Pozi Joist deflection and cantilevers

[BartW]

Hi,

I am specifying the intermediate floor pozi joists for our build, and read on Mitek's website the max permissible deflection is length / 250 = mm. Their product brochure seems to suggest various joists choices and sizes with varied spacing and deflection. Is there a practical maximum below the manufacturer's guidelines that the SE is likely to accept?  I am having a 6 metre unsupported span above the living room, and I am trying to foresee the route (and the joist heights) I might have to take. 

 

Also, is there a robust cantilever detail that one would be able to suggest for the front corner first floor bedroom and "porch" overhangs? The house will be built out of Durisol ICF. I am trying to predict and resolve any structural engineering issues to ensure I will not have to later change my planning due to prohibitive cost, or complexity of one solution over another.

 

 

[Russell griffiths]

I believe at 6 m you will end up with one of three things

a steel beam in the centre to cut the span down. 

Joists so deep that they make the floor buildup unacceptable 

a bouncy floor.

 

I bet you end up with number one. 

 

When I was looking into all different types of joist the 5m figure cropped up all the time, anything over this just started to get so complicated to engineer out the deflection. 

[ProDave]

We have a 5M span with just posi joists and a strong back and it is fine, but I sense it is a big step up from that to 6M

[Thorfun]

I asked the TF company to design our first floor joists to an 8mm deflection which they did at no extra cost. our floors should be solid as a rock as the joists are 300mm centres.

[Gone West]

Our kitchen is 5.9m with metal web joists. IIRC the joists are 250mm deep with 122mm chords and are spaced at 350mm centres. There is zero discernible deflection.

[Russell griffiths]

9 minutes ago, PeterStarck said:

Our kitchen is 5.9m with metal web joists. IIRC the joists are 250mm deep with 122mm chords and are spaced at 350mm centres. There is zero discernible deflection.

Hi peter, was there a reason you couldn’t break this span in two with a beam in the centre, those joists at that size and spacing would have been a significant cost. 

 

Or do you think the extra cost for the larger timbers was better than the price of a steel. 

[Mr Punter]

8mm deflection is good.  They will often spec 12mm, which is the maximum for NHBC / mass produced cheap housing and can feel bouncy.

[Thorfun]

16 minutes ago, Mr Punter said:

8mm deflection is good.  They will often spec 12mm, which is the maximum for NHBC / mass produced cheap housing and can feel bouncy.

yep. it was initially 12mm but after reading some threads on here I decided I didn't want to take the risk of a bouncy floor and asked them to reduce it to 8mm deflection.

 

another win for this amazing forum!

[BartW]

So,

when I specify 5700mm span between the Utility and the extension wall, that is just under 5700mm. 254mm high achieves just under 12mm @ 400mm centres. Equally, a 304mm gets it down to 8mm deflection. 

 

I agree that perhaps introducing a bean midspan could take away some of the loading. As for cost, I suppose handling a 3m joist is more of a two people job than handling 6metres. Obviously the contrary goes for the steel beams depending on their size x weight.

 

Handy calculator can be seen here:

 

https://www.mitek.co.uk/span-calculator/

 

 

[Oz07]

I've had 6m 304mm pozis before. Can't comment on deflection as I had a stud partition in the middle. Probably isn't right to do that but it was all on the drawing and apart from leaving a gap at top of stud, it's always going to reduce deflection a bit

[trialuser]

Just another set of numbers. I have easijoists, 6345 long, (approx 6050 clear span) 304 x 147, 400mm centres, 8.4mm deflection, slight bounce if you jump, otherwise fine. Partitions and ceilings not yet in place. Not sure if these will make it better or worse. Cost for approx 17M run was just under 4.5K inc vat and delivery

[BartW]

40 minutes ago, trialuser said:

Just another set of numbers. I have easijoists, 6345 long, (approx 6050 clear span) 304 x 147, 400mm centres, 8.4mm deflection, slight bounce if you jump, otherwise fine. Partitions and ceilings not yet in place. Not sure if these will make it better or worse. Cost for approx 17M run was just under 4.5K inc vat and delivery

hi, mine is going to be weighted down by Lewis + screed, so will probably make it a bit more rigid, and quiet. Sound transfer between my floors is my paranoia, having seen some new developments recently. 
 

I didn’t understand the 17M run. Was it 17m2? If so, I might be quicker and cheaper to consider steels to break things up. The downside then is running of services that becomes particularly inconvenient. Water and electricity being the easy ones...

[trialuser]

Sorry, floor size is approx 6M wide x 17M long. I used Stevenson and Kelly as supplier.

[Russell griffiths]

If you are putting Lewis deck and screed I think you have just moved the goal posts into another universe. 

[BartW]

1 hour ago, trialuser said:

Sorry, floor size is approx 6M wide x 17M long. I used Stevenson and Kelly as supplier.

that’s not bad then. I guesstimated similar budget for my 80m2 of cover. Admittedly, much higher joist profile. 

