using span tables as1684 2
TRANSCRIPT
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Timber Framing
Using AS 1684.2 Span Tables
Understanding
AS1684Residential Timber
Framed Construction
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the timber framing standard
AS 1684 Residential timber-framed construction
Currently you should be using the 2006 Edition
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the timber framing standard
• design or check construction details, • determine member sizes, and• bracing and fixing requirements
for timber framed construction in non-cyclonic areas (N1 – N4)
AS 1684 Residential timber-framed construction
Provides the building industry with procedures that can be used to determine building practice, to
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AS 1684.2 – CD Span Tables
Unseasoned softwood: F5, F7
Seasoned softwood: F5, F7, F8, MGP10, MGP12, MGP15,
Unseasoned hardwood: F8, F11, F14, F17
Seasoned hardwood: F14, F17, F27
Contains a CD of Span Tables (45 sets in all) for wind zones N1/N2, N3 and N4 for the following timber stress grades:
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Timber Framed Construction
Each set of Span Tables contains 53 separate design tables
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Using AS 1684 you should be able to design or check
virtually every member in a building constructed
using timber framing
Timber Framed Construction
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Floor joists
Bearers Stumps or piles
Wall frame
Wall stud
Lintel
Flooring
Floor joists
Internal cladding
External cladding
Ceiling battens First floor wall frames
Flooring Hanging beams
Ceiling battensCeiling
RaftersRidge beam
BattensRoofing
Ceiling
Timber Framed Construction
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AS1684 Scope & Limitations
Where can AS1684 be used?
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AS1684 Limitations - Physical
16.0 m max.W
16.0
m m
ax.
W
Plan: rectangular, square or “L”-shapedStoreys: single and two storey constructionPitch: 35o max. roof pitchWidth: 16m max. (Between the “pitching points” of the roof,
ie excluding eaves)•x•.
16.0 m m
axW
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Width
16.0 m m ax.
Pitching Point of main roof.
Pitching Point of garage roof.
Pitching Point of verandah orpatio roof.
Pitching Point of main roof.
16.0 m max. 16.0 m m ax.
Main houseGarage Verandahor Patio
The geometric limits of the span tables often will limit these widths.
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Wall Height
The maximum wall height shall be 3000 mm (floor to ceiling)
as measured at common external walls,
i.e. not gable or skillion ends.
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Design Forces on Buildings
LIVE LOADS (people, furniture etc.)
DEAD LOAD (structure)
Construction loads (people, materials)
DEAD LOAD (structure)
Suction
Internal pressure
Suction (uplift)
Wind
(a) Gravity loads (b) Wind loads
AS1684 can be used to design for Gravity Loads (dead & live) and wind loads.
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Wind Classification
Non-Cyclonic Regions A & B only
N1 - W28N 100km/h gust
N2 - W33N 120km/h gust
N3 - W41N 150km/h gust
N4 - W50N 180km/h gust
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Wind Classification
• Building height
• Geographic (or wind) region (A for Victoria)
• Terrain category (roughness of terrain)
• Shielding classification (effect of surrounding objects)
• Topographic classification (effect of hills, ridges, etc)
Wind Classification is dependant on :
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Wind Classification - Simple Reference
Geographic Region A
Site Location Below top 1/3 Top 1/3 of hill of hill or ridge or ridge
Suburban site1. Not within two rows from• The city or town perimeter as estimated 5 years hence• Open areas larger than 250,000m2
2. Less than 250m from• The sea or • open water wider than 250m3.Within two rows from• The city or town perimeter as estimated 5 years hence• Open areas larger than 250,000m2
Rural areas
N1
N2 N3
N2
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• Design fundamentals & basic terminology
• Roof framing• Wall framing• Floor framing
Click on arrow to move to section required
Using AS1684.2 Span Tables
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Design Fundamentals&
Basic Terminology
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Floor joists
Bearers Stumps or piles
Wall frame
Wall stud
Lintel
Flooring
Floor joists
Internal cladding
External cladding
Ceiling battens First floor wall frames
Flooring Hanging beams
Ceiling battensCeiling
RaftersRidge beam
BattensRoofing
Ceiling
Design Fundamentals NOTE
While you might build from the Bottom – Up
You design from the Roof – Down
As loads from above can impact on members below – so
start with the roof and work down to the ground level
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• As a general rule it is necessary to increase the timber member size when:– Load increases (a function of dead, live, wind loads)– Span increases (a function of load paths across openings)– Indirect load paths occur (e.g. cantilevers and offsets)
• It is possible to decrease timber member size when:– Sharing loads across many members– Using members with higher stress grades
Design Fundamentals
• Understanding the concept of a ‘load path’ is critical. Loads need to be supported down the building to the ground Indirect Load path
due to cantilever
Roof Load
Ground level
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Load distribution
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Loads distributed equally between Points of support.
