RESIDENTIAL FOOTING DESIGN - TMK Engineering

1 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 Copyright TMK Consulting Engineers 2012 OVERVIEW RESIDENTIAL FOOTING DESIGN 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 Copyright TMK Consulting Engineers 2012 RESIDENTIAL FOOTING SYSTEMS USED IN SOUTH AUSTRALIA RESIDENTIAL FOOTING DESIGN 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.

2 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 TECHNICAL PUBLICATION TIMELINE RESIDENTIAL FOOTING DESIGN Author 1940 s 1950 s 1960 s 1970 s 1980 s 1990 s 2000 s Thornthwaite (CSIRO) de Bruijn (South Africa) 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)

3 Mathematical Methods of Analysis and DESIGN Copyright TMK Consulting Engineers 2012 KEY INFLUENCES ON DESIGN PRACTICE IN AUSTRALIA RESIDENTIAL FOOTING DESIGN Author 1940 s 1950 s 1960 s 1970 s 1980 s 1990 s 2000 s Thornthwaite (CSIRO) de Bruijn (South Africa) 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) Mathematical Methods of Analysis and DESIGN * * * Copyright TMK Consulting Engineers 2012 GUIDELINES PANEL, AS 2870 & FOOTING DESIGN AT TMK RESIDENTIAL FOOTING DESIGN 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.

4 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.

5 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 IDEALIZED MOUND SHAPES AS 2870 (WALSH METHOD) RESIDENTIAL FOOTING DESIGN 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 IDEALIZED MOUND SHAPES AS 2870 (WALSH METHOD) RESIDENTIAL FOOTING DESIGN Modified Wf Factor (UniSA) Copyright TMK Consulting Engineers 2012 AS 2870 DIFFERENTIAL MOVEMENT LIMITS RESIDENTIAL FOOTING DESIGN Copyright TMK Consulting Engineers 2012 OVERVIEW OF THE DESIGN PROCESS AT TMK RESIDENTIAL FOOTING DESIGN 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.

6 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.

7 Copyright TMK Consulting Engineers 2012 SOIL VERTICAL MOVEMENT BOREHOLE DATA INPUT RESIDENTIAL FOOTING DESIGN Copyright TMK Consulting Engineers 2012 SOIL VERTICAL MOVEMENT SUMMARY OF CALCULATIONS RESIDENTIAL FOOTING DESIGN NOTE: In this example, there is no effect due to trees. Copyright TMK Consulting Engineers 2012 SOIL VERTICAL MOVEMENT EFFECT OF TREES RESIDENTIAL FOOTING DESIGN NOTES: Three input parameters are required: expected height of the mature trees; distance from the FOOTING system to the tree or trees; tree factor Di /HT, which depends on the number and arrangement of trees involved in the DESIGN .

8 Copyright TMK Consulting Engineers 2012 SOIL VERTICAL MOVEMENT SUMMARY OF CALCULATIONS RESIDENTIAL FOOTING DESIGN 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 and respectively. Copyright TMK Consulting Engineers 2012 TMK FOOTING POLICY RESIDENTIAL FOOTING DESIGN 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.

9 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. Copyright TMK Consulting Engineers 2012 TMK FOOTING POLICY RESIDENTIAL FOOTING DESIGN Copyright TMK Consulting Engineers 2012 FOOTING DESIGN SPREADSHEET SOIL PARAMETERS RESIDENTIAL FOOTING DESIGN 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.

10 Copyright TMK Consulting Engineers 2012 FOOTING DESIGN SPREADSHEET CONCRETE PROPERTIES RESIDENTIAL FOOTING DESIGN NOTES: The concrete strength is assumed to be 20 MPa in all DESIGN cases; The program assumes that all internal beams have the same cross-section. Copyright TMK Consulting Engineers 2012 FOOTING DESIGN SPREADSHEET DESIGN SUMMARY RESIDENTIAL FOOTING DESIGN NOTE: Ligatures must be specified in the FOOTING Construction Report in accordance with TMK s RESIDENTIAL FOOTING DESIGN Policy. Copyright TMK Consulting Engineers 2012 FOOTING DESIGN SPREADSHEET LOAD CALCULATIONS RESIDENTIAL FOOTING DESIGN Copyright TMK Consulting Engineers 2012 FOOTING DESIGN SPREADSHEET TYPICAL RESULTS PAGE RESIDENTIAL FOOTING DESIGN Copyright TMK Consulting Engineers 2012 SUMMARY OF THE FOOTING DESIGN METHOD RESIDENTIAL FOOTING DESIGN In South Australia, DESIGN calculations for domestic FOOTING began in the late 1970 s, using a DESIGN model originally proposed by Lytton of CSIRO.