1. As 2870 Residential Slabs And Footings Pdf Free
2. As 2870 Residential Slabs And Footings Pdf File

. 1.2k DownloadsAbstractThis study investigates the soil–slab interaction for waffle slabs supporting residential structures on highly expansive soils. Interaction modelling techniques are reviewed, and the implications of the modelling assumptions typically employed are discussed. More realistic modelling assumptions are proposed, and their effects are investigated. For this purpose, advanced incremental/inelastic FE models are developed in OpenSees to capture the slab structural response during the history of soil movement in heave condition.

Soil profile (mound shape), soil stiffness profile and soil–slab contact are updated corresponding to growing mound. The study provides an insight into the resulting changes in bending moment and deformation demands on such slabs. It is found that the conventional assumption of a constant soft soil stiffness coupled with a stepped transition to that of hard soil is generally unconservative. The analyses also suggest that predefining a critical scenario and disregarding the history of loading is not necessarily conservative. Raft foundations (waffle raft/stiffened raft with or without deep edge beam) are commonly used, in a number of places around the world, as the preferred foundation system for supporting structures. In Australia, waffle rafts are commonly constructed to support relatively light residential structures (houses).Slabs are primarily designed to limit the differential movement of the structure considering the gravity loads that need to be safely transferred to ground and also the potential movements of underneath reactive soil. Reactive soil can swell/shrink with changes in moisture content, and this could happen due to several reasons, including: (1) seasonal/natural causes, such as rain, evaporation, and effect of adjacent trees; and (2) other causes, such as pipe leakage, poor or faulty drainage systems, and poor surface water management during and after construction.In Australia, AS2870-2011  is the relevant standard for analysis, design and construction of residential slabs and footings.

Unfortunately, there have been some recent reports of damages to hundreds of houses supported by waffle slabs (designed to AS2870) on highly reactive soils of Western suburbs of Melbourne, Victoria, Australia. Site investigations have confirmed excessive slab differential movements as being responsible for reported cracking and damages to houses beyond their serviceability limits. This study investigates some relevant aspects of current design practice and philosophy with the focus on soil–slab interaction analysis and the performance of waffle slabs. Background of soil–slab interaction. The elegant beam on mound model as proposed by Lytton comprised of the assumptions of a rigid beam resting on a parabolic mound, as shown in Fig. Length X or ‘liftoff point’ could be calculated based on equilibrium of applied loads and resulting soil reaction (calculated as the product of soil stiffness K by the area of the depressed soil as shaded ‘A’ in this figure). Estimated X value could then be used to calculate the centroid of the depressed mound (located at 3X/8) and to estimate the moment required at the critical location of the beam which was typically assumed to be at mid-span.

Figure shows the variation of soil stiffness with gravimetric moisture content (GMC) as obtained from the laboratory tests of typical reactive soil/clay of interest which was collected from a monitoring site in one of the western suburbs of Melbourne. The tests/curves were obtained using consolidation and power law methods. A polynomial of the second order was then obtained from the test results as the most relevant curve approximating the soil stiffness across the entire range of moisture content. Taking GMC as a measure of swell, this curve further suggests that a linear variation of stiffness (as opposed to stepped stiffness) could be a simplified yet relevant representation of swelling soil stiffness.

The soil is modelled using the classical Winkler springs and the co-operative width to consider the shear contribution of the soil outside the contact boundaries. “Zero-Length Elements” and “No Tension Materials” (as available in OpenSees) were employed for this purpose. A special algorism was also developed to monitor and detect the separation of the beam from ground (as the mound height is increased incrementally) and to enforce an updated boundary condition for the beam before the next increment of heaving. It should be noted that there are still uncertainties regarding the actual shape of mound profiles as may be inferred from the comparisons of Walsh and Mitchells models, yet both recognised by AS2870-11 (See Fig. This may also be inferred from continuing attempts for refinements of the shape of such profiles, e.g. and the ongoing research at Swinburne University of Technology which is investigating the changing trend of such profiles with prevailing climate condition. Given that, it is believed that the employed simplified mound profiles are reasonably/adequately representative of the two well-known set of profiles.

