Residential Footing Design

RESIDENTIAL FOOTING DESIGN Presented by Dr David Payne & Trevor Kokkinakis to Australian Institute of Building Surveyors South Australian Chapter 6 June 2012

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RESIDENTIAL FOOTING DESIGN

OVERVIEW

•

A brief history of footing design in South Australia

• Domestic footing design methods • TMK’s approach to Residential Footing Design

• Research into extending current knowledge • Future design methods for mid-rise residential buildings

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RESIDENTIAL FOOTING DESIGN

RESIDENTIAL FOOTING SYSTEMS USED IN SOUTH AUSTRALIA Footing Type

1836+

1950’s

1960’s

1970’s

1980’s

1990’s

2000’s

Unreinforced Stone Reinforced Concrete Strip Beams Reinforced Raft Pier and Beam Deep Grillage Raft Waffle Pod Raft

From the time of first European settlement in 1836 until about 1950, house footings in South Australia, if they existed, were often built from unreinforced “blue stone”, a shale or slate type of material. The relatively soft “Mount Gambier stone”, a weak limestone material, was occasionally used as well. With improvements in concrete technology after World War II and the availability of pre-mixed concrete, house footings changed to reinforced concrete strip beams. The SA Department of Mines provided general recommendations on footing sizes for the Adelaide metropolitan area. Reinforced raft footings began to appear in the early 1960’s. Copyright © TMK Consulting Engineers 2012

RESIDENTIAL FOOTING DESIGN

TECHNICAL PUBLICATION TIMELINE Author

1940’s

1950’s

1960’s

1970’s

1980’s

1990’s

2000’s

Thornthwaite (CSIRO) de Bruijn (South Africa)

Mathematical Methods of Analysis and Design

Aitchison (CSIRO) Lytton (CSIRO / USA) Woodburn (CSIRO / U of A) Holland (Swinburne Tech) Wray (CSIRO / U of Texas) Richards (CSIRO) Walsh (CSIRO) Cameron (CSIRO / UniSA) Fargher (Adelaide)

Arnold (U of A) Mitchell (Adelaide) Pile (UniSA) Payne (Adelaide) AS 2870 (SAA) Jaksa (U of A) Fityus (U Newcastle) Copyright © TMK Consulting Engineers 2012

RESIDENTIAL FOOTING DESIGN

KEY INFLUENCES ON DESIGN PRACTICE IN AUSTRALIA Author

1940’s

1950’s

1960’s

1970’s

1980’s

1990’s

2000’s

Thornthwaite (CSIRO) de Bruijn (South Africa)

Mathematical Methods of Analysis and Design

Aitchison (CSIRO) Lytton (CSIRO / USA) Woodburn (CSIRO / U of A) Holland (Swinburne Tech) Wray (CSIRO / U of Texas) Richards (CSIRO)

*

Walsh (CSIRO)

*

Fargher (Adelaide)

*

Mitchell (Adelaide)

Cameron (CSIRO / UniSA)

Arnold (U of A)

Pile (UniSA) Payne (Adelaide) AS 2870 (SAA) Jaksa (U of A) Fityus (U Newcastle)

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RESIDENTIAL FOOTING DESIGN

GUIDELINES PANEL, AS 2870 & FOOTING DESIGN AT TMK In 1979, Engineers Australia formed a Guidelines Panel in South Australia, whose brief was to collate all existing information on the design of house footings and to publish a “guidelines” document that all engineers involved in the design of residential footings in South Australia must comply with. Following several years of discussion and amendments to the Guidelines, and with a considerable amount of input from engineers throughout Australia, AS 2870 was eventually issued in 1986. The current version is AS 2870 – 2011. AS 2870 provides tables of “deemed-to-comply” footing sizes that can be used without a need to carry out structural calculations. Alternatively, a residential raft footing can be designed by calculation, provided the designer uses either the Walsh Method or the Mitchell Method. AS 2870 recommends several of the key design parameters for both of these design methods. TMK has chosen to use the Walsh Method in conjunction with its own Residential Footing Design Policy for all residential footing designs produced in its office. Copyright © TMK Consulting Engineers 2012