Of the total load on MEMBER X, half (2000mm) will be supported by the beam or wall at A and half (2000mm) will be supported by the beam or wall at B.
BA
MEMBER X
Loads distributed
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Beam A will carry 1000 mm of loadBeam B will carry 3000mm
(1000 mm plus the 2000 mm on the other side)Beam C will carry 2000 mm
If MEMBER X is supported at 3 or more points, it is assumed that half the load carried by the spans either side of supports will be equally distributed.
A B C
MEMBER X
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Span & Spacing
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Terminology - Span and Spacing
Spacing The centre-to-centre distance between structural members, unless otherwise indicated.
J o is t s s p a c in g( c e n t r e - l in e t o c e n t r e - l i n e )
B e a r e r s p a c in g( c e n t r e - l i n e to c e n t r e - l in e )
J o is t s s p a n ( b e t w e e n i n te r n a lf a c e s o f s u p p o r t m e m b e rs )
Bearers and Floor joists
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Terminology - Span and Spacing
Span The face-to-face distance between points capable of giving full support to structural members or assemblies.
J o is t s s p a c in g( c e n t r e - l i n e to c e n t r e - l i n e )
B e a r e r s p a c in g( c e n t r e - l i n e to c e n t r e - l in e )
J o is t s s p a n ( b e t w e e n i n te r n a lf a c e s o f s u p p o r t m e m b e rs )
Bearers and Floor joists
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Terminology - Single Span
The span of a member supported at or near both ends with no immediate supports.
S i n g l e s p a n
S i n g le s p a n S i n g le s p a n
S a w c u t J o in t o r l a p
Joint or saw cut over supports
This includes the case where members are partially cut through over intermediate supports to remove spring.
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The term applied to members supported at or near both ends and at one or more intermediate points such that no span is greater than twice another.
C o n t i n u o u s s p a n
C o n t i n u o u s s p a n
NOTE: The design span is the average span unless one span is more than 10% longer than another, in which case the design span is the longest span.
Terminology - Continuous Span
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• Span 1 (2000mm) Span 2 (3925mm)
1/3 (2000mm)
The centre support must be wholly within
the middle third.
Span 2 is not to be greater than twice Span 1.This span is used to determine the size using the continuous span tables.
6000mm1/3 (2000mm) 1/3 (2000mm)
75mm 75mm 75mm
Example: Continuous Span
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Rafter
Terminology – Rafter Span and Overhang
Rafter spans are measured as the distance between points of support along the length of the rafter and not as the horizontal projection of this distance.
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Loadbearing wallA wall that supports roof or floor loads, or both roof and floor loads.
The main consideration for a non-loadbearing internal wall is its stiffness. i.e. resistance to movement from someone leaning on the wall, doors slamming shut etc.
Terminology – Wall Construction
Non-loadbearing walls A non-loadbearing internal wall does not support roof or floor loads but may support ceiling loads and act as a bracing wall.
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Ridge board
Rafters & Ce ilin g Jo ist m ust befixed togeth er a t the p itch in g po in ts
Ceiling jo is t
Ra fte r
otherw ise there is no th in g to stopth e w a lls fro m spreading
and th e roof from collapsing
Ce iling jo ist(C olla r Tie )
Ra fte r
R idge board
T his m eth od of roo f co nstruc tio n is n o t covered by A S1684
Coupled roof
Terminology – Roof Construction
When the rafters are tied together by ceiling joists so that they cannot spread the roof is said to be coupled
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Non-coupled roof A pitched roof that is not a coupled roof and includes cathedral roofs and roofs constructed using ridge and intermediate beams.
