As 2870 Residential Slabs And Footings Pdf File

AS 2870—2011 2870—2011
A S 2 8 7 0 — 2 0 1 1
Australian Standard . m o c . l a b o l g i a s . e r o t s o f n i / / : p t t h t a n o i s r e v l l u f e h t s s e c c A . e l p m a s e g a p 7 e e r f a s i s i h T
®
Residential slabs and footings
This Australian Standard® was prepared by Committee BD-025, Residential Slabs and Footings. It was approved on behalf of the Council of Standards Australia on 20 December 2010. This Standard was published on 17 January 2011.
The following are represented on Committee BD-025: • • • • • •
. m o c . l a b o l g i a s . e r o t s o f n i / / : p t t h t a n o i s r e v l l u f e h t s s e c c A . e l p m a s e g a p 7 e e r f a s i s i h T
• • • • • • • • • • •
Australian Building Codes Board Australian Chamber of Commerce and Industry Australian Geomechanics Society Australian Institute of Building Surveyors Cement Concrete and Aggregates Australia Concrete Masonry Association of Australia Construction Industry Advisory Council Engineers Australia Foundations and Footings Society of Australia Housing Industry Association Master Builders Australia National Timber Development Council Plastics and Chemicals Industries Association Steel Reinforcement Institute of Australia Think Brick Australia University of Newcastle University of South Australia
This Standard was issued in draft form for comment as DR AS 2870. Standards Australia wishes to acknowledge the participation of the expert individuals that contributed to the development of this Standard through their representation on the Committee and through the public comment period.
Australian Standards® are living documents that reflect progress in science, technology and systems. To maintain their currency, all Standards are periodically reviewed, and new editions are published. Between editions, amendments may be issued. Standards may also be withdrawn. It is important that readers assure themselves they are using a current Standard, which should include any amendments that may have been published since the Standard was published. Detailed information about Australian Standards, drafts, amendments and new projects can be found by visiting Standards Australia welcomes suggestions for improvements, and encourages readers to notify us immediately of any apparent inaccuracies or ambiguities. Contact us via email at , or write to Standards Australia, GPO Box 476, Sydney, NSW 2001.
AS 2870—2011
Australian Standard . m o c . l a b o l g i a s . e r o t s o f n i / / : p t t h t a n o i s r e v l l u f e h t s s e c c A . e l p m a s e g a p 7 e e r f a s i s i h T
®
Residential slabs and footings
Originated as AS 2870—1986. Previous editions AS 2870.1—1988 and AS 2870.2—1990. Revised, amalgamated and redesignated AS 2870—1996. AS 2870—1996 and AS 2870 Supp 1—1996 revised and published as AS 2870—2011.
COPYRIGHT
© Standards Australia Limited All rig hts are res erv ed. No par t o f t his wor k m ay be rep rod uce d o r c opie d i n a ny for m o r b y any means, electronic or mechanical, including photocopying, without the written permission of the publisher, unless otherwise permitted under the Copyright Act 1968. Published by SAI Global Limited under licence from Standards Australia Limited, GPO Box 476, Sydney, NSW 2001, Australia ISBN 978 0 7337 9756 9
2
AS 2870 —201 1
PREFACE
This Standard was prepared by the Standards Australia Committee BD-025, Residential Slabs and Footings, to supersede AS 2870—1996. The objective of this Standard is to specify performance criteria and specific designs for footing systems for foundation conditions commonly found in Australia and to provide guidance on the design of footing systems by engineering principles. This Standard places particular emphasis on design for reactive clay sites susceptible to significant ground movement due to moisture changes. The Standard takes account of the following:
. m o c . l a b o l g i a s . e r o t s o f n i / / : p t t h t a n o i s r e v l l u f e h t s s e c c A . e l p m a s e g a p 7 e e r f a s i s i h T
(a)
Swelling and shrinkage movements of reactive clay soils due to moisture changes.
(b)
Settlement of compressible soils or fill.
(c)
Distribution to the foundation of the applied loads.
(d)
Tolerance of the superstructure to movement.
Notes are included for clarification and general advice only and are not part of the mandatory provisions of the Standard. Changes to the previous edition are as follows: (a)
Revision of the overall Standard.
(b)
Site Class H split into Classes H1 and H2.
(c)
New Appendix H Guide to design of footings for trees.
The terms ‘normative' and ‘informative' have been used in this Standard to define the application of the appendix to which they apply. A ‘normative' appendix is an integral part of a Standard, whereas an ‘informative' appendix is only for information and guidance. The Figures in this Standard are intended to show only the structural proportions of the footing system. All other details are purely illustrative. Commentary to this Standard has been included at the back of this document. The Commentary is for information and advice only, and does not form part of the mandatory body of the Standard. The layout of the Commentary follows that of the Standard. The numbering differs only in that its clauses, figures and tables are prefixed by the letter ‘C', e.g. Clause C3.2.1 of this Commentary refers to Clause 3.2.1 of the Standard. Where there is no commentary to a Clause of the Standard it does not appear, therefore the Clause numbers in this Commentary are not consecutive. References to various publications and papers are listed as the last item of the Section or Appendix in which they occur. Section C7 provides recommendations not given in the Standard. The Commentary is for information and advice only.
