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Concepts in Australian Standard 1684 Residential Timber

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Concepts in Australian Standard 1684 Residential Timber Framed Construction What s in this presentation: Overview What s in AS1684 Design flow chart – PowerPoint PPT presentation

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Title: Concepts in Australian Standard 1684 Residential Timber


1
Concepts in Australian Standard 1684 Residential
TimberFramed Construction
  • Whats in this presentation
  • Overview
  • Whats in AS1684
  • Design flow chart
  • Appreciation of loads
  • Responsive measures to loads on buildings
  • Choosing framing members to resist loads
  • Using span tables (an example)
  • Bracing
  • Tie Down

2
Overview
  • AS1684 is the main document used in the housing
    industry to design and construct timber floor,
    wall and roof framing
  • Most building specifications call it up as a
    mandatory compliance standard
  • AS1684 comes in 4 volumes to suit different users
  • Part 1 - Design criteria (principally for
    structural engineers)
  • Part 2 Non-cyclonic areas (framing solutions
    for non-cyclonic wind areas)
  • Part 3 Cyclonic areas (framing solutions for
    cyclonic wind areas)
  • Part 4 Simplified non-cyclonic areas
    (oriented towards simple situations
    and novice users)

3
Whats in AS1684
  • Definitions of framing members
  • Spanning concepts (e.g. simple spans, continuous
    spans, rafter spans)
  • Geometric limits to building sizes handled under
    the standard
  • Wind load limits (e.g. cyclonic, non-cyclonic)
  • Dead load limits (e.g. loads on floor, wall and
    roof structures)
  • Details on floor, wall and roof framing systems
  • Bracing requirements
  • Fixing and tie-down requirements
  • The above helps the user choose the right timber
    member and framing system using span tables and
    tables for fixings and bracing

4
Process for designing timber framing
Design wind classification
Consider preliminary location and extent of
bracing and tie-down systems, and modify framing
layout if required
Appreciation of loads including dead, live and
wind loads
Establish basic frame layout and method of
constructing floor, wall and roof frame including
load paths, cantilevers and offsets
Determine individual member sizes
Design bracing system
Responsive measures
Design tie down and other connection requirements
5
Loads on Residential Buildings
  • AS1684 deals with three generic types of loads on
    buildings
  • Gravity Dead Loads
  • Gravity Live Loads
  • Wind loads
  • More than one type of load can be acting on a
    building at the same time
  • Span tables in AS1684 make sure appropriate
    timber members are chosen to resist the loads

Click on the picture to see a video
6
Appreciation of Dead Loads
  • Dead Loads are the forces arising from the weight
    of the building components e.g. roof tiles, roof
    framing and ceiling materials
  • These are felt by the structure all of the time
    but vary according to construction type e.g.
  • A tiled roof with battens, a plasterboard ceiling
    and insulation is approximately 75 kg/m2
  • A sheet metal roof with softwood ceiling and
    insulation is only 20 kg/m2
  • Dead loads impact during construction and during
    the serviceable life of the building

7
Appreciation of Live Loads
  • Live Loads are the forces arising from the weight
    of people using the building plus moveable
    equipment and furniture. These loads are felt
    some of the time by the structure according to
    usage
  • Live loads impact on the building during
    construction and during its serviceable life

8
Appreciation of Wind Loads
  • In general terms, wind loads create the most
    critical loads on houses
  • Wind loads can create downward pressure on the
    structure, or suction that lifts upwards
  • As wind speed increases so does wind load this
    load is spread over the area of the building
    exposed to the wind
  • For the purpose of design, these loads are
    resolved into
  • horizontal loads on walls and roofs
  • Vertical uplift or downward loads on ceilings and
    roofs

9
Horizontal Wind Loads Cause Racking Forces
  • Racking causes distortion to the shape of the
    building
  • Bracing requirements specified in AS1684 are used
    to resist racking forces

10
Horizontal Wind Loads Cause Shear Forces
  • Horizontal wind loads can cause a shear action at
    floor levels.
  • Fixings must be strong enough to resist these
    forces

11
Vertical Wind Loads Cause Uplift Forces
  • When wind passes over a roof it can cause a
    suction. When it gains access to the interior
    it can cause an uplift
  • Light sheet metal roofs are prone to this problem
    but the effect is less of an issue for heavy
    tiled roofs
  • Tie down fixings are important to stop this effect

12
Vertical and Horizontal Wind Loads Cause Rotation
Forces
  • Rotation forces are a concern because they can
    effect entire storeys in strong winds
  • As with uplift forces, tie down fixings are used
    to resist such forces

13
Determining Wind Loads
  • Wind loads can be determined using Australian
    Standard AS1170.2 or a simplified version for
    housing, AS4055
  • This determines which volume of AS1684 is to be
    used for selecting framing members
    (cyclonic/non-cyclonic)
  • In using the wind load standards, buildings in
    protected areas will be subject to less wind load
    than those on exposed sites
  • To calculate the wind load that the building is
    likely to feel, the basic speeds are adjusted for
    factors such as height, shielding and terrain type