It is noted that Fig. Is merely intended to illustrate the extent of variations in currently accepted mound profiles, for a given set of parameters importantly L (slab Length) and Hs (depth of design suction change for Melbourne—AS2870-2011). The curves in Fig. Are not used directly in any of the models analysed in this study. Instead, a simplified, yet comparable, version of mound profiles was employed as outlined earlier and shown in Fig. Model#4 This model parameterises the effect of soil shear deformation on overall foundation stiffness through a concept known as “co-operative width” as introduced by Walsh and Cameron.

Walsh and Cameron proposed that in a simple spring model representing foundation, the extra stiffness of foundation associated with soil shear deformation can be represented by an extra (200 mm) length or width of footing at the locations where there is a contact discontinuity. Walsh assumed that the discontinuity between soil (foundation) and slab (footing) occurs merely outside the boundary of slab and hence included single-sided 200-mm co-operative width for the edge beams only (see Fig.

This assumption was employed in Models 1–3, but it is refined in Model 4 which considers the co-operative width for both internal and edge beams in waffle slabs (see Fig. Fig. 11Top view of soil–slab contact; effective contact area a corresponding to Models 1–3; b corresponding to Model 4 and c edge heave assumption corresponding to Model 5Model 4 is intended to demonstrate that underestimating the effective contact area between ground and slab could be translated into more soil depression, for a given heave magnitude, and less deformation demand on slab which is unconservative.Model#5 This model is similar to Model 2 except that only the soil underneath of the edge beam is heaving (see Fig.

C) to simulate a case of concentrated heave that may occur due to causes other than seasonal/uniform moisture change (e.g. Pipe leakage). Figures and show the deflection profiles and bending moment diagrams, respectively, corresponding to the first four models developed. Considering the 30-mm limit for slab differential deflection (SDD) and the section actual yield strength (My = 42kN.m), it can be seen that the analysed slab is generally more vulnerable to failure due to excessive deflection than strength inadequacy (yielding).

It should be noted that in the results shown in Fig., the effect of creep is not included. Creep effect may be considered as suggested later in this study. Fig. 13Bending moment profiles/envelopes of the analysed slab as obtained from Swinburne FE modelsBy comparing the results corresponding to Model 2 against the reference model (Model 1) in Fig., it could be seen that the analysed slab sustains more deflection in Model 2 which could be attributed to the history of loading (heaving). It is noted that at Ym = 50 mm, both models have the same boundary profile (heave shape) and the same soil stiffness profile.

However, the slab in Model 2 endures some cracking corresponding to growing heave with decreasing stiffness magnitude. That would be translated into having a softer, non-linearly responding slab (at least in part) by the time the heave height reaches Ym = 50 mm. The bending moment demand is also greater in Model 2 as compared with Model 1 (see Fig. ).By further refining the stiffness profile from a stepped transition to a more realistic linear transition, as discussed earlier and shown in Figs. And, respectively, the deflection demand imposed on a given slab is further increased. This could be seen if Model 3 is compared against Model 4 in Fig.

(all other parameters are the same in the two models). This is explained by the fact that in general the stiffer is the soil; the lesser is the penetration of slab into heaving soil (soil depression) and the greater is the deformation demand on slab.

This might be better understood if one considers that the moisture-induced heaving action of soil, which can push against the slab in an upward direction, has to be transformed into a combination of (1) slab differential deflection, (2) the depression of soil under the slab which is also referred to as slab penetration into soil, and (3) any uniform movements of entire slab depending on the slab size, magnitude of restraining gravity loads and etc. There is usually a trade-off between the three components.Increasing the effective contact area between soil and slab as considered in Model 4 compared with previous models is seen to further increase the deflection and bending moment demands imposed on slab.

