Understanding AS1684 Residential Timber Framed Construction

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Understanding AS1684
Residential Timber Framed Construction
Timber Framing Using AS 1684.2 Span Tables
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the timber framing standard

• Currently you should be using the 2006 Edition

AS 1684 Residential timber-framed construction
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the timber framing standard
Provides the building industry with procedures
that can be used to determine building practice,
to

• 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
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AS 1684.2 CD Span Tables
Contains a CD of Span Tables (45 sets in all)
for wind zones N1/N2, N3 and N4 for the following
timber stress grades
Unseasoned softwood F5, F7 Seasoned softwood
F5, F7, F8, MGP10, MGP12,
MGP15, Unseasoned hardwood F8, F11, F14,
F17 Seasoned hardwood F14, F17, F27
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Timber Framed Construction
Each set of Span Tables contains 53 separate
design tables
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Timber Framed Construction
Using AS 1684 you should be able to design or
check virtually every member in a building
constructed using timber framing
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Timber Framed Construction
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AS1684 Scope Limitations
Where can AS1684 be used?
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AS1684 Limitations - Physical

• Plan rectangular, square or L-shaped
• Storeys single and two storey construction
• Pitch 35o max. roof pitch
• Width 16m max. (Between the pitching points of
  the roof, ie excluding eaves)

• .

• x

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Width
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
(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
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Using AS1684.2 Span Tables

• Design fundamentals basic terminology
• Roof framing
• Wall framing
• Floor framing

Click on arrow to move to section required
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Design Fundamentals Basic Terminology
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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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Design Fundamentals

• Understanding the concept of a load path is
  critical. Loads need to be supported down the
  building to the ground

• 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

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Load distribution
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Loads distributed
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.
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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
C
B
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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.  
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.
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.
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Terminology - Continuous Span
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.
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.
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Example Continuous Span
6000mm
1/3
(2000mm)
1/3
(2000mm)
1/3
(2000mm)
The centre support
must be wholly within
the middle third.

• Span 1 (2000mm)

Span 2 (3925mm)
75mm
75mm
75mm
Span 2 is not to be greater than twice Span 1.
This span is used to determine the size using
the continuous span tables.
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Terminology Rafter Span and Overhang
   Rafter
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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Terminology Wall Construction
Loadbearing wallA wall that supports roof or
floor loads, or both roof and floor loads.
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.

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.
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Terminology Roof Construction
Coupled roof
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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Terminology Roof Construction
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.
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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

• Roof shapes are made to cover the footprint while
  also providing sloping planes able to shed water

Skillion
Gable (Cathedral or flat ceiling)
Hip

• Common roof shapes used to cover the required
  area are shown above

Dutch Hip (or Dutch Gable)
Hip and valley
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Typical Roof Framing Members
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Transferring Loads to Pitched Roofs
Support wall
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Batten Design
Typical Process

• 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

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Batten Design
Step 5 Look up Volume 2 of AS1684 (i.e.
non-cyclonic winds N1 N2) and go to the batten
span tables

• Step 6 Choose a table reflecting your preferred
  stress grade

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
Simplify table

• 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
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Rafter Design
Scenario - Rafters for a Cathedral Roof

• 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

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

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• Step 6 Look up Volume 2 of AS1684 (N1 N2)

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
Step 11 Read off the rafter size 90x45mm
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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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Rafter Size
Simplify table
Maximum Rafter or Purlin Span Overhang (mm)

• 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
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Ceiling Joist Design
Ridgeboard
Rafter
Ceiling Joist

• 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
Simplify table

• Inputs required
• Wind Classification N2
• Stress Grade F17
• Overbatten No
• Single or Continuous Span Single
• Joist Spacing 450 mm
• Ceiling Joist Span 3600 mm

A 120 x 45mm F17 ceiling joist is adequate
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Ridgeboard
OTHER MEMBERS AND COMPONENTS
Some members do not have to be designed using
span tables they are simply called up or
calculated based on members framing into them
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Roof Member - Load Impacts
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
Roof Load Width (RLW)
Ceiling Load Width (CLW)
Roof area supported
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Roof Load Width(RLW)
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RLW - Roof Load Width

• 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

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RLW - Roof Load Width
Roof Load Widths are measured on the rake of the
roof.
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RLW - Roof Load Width
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RLW - Roof Load Width
RLW wall A
Trusses
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RLW - Roof Load Width
Without ridge struts
RLW wall A
RLW wall B
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RLW - Roof Load Width
With ridge struts
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Ceiling Load Width(CLW)
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CLW - Ceiling Load Width
Ceiling load width (CLW) is the width of ceiling
that contributes ceiling load to a supporting
member (it is usually measured horizontally).
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CLW - Ceiling Load Width

• CLW is used as an input to Span Tables for
• hanging beams, and
• strutting/hanging beams

Strutting/Hanging Beam
Hanging Beam
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CLW - Ceiling Load Width
FIGURE  2.12   CEILING LOAD WIDTH (CLW)
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CLW - Ceiling Load Width
FIGURE  2.12   CEILING LOAD WIDTH (CLW)
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Roof Area Supported
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Roof Area Supported
EXAMPLE The STRUTTING BEAM span table requires
a Roof Area Supported (m2) input.
The strutting beam shown supports a single strut
that supports an underpurlin.
A
A/2
The area required, is the roof area supported
by the strut. This is calculated as follows-
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)
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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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F17
Simplify table

• 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
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Wall Framing
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Wall Framing
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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
Simplify table

• 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

70 x 35mm MGP10 wall studs are adequate
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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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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

2 x 35x 70mm MGP10 top plates are adequate
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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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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

A 140 x 35mm F17 Lintel is adequate
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Floor Framing
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Floor Members
Floor bearers
Floor joists
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Floor Bearers

• Bearers are commonly made from hardwood or
  engineered timber products and are laid over
  sub-floor supports

• Bearers are sized according to span and spacings
  typically a 1.8m (up to to 3.6m) grid

Bearer Span
Bearer Spacing
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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 A 1900mm
FLW A (x/2) a
FLW B 3000mm
FLW B (xy)/2
FLW C 2000mm
FLW C y/2
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Bearer Floor Joist Design Example
Simple rectangular shaped light-weight home
Floor joists
Bearers

• Gable Roof (25o pitch)
• Steel Sheet (20 kg/m2)
• Wind Speed N2
• Wall Height 2400 mm

Elevation
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Bearer Design Example
25o
roof load
and floor load
Floor Joists at 450mm crs
Bearer A
supports both
1800
3600
Section
Floor Load Width (FLW) Bearers at 1800mm crs FLWA
1800/2 900mm
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Bearer Design Example
Roof Load Width (FLW)
RLW 1986 mm (say 2000 mm) 496 mm (say 500 mm)
Total RLW On Wall A 2500 mm
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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 Size
Simplify table
2 x 90 x 35mm F17 members joined together are
adequate

• 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

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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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Joist Size
90 x 35mm F17 floor joists at 450mm crs are
adequate

• 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

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Understanding AS1684
Residential Timber Framed Construction
Timber Framing Using AS 1684.2 Span Tables
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