The purpose of my blog is to support a growing group of people in the engineering community who feel that the efficacy of Australian Standard AS2870 – ‘Residential slabs and footings’, is unsatisfactory. Readers interested in supporting this group may do so by posting a comment at the bottom of the page, or send me an Email. Either way, I will never disclose your personal details to any person or entity, unless required to do so by law.
If you’re not an engineer and think you’ve landed on this site by mistake, please don’t quit immediately. In paragraph 2 below, under the heading ‘Systemic issues’, you’ll read that many new home-owners become innocent victims of premature house slab failures caused by application of a faulty Australian Standard. You might also like to read some of the common sense dot items further down the page.
Coincidentally with the introduction of AS2870 in 1986, I started a consulting structural engineering practice in Brisbane’s northern suburbs and beyond, specializing in structural design of residential slabs and footings. Because Local Authorities required professional engineer certification of designs, I diligently assessed the structural adequacy of standard designs prescribed in AS 2870. Rudimentary structural engineering calculations raised my initial concern that many standard slabs were not strong enough to withstand the foundation movements they had supposedly been designed to withstand. Subsequent in-depth research confirmed my initial findings, and revealed that the theory under-pinning the prescription of standard slabs is fundamentally flawed.
Background of AS2870
Prior to the introduction of AS2870 the construction of residential footings was largely determined by builders, often with relatively minor input from Local Authorities or engineers. Slab-on-ground construction in those days consisted of a shallow perimeter strip footing, a bricked up base, and an in-fill slab. Steel reinforcement was minimal, just enough to control concrete shrinkage cracking.
Around the 1960s Local Authorities started to exercise a more regulatory and supervisory role, probably because slab-on-ground constructions were becoming popular and there was a growing awareness of structural damage due to foundation movement. Many Local Authorities began accumulating site details in the form of local maps, using the past performance of houses constructed in areas of similar site characteristics as a guide. Some Local Authorities began specifying the type and size of slabs and footings they deemed appropriate to those characteristically similar building sites. In so doing, those Local Authorities became potentially legally liable in the event of subsequent structural damage.
The introductory edition of AS2870 ambitiously claimed that adherence to it in engineering practice of the subject would reduce future litigation for compensation for structural damage in newly built houses caused by premature slab and footing failures. There is, however, a growing body of evidence this has not been the case. The Queensland Building Services Authority (QBSA) reported in May 1998, ‘A large percentage of damage compensation claims involved subsidence problems’. A subsequent QBSA press release revealed, ‘The cost of subsidence problems was doubling each year’. The Victorian Building Authority (VBA) reported in May 2014, ‘Research found that 5.3 percent of dwellings built in an area of Melbourne’s western suburbs between 2003 and 2011 showed some form of distress (such as cracks in the floor and walls)’, attributed to slab heave and ‘The key issues relating to slab heave were associated with deficiencies in the storm water drainage systems of the dwellings’. The Age newspaper published an article on 08 June 2014, ‘Estimates suggest up to 4,300 homes in Wyndham, Melton and Hume Local Government areas may be suffering from slab heave’. Incredulously, the two State Authorities did not, or could not, agree on whether the structural damage problems are due to heave or subsidence. Slab heave
Only a few years after it was introduced, AS2870 was split into two documents, ‘Residential slabs and footings – Construction’ (Designated AS 2870.1-1988), and ‘Residential slabs and footings – Guide to design by engineering principles’ (Designated AS2870.2-1990). Waffle raft slabs were first introduced into the 1988 version. The 1988 and 1990 versions were revised, amalgamated and re-designated AS 2870-1996. The current edition is designated AS2870-2011. As of about 2011, a vast majority of new houses are built on waffle raft slabs.
AS 2870 became a mandatory document when the Building Code of Australia (BCA) moved to a performance based document in 1996, resulting in the National Construction Code (NCC) in 2011. This means that all parties involved in the design, construction and maintenance of residential slabs and footings must comply with it. This move raised two major issues:
- Many practising engineers do not design slabs and footings in the true sense of the term, they simply select a standard design prescribed in Section 3 of AS2870, comfortable in the knowledge that their selections are deemed to comply. This widespread practice raises the issue of the engineering community’s professional duty of care to home-owners.