 

 

50 minutes ago, Russell griffiths said:

If you are putting Lewis deck and screed I think you have just moved the goal posts into another universe. 

Are you saying much higher joist profile due to weight per m2?

[Gone West]

6 hours ago, Russell griffiths said:

Hi peter, was there a reason you couldn’t break this span in two with a beam in the centre, those joists at that size and spacing would have been a significant cost. 

 

Or do you think the extra cost for the larger timbers was better than the price of a steel. 

Our house has been built with an I-beam timber portal frame and the joists were included as part of the overall cost.

[Russell griffiths]

2 hours ago, BartW said:

that’s not bad then. I guesstimated similar budget for my 80m2 of cover. Admittedly, much higher joist profile. 

 

 

Are you saying much higher joist profile due to weight per m2?

I would have thought you will need to know the load that the Lewis deck and screed will impose. 

 

The floor designer will work to a normal house loading unless told otherwise. 

 

So they will work to a standard design that would include a chipboard floor then floor covering then standard furniture for a standard habitable room.

 

If you told them your having a full size snooker table up there, or a jacuzzi then that would move the design into another dimension, I believe it would be the same for the screed, it might not be a lot but it’s all very relevant. 

 

Completely different scenario but I’ve just had downstairs floor screeded and they used 26 tonnes of screed.

 

So it all needs taking into account.   

[BartW]

8 minutes ago, Russell griffiths said:

I would have thought you will need to know the load that the Lewis deck and screed will impose. 

 

The floor designer will work to a normal house loading unless told otherwise. 

 

So they will work to a standard design that would include a chipboard floor then floor covering then standard furniture for a standard habitable room.

 

If you told them your having a full size snooker table up there, or a jacuzzi then that would move the design into another dimension, I believe it would be the same for the screed, it might not be a lot but it’s all very relevant. 

 

Completely different scenario but I’ve just had downstairs floor screeded and they used 26 tonnes of screed.

 

So it all needs taking into account.   

 

 

Yes, that is all very relevant what you are saying, thanks

 

Perhaps I panicked thinking I might suddenly end up with 421mm high joists otherwise  

 

Probably best if I stipulate the cost difference of taller single length joists vs RSJ half way through the middle + shorter joists of a smaller height. Then the handling of the latter, which could be (arguably) a simpler task as well.

 

 

Anyone could share some knowledge on the cantilevers at the front please?

 

I hope overcomplicated steel structures may not be necessary. 

 

[Mr Punter]

The cantilevers are one for your architect / engineer.  Maybe Durisol can give you a steer.

[BartW]

1 hour ago, Mr Punter said:

The cantilevers are one for your architect / engineer.  Maybe Durisol can give you a steer.

Yes they gave me a contact for SE practice. I have just not had them do any work as yet, whilst I have been finalising the design and resolving the obvious oversights that would otherwise lead to over-specifying of connection details that would later be changed or followed. Both usually at cost. 
 

Velox do an intermediate floor system that in theory might be able to deal with the overhang on its own, but I am not sure if mixing the two would be a good idea. 

[Russell griffiths]

I have got some fairly serious cantilever sections holding my roof up, it is all formed out of the concrete core of the icf,

so lots of reinforcement inside the icf then a concrete beam was formed using shuttering plywood, pored the concrete and removed the plywood leaving a concrete beam poking out. 

 

Mine are just straight out arms but I have seen a ring beam configuration done the same way. 

Edited November 16, 2020 by Russell griffiths

[Russell griffiths]

[PeterW]

1 hour ago, BartW said:

Yes they gave me a contact for SE practice. I have just not had them do any work as yet, whilst I have been finalising the design and resolving the obvious oversights that would otherwise lead to over-specifying of connection details that would later be changed or followed. Both usually at cost. 
 

. 

You need an SE all over this as your Lewis Deck will change all the design criteria of the joists and the connection issues with the walls. Durisol usually uses a system of wall plates drilled to the concrete cores - you will need something much more than that as deflection won’t be your issue, sheer force on the wall plates will be. 
 

In terms of that overhang, I would get it sized in steel and change the upper floor construction method to timber frame and lighten the load on the cantilever over that section as concrete cored Durisol is very heavy. 

[BartW]

1 hour ago, PeterW said:

You need an SE all over this as your Lewis Deck will change all the design criteria of the joists and the connection issues with the walls. Durisol usually uses a system of wall plates drilled to the concrete cores - you will need something much more than that as deflection won’t be your issue, sheer force on the wall plates will be. 
 

In terms of that overhang, I would get it sized in steel and change the upper floor construction method to timber frame and lighten the load on the cantilever over that section as concrete cored Durisol is very heavy. 

 

I agree, the wall plates will have a job and a half over that open plan there.

 

As for the timber on the upper floor, perhaps I could do a timber construction only over that open area where the overhang is... Although the point of the build was a well built monolithic construction initially.

 

 

 

[PeterW]

Yes that was my point - just lighten the cantilever area and leave the rest as Durisol.