By more realistic representation of the expected contact area, the tributary area of the soil represented by each spring is increased. This would mean stiffer springs which in turn put more demand on slab. Figure compares the slab differential deflection changes as a function of mound height (Y) for Models 2 and 5. The results suggest that the two models are comparable with Model 5 being slightly more demanding on the slab despite the edge beam heave assumption corresponding to Fig. This suggests the vulnerability of the standard slabs to concentrated heaving. However, it is to be noted that the stiffening effect of adjacent beams would have a desirable effect which could partly offset the additional deformation expected due to creep.

Fig. 14The history of peak demand on slab (i.e. Differential deflection) with developing mound in Models 2 and 5Figure also demonstrates that the combination of maximum mound height (Ym = 50) and soft soil stiffness of 1 MPa/m does not necessarily make the most critical scenario as far as the demands on slab are concerned. The results shown in this figure suggest that the differential deflection demand on the analysed slab could be even more severe at the heave height of Y = 40 coupled with relevant soil stiffness. In the absence of dedicated research on the creep effect for waffle slab on reactive soil, it is suggested that the estimated slab differential deflection presented by the models in this study be scaled up by a factor of 1.5. This is approximately as conservative as the measure proposed by AS2870-11 which recommends an interaction analysis using a reduced modulus of elasticity (Er = 15,000 MPa) for N20 concrete. Considering that N20 concrete has the mean Ec value of 22,500 MPa as per AS3600-2009  , the Er/Ec would be 0.67.

A reduction in modulus of elasticity, by a factor of 0.67, suggests an increase in slab deflection ∆ by a factor of 1.5 as the two parameters are inversely related. This is evident in elastic analysis in general and could also be seen in the simplified design equation as proposed by Walsh as is given below. (1)It should be mentioned that the above creep factor is not to be applied to the required bending moment as it is believed that the presented BM envelopes represent a realistically conservative ultimate demand. Employing a reduced E value would mean that the history of interaction, extent of cracking and bending moment development is altered to some extent. This technique is generally translated into an increased deflection (which is favourable considering the creep consideration) and underestimating the bending moment demand if the middle section of the beam is in contact with ground before yielding. A comprehensive inelastic FE model was developed in OpenSees for the analysis of waffle slabs on reactive soil (edge heave condition).The developed model simulates the action of expansive soil pushing against the slab.

As 2870 Residential Slabs And Footings Pdf Free

It records the history of soil–slab interaction, while the swelling soil under the slab edges grows vertically and progresses horizontally under the slab.Several models were developed to study the implication of typical assumptions conventionally employed in soil–slab interaction analysis. This includes soil stiffness profile, effective contact between soil and slab and so on.It was demonstrated that the assumption of a single-stiffness profile (i.e., 1 MPa/m for the soft soil and 5 MPa/m for the hard soil) corresponding to ultimate mound height is not necessarily conservative.

As 2870 Residential Slabs And Footings Pdf File

In this study, gradual mound development and corresponding soil stiffness reduction were modelled to simulate the history of soil–slab interaction.It was demonstrated that the assumption of stepped transition from soft to hard soil is not conservative, and a linear transition may be considered as a more realistic and conservative substitution.It was demonstrated that the effective contact is a key factor in interaction analysis. It is recommended that the 200-mm co-operative width be considered on both sides of all internal and edge beams.In the analyses presented here, the effects of structural stiffness in restraining the slab movement are not considered. Optimal design of waffle slabs would require the inclusion of structure into interaction analysis (i.e.

A soil–slab–structure interaction analysis). Cite this article as:Fardipour, M., Gad, E., Sivagnanasundram, S. (2016) 1: 20. Received 13 March 2016. Accepted 21 June 2016.

First Online 06 July 2016. DOI Publisher Name Springer International Publishing. Print ISSN 2364-4176. Online ISSN 2364-4184.This article is published under an open access license.Please check the 'Copyright Information' section for details of this license andwhat re-use is permitted.If your intended use exceeds what is permitted by the license or ifyou are unable to locate the licence and re-use information,please contact the.