3
AS 2870 —201 1
CONTENTS
Page
. m o c . l a b o l g i a s . e r o t s o f n i / / : p t t h t a n o i s r e v l l u f e h t s s e c c A . e l p m a s e g a p 7 e e r f a s i s i h T
SECTION 1 SCOPE AND GENERAL 1.1 SCOPE ............................................................ 5 1.2 APPLICATION ...................................................... 5 1.3 PERFORMANCE OF FOOTING SYSTEMS ............................... 6 1.4 DESIGN CONDITIONS............................................... 7 1.5 DEEMED-TO-COMPLY STANDARD DESIGNS ........................... 8 1.6 ARTICULATION REQUIREMENTS..................................... 8 1.7 NORMATIVE REFERENCES.......................................... 8 1.8 DEFINITIONS ...................................................... 9 1.9 NOTATION ....................................................... 13 1.10 REINFORCEMENT DESIGNATION.................................... 15 1.11 INFORMATION IN DOCUMENTS ..................................... 16 SECTION 2 SITE CLASSIFICATION 2.1 GENERAL ........................................................ 17 2.2 METHODS FOR SITE CLASSIFICATION............................... 18 2.3 ESTIMATION OF THE CHARACTERISTIC SURFACE MOVEMENT ........ 20 2.4 SITE INVESTIGATION REQUIREMENTS .............................. 23 2.5 ADDITIONAL CONSIDERATIONS FOR SITE CLASSIFICATION........... 24 SECTION 3 STANDARD DESIGNS 3.1 SELECTION OF FOOTING SYSTEMS.................................. 26 3.2 STIFFENED RAFT .................................................. 28 3.3 FOOTING SLAB.................................................... 31 3.4 WAFFLE RAFTS................................................... 33 3.5 STIFFENED SLAB WITH DEEP EDGE BEAM........................... 35 3.6 STRIP FOOTINGS.................................................. 36 3.7 REINFORCEMENT EQUIVALENCES .................................. 39 3.8 SUSPENDED CONCRETE FLOORS IN ONE-STOREY CONSTRUCTION ..... 40 3.9 FOOTING SYSTEMS FOR TWO-STOREY CONSTRUCTION WITH SUSPENDED CONCRETE FLOOR ................................................ 40 3.10 FOOTINGS FOR CONCENTRATED LOADS............................. 40 SECTION 4 DESIGN BY ENGINEERING PRINCIPLES 4.1 GENERAL ........................................................ 41 4.2 DESIGN CRITERIA ................................................. 41 4.3 DESIGN OF FOOTING SYSTEMS..................................... 41 4.4 STIFFENED RAFT FOOTING SYSTEMS................................ 41 4.5 SIMPLIFIED METHOD FOR RAFT DESIGNS............................ 43 4.6 DESIGN OF FOOTING SYSTEMS OTHER THAN STIFFENED RAFTS....... 44 4.7 FOOTING SYSTEMS FOR REINFORCED SINGLE–LEAF MASONRY WALLS ........................................................... 45 4.8 DESIGN FOR PILED OR PIERED FOOTING SYSTEMS ................... 45 SECTION 5 DETAILING REQUIREMENTS 5.1 GENERAL ........................................................ 46 5.2 DRAINAGE DESIGN REQUIREMENTS ................................ 46 5.3 REQUIREMENTS FOR RAFTS AND SLABS............................. 47 5.4 REQUIREMENTS FOR PAD AND STRIP FOOTINGS ..................... 53
AS 2870 —201 1
5.5 5.6
4
REQUIREMENTS IN AGGRESSIVE SOILS.............................. 54 ADDITIONAL REQUIREMENTS FOR CLASSES M, H1, H2 AND E SITES........................................................... 58
SECTION 6 CONSTRUCTION REQUIREMENTS 6.1 GENERAL ........................................................ 60 6.2 PERMANENT EXCAVATIONS ....................................... 60 6.3 TEMPORARY EXCAVATIONS ....................................... 60 6.4 CONSTRUCTION OF SLABS......................................... 60 6.5 CONSTRUCTION OF STRIP AND PAD FOOTINGS....................... 66 6.6 ADDITIONAL REQUIREMENTS FOR MODERATELY, HIGHLY AND EXTREMELY REACTIVE SITES...................................... 66
. m o c . l a b o l g i a s . e r o t s o f n i / / : p t t h t a n o i s r e v l l u f e h t s s e c c A . e l p m a s e g a p 7 e e r f a s i s i h T
APPENDICES A FUNCTIONS OF VARIOUS PARTIES.................................. 68 B FOUNDATION PERFORMANCE AND MAINTENANCE................... 69 C CLASSIFICATION OF DAMAGE DUE TO FOUNDATION MOVEMENTS .... 72 D SITE CLASSIFICATION BY SOIL PROFILE IDENTIFICATION ............. 74 E STUMP PAD SIZES, BRACED STUMP UPLIFT HORIZONTAL LOAD CAPACITY ........................................................ 79 F SOIL STRUCTURE INTERACTION ANALYSIS FOR STIFFENED RAFTS .... 83 G DEEP FOOTINGS .................................................. 86 H GUIDE TO DESIGN OF FOOTINGS FOR TREES......................... 90 I BIBLIOGRAPHY................................................... 93 COMMENTARY TO AS 2870—2011........................................... 96
5
AS 2870 —201 1
STANDARDS AUSTRALIA
Australian Standard Residential slabs and footings
S E C T I O N 1.1
. m o c . l a b o l g i a s . e r o t s o f n i / / : p t t h t a n o i s r e v l l u f e h t s s e c c A . e l p m a s e g a p 7 e e r f a s i s i h T
1
S C O P E
A N D
G E N E R A L
SCOPE
This Standard sets out the criteria for the classification of a site and the design and construction of a footing system for a single dwelling house, townhouse or similar structure which may be detached or separated by a party wall or common wall, but not situated vertically above or below another dwelling, includin g buildings classified as Class 1 and Class 10a in the Building Code of Australia. The Standard may also be used for other forms of construction, including some light industrial, commercial and institutional buildings if they are similar to houses in size, loading and superstructure flexibility. The footing systems for which designs are given include slab on ground, stiffened rafts, waffle rafts, strip footings, pad footings and piled footings. NOTE : This Sta ndard give s no advi ce on deta ili ng of the connect ion of supers truc tures to the footing systems for wind loads or earthquake loads.