A non-coupled roof relies on ridge and intermediate beams to support the centre of the roof. These ridge and intermediate beams are supported by walls and/or posts at either end.
R id g e B e a m
R a fte r In te rm e d ia te B e a m
Terminology – Roof Construction
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Roof Framing
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Typical Basic Roof Shapes
• The footprint of a building generally consists of a rectangular block or multiple blocks joined together
Hip
Skillion
• Common roof shapes used to cover the required area are shown above
• Roof shapes are made to cover the footprint while also providing sloping planes able to shed water
Gable (Cathedral or flat ceiling)
Dutch Hip (or Dutch Gable)
Hip and valley
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Typical Roof Framing Members
R id g e b o a rd
C o l la r t i e
U n d e r p u r l i n
S t ru tS t r u t t i n g b e a m
C e i l i n g jo i s tS t ru t
R a f te r
To p p l a te To p p l a te
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Transferring Loads to Pitched Roofs
2. Battens - take roofing loads and transfers them to the rafters/trusses
1. Roofing materials - take live/dead/wind loads and transfers them to the battens
3. Rafters – take batten loads and transfers them to the support structure below e.g. walls
Support wall
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Batten Design
Step 1: Determine the wind classification to factor in wind loads – for the example assume noncyclonic winds (N1 or N2)
Step 2: Determine type of roof - tiled roof or sheet
Step 3: Determine the batten spacing – typically 330mm for tiles, or 450, 600, 900, 1200mm sheet
Step 4: Determine the batten span – this will be the supporting rafter spacing
BattenSpan
BattenSpacing
Typical Process
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Batten Design
Step 6: Choose a table reflecting your preferred stress grade
BattenSpan
BattenSpacing
Step 5: Look up Volume 2 of AS1684 (i.e. non-cyclonic winds N1 & N2) and go to the batten span tables
Step 7: Determine which column in the table to select using the previous “batten spacing” and “batten span” assumptions
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Roof Batten Design Example
Inputs required
• Wind Classification = N2• Timber Stress Grade = F8• Roof Type = Steel Sheet (20
kg/m2)• Batten Spacing = 900 mm• Batten Span = 900 mm
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Roof Batten Size
Inputs required• Wind Classification = N2• Timber Stress Grade = F8• Roof Type = Steel Sheet (20 kg/m2)• Batten Spacing = 900 mm• Batten Span = 900 mm
A 38 x 75mm F8 Batten Is adequate
Simplify table
2006
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Rafter Design
Step 1: Determine the wind classification to factor in wind loads – for the example assume noncyclonic winds (N1 or N2)
Step 2: Determine dead/live loads on rafters – for the example assume loads are as for a tiled roof with battens e.g. 60kgs/m2
Step 3: Determine the rafter span – for the example assume a 2100mm single rafter span
Ridge beam
Overhang
Rafter span
RafterSpacing
Step 4: Determine the rafter overhang which creates a cantilever span adding extra load – for the example assume a 500mm overhang
Step 5: Determine the rafter spacing as this determines how much roof loads are shared between rafters – for the example assume a 600mm spacing
Scenario - Rafters for a Cathedral Roof
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Step 6 Look up Volume 2 of AS1684 (N1 & N2)
Step 11 Read off the rafter size – 90x45mm
Step 7 Choose a table reflecting your preferred stress grade
Step 8 Determine which column in the table to select using the previous “rafter spacing” and “single span” assumptions
Step 9 Go down the column until reaching the assumed rafter span and overhang – 2100 and 500mm
Step 10 Check the spans work with the assumed roof load of 60kgs/m2
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Rafter Design Example
Inputs required
• Wind Classification = N2• Stress Grade = F8• Rafter Spacing = 900 mm• Rafter Span = 2200 mm• Single or Continuous Span= Single• Roof Mass (Sheet or Tile) = Steel Sheet
(20 kg/m2)
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Simplify table
