Using span tables as1684 2

Timber Framing

Using AS 1684.2 Span Tables

Understanding

AS1684Residential Timber

Framed Construction

the timber framing standard

AS 1684 Residential timber-framed construction

Currently you should be using the 2006 Edition

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

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:

Timber Framed Construction

Each set of Span Tables contains 53 separate design tables

Using AS 1684 you should be able to design or check

virtually every member in a building constructed

using timber framing


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


AS1684 Scope & Limitations

Where can AS1684 be used?

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

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.

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.

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.

Wind Classification

Non-Cyclonic Regions A & B only

N1 - W28N 100km/h gust




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 :

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

• Design fundamentals & basic terminology

• Roof framing• Wall framing• Floor framing

Click on arrow to move to section required

Using AS1684.2 Span Tables

Design Fundamentals&

Basic Terminology

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

• 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

Load distribution

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

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

Span & Spacing

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

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

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.

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

• 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

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.

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.

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

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

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

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

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

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

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

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

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

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

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

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)

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

Ceiling Joist Design

Ridgeboard

Ceiling Joist

Rafter

Design variables• Timber Stress Grade• Ceiling Joist Spacing• Ceiling Joist Span• Single or Continuous Span

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

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

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

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

Roof Load Width(RLW)

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

A

B

3000

1500 1500

Roof Load Widths are measured on the rake of the roof.


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 '.


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.

*


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

Ceiling Load Width(CLW)

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

CLW is used as an input to Span Tables for

• hanging beams, and• strutting/hanging beams


Hanging beam

Ceiling jo ist

Hanging beam span

'x '

Hanging Beam Strutting/Hanging Beam

Strutting beam span

Ridgeboard

Underpurlin

Strutting beam

Roof strut

CLW Hanging beam D =2x

A B C

ED

x y

CLW CLW


FIGURE 2.12 CEILING LOAD WIDTH (CLW)

CLW Strutting/Hanging beam E =2y

A B C

D E

yx

CLWCLW

FIGURE 2.12 CEILING LOAD WIDTH (CLW)


Roof Area Supported

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 =

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)

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

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

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)

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

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)

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

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)

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

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

• 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

Floor Load Width(FLW)

‘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

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

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


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)


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

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

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

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

Timber Framing

Using AS 1684.2 Span Tables

Understanding

AS1684Residential Timber

Framed Construction

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Using span tables as1684 2

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