For design purposes, the life of the structure is taken to be 50 years. NOTE S: 1
This Standard has been widely used for a number of years for the economical design of footings and slabs. Economical designs that avoid significant damage are practicable only if the soil moisture content of the foundation material under the footing or slab is stable or within reasonable limits of stability over the design life of the house or structure. For all sites (in particular sites with reactive soils) drainage and soil moisture conditions around the building need to be managed to avoid abnormal moisture conditions, as outlined in Clause 1.3.3, which may result in building damage.
2
Site management recommendations are given in Appendix B.
3
Where slab on ground construction is used for long slabs and large houses, particular consideration in design may be needed to avoid significant damage.
4
Information on earthquake actions is included in AS 1170.4. Information on wind actions is included in AS/NZS 1170.2 and AS 4055.
1.2
APPLICATION
To comply with this Standard— (a)
all sites shall be classified in accordance with Section 2; and
(b)
footing system design shall be by either—
(c)
(i)
prescribing a standard design in accordance with Section 3; or
(ii)
applying the engineering principles described in Section 4; and
all design and construction shall comply with Sections 5 and 6.
Residential footing system design, detailing and construction shall also comply with AS 3600 except that, where in conflict, this Standard (AS 2870) shall take precedence. NOTE : The fun ctions of the various part ies inc lude d in the desi gn and constr uction of res idential slabs and footings are normally as described in Appendix A.
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© Standards
Australia
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AS 2870-2011, Residential slabs and footings . m o c . l a b o l g i a s . e r o t s o f n i / / : p t t h t a n o i s r e v l l u f e h t s s e c c A . e l p m a s e g a p 7 e e r f a s i s i h T
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FOOTING SYSTEMS Reference - As 2870 - Residential slabs and footings - construction and BCA. Footings transfer building loads onto foundations. House design and shape must be designed before footing design can occur. AS2870 is based upon inspecting site to determine soil type and designing footing to suit both foundation and structure to be built. Footing construction New footings are constructed from reinforced concrete in accordance with designs set out in AS 2870 or engineering principles. Steel is extremely strong in tension and compression but it rusts. Concrete is extremely strong in compression but relatively relativel y weak in tension. It covers the steel and protects it from moisture in the ground. Together they produce a composite material which is strong in compression and tension, easily shaped and durable. Steel is always placed in areas of tension in all reinforced concrete members to stop the concrete cracking. For domestic (residential housing) purposes steel is available in the following forms: Trench mesh -e.g. previously 3 - 8TM (now (now called L8) - 3 bars of 8mm diameter diameter steel connected by cross wires to form a fabric laid in trenches for strip footings. No and diameter of bars varies. Comes in 6m lengths (strength of 300 Mpa). Slab fabric -e.g.previously called F62 (now SL 62)= fabric 6mm bars welded together in a 200mm square grid. Sheet size 2.4m 2 .4m X 6.0m. Available as F62, F72, F 72, F82, F92, F1018, etc. (strength of 300 Mpa). Bars -e.g. R10 = round 10mm diameter bar. -e.g. Y16 (now called N16)= deformed bar 16mm diameter - (Y= high yield strength of 410 Mpa +) •
•
•
Bars can be ordered cogged (bent) to suit su it but must be transportable. Basic types of footings Common details Min . strength concrete 20 Mpa. Nominal aggregate size 20mm. Pad footings Also called blob footings. Is a solid mass of concrete ( no reo) laid in ground to support brick, timber or steel piers / posts. Commonly used to support timber floor frames. With reo and engineering design can be used to support suspended concrete floors . Details: Brickwork not acceptable Reo (if used) requires 40mm concrete cover. Suitable for A, S, M, H class sites. Sizes for pads is given in AS1684 Timber Framing code - size subject to area and load of floors. Minimum 400 x 400 x 200 high.
Isolated brick piers on pad footings Strip footing Reinforced strip of concrete laid in trench in ground. Used to support continuous brick walls. Typically 300mmm deep x 300 - 400mm wide. Process: Dig trench with backhoe or by hand. Tie up reinforcing cage. Lay reinforcing (reo) in trench. Support reo cage to ensure required concrete cover all round. Pour concrete and allow to cure before loading. Details: Reo requires 40mm concrete cover. Lapping of bars min. 500mm or full width at T and L intersections. Stepping techniques - see As 2870 Clause 5.4.3 Suitable for A, S, M, H class sites.
