Browse Forums Building A New House Re: Engineered slab, flat plate? 2Apr 09, 2021 5:39 pm In most cases, flat slab is the best slab you can ever get, as your slab will be acting as one big footing and won't require any piers as it will be generally floating (not popular in Australia due to costs, but quite popular in colder countries). Why your site is classified as P? The main drawback is that it will require a lot more concrete and reinforcement as well as compaction/compaction testing, however, you might save a lot on the piers and preparation earthworks. Under the slab you will require 150 mm of compacted sand followed by 150-200 mm of 20-40 mm gravel, well compacted and tested for compaction (a layer of geotextile will be typically needed between sand and gravel as well as between sand and natural soil to prevent sand from being washed away). You might typically require a perimeter drainage installed around your slab, too. The slab itself will be usually at least 200 mm thick (typically goes to 250-300 mm) and if it is thicker than 200 mm it will require two layers of reinforcement mesh. It will be better from energy efficiency (when used in passive solar environment) than your typical concrete slab as it will be able to store more energy. Re: Engineered slab, flat plate? 3Apr 09, 2021 5:47 pm Thanks for the detailed reply alexp79! Sounds like it's a good way to go, although the engineer has said it'll still need to be on piers? And also said it'd require less concrete than the raft slab with piers making it cheaper overall.. Re: Engineered slab, flat plate? 4Apr 09, 2021 5:55 pm Cala90 Thanks for the detailed reply alexp79! Sounds like it's a good way to go, although the engineer has said it'll still need to be on piers? And also said it'd require less concrete than the raft slab with piers making it cheaper overall.. Well, ask him what is the thickness of the slab he wants to propose? I haven't seen a flat panel slab thinner than 200 mm and I don't think it makes any sense. You can easily run calculation to understand how much concrete you would need and compare side by side with raft slab footings + slab. Having it on piers might not be a good idea as piers will be creating a point pressure to the floating flat panel slab and hence partially undermine the actual benefit/idea of having a floating slab altogether as the main idea is that the slab fully rests on the ground and hence distributes the pressure evenly across its complete surface. In fact, it is usually considered as a legit alternative to piering on highly reactive soils. I am quite Cornellengineers can comment here too. Re: Engineered slab, flat plate? 5Apr 09, 2021 6:28 pm alexp79 Cala90 Thanks for the detailed reply alexp79! Sounds like it's a good way to go, although the engineer has said it'll still need to be on piers? And also said it'd require less concrete than the raft slab with piers making it cheaper overall.. Well, ask him what is the thickness of the slab he wants to propose? I haven't seen a flat panel slab thinner than 200 mm and I don't think it makes any sense. You can easily run calculation to understand how much concrete you would need and compare side by side with raft slab footings + slab. Having it on piers might not be a good idea as piers will be creating a point pressure to the floating flat panel slab and hence partially undermine the actual benefit/idea of having a floating slab altogether as the main idea is that the slab fully rests on the ground and hence distributes the pressure evenly across its complete surface. In fact, it is usually considered as a legit alternative to piering on highly reactive soils. I am quite Cornellengineers can comment here too. Thanks again! Not sure if he’s proposing piers as the soil test says it needs to be on piers. I’ll have to speak to him again on Monday and get some more information. Re: Engineered slab, flat plate? 6Apr 12, 2021 8:26 pm cornellengineers alexp79 Well, ask him what is the thickness of the slab he wants to propose? I haven't seen a flat panel slab thinner than 200 mm and I don't think it makes any sense. You can easily run calculation to understand how much concrete you would need and compare side by side with raft slab footings + slab. Having it on piers might not be a good idea as piers will be creating a point pressure to the floating flat panel slab and hence partially undermine the actual benefit/idea of having a floating slab altogether as the main idea is that the slab fully rests on the ground and hence distributes the pressure evenly across its complete surface. In fact, it is usually considered as a legit alternative to piering on highly reactive soils. I am quite Cornellengineers can comment here too. The first thing I asked is "Why is this a Class P site?" and alexp79 has already said that. Soil testers tend to classify P sites for fill, soft soil, trees within proximity and slope issues. If the land is filled and the fill will continue to consolidate then yes, bored piers and a slab system (of some sort) is the normal way to go. it doesn't need to be a flat slab but it can be if it is designed. If there are trees close by, then maybe bored piers and some sort of slab system. If the soil has an element of clayiness and any of the other combinations of problems then piers and flat slab system is an option - but only on the proviso that the entire slab is isolated from the clayey soil (loosened clay, clay wet to the full depth of reactive movement, or crushable cardboard forms or permanent formwork). All of these isolation options are really hard to do well. Clay soil swells when it gets wet. Water running under a flat slab system and into bored pier holes could cause slab heave that it would be very difficult (but not impossible) to design a flat slab for. Thanks for the information Cornellengineers. Having a read of the soil report, it's classed as a P site due to "adverse moisture conditions" with poor site drainage. Engineering timber is certainly a less fuss option, times cheaper to supply and install and better withstands humidity. 1 15839 Thanks Simon, I guess I'm no concerned with the volume of the noise rather that dead and hollow sound and feel that is associated with floating floors. But I'm not sure… 3 6085 I would suggest you simply go and talk to council about your proposal, most councils are getting onboard with ancillary dwellings. There is no benefit in keeping it… 1 4069 |