14
Other Loads
  • Other types of loads that may affect buildings
    include
  • Earthquake loads
  • Snow loads (a heavier than usual live load)
  • Snow loads have been determined as not being
    critical in the design of domestic structures
    under AS1684 (specific applications may require
    special consideration)
  • Similarly, earthquake is also considered not
    critical

15
Responsive Measures to Loads on Buildings
  • There are three basic responses to resisting the
    previous loads
  • Choosing framing members to resist loads
  • Bracing requirements
  • Tie down requirements
  • Each is discussed (as follows)

16
Choosing Framing Members to Resist Loads
  • 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

Click to see a video
Indirect Load path due to cantilever
  • AS1684 puts the above factors into practice using
    span tables to select the right size members

17
Spanning Situations
  • Before using span tables in AS1684 it is
    important to understand the difference between
    single and continuous span elements - this is
    taken into account when selecting members from
    span tables
  • Single span elements only run between two points
    e.g. a 4.0m span means a 4.0m beam is used
  • Continuous span elements are longer - they span
    the same 4.0m distance but then go onto one or
    more extra spans e.g. extra 4.0m spans

18
  • From a structural point of view, the continuous
    span element has an advantage over the single
    span element a load on one span, forces some of
    the second span in the opposite direction e.g. as
    shown by the upward effect below
  • This means the continuous span element doesnt
    need to be quite as big as the single span element

19
Load Bearing Versus Non-Load Bearing Walls
  • Another issue influencing the use of span tables
    in AS1684 is the difference between the terms
    Load bearing and Non-load bearing walls
  • Load bearing refers to a wall that supports roof
    or floor loads, or both
  • Non-load bearing refers to walls that dont
    support the above loads but may support ceiling
    loads and be used as bracing walls

Click above to see a video
20
Using AS1684 Span Tables to Choose Member Sizes
an example
  • 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

21
  • Step 6 Look up Volume 2 of AS1684 (i.e.
    non-cyclonic winds N1 N2) and go to the rafter
    span tables
  • Step 7 Choose a table reflecting your preferred
    stress grade plus needs such as
    seasoned/unseasoned, softwood/hardwood in this
    example assume MGP10 seasoned softwood
  • 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

22
A Snapshot of What Bracing does
After choosing member sizes to make up floor,
wall and roof frames, bracing is required to
stabilises the three dimensional structure.
Bracing for each of these elements is linked so
forces can be transferred down the structure to
the ground
23
Types of Bracing
  • There are three generic types of bracing
  • Sheet bracing - typically plywood, hardboard or
    other sheet products
  • Cross bracing - typically strap metal or solid
    timber lengths
  • One directional bracing typically metal angle
    or timber lengths

Gable end bracing
Cross or sheet
bracing
Cross or sheet
bracing
Some types of bracing provide more resistance to
racking forces than others.
Subfloor cross-bracing,
cantilevered stumps or
bracing wall
Wind
Examples of Bracing
Click on the picture to see a video
24
Bracing Addresses the Problem of Plane Distortion
  • Roof, wall and floor planes try to distort as a
    result of wind and other forces causing
    rectangular shapes to become parallelograms

25
  • The various types of bracing mentioned earlier,
    act to stop distortion the diagrams show this
    using strap cross bracing which works in tension
  • If the braces are only fixed at the ends, the mid
    regions can still move sideways
  • If the bracing is fixed to every element it
    crosses (as shown in the lower figure) the forces
    arising can be transferred to the supporting
    structure and the whole assembly can be kept
    square.

26
Typical Bracing Procedure
  • Determine the wind classification (detailed
    calculations are required for the calculation of
    wind pressure on the building)
  • Determine the area of elevation affected
    (detailed calculations also required for the
    calculation of racking forces)
  • Design the bracing system (choose the type of
    bracing and sufficient units to match racking
    forces)
  • Check for even distribution and spacing of
    bracing
  • Check the consistency of bracing connections
    between roof/ceiling/wall/floor/subfloor

27
Fixing and Tie Down
  • All fixings at joints must be able to resist
    gravity, uplift and horizontal forces
  • Generic types of fixing include
  • Nails
  • Straps
  • Bolts
  • Screws
  • Coach screws
  • Framing anchors
  • Steel washers may assist the above

28
Fixing and Tie Down Requirements
  • The size and spacing of connectors varies to suit
    different joints, spans and loading scenarios
  • Wind speed is a particularly important in
    dictating fixing and tie down requirements
  • Rules and selection tables are in AS1684

Click on the picture to see a video
29
Common Fixing and Tie Down Scenarios
  • Common joints requiring specific attention to tie
    down include
  • Battens to rafters (or trusses)
  • Rafters (or trusses) to top plates
  • Top plates to studs
  • Studs to bottom plates
  • Bottom plates to floor structures

30
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