RESIDENTIAL FOOTING DESIGN

IDEALIZED MOUND SHAPES – AS 2870 (WALSH METHOD)

NOTE: The Walsh Method assumes that the behaviour of the raft footing can be satisfactorily modelled as a one-way reinforced slab system. The Walsh Method should not be used if the designer believes that this assumption does not apply. Copyright © TMK Consulting Engineers 2012

RESIDENTIAL FOOTING DESIGN

IDEALIZED MOUND SHAPES – AS 2870 (WALSH METHOD)

Modified Wf Factor (UniSA)

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RESIDENTIAL FOOTING DESIGN

AS 2870 – DIFFERENTIAL MOVEMENT LIMITS

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RESIDENTIAL FOOTING DESIGN

OVERVIEW OF THE DESIGN PROCESS AT TMK Continuous soil samples are taken from a new or existing house site and the soil properties are determined in the laboratory by an experienced soil logger in accordance with AS 2870 – 2011; The Characteristic Surface Movement (ys) is calculated using the soil log data; If the design is affected by trees, either existing or new, and either on the site or on an adjacent site, the designer must calculate the estimated impact of the trees on the structure for which the footings are being designed; Using this information, TMK’s designer then determines the smallest footing that can be accepted on that particular site for the type of house planned for the site. Compliance with TMK’s Footing Design Policy is closely linked to AS2870 – 2011 and uses proprietary information collected by Trevor Kokkinakis over his more than 25 years experience in the design of house footings in South Australia;

Finally, each design is checked mathematically using TMK’s new in-house design spreadsheet which uses an updated version of Dr Paul Walsh’s DUBAL program, originally written in the late 1970’s when Walsh worked at the CSIRO in Melbourne. Copyright © TMK Consulting Engineers 2012

RESIDENTIAL FOOTING DESIGN

SOIL VERTICAL MOVEMENT – BOREHOLE DATA INPUT

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RESIDENTIAL FOOTING DESIGN

SOIL VERTICAL MOVEMENT – SUMMARY OF CALCULATIONS

NOTE:

In this example, there is no effect due to trees.

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SOIL VERTICAL MOVEMENT – EFFECT OF TREES

NOTES:

Three input parameters are required: 1. The expected height of the mature trees; 2. The distance from the footing system to the tree or trees; 3. The tree factor Di /HT, which depends on the number and arrangement of trees involved in the design.

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RESIDENTIAL FOOTING DESIGN

SOIL VERTICAL MOVEMENT – SUMMARY OF CALCULATIONS

NOTE:

This example shows a summary including the effects of trees. The influence of the tree group is separated from the free soil swell and is not reduced by the centre heave and edge heave factors of 0.7 and 0.5 respectively.

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RESIDENTIAL FOOTING DESIGN

TMK FOOTING POLICY TMK have adopted a set of policies (“rules”) which place “lower-bound” limits on the size of footings and the reinforcement which can be specified for them. The rules which set these “lower bounds” are encapsulated in what is known as the “TMK Footing Policy”. TMK have developed a spreadsheet calculator which enables a designer to check the Footing Policy limits for a particular type of construction on a particular site.

The TMK Footing Policy MUST be checked for EVERY footing designed in the office.

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RESIDENTIAL FOOTING DESIGN

TMK FOOTING POLICY

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RESIDENTIAL FOOTING DESIGN

FOOTING DESIGN SPREADSHEET – SOIL PARAMETERS

NOTES:

• The Characteristic Surface Movement, ys, must be calculated separately; • The Design Surface Movement, yt, due to the effects of trees, must be calculated separately; • The values of the swell stiffness are left to the discretion of the designer, within the ranges specified in AS 2870. Copyright © TMK Consulting Engineers 2012

RESIDENTIAL FOOTING DESIGN

FOOTING DESIGN SPREADSHEET – CONCRETE PROPERTIES

NOTES: • The concrete strength is assumed to be 20MPa in all design cases; •The program assumes that all internal beams have the same cross-section.