Strip footing trench with trench mesh reinforcement
Strip footing after pouring of concrete
Pier and Beam This system of footing basically a post and lintel method of load support. This concept permeates almost all structural elements of building. Its basic premise is that the lintel (horizontal member) carries a load from above and spreads it horizontally to the posts (vertical members). The posts then pass the load to another supporting element or the foundation material. The beam (lintel) is a strip footing which is deeper than it is wide. It is constructed in the same as a strip footing. The pier (post) is a vertical cylinder of normally unreinforced concrete (up to 3.0 m deep) which is made by drillling a hole in the ground to the depth required to find a suitable ABP or pass below the reactive zones of a reactive soil. The piers supports the beam at approximately 1800 - 2400 mm centres. Piles or piers may also be used in all forms of slabs on ground to find adequate ABP or bypass reactive areas. The piers may or may not be tied to the beam by reo (see your engineer for details). Process: Drill pier holes as directed by engineer Fill piers with concrete to level which coincides with bottom of beam then construct beam as per strip footing. Piers are sometimes belled (enlarged) on the end to resist upheaval on reactive sites or reduce pressure by increasing surface contact area. In highly reactive sites beam may require slip joint (2 layers of plastic membrane) to allow soil to slip past beam. Often also utilizes compressible material (foam, corrugated steel, etc) under beam to accommodate ground heave. Ground level Poured strip footing (beam) Piers (posts)
Pile and Beam Piles perform the same function as piers and piers are often called piles. The pile and beam system is identical to pier and beam except for the piles. Piles are preformed units of timber (with steel collars or caps), reinforced concrete or steel which are hammered into the ground much the same as a nail is hammered into timber. When piles are used in clusters ( a group) for large buildings a pile cap (pad footing) is often poured on top to carry the load of the beam or slab. When being hammered piles stop due to : Friction on the sides and end of the pile; or End resistance when the pile hits a very strong or hard foundation. A 1000kg percussion hammer is often used to hammer the piles. Damage to neighbouring buildings from vibration or ground heave is of concern. Piles are often used where collapsing soils exist on the site and drilling pier holes would result in collapsing holes Raft slabs All slabs cast on the ground are considered as raft slabs as they float on the soil. They vary in design to suit the type of foundation material. Slab on Ground ( SOG) Consists of a flat slab sitting on a perimeter beam (similar to a strip footing). Poured as one integral unit. Is used primarily in conjunction with trussed roofs which impart no load to the internal area of the building. Process: Use excavator to cut and fill site as necessary to provide flat building platform. Setout formwork Dig trenches for perimeter beam with backhoe or by hand. Install drainage pipes. Place blinding layer of sand. Pest spray the sand if required. Install vapour barrier Place reinforcement Pour concrete and allow to cure before loading. Details: Suitable for A & S class sites. Reo used- slab fabric over entire area of slab; trench mesh in bottom of edge beam. Reo cover required: Slab fabric where protection from moisture is provided by structure - 20mm Trench mesh at bottom of edge beam protected by vapour barrier - 30mm Trench mesh at bottom of edge beam not protected by vapour barrier - 40mm Uses edge rebate to stop horizontal moisture penetration at floor level and assist flashing system. Internal load bearing walls require the slab to be thickened by 50mm under the wall. Setdowns in top of slab matched by setdown in soffit of slab (special care with reo). Height of Finished Floor Level above ground level determined by: Height of surcharge gully Possibility of local flooding Termite protection method Effects of cut and fill Foundation type and use of perimeter paving.
Slab on ground viewed from underneath showing flat slab on edge beams. Diagram on right hand side shows detail of one corner of a slab with a dropped edge beam. The interior of the slab also rests on soil as do the perimeter beams.
Footing slab Identical to slab on ground except edge beam is poured separately from slab. Useful on sloping sites or where soil is soft or collapsing. May utilize brickwork between edge beam and slab on low sides of site. R10 ligatures @ 500 centres used to tie edge beam to slab.
Stiffened raft slab This is an extension of the slab on ground and is much stronger. It utilizes internal beams @ 3.0 - 6.0 m centres in addition to the edge beams to provide a stiff grid of beams. Suited to class M & H sites. Process and details identical to those for SOG.
Diagram showing grid layout of internal and external beams. The shaded section depicts the area of the slab only 100mm thick whilst the lighter areas are beams typically 400mm deep.
Waffle raft slab This is the strongest form of standard slab. Utilizes a closely spaced grid of 110 wide internal beams @ 1090mm max. centres in both directions. Edge beams are min 150 high x 300 min wide. Beams are formed by used of expanded polystyrene blocks set apart by spacers which also act as reo chairs. Slab is formed above ground so eliminating trench excavation. Uses N high strength bars instead of trench mesh in beams and uses standard slab fabric over slab area. Slab may be 85 thick. Advantage is accurate concrete quantity estimates. Services cut in through waffle forms. Suited to A, S, M, H class sites.
