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17 thoughts on “Building and Construction Forum”

  1. Hi All,

    It’s good to see waffle slab issue is being discussed. I’ve personally designed and signed off literally thousand of waffle slab in Victoria. So far only encountered 2 failures due to stormwater pipe bursting.

    Hopefully not getting shot down here, but I don’t the the issue lies in waffle design itself. But actually lies in understanding current and future soil conditions. I’m no geotech specialist, but majority of soil tests conducted via visual tactile method which is acceptable, however has it’s flaw if local knowledge is not presence while using this method. Example of slab failure case https://www.austlii.edu.au/cgi-bin/viewdoc/au/cases/vic/VCAT/2014/1502.html?context=1;query=Metricon%20softley;mask_path=
    2 expert witnesses present ys calculation that is so different (see point 83) even after gone through lab test.

    Hence, in designing slab in my opinion would be great understanding of local area soil behaviours and external factors will greatly help determine economical slab design for clients. Merely throwing and beefing up slab design does help, however, it defeat the purpose of client coming to us for best design advice rather than conservative.

    There is also topic about tree/s where AS2870 present 2 ways of working out the impact (SA or VIC methods) and etc. I can go on forever, but I believe to really benefits home owners long term the discussion of footing design process need to start with land developer through to home owners.

  2. Hi Matt, our builder has advised us that a waffle pod slab will be the best option for our new two storey dwelling, and has provided an engineer’s slab and footing design for the site. I’m concerned that the engineer didn’t visit the site and is unaware of the shape and slope of the site. The house pad is proposed to sit on existing compacted fill which sits on sloping rocky natural ground. The fill depth varies across the site between 500mm to 2000mm. I think the slab, whatever type, should be piered to natural ground but how is that decision made? Regards, David

    1. Hi Dave
      I personally welcome feedback from clients that have historical knowledge of a site and I’d prefer to receive that feedback before the project progresses so far that something could go wrong. I don’t have the opportunity to inspect every site that I generate a design for so I use soil tests, compaction tests, contour plans, Google street view, Dial Before You Dig and historical data to analyse sites for risks. Take the opportunity to raise your concerns about the site with your builder and keep a record of the response you receive. I hope this helps.
      Matt

  3. Hi Guys

    I am planning to have a 182cm(L) x 73cm(w) x 76cm(H) aquarium in my apartment. I am on the first floor and concrete is about 8inch thick. I am going to place is against the back wall. Just wondering if the concrete would hold this. Because tank will be 1000kg plus another 200-300 including aquarium stand + hood

    Thanks
    Hari

    1. Hi Hari
      Thanks for your question.

      1000kg as a concentrated load is a lot higher than a residential slab is normally designed for so I would recommend against installing your tank before you get the slab checked by a local structural engineer. They will probably want to know how the slab is reinforced and confirm that the wall near the tank is a load-bearing wall (it could be a one way slab spanning parallel with the wall).

      Best of luck
      Matthew Cornell
      Cornell Engineers

      1. Hi Matthew

        Thank you so very much for your reply. The weight going to spread across 6 foot by 2.5 foot and since i am going to place it against a wall ( bare wall ) where the apartment complex ends. As my apartment is the last one and that wall is pretty thick and on the 1st floor and there are 2 floors above me. Not sure if these information helps but I thought I would share though.
        Also the building was built in late 80’s and very specious and have a bath tube in my bathroom too.
        Sorry I am just trying to give you details as much as I could thinking it would help you to shed some light.

        Thanks heaps
        Hari

  4. Hi Matt,

    Further to my question above, please note that the holes referred to above are for suspending a load, viz. a partition, a strong room wall etc.

    Best regards.

    M I Alam.

    1. Hi M I Alam
      The waffle slabs we refer to on our website are the ground slab system commonly referred to as waffle slabs, not the suspended slab style. Depending on the load being suspended from the suspended slab, the T part would be my first choice too but small loads could be suspended from the slab itself.

      1. Dear Matt,

        Thanks for your response.
        Let me try to rephrase my question.
        I understand from my structual engineer colleague that the only part in a waffle slab where any hole may be drilled is the bottom of the ‘T’ part. Size and depth of hole (as well as of the bolt/screw) are also relevant questions.
        I would appreciate some guidelines on the above.
        Best regards.

  5. I have been researching stiffened raft and waffle raft slabs for many years, concluding that the AS2870 model is seriously flawed. More specifically, both stiffened raft and waffle raft designs prescribed in AS 2870 are nowhere strong enough to sustain the surface movements they have supposedly been designed to to withstand. Find out more at http://as2870.raftslabs.com.au

  6. Matt,

    Very interesting to hear your views on Waffle Slabs. We have engineered over 2000 single residential houses in NSW and VIC (all non-cyclonic). We, also, have never engineered a Waffle Slab. Nor do we ever plan on it.

    Out of interest we do not believe that Stiffened Raft Slabs and piering are a good mix. We recommend that if piers are required, the slab should be isolated from the ground (with void formers) and engineered as a fully suspended concrete slab.

    1. Hi Damian

      Thanks for your feedback and interesting comment about isolating raft slabs when used with piers. What is the problem you are trying to overcome when you do that? Slab heave under the building and tensile forces in the piers? The ‘latest’ method of rectification for waffle slabs is somewhat similar – underpins throughout and jack the waffle slab to isolate it from the ground. I’ve always been wary of bored piers on reactive site but haven’t been made aware of any problems. I remember one engineer used to try and isolate the sides of bored piers from reactive ground but the detail just confused the builder and the isolation effect was never realised.

      Pretty impressed with your community involvement. I have similar ‘one day’ plans for classroom involvement but haven’t achieved. Good stuff!
      http://cantileverengineers.com.au/community/
      Matt Cornell

    2. Hi Damien
      there’s nothing wrong with a waffle slab if its done properly, unfortunately a lot of contractors are in such a hurry to put them in, and move to the next job, workmanship suffers, I think the thing with them that’s critical for strength is to mechanically vibrate every beam, a
      time consuming job.
      I have put down hundreds of them over the years, and none of the ones that I have done that I know
      have ever failed.
      I have heard some horror stories of those that have failed, most likely due to poor workmanship.
      With your comment on piers, the only thing I would say is that they are all put in at a uniform depth to give them an equal chance of moving evenly, if they are place at varying depths this wont happen due to varying moisture content of the soil

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