Moggaman Posted April 5, 2019 Share Posted April 5, 2019 Hi, I realise this is a question for a SE. I am only looking for guidance as I am only planning my build at this stage. I want to have a hollowcore/precast first floor. I have a proposed span across the kitchen of 5.2m from inside edge of wall to inside edge of wall. Is this too much for a 150mm precast to span or will i need to span a beam the other way as shown in red and place them into the web of that. I realise the wall internally is only shown as a 100mm but that will be a 215mm.. The design upstair is simple..rooms in the roof style all over . Thanks M Link to comment Share on other sites More sharing options...
Russell griffiths Posted April 6, 2019 Share Posted April 6, 2019 Send your drawings to a precast company they will have all the software to run the numbers not it the same but my block n beam floor came in at 5m and they just enlarged the beam size. Any reason you need to stick to 150mm. Link to comment Share on other sites More sharing options...
bassanclan Posted April 6, 2019 Share Posted April 6, 2019 I spanned a 5m floor with precast beams (insulates block and beam). The engineer specified 150mm but I opted to over engineer and went for the 215mm. I'm glad I did as there is still a small amount of movement if someone jumps around on the floor. It is within tolerances, but I would have been mad if I'd had the 4-5mm movement allowed within the engineering calcs. Check the enginnering tolerances and ideally see how it behaves in the real world before agreeing to the 150mm Link to comment Share on other sites More sharing options...
PeterW Posted April 6, 2019 Share Posted April 6, 2019 Going to 215 will lead to a very thick floor if you’re considering any insulation and screed as you’ll be up to 350mm or more. Other option is use something like Thermofloor and get an EPS / Steel insulation layer with reinforced concrete above. Link to comment Share on other sites More sharing options...
Simplysimon Posted April 6, 2019 Share Posted April 6, 2019 8 hours ago, bassanclan said: I spanned a 5m floor with precast beams (insulates block and beam). The engineer specified 150mm but I opted to over engineer and went for the 215mm. I'm glad I did as there is still a small amount of movement if someone jumps around on the floor. It is within tolerances, but I would have been mad if I'd had the 4-5mm movement allowed within the engineering calcs. Check the enginnering tolerances and ideally see how it behaves in the real world before agreeing to the 150mm i've just put a 6350mm span in with double beams and am shocked by the amont of movement in them. had i known how much deflection they would have, i would have put in a dwarf wall and halved the span. 1 Link to comment Share on other sites More sharing options...
Sue B Posted April 6, 2019 Share Posted April 6, 2019 Our last bedroom was over 6m across and we had block and beam floor. The spec came back to triple beam per one row of blocks. The floor still moved more than I felt comfortable with. The kitchen was underneath and it was one of the main reasons not to have an extractor hanging from the ceiling over the peninsular where the hob was - I didn’t want any more weight on the floor. If I’d have realised what the flex was going to be like, I would have asked the SE for more steel which I would presume would fix the problem. 1 Link to comment Share on other sites More sharing options...
Alexphd1 Posted April 6, 2019 Share Posted April 6, 2019 I am surprised by the comments with bounce on block and beam, I always thought it would have been a solid feeling floor (we don't use block and beam up here) . Link to comment Share on other sites More sharing options...
Sue B Posted April 6, 2019 Share Posted April 6, 2019 Me too - I expected a completely solid feel to the floor. This was the only room where there was any noticeable flex. Link to comment Share on other sites More sharing options...
bassanclan Posted April 6, 2019 Share Posted April 6, 2019 It is definitely not solid. I would suggest going to a house with similar specifications of floor before committing yourself 1 Link to comment Share on other sites More sharing options...
Jeremy Harris Posted April 6, 2019 Share Posted April 6, 2019 One thing to remember with block and beam is that the dead load is pretty high, due to its inherently high mass. A lighter floor build up may well end up being stiffer, just because the total load (dead and live loads) may well be a fair bit lower, so the overall deflection will be lower for a given area moment of inertia of any support beams/joists. 1 Link to comment Share on other sites More sharing options...
Adam2 Posted April 6, 2019 Share Posted April 6, 2019 Some of the hollowcore suppliers have span tables on their website which may help. I sent my planning drawings with some additional notes out to 2 if them to get initial ideas on cost. But having read the above posts will check with the SE on deflection. Is it beneficial to use some mesh in the screed over hollowcore to help reduce any deflection? 1 Link to comment Share on other sites More sharing options...
bassanclan Posted April 6, 2019 Share Posted April 6, 2019 In my case it was Hanson jet floor polystyrene blocks. Mesh won't make any odds to deflection Link to comment Share on other sites More sharing options...
Moggaman Posted April 7, 2019 Author Share Posted April 7, 2019 On 06/04/2019 at 08:54, Russell griffiths said: Send your drawings to a precast company they will have all the software to run the numbers not it the same but my block n beam floor came in at 5m and they just enlarged the beam size. Any reason you need to stick to 150mm. I am trying to keep the first floor to 300mm thick in total max... ridge height is set. Can’t go any higher . Link to comment Share on other sites More sharing options...
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