Egg crate layout of closely spaced beams in waffle slab construction

AS 2870—2011 2870—2011
A S 2 8 7 0 — 2 0 1 1
Australian Standard . m o c . l a b o l g i a s . e r o t s o f n i / / : p t t h t a n o i s r e v l l u f e h t s s e c c A . e l p m a s e g a p 7 e e r f a s i s i h T
®
Residential slabs and footings
This Australian Standard® was prepared by Committee BD-025, Residential Slabs and Footings. It was approved on behalf of the Council of Standards Australia on 20 December 2010. This Standard was published on 17 January 2011.
The following are represented on Committee BD-025: • • • • • •
. m o c . l a b o l g i a s . e r o t s o f n i / / : p t t h t a n o i s r e v l l u f e h t s s e c c A . e l p m a s e g a p 7 e e r f a s i s i h T
• • • • • • • • • • •
Australian Building Codes Board Australian Chamber of Commerce and Industry Australian Geomechanics Society Australian Institute of Building Surveyors Cement Concrete and Aggregates Australia Concrete Masonry Association of Australia Construction Industry Advisory Council Engineers Australia Foundations and Footings Society of Australia Housing Industry Association Master Builders Australia National Timber Development Council Plastics and Chemicals Industries Association Steel Reinforcement Institute of Australia Think Brick Australia University of Newcastle University of South Australia
This Standard was issued in draft form for comment as DR AS 2870. Standards Australia wishes to acknowledge the participation of the expert individuals that contributed to the development of this Standard through their representation on the Committee and through the public comment period.
Australian Standards® are living documents that reflect progress in science, technology and systems. To maintain their currency, all Standards are periodically reviewed, and new editions are published. Between editions, amendments may be issued. Standards may also be withdrawn. It is important that readers assure themselves they are using a current Standard, which should include any amendments that may have been published since the Standard was published. Detailed information about Australian Standards, drafts, amendments and new projects can be found by visiting Standards Australia welcomes suggestions for improvements, and encourages readers to notify us immediately of any apparent inaccuracies or ambiguities. Contact us via email at , or write to Standards Australia, GPO Box 476, Sydney, NSW 2001.
AS 2870—2011
Australian Standard . m o c . l a b o l g i a s . e r o t s o f n i / / : p t t h t a n o i s r e v l l u f e h t s s e c c A . e l p m a s e g a p 7 e e r f a s i s i h T
®
Residential slabs and footings
Originated as AS 2870—1986. Previous editions AS 2870.1—1988 and AS 2870.2—1990. Revised, amalgamated and redesignated AS 2870—1996. AS 2870—1996 and AS 2870 Supp 1—1996 revised and published as AS 2870—2011.
COPYRIGHT
© Standards Australia Limited All rig hts are res erv ed. No par t o f t his wor k m ay be rep rod uce d o r c opie d i n a ny for m o r b y any means, electronic or mechanical, including photocopying, without the written permission of the publisher, unless otherwise permitted under the Copyright Act 1968. Published by SAI Global Limited under licence from Standards Australia Limited, GPO Box 476, Sydney, NSW 2001, Australia ISBN 978 0 7337 9756 9
2
AS 2870 —201 1
PREFACE
This Standard was prepared by the Standards Australia Committee BD-025, Residential Slabs and Footings, to supersede AS 2870—1996. The objective of this Standard is to specify performance criteria and specific designs for footing systems for foundation conditions commonly found in Australia and to provide guidance on the design of footing systems by engineering principles. This Standard places particular emphasis on design for reactive clay sites susceptible to significant ground movement due to moisture changes. The Standard takes account of the following:
. m o c . l a b o l g i a s . e r o t s o f n i / / : p t t h t a n o i s r e v l l u f e h t s s e c c A . e l p m a s e g a p 7 e e r f a s i s i h T
(a)
Swelling and shrinkage movements of reactive clay soils due to moisture changes.
(b)
Settlement of compressible soils or fill.
(c)
Distribution to the foundation of the applied loads.
(d)
Tolerance of the superstructure to movement.
Notes are included for clarification and general advice only and are not part of the mandatory provisions of the Standard. Changes to the previous edition are as follows: (a)
Revision of the overall Standard.
(b)
Site Class H split into Classes H1 and H2.
(c)
New Appendix H Guide to design of footings for trees.
The terms ‘normative' and ‘informative' have been used in this Standard to define the application of the appendix to which they apply. A ‘normative' appendix is an integral part of a Standard, whereas an ‘informative' appendix is only for information and guidance. The Figures in this Standard are intended to show only the structural proportions of the footing system. All other details are purely illustrative. Commentary to this Standard has been included at the back of this document. The Commentary is for information and advice only, and does not form part of the mandatory body of the Standard. The layout of the Commentary follows that of the Standard. The numbering differs only in that its clauses, figures and tables are prefixed by the letter ‘C', e.g. Clause C3.2.1 of this Commentary refers to Clause 3.2.1 of the Standard. Where there is no commentary to a Clause of the Standard it does not appear, therefore the Clause numbers in this Commentary are not consecutive. References to various publications and papers are listed as the last item of the Section or Appendix in which they occur. Section C7 provides recommendations not given in the Standard. The Commentary is for information and advice only.