Rafter Size
Inputs required• Wind Classification = N2• Stress Grade = F8• Single or Continuous Span = Single• Rafter Spacing = 900 mm• Rafter Span = 2200 mm• Roof Mass (Sheet or Tile) = Steel
Sheet (20 kg/m2)
A 100 x 50mm F8 rafter
is adequate
Maximum Rafter or Purlin Span & Overhang (mm)
At least 2200mm
2006
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Ceiling Joist Design
Ridgeboard
Ceiling Joist
Rafter
Design variables• Timber Stress Grade• Ceiling Joist Spacing• Ceiling Joist Span• Single or Continuous Span
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Ceiling Joist Design Example
Inputs required
• Wind Classification = N2• Stress Grade = F17• Overbatten = No• Single or Continuous Span= Single • Joist Spacing = 450 mm• Ceiling Joist Span = 3600 mm
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Ceiling Joist Size
Inputs required• Wind Classification = N2• Stress Grade = F17• Overbatten = No• Single or Continuous Span = Single• Joist Spacing = 450 mm• Ceiling Joist Span = 3600
mm
Simplify table
A 120 x 45mm F17 ceiling joistis adequate
At least 3600mm
2006
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Some members do not have to be designed using span tables
they are simply called up or calculated based on members
framing into them
Member Application Minimum size (mm)
Unstrutted ridge in coupled roof Depth not less than length of the rafter plumb cut 19 thick
Strutted ridge in coupled roof with strut spacing not greater than 1800 mm
Depth not less than length of the rafter plumb cut 19 thick
Ridgeboards
Strutted ridge in coupled roof with strut spacing greater than 1800 to 2300 mm
Depth not less than length of the rafter plumb cut 35 thick
Stress grade F11/MGP15 minimum and no less than rafter stress grade
50 greater in depth than rafters 19 thick (seasoned) or 25 thick
(unseasoned) Hip rafters
Stress grades less than F11/MGP15 50 greater in depth than rafters min. thickness as for rafters
Valley rafters Minimum stress grade, as for rafters 50 greater in depth than rafters with thickness as for rafter (min. 35)
Valley boards See Note 19 min. thick width to support valley gutter
Struts to 1500 mm long for all stress grades 90 45 or 70 70
Roof struts (sheet roof) Struts 1500 to 2400 mm long for all
stress grades 70 70
OTHER MEMBERS AND COMPONENTS
Ridgeboard
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Roof Member - Load Impacts
Roof Load Width (RLW)
The loads from roof members often impact on the design of members lower down in the structure.
This impact can be determined from the following load sharing calculations
Ceiling Load Width (CLW)
Roof area supported
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Roof Load Width(RLW)
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RLW is the width of roof that contributes roof load to a supporting member – it is used as an input to Span Tables for
• Floor bearers• Wall studs• Lintels• Ridge or intermediate beams• Verandah beams
RLW - Roof Load Width
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A
B
3000
1500 1500
Roof Load Widths are measured on the rake of the roof.
RLW - Roof Load Width
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RLW - Roof Load Width
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Trusses
ayx
2
RLW wall A = byx
2
RLW wall B =
x ya b
A B
The roof loads on trusses a re d istr ibu ted equally betw een w a lls 'A ' and 'B '.
RLW - Roof Load Width
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Without ridge struts
ax
2RLW wall A = by
2
RLW wall B =
R LW R LW
R LW
213
x ya b
RLW RLW
A B
** For a pitched roof without ridge struts, it is assumed that some of the load from the un-supported ridge will travel down the rafter to walls 'A' and 'B'. The RLW's for walls A & B are increased accordingly.
*
RLW - Roof Load Width
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With ridge struts
2x
Underpurlin 1 =
x yRLW RLW
BCA
ba 1 2 3
RLW
‘RLW’ - Roof Load Width
Underpurlin 2 = 3y
Underpurlin 3 = 3y
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Ceiling Load Width(CLW)
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Ceiling load width (CLW) is the width of ceiling that contributes ceiling load to a supporting member (it is usually measured horizontally).