- Many failure investigators identify what are called ‘Abnormal soil moisture conditions’ as the cause of structural damage, and under Section 1.3.3.(ii) in AS2870 the home-owner is the responsible party for these conditions developing after practical completion of their homes. This has left many new home-owners potentially facing huge structural damage repair costs, and with no avenue of recourse to litigate for compensation after their homes have suffered significant structural damage. Abnormal moisture conditions alone do not cause structural damage. What matters is how much the soil moisture content has changed. Whilst there is no doubt that excessive foundation movement is the principal cause of structural damage, investigators are not in a position to determine the quantum of soil moisture changes that actually caused the damage. Nevertheless, bearing in mind that site characteristic surface movements, commonly called ys values, are defined and determined in accordance with AS 2870 as occurring between extreme moisture changes that have less than 5% chance of being exceeded in 50 years, it is surprising that most, if not all investigators to date have not looked for explanations of structural damage other than abnormal moisture conditions. The fact that the main symptom of structural damage is significant cracking of the slab and walls suggests that insufficient strength is the most likely explanation of structural damage. However, damage investigators are reasonably entitled to expect that standard deemed-to-comply slab design solutions have sufficient strength to sustain foundation movements at the design levels prescribed in AS 2870. Unfortunately, as the following link proves beyond reasonable doubt, this is not the case. ys Capacities
Structural engineering issues
- The performance of residential slabs and footings involves many variables, yet the original author(s) of AS 2870 took the ambitious step of prescribing slab design solutions requiring only two input parameters – Site Class and Type of Construction.
- Applicable ranges of characteristic ys values in Table 2.3 of AS2870 are too broad. For example, if a house slab for a Class H1 site has been designed for a ys of 40mm (Bottom of the range for Class H1), it would not be adequate for a ys of 60mm (Top of the range for Class H1). Conversely, if it has been designed for a ys of 60mm it would be overly conservative for a ys of 40mm.
- Deemed to comply slab design solutions are independent of floor plan dimensions. By the same argument as in the previous dot item, if a house slab has been designed for a single storey cottage measuring 12m by 10m, it would not be adequate for a double storey mansion measuring 15m by 12m on the ground floor.
- Section 4 in AS 2870 is under the title ‘Design by Engineering Principles’, but the graph of movement ratio versus unit stiffness in Figure 4.1 is fundamentally wrong. Clause 4.5.1 in AS2870 suggests this graph covers a huge range of design parameters. Actually, it appears that the graph is an empirical fit to standard slab design solutions in Figures 3.1 and 3.4 in AS2870, taking the logarithm of values on the horizontal axis to get a straight line. Admittedly the term within the brackets of the logarithm is the sum of the second moments of area of the gross (that is, not cracked) cross-sections of the webs of the stiffening beams divided by the overall width of the slab, but to call this unit stiffness is misleading, and taking the logarithm of a number involving units of measurement breaches fundamental principles of mathematics.
- Deemed to comply slab design solutions are independent of soil stiffness. Continuing on from the previous dot item, it would appear that the concepts of slab stiffness and soil stiffness are not well known or understood by practitioners. This comes as no surprise, because Section F2 in AS2870 makes the bold statement ‘The computed forces and displacements are generally not particularly sensitive to the value of soil stiffness used except for certain edge heave situations’. This statement is a direct consequence of the fundamentally flawed AS2870 model. The ultimate strength model proves beyond reasonable doubt that for the same ys values stiffer soils require stronger slabs. However, it seems likely that stiffer soils have lower ys values, but a lot of testing will be required before this can be relied upon.
- The AS 2870 design model is based on the analysis of a one-way action beam model for what is obviously a two-way action plate problem.
I made a contribution to a ‘Discussion Session on the Content an Application of AS2870-1996’ in the News Journal of the Australian Geomechanics Society AGS December 1997. My recommendation then was, and still is, to replace the prescriptive design solutions in AS2870 with a prescriptive process for structural engineers to arrive at slab design solutions via the application of appropriate engineering principles. In my paper presented to the AGS Vic 2017 Symposium, ‘Reactive clays and light structures’, I recommended changing to the traditional ultimate strength design model. This change would align AS2870 with other similar Standards such as the concrete structures code (AS3600), the steel structures code (AS4100), and the timber structures code (AS1720). Ultimate strength designs will, over time, reduce the unsustainable incidence of premature failures of residential raft slabs, the waffle raft variety more so than the stiffened raft variety.