3
AS 2870 —201 1
CONTENTS
Page
. m o c . l a b o l g i a s . e r o t s o f n i / / : p t t h t a n o i s r e v l l u f e h t s s e c c A . e l p m a s e g a p 7 e e r f a s i s i h T
SECTION 1 SCOPE AND GENERAL 1.1 SCOPE ............................................................ 5 1.2 APPLICATION ...................................................... 5 1.3 PERFORMANCE OF FOOTING SYSTEMS ............................... 6 1.4 DESIGN CONDITIONS............................................... 7 1.5 DEEMED-TO-COMPLY STANDARD DESIGNS ........................... 8 1.6 ARTICULATION REQUIREMENTS..................................... 8 1.7 NORMATIVE REFERENCES.......................................... 8 1.8 DEFINITIONS ...................................................... 9 1.9 NOTATION ....................................................... 13 1.10 REINFORCEMENT DESIGNATION.................................... 15 1.11 INFORMATION IN DOCUMENTS ..................................... 16 SECTION 2 SITE CLASSIFICATION 2.1 GENERAL ........................................................ 17 2.2 METHODS FOR SITE CLASSIFICATION............................... 18 2.3 ESTIMATION OF THE CHARACTERISTIC SURFACE MOVEMENT ........ 20 2.4 SITE INVESTIGATION REQUIREMENTS .............................. 23 2.5 ADDITIONAL CONSIDERATIONS FOR SITE CLASSIFICATION........... 24 SECTION 3 STANDARD DESIGNS 3.1 SELECTION OF FOOTING SYSTEMS.................................. 26 3.2 STIFFENED RAFT .................................................. 28 3.3 FOOTING SLAB.................................................... 31 3.4 WAFFLE RAFTS................................................... 33 3.5 STIFFENED SLAB WITH DEEP EDGE BEAM........................... 35 3.6 STRIP FOOTINGS.................................................. 36 3.7 REINFORCEMENT EQUIVALENCES .................................. 39 3.8 SUSPENDED CONCRETE FLOORS IN ONE-STOREY CONSTRUCTION ..... 40 3.9 FOOTING SYSTEMS FOR TWO-STOREY CONSTRUCTION WITH SUSPENDED CONCRETE FLOOR ................................................ 40 3.10 FOOTINGS FOR CONCENTRATED LOADS............................. 40 SECTION 4 DESIGN BY ENGINEERING PRINCIPLES 4.1 GENERAL ........................................................ 41 4.2 DESIGN CRITERIA ................................................. 41 4.3 DESIGN OF FOOTING SYSTEMS..................................... 41 4.4 STIFFENED RAFT FOOTING SYSTEMS................................ 41 4.5 SIMPLIFIED METHOD FOR RAFT DESIGNS............................ 43 4.6 DESIGN OF FOOTING SYSTEMS OTHER THAN STIFFENED RAFTS....... 44 4.7 FOOTING SYSTEMS FOR REINFORCED SINGLE–LEAF MASONRY WALLS ........................................................... 45 4.8 DESIGN FOR PILED OR PIERED FOOTING SYSTEMS ................... 45 SECTION 5 DETAILING REQUIREMENTS 5.1 GENERAL ........................................................ 46 5.2 DRAINAGE DESIGN REQUIREMENTS ................................ 46 5.3 REQUIREMENTS FOR RAFTS AND SLABS............................. 47 5.4 REQUIREMENTS FOR PAD AND STRIP FOOTINGS ..................... 53
AS 2870 —201 1
5.5 5.6
4
REQUIREMENTS IN AGGRESSIVE SOILS.............................. 54 ADDITIONAL REQUIREMENTS FOR CLASSES M, H1, H2 AND E SITES........................................................... 58
SECTION 6 CONSTRUCTION REQUIREMENTS 6.1 GENERAL ........................................................ 60 6.2 PERMANENT EXCAVATIONS ....................................... 60 6.3 TEMPORARY EXCAVATIONS ....................................... 60 6.4 CONSTRUCTION OF SLABS......................................... 60 6.5 CONSTRUCTION OF STRIP AND PAD FOOTINGS....................... 66 6.6 ADDITIONAL REQUIREMENTS FOR MODERATELY, HIGHLY AND EXTREMELY REACTIVE SITES...................................... 66
. m o c . l a b o l g i a s . e r o t s o f n i / / : p t t h t a n o i s r e v l l u f e h t s s e c c A . e l p m a s e g a p 7 e e r f a s i s i h T
APPENDICES A FUNCTIONS OF VARIOUS PARTIES.................................. 68 B FOUNDATION PERFORMANCE AND MAINTENANCE................... 69 C CLASSIFICATION OF DAMAGE DUE TO FOUNDATION MOVEMENTS .... 72 D SITE CLASSIFICATION BY SOIL PROFILE IDENTIFICATION ............. 74 E STUMP PAD SIZES, BRACED STUMP UPLIFT HORIZONTAL LOAD CAPACITY ........................................................ 79 F SOIL STRUCTURE INTERACTION ANALYSIS FOR STIFFENED RAFTS .... 83 G DEEP FOOTINGS .................................................. 86 H GUIDE TO DESIGN OF FOOTINGS FOR TREES......................... 90 I BIBLIOGRAPHY................................................... 93 COMMENTARY TO AS 2870—2011........................................... 96
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AS 2870 —201 1
STANDARDS AUSTRALIA
Australian Standard Residential slabs and footings
S E C T I O N 1.1
. m o c . l a b o l g i a s . e r o t s o f n i / / : p t t h t a n o i s r e v l l u f e h t s s e c c A . e l p m a s e g a p 7 e e r f a s i s i h T
1
S C O P E
A N D
G E N E R A L
SCOPE
This Standard sets out the criteria for the classification of a site and the design and construction of a footing system for a single dwelling house, townhouse or similar structure which may be detached or separated by a party wall or common wall, but not situated vertically above or below another dwelling, includin g buildings classified as Class 1 and Class 10a in the Building Code of Australia. The Standard may also be used for other forms of construction, including some light industrial, commercial and institutional buildings if they are similar to houses in size, loading and superstructure flexibility. The footing systems for which designs are given include slab on ground, stiffened rafts, waffle rafts, strip footings, pad footings and piled footings. NOTE : This Sta ndard give s no advi ce on deta ili ng of the connect ion of supers truc tures to the footing systems for wind loads or earthquake loads.
