

57 AS 1684 SECTION 8 - RACKING AND SHEAR FORCES 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 )

58 AS 1684 SECTION 8 - RACKING AND SHEAR FORCES 58 EXAMPLE: cont’d 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 AS 1684 SECTION 8 - RACKING AND SHEAR FORCES 59 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 Wall capacity and height modification pg 147

60 AS 1684 SECTION 8 - RACKING AND SHEAR FORCES 60 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 Length and capacity for plywood bracing walls

61 AS 1684 SECTION 8 - RACKING AND SHEAR FORCES 61 Bracing shall be approximately evenly distributed and shall be provided in both directions (see Figure 8.5) Location and distribution of bracing Bracing shall initially be placed in external walls and where possible at the corners of the building.

62 AS 1684 SECTION 8 - RACKING AND SHEAR FORCES 62 FIGURE 8.5 LOCATION OF BRACING

63 AS 1684 SECTION 8 - RACKING AND SHEAR FORCES 63 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) Spacing of bracing walls in single storey or upper storey of two storey construction A3

64 AS 1684 SECTION 8 - RACKING AND SHEAR FORCES 64 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 Spacing of bracing walls in single storey or upper storey of two storey construction A3

65 AS 1684 SECTION 8 - RACKING AND SHEAR FORCES 65 NOTE: A ceiling depth of 16 m is to be used for all ceiling depths greater than 16 m. N3

66 AS 1684 SECTION 8 - RACKING AND SHEAR FORCES 66 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) Spacing of bracing walls in single storey or upper storey of two A3

67 AS 1684 SECTION 8 - RACKING AND SHEAR FORCES 67 FIGURE 8.6 SPACING OF BRACING

68 AS 1684 SECTION 8 - RACKING AND SHEAR FORCES 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 AS 1684 SECTION 8 - RACKING AND SHEAR FORCES 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

70 AS 1684 SECTION 8 - RACKING AND SHEAR FORCES 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 AS 1684 SECTION 8 - RACKING AND SHEAR FORCES 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 AS 1684 SECTION 8 - RACKING AND SHEAR FORCES 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 AS 1684 SECTION 8 - RACKING AND SHEAR FORCES 73 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 Fixing of top of bracing walls

74 AS 1684 SECTION 8 - RACKING AND SHEAR FORCES 74 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 pg Fixing of top of bracing walls

75 AS 1684 SECTION 8 - RACKING AND SHEAR FORCES 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.

76 AS 1684 SECTION 8 - RACKING AND SHEAR FORCES 76

77 AS 1684 SECTION 8 - RACKING AND SHEAR FORCES 77 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 AS 1684 SECTION 8 - RACKING AND SHEAR FORCES 78 Connection used equals the total brace capacity. Refer to table 8.22 pg 155

79 AS 1684 SECTION 8 - RACKING AND SHEAR FORCES 79 Connections used equals the total brace capacity. Refer to table 8.22 pg153

80 AS 1684 SECTION 8 - RACKING AND SHEAR FORCES 80

81 AS 1684 SECTION 8 - RACKING AND SHEAR FORCES 81

82 AS 1684 SECTION 8 - RACKING AND SHEAR FORCES 82

83 AS 1684 SECTION 8 - RACKING AND SHEAR FORCES 83 A3 A4

84 AS 1684 SECTION 8 - RACKING AND SHEAR FORCES 84 (b)

85 AS 1684 SECTION 8 - RACKING AND SHEAR FORCES 85

86 AS 1684 SECTION 8 - RACKING AND SHEAR FORCES 86

87 AS 1684 SECTION 8 - RACKING AND SHEAR FORCES 87 A3 A4

88 AS 1684 SECTION 8 - RACKING AND SHEAR FORCES 88 A4

89 AS 1684 SECTION 8 - RACKING AND SHEAR FORCES 89

90 AS 1684 SECTION 8 - RACKING AND SHEAR FORCES 90

91 AS 1684 SECTION 8 - RACKING AND SHEAR FORCES 91 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 pg Fixing of bottom of bracing walls pg 155

92 AS 1684 SECTION 8 - RACKING AND SHEAR FORCES 92 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 Fixing of bottom of bracing walls

93 AS 1684 SECTION 8 - RACKING AND SHEAR FORCES 93 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 Fixing of bottom of bracing walls

94 AS 1684 SECTION 8 - RACKING AND SHEAR FORCES 94 NOTES: 1Some bracing wall systems require fixings to be full-length anchor rods, that is from the top plate to the floor frame or concrete slab. 2The 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. 3Where 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.

95 AS 1684 SECTION 8 - RACKING AND SHEAR FORCES 95

96 AS 1684 SECTION 8 - RACKING AND SHEAR FORCES 96

97 AS 1684 SECTION 8 - RACKING AND SHEAR FORCES 97

98 AS 1684 SECTION 8 - RACKING AND SHEAR FORCES 98 A4

99 AS 1684 SECTION 8 - RACKING AND SHEAR FORCES 99

100 AS 1684 SECTION 8 - RACKING AND SHEAR FORCES 100

101 AS 1684 SECTION 8 - RACKING AND SHEAR FORCES 101

102 AS 1684 SECTION 8 - RACKING AND SHEAR FORCES 102 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 Roof Bracing pg Pitched roofs (coupled and non- coupled roofs)

103 AS 1684 SECTION 8 - RACKING AND SHEAR FORCES 103 ( 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 Pitched roofs (coupled and non- coupled roofs)

104 AS 1684 SECTION 8 - RACKING AND SHEAR FORCES 104 FIGURE 8.9 GABLE ROOF BRACING


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