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RESIDENTIAL FOOTING DESIGN

FOOTING DESIGN SPREADSHEET – DESIGN SUMMARY

NOTE: Ligatures must be specified in the Footing Construction Report in accordance with TMK’s Residential Footing Design Policy.

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RESIDENTIAL FOOTING DESIGN

FOOTING DESIGN SPREADSHEET – LOAD CALCULATIONS

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RESIDENTIAL FOOTING DESIGN

FOOTING DESIGN SPREADSHEET – TYPICAL RESULTS PAGE

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RESIDENTIAL FOOTING DESIGN

SUMMARY OF THE FOOTING DESIGN METHOD In South Australia, design calculations for domestic footing began in the late 1970’s, using a design model originally proposed by Lytton of CSIRO ; Australian Standard AS 2870, first issued in 1986, provides tables of “deemed-to-comply” footing sizes that can be used without a need to carry out structural calculations. Alternatively, a residential raft footing can be designed by calculation, provided the designer uses either the Walsh Method or the Mitchell Method. AS 2870 recommends several of the key design parameters for both of these design methods in the “Informative” Appendix F; TMK designers use a purpose-written spreadsheet to calculate the design soil movement for a site, including the effects of trees if required, all in accordance with AS 2870 – 2011; TMK uses the Walsh Method in conjunction with its own Residential Footing Design Policy for all of its residential footing designs.

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RESIDENTIAL FOOTING DESIGN

RESEARCH AND DEVELOPMENT PROJECT In 2009, TMK has commenced a research and development project aimed at improving current knowledge on the way reactive soils interact with buildings. The first phase, which is complete, has involved the development of the TMK Design Spreadsheet, incorporating the Walsh Design Method. The second phase is underway and involves the development of a practical way of modelling the interaction between a soil undergoing vertical movement and a structure supported by that soil. A structure can be of any size and shape and the soil model includes the effects of the various layers in the soil profile. A paper intended for journal publication has been prepared and is currently undergoing “peer review”, initially at the University of South Australia. The third phase, which is planned to commence in late 2013, will build on work published by David Payne in 1991 and 1992, and will produce a design method for footings of mid-rise, large-plan commercial and general industrial buildings. At the end of the project, TMK plans to have the capability of modelling and designing these structures in 3D, thus extending the design tools currently available to professional engineers through AS 2870. Copyright © TMK Consulting Engineers 2012

RESIDENTIAL FOOTING DESIGN

SPECIAL CONSIDERATION - SOIL PROFILES WITH BCa HORIZON

Soil profiles with calcareous material in the upper layers and moderate to highly reactive layers at depth can require special consideration – beyond AS 2870 - 2011. Copyright © TMK Consulting Engineers 2012

RESIDENTIAL FOOTING DESIGN

SUMMARY TMK has embarked on a research and development project aimed at improving current knowledge on the way reactive soils interact with buildings. From the completed first phase of the project, TMK designers now use a purpose-written spreadsheet to calculate the design soil movement for a site, including the effects of trees if required, all in accordance with AS 2870 – 2011. The spreadsheet uses the Walsh Method in conjunction with its own Residential Footing Design Policy for all of its residential footing designs. The second phase of the research project is underway. A paper intended for publication in an international journal has been drafted and is currently undergoing “peer review”, initially at the University of South Australia. The third phase will commence in late 2013 and will extend the design tools currently available to professional engineers through AS 2870 into 3D. In particular, the research project will allow TMK to take better account of the effects of soil profiles with calcareous material in the upper layers and moderate to highly reactive layers at depth. Copyright © TMK Consulting Engineers 2012

RESIDENTIAL FOOTING DESIGN

RESIDENTIAL FOOTING DESIGN

COMMENTS AND QUESTIONS

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RESIDENTIAL FOOTING DESIGN