For design purposes, the life of the structure is taken to be 50 years. NOTE S: 1
This Standard has been widely used for a number of years for the economical design of footings and slabs. Economical designs that avoid significant damage are practicable only if the soil moisture content of the foundation material under the footing or slab is stable or within reasonable limits of stability over the design life of the house or structure. For all sites (in particular sites with reactive soils) drainage and soil moisture conditions around the building need to be managed to avoid abnormal moisture conditions, as outlined in Clause 1.3.3, which may result in building damage.
2
Site management recommendations are given in Appendix B.
3
Where slab on ground construction is used for long slabs and large houses, particular consideration in design may be needed to avoid significant damage.
4
Information on earthquake actions is included in AS 1170.4. Information on wind actions is included in AS/NZS 1170.2 and AS 4055.
1.2
APPLICATION
To comply with this Standard— (a)
all sites shall be classified in accordance with Section 2; and
(b)
footing system design shall be by either—
(c)
(i)
prescribing a standard design in accordance with Section 3; or
(ii)
applying the engineering principles described in Section 4; and
all design and construction shall comply with Sections 5 and 6.
Residential footing system design, detailing and construction shall also comply with AS 3600 except that, where in conflict, this Standard (AS 2870) shall take precedence. NOTE : The fun ctions of the various part ies inc lude d in the desi gn and constr uction of res idential slabs and footings are normally as described in Appendix A.
ww w.s tan dard s.o rg. au
© Standards
Australia
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AS 2870-2011, Residential slabs and footings . m o c . l a b o l g i a s . e r o t s o f n i / / : p t t h t a n o i s r e v l l u f e h t s s e c c A . e l p m a s e g a p 7 e e r f a s i s i h T
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FOOTING SYSTEMS Reference - As 2870 - Residential slabs and footings - construction and BCA. Footings transfer building loads onto foundations. House design and shape must be designed before footing design can occur. AS2870 is based upon inspecting site to determine soil type and designing footing to suit both foundation and structure to be built. Footing construction New footings are constructed from reinforced concrete in accordance with designs set out in AS 2870 or engineering principles. Steel is extremely strong in tension and compression but it rusts. Concrete is extremely strong in compression but relatively relativel y weak in tension. It covers the steel and protects it from moisture in the ground. Together they produce a composite material which is strong in compression and tension, easily shaped and durable. Steel is always placed in areas of tension in all reinforced concrete members to stop the concrete cracking. For domestic (residential housing) purposes steel is available in the following forms: Trench mesh -e.g. previously 3 - 8TM (now (now called L8) - 3 bars of 8mm diameter diameter steel connected by cross wires to form a fabric laid in trenches for strip footings. No and diameter of bars varies. Comes in 6m lengths (strength of 300 Mpa). Slab fabric -e.g.previously called F62 (now SL 62)= fabric 6mm bars welded together in a 200mm square grid. Sheet size 2.4m 2 .4m X 6.0m. Available as F62, F72, F 72, F82, F92, F1018, etc. (strength of 300 Mpa). Bars -e.g. R10 = round 10mm diameter bar. -e.g. Y16 (now called N16)= deformed bar 16mm diameter - (Y= high yield strength of 410 Mpa +) •
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Bars can be ordered cogged (bent) to suit su it but must be transportable. Basic types of footings Common details Min . strength concrete 20 Mpa. Nominal aggregate size 20mm. Pad footings Also called blob footings. Is a solid mass of concrete ( no reo) laid in ground to support brick, timber or steel piers / posts. Commonly used to support timber floor frames. With reo and engineering design can be used to support suspended concrete floors . Details: Brickwork not acceptable Reo (if used) requires 40mm concrete cover. Suitable for A, S, M, H class sites. Sizes for pads is given in AS1684 Timber Framing code - size subject to area and load of floors. Minimum 400 x 400 x 200 high.
Isolated brick piers on pad footings Strip footing Reinforced strip of concrete laid in trench in ground. Used to support continuous brick walls. Typically 300mmm deep x 300 - 400mm wide. Process: Dig trench with backhoe or by hand. Tie up reinforcing cage. Lay reinforcing (reo) in trench. Support reo cage to ensure required concrete cover all round. Pour concrete and allow to cure before loading. Details: Reo requires 40mm concrete cover. Lapping of bars min. 500mm or full width at T and L intersections. Stepping techniques - see As 2870 Clause 5.4.3 Suitable for A, S, M, H class sites.
