Welcome to my engineering blog Frank Van der Woude B. Your comments will be very much appreciated. Please use the form at the bottom of any page if you allow me to post your comment, otherwise please Email a private comment. Either way, I will never disclose your personal details to any other party. Introduction AS regulates site classification, design, construction and maintenance of residential slabs and footings.
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The following paragraphs arguably identify edge subsidence as the most severe. From the day the slab is constructed, it effectively seals the soil beneath it against moisture changes. Thereafter, normal weather-induced soil moisture changes happen only outside the soil-slab domain. Therefore, foundation movements due to normal weather-induced soil moisture changes can only be either edge heave or edge subsidence, but not centre heave.
When construction of the house is practically complete, the static weight of the house, most of which is supported on the edges of the slab, has pushed the slab deeper into soil at the edges than in the centre, distorting the slab in a dome shape. It takes a long period of wet weather for swelling soil outside the soil-slab domain to heave the edges of the slab to level with the centre, and more continuing wet weather to distort the slab into a dish shape deep enough to cause damage.
Moreover, dishing distortion produces invisible cracking on the bottom of the slab stiffening ribs. Cracking of masonry walls is unlikely, because the outside walls tend to tilt inwards when the slab is dished, putting them in compression from the top down. Compression in the outside walls also provides some physical restraint against dishing distortion of the slab. In dry weather after construction of the house is practically complete, shrinking soil outside the soil-slab domain immediately exacerbates the initial doming distortion of the slab due to the static weight of the house.
Relative to edge heave, it takes lot less subsiding soil movement to distort the slab from the initial shallow dome shape into one high enough to cause damage. Moreover, doming distortion of the slab produces visible cracking on the top of the slab. Cracking of masonry walls is very likely, because the outside walls tend to tilt outwards when the slab is domed, putting them in tension from the top down, and it is a well-known fact that masonry walls crack at relatively low levels of tension.
AS 2870-2011 Residential slabs and footings
Therefore, many practising engineers do not design slabs and footings in the true sense of the term, they simply select a design solution prescribed in AS, comfortable in the knowledge that their selections are deemed-to-comply. Applicable ranges of characteristic Ys values in Table 2. Deemed-to-comply slab design solutions are independent of soil stiffness. Deemed-to-comply slab design solutions are independent of floor plan dimensions and floor plan shape. In a nutshell Mound profiles The flawed AS model boils down to using a static analysis method to analyze the dynamic effect of foundation movement. AS recommends the profiles shown at right. It can be demonstrated from fundamental principles that this static analysis corresponds to superimposing a prefabricated and loaded structure on a building platform that has been shaped into the assumed mound profile, and then analyzing the combined static effects of dead load, live load, and assumed foundation movement load.
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However, there is a growing body of evidence denying this claim. Most, if not all VBA reported slab heave problems occurred within a few years after construction, some even within a few months. Why are so many slabs experiencing structural damage prematurely? Who is responsible for abnormal moisture conditions? AS suggests that site classifiers, qualified engineers, designers and builders are responsible for abnormal moisture conditions existing prior to or resulting from building construction, and home-owners are responsible for abnormal moisture conditions developing after construction.