CLW - Ceiling Load Width
A B
xCLW
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CLW is used as an input to Span Tables for
• hanging beams, and• strutting/hanging beams
CLW - Ceiling Load Width
Hanging beam
Ceiling jo ist
Hanging beam span
'x '
Hanging Beam Strutting/Hanging Beam
Strutting beam span
Ridgeboard
Underpurlin
Strutting beam
Roof strut
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CLW Hanging beam D =2x
A B C
ED
x y
CLW CLW
CLW - Ceiling Load Width
FIGURE 2.12 CEILING LOAD WIDTH (CLW)
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CLW Strutting/Hanging beam E =2y
A B C
D E
yx
CLWCLW
FIGURE 2.12 CEILING LOAD WIDTH (CLW)
CLW - Ceiling Load Width
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Roof Area Supported
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The sum of, half the underpurlin spans either
side of the strut (A/2), multiplied by
the sum of half the rafter spans either side of
the underpurlin (B/2)
Roof Area SupportedEXAMPLE: The STRUTTING BEAM span table requires a ‘Roof Area Supported (m2)’ input.
The strutting beam shown supports a single strut that supports an underpurlin.
The ‘area required’, is the roof area supported by the strut. This is calculated as follows:-
BB/2
AA/2
Stru tting Beam Span
Strutting Beam
Underpurlin
Strut
2B
2A
Roof Area Supported =
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Strutting Beam Design Example
Inputs required
• Wind Classification = N2• Stress Grade = F8• Roof Area Supported = 6m2
• Strutting Beam Span = 2900 mm• Single or Continuous Span= Single• Roof Mass (Sheet or Tile) = Steel Sheet
(20 kg/m2)
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Inputs required• Wind Classification = N2• Stress Grade = F17• Single or Continuous Span = Single• Roof Mass (Sheet or Tile) = Steel
Sheet (20
kg/m2)• Roof Area Supported = 6m2
• Strutting Beam Span = 2900 mm
2 x 140 x 45mm F17 members are
adequate
F17
Simplify tableAt least 2900mm
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Top plate
Wall Framing
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Wall Framing
T im b e r o r m e t a l b r a c in gTo p p la t e
B o t t o m p la te
J a c k s tu d
J a m b s t u d
W a l l i n te r s e c t io n
N o g g in g
C o m m o n s t u d
L i n te l
S h e e t b r a c in g
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Wall Studs Design Example
Inputs required
• Wind Classification = N2• Stress Grade = MGP10• Notched 20 mm = Yes• Stud Height = 2400 mm• Rafter/Truss Spacing = 900 mm• Roof Load Width (RLW) = 5000 mm• Stud Spacing = 450 mm• Roof Type = Steel Sheet (20
kg/m2)
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Wall Stud Size
Inputs required• Wind Classification = N2• Stress Grade = MGP10• Notched 20 mm = Yes• Stud Spacing = 450 mm• Roof Type = Steel Sheet (20 kg/m2)• Rafter/Truss Spacing = 900 mm• Roof Load Width (RLW) = 5000 mm• Stud Height = 2400 mm
Simplify tableAt least 5000mm
70 x 35mm MGP10 wall studs
are adequate
2006
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Top Plate Design Example
Inputs required
• Wind Classification = N2• Stress Grade = MGP10• Rafter/Truss Spacing = 900 mm• Roof Load Width (RLW) = 5000 mm• Stud Spacing = 450 mm• Roof Type = Steel Sheet (20
kg/m2)
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Simplify table
Top Plate Size
Inputs required• Wind Classification = N2• Stress Grade = MGP10• Roof Type = Steel Sheet (20 kg/m2)• Rafter/Truss Spacing = 900 mm• Tie-Down Spacing = 900 mm• Roof Load Width (RLW) = 5000 mm• Stud Spacing = 450 mm
At least 5000mm
2 x 35x 70mm MGP10 top plates
are adequate
2006
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Wall Lintel Design Example
Inputs required
• Wind Classification = N2• Stress Grade = F17• Opening size = 2400 mm• Rafter/Truss Spacing = 900 mm• Roof Load Width (RLW) = 2500 mm• Roof Type = Steel Sheet (20