Strip footing trench with trench mesh reinforcement
Strip footing after pouring of concrete
Pier and Beam This system of footing basically a post and lintel method of load support. This concept permeates almost all structural elements of building. Its basic premise is that the lintel (horizontal member) carries a load from above and spreads it horizontally to the posts (vertical members). The posts then pass the load to another supporting element or the foundation material. The beam (lintel) is a strip footing which is deeper than it is wide. It is constructed in the same as a strip footing. The pier (post) is a vertical cylinder of normally unreinforced concrete (up to 3.0 m deep) which is made by drillling a hole in the ground to the depth required to find a suitable ABP or pass below the reactive zones of a reactive soil. The piers supports the beam at approximately 1800 - 2400 mm centres. Piles or piers may also be used in all forms of slabs on ground to find adequate ABP or bypass reactive areas. The piers may or may not be tied to the beam by reo (see your engineer for details). Process: Drill pier holes as directed by engineer Fill piers with concrete to level which coincides with bottom of beam then construct beam as per strip footing. Piers are sometimes belled (enlarged) on the end to resist upheaval on reactive sites or reduce pressure by increasing surface contact area. In highly reactive sites beam may require slip joint (2 layers of plastic membrane) to allow soil to slip past beam. Often also utilizes compressible material (foam, corrugated steel, etc) under beam to accommodate ground heave. Ground level Poured strip footing (beam) Piers (posts)
Pile and Beam Piles perform the same function as piers and piers are often called piles. The pile and beam system is identical to pier and beam except for the piles. Piles are preformed units of timber (with steel collars or caps), reinforced concrete or steel which are hammered into the ground much the same as a nail is hammered into timber. When piles are used in clusters ( a group) for large buildings a pile cap (pad footing) is often poured on top to carry the load of the beam or slab. When being hammered piles stop due to : Friction on the sides and end of the pile; or End resistance when the pile hits a very strong or hard foundation. A 1000kg percussion hammer is often used to hammer the piles. Damage to neighbouring buildings from vibration or ground heave is of concern. Piles are often used where collapsing soils exist on the site and drilling pier holes would result in collapsing holes Raft slabs All slabs cast on the ground are considered as raft slabs as they float on the soil. They vary in design to suit the type of foundation material. Slab on Ground ( SOG) Consists of a flat slab sitting on a perimeter beam (similar to a strip footing). Poured as one integral unit. Is used primarily in conjunction with trussed roofs which impart no load to the internal area of the building. Process: Use excavator to cut and fill site as necessary to provide flat building platform. Setout formwork Dig trenches for perimeter beam with backhoe or by hand. Install drainage pipes. Place blinding layer of sand. Pest spray the sand if required. Install vapour barrier Place reinforcement Pour concrete and allow to cure before loading. Details: Suitable for A & S class sites. Reo used- slab fabric over entire area of slab; trench mesh in bottom of edge beam. Reo cover required: Slab fabric where protection from moisture is provided by structure - 20mm Trench mesh at bottom of edge beam protected by vapour barrier - 30mm Trench mesh at bottom of edge beam not protected by vapour barrier - 40mm Uses edge rebate to stop horizontal moisture penetration at floor level and assist flashing system. Internal load bearing walls require the slab to be thickened by 50mm under the wall. Setdowns in top of slab matched by setdown in soffit of slab (special care with reo). Height of Finished Floor Level above ground level determined by: Height of surcharge gully Possibility of local flooding Termite protection method Effects of cut and fill Foundation type and use of perimeter paving.
Slab on ground viewed from underneath showing flat slab on edge beams. Diagram on right hand side shows detail of one corner of a slab with a dropped edge beam. The interior of the slab also rests on soil as do the perimeter beams.
Footing slab Identical to slab on ground except edge beam is poured separately from slab. Useful on sloping sites or where soil is soft or collapsing. May utilize brickwork between edge beam and slab on low sides of site. R10 ligatures @ 500 centres used to tie edge beam to slab.
Stiffened raft slab This is an extension of the slab on ground and is much stronger. It utilizes internal beams @ 3.0 - 6.0 m centres in addition to the edge beams to provide a stiff grid of beams. Suited to class M & H sites. Process and details identical to those for SOG.
Diagram showing grid layout of internal and external beams. The shaded section depicts the area of the slab only 100mm thick whilst the lighter areas are beams typically 400mm deep.
Waffle raft slab This is the strongest form of standard slab. Utilizes a closely spaced grid of 110 wide internal beams @ 1090mm max. centres in both directions. Edge beams are min 150 high x 300 min wide. Beams are formed by used of expanded polystyrene blocks set apart by spacers which also act as reo chairs. Slab is formed above ground so eliminating trench excavation. Uses N high strength bars instead of trench mesh in beams and uses standard slab fabric over slab area. Slab may be 85 thick. Advantage is accurate concrete quantity estimates. Services cut in through waffle forms. Suited to A, S, M, H class sites.
Egg crate layout of closely spaced beams in waffle slab construction