kg/m2)
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Simplify table
Lintel Size
Inputs required• Wind Classification = N2• Stress Grade = F17• Roof Type = Steel Sheet (20 kg/m2)• Roof Load Width (RLW) = 2500 mm• Rafter/Truss Spacing = 900 mm• Opening size = 2400 mm
Use 3000mm
Use 1200mm
A 140 x 35mm F17 Lintel isadequate
2006
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Floor Framing
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Floor Members
Bearers
Platform floor sheets
Joists
Continuous (strip) footings
Bearer lines
Engaged supports
Isolated supports
Isolated (pad) footing
Perimeter brickwork
Bearers
Platform floor sheets
Joists
Continuous (strip) footings
Bearer lines
Engaged supports
Isolated supports
Isolated (pad) footing
Perimeter brickwork
Floor bearers
Floor joists
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• Bearers are commonly made from hardwood or engineered timber products and are laid over sub-floor supports
Floor Bearers
Bearer spanBearer spacing
• Bearers are sized according to span and spacings – typically a 1.8m (up to to 3.6m) grid
BearerSpan
BearerSpacing
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Floor Load Width(FLW)
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‘FLW’ Floor Load Width Example
If x = 2000mm y = 4000mm a = 900mm
FLW C = 2000mm
FLW B = 3000mm
FLW A = 1900mm
FLW C =y/2
FLW B =(x+y)/2
FLW A = (x/2) +a
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Bearer & Floor Joist Design Example
• Gable Roof (25o pitch)• Steel Sheet (20 kg/m2)• Wind Speed = N2• Wall Height = 2400 mm4500
Simple rectangular shaped light-weight home
Elevation
3600
Section
Floor joistsBearers
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Bearer Design Example
3600
Section
Bearer A
roof load andfloor loadsupports both
1800
Floor Load Width (FLW) Bearers at 1800mm crsFLWA = 1800/2 = 900mm
25o
Floor Joistsat 450mm crs
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Bearer Design Example
x ya b
RLW RLW
A B
ayx
2
RLW wall A =
RLW = 1986 mm (say 2000 mm) + 496 mm (say 500 mm)
Total RLW On Wall A = 2500 mm
Roof Load Width (FLW)
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Bearer Design Example
Inputs required
• Wind Classification = N2• Stress Grade = F17• Floor Load Width (FLW) at A = 900 mm
Roof Load Width (RLW) = 2500 mm• Single or Continuous Span= Continuous• Roof Mass (Sheet or Tile) = Steel Sheet
(20 kg/m2)• Bearer Span = 1800mm
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Bearer Size2006
Inputs required• Wind Classification = N2• Stress Grade = F17• Floor Load Width (FLW) at A = 900 mm• Roof Mass (Sheet or Tile) = Steel
Sheet (20 kg/m2) Single or Continuous Span = Continuous
• Roof Load Width (RLW) = 2500 mm
• Bearer Span = 1800mm
Simplify table
Use 1200mm
table
Use 4500mm
2 x 90 x 35mm F17 members joined
together are adequate
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Floor Joist Design Example
Inputs required
• Wind Classification = N2• Stress Grade = F17• Roof Load Width (RLW) = 0 mm (just
supporting floor loads)• Single or Continuous Span = Continuous (max 1800)• Roof Type = Steel Sheet (20 kg/m2)• Joist Spacing = 450 mm
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Simplify table
Joist Size
2006
Inputs required• Wind Classification = N2• Stress Grade = F17• Joist Spacing = 450 mm• Roof Type = Steel Sheet (20 kg/m2)• Single or Continuous Span = Continuous (max
1800)• Roof Load Width (RLW) = 0 mm• Joist span = 1800mm
At least 1800mm
90 x 35mm F17 floor joists at 450mm crs
are adequate
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Timber Framing
Using AS 1684.2 Span Tables
Understanding
AS1684Residential Timber
Framed Construction
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