Floor bounce - PosiJoist Floor Vibration checks

[jamiehamy]

  On 17/01/2018 at 12:24, Barney12 said:

 

I'm in no way an expert but can tell you that my strong backs are fixed using a multitude of gas nailed ring shanks to every joist.

 

We actually had a slight squeak in the master bedroom floor when first installed. I pointed it out to Brendan (MBC) and he found that the guys had missed nailing a section of joists to the strong back down the centre of the room. A few minutes of Paslode action later and the squeak was gone.

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  On 17/01/2018 at 16:14, ProDave said:

I suspect in that picture they are not yet fixed.  You have to slot them in at that stage as it will be impossible later, but mine were not actually fixed to the joists until some time later.

 

Is it possible that someone forgot to attach them?

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When I did ours I nailed them initially but didn't feel they were tight enough hence coach bolting. Difference when I bolted them in tight was noticeable.

 

@readiescards. Any down lighters nearby you could use to try get a glimpse? 

[readiescards]

No down lights but a central rose might just let me poke a small camera in

[ProDave]

Looking again those strong backs look a bit small. Ours just abut fill the gap between the top and bottom chord.

[Jeremy Harris]

  On 17/01/2018 at 22:52, ProDave said:

Looking again those strong backs look a bit small. Ours just abut fill the gap between the top and bottom chord.

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Ours were around that size, and those look to be around the same size as given in the standard details:  http://www.mitek.co.uk/Products/Posi-Joist/Standard-Details/

[Mr Punter]

When my floors use engineered joists I specify that the maximum acceptable deflection must be the lesser of <8mm or 0.002 span,

 

NHBC allow 12mm and 0.003 span, which I feel is very bouncy, but is probably aimed at house bashers with more of an eye on costs than customer experience.

[Simplysimon]

i have to say the trimmer looks to be quite thin compared to the trimming joists, i'd have thought it would be the same. any pics of fixing of joist ends where fixed to chimney wall?

[Simplysimon]

looking at chimney wall, are joists sitting on inner leaf, opposite wall is hangered to block inner leaf.

[readiescards]

Using a spinning laser level on a trippod thingy I reckon there is a 4mm sag in the middle of the room.  Bit tricky to be sure as measuring with carpet and underlay in place.  Took plenty of measurements.

 

Anyone else with 4.5m posi-joists able to tell me if they likewise have any sag?

 

Thanks Paul

 

 

 

 

 

 

[readiescards]

  On 18/01/2018 at 19:22, Simplysimon said:

looking at chimney wall, are joists sitting on inner leaf, opposite wall is hangered to block inner leaf.

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This helpful?

[Simplysimon]

@readiescards, the only thing i can see is that the trimmer is not as deep as the joists and is thinner in section. i don't have much experience with posi joists, but do know that joists shoudn't sag!

timber joists should be fitted with a camber (if it has one) uppermost so that loading will level it. i would have had a couple of rows of strutting in 4.5m i presume that's what the strongback is for

[Declan52]

The hangers that hold the trimmer have very few fixings in them due to where they are placed. Only a few on top and maybe on the bottom of the clip if has been bent up as it's not showing on the bottom of the next joist. Could it be just the actual hangers are flexing instead of the joists. 

I think your going to have to lift the floor and figure out is the joists flexing, the trimmer piece or the hangers. Once you know this then it should be easier fixed.

[ProDave]

  On 20/01/2018 at 20:28, readiescards said:

Using a spinning laser level on a trippod thingy I reckon there is a 4mm sag in the middle of the room.  Bit tricky to be sure as measuring with carpet and underlay in place.  Took plenty of measurements.

 

Anyone else with 4.5m posi-joists able to tell me if they likewise have any sag?

 

Thanks Paul

 

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With no live load (just dead load) mine at 4.8 metres are dead flat as far as I can tell. I don't know if they were pre cambered or not.

 

I have to say,  yours look very thin.  Mine are 100mm wide each, probably more like the two either side of your fireplace that are presumably thicker to take the extra weight of the pair in front of the fireplace.

[PeterW]

Backing block is missing out of the trimmer where it meets the joists either side. 

 

Looks like the joists may have been installed back to front - usually there is a solid backer for the trimmer to sit against in the posijoist where the hanger goes. Without it, the trimmer will twist - doesn’t look to be double thickness either - and the centre posijoists will move. 

 

You could take the flooring up in front of the chimney section upstairs and do two things 

- pack behind the trimmer with a solid timber the full depth of the ceiling void 

- concrete screw through the trimmer to the chimney blocks behind. 

 

This would stop the trimmer twisting and give it some lateral strength. 

 

[jamiehamy]

Obviously can't zoom in but it looks like the trimmer, which takes the ends of two whole joists, could literally be supported buy only a couple of nails. Again, can't see properly but the tops of the joists may not be flush with the rest at that point. I might be getting a bit hypothetical but that 'problem' could have been sorted by using the floor screwing to help bring the joists up a few mm which would be the source of the extreme bounce. The bounce being the floor boards flexing, not the joists (the two potentially offering little support at that end). 

 

As an experiment, could you try propping under the chimney area directly below? And seeing if that localised support solves the problem before taking anything out? Some 4x2, osb on the plasterboard to spread weight and some packers to tighten it in? If locals liked propping sorts it, you will have confidence before anyone takes up any flooring. 

[Gone West]

  On 20/01/2018 at 20:28, readiescards said:

 

Anyone else with 4.5m posi-joists able to tell me if they likewise have any sag?

 

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The span in my kitchen is 5.9m and there isn't any sag. I am surprised because the cords of your joists seem narrow for the span.

 

 

 

[readiescards]

  On 21/01/2018 at 08:38, PeterStarck said:

The span in my kitchen is 5.9m and there isn't any sag. I am surprised because the cords of your joists seem narrow for the span.

 

 

 

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400mm centres too?

[Oz07]

Is the sag static or moving depending on load? If it's a permanent sag could you not lift the underlay and carpet and latex floor?

[Gone West]

  On 21/01/2018 at 08:53, readiescards said:

400mm centres too?

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The centres varies across the floor along with the cord width. Here's a copy of my plans, if it helps, if they are readable.

 

Metal Web Joists Plan1.PDF  Fetching info...

Metal Web Joists Plan2.PDFFetching info...

Edited January 22, 2018 by PeterStarck
To clarify attachments.

[ProDave]

  On 21/01/2018 at 09:17, PeterStarck said:

The centres varies across the floor along with the cord width. Here's a copy of my plans, if it helps, if they are readable.

 

Metal Web Joists Plan1.PDF  Fetching info... Metal Web Joists Plan2.PDFFetching info...

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I only see the joist sizes not the layout plan?

 

As has been said, your joists look small. For my 4.8 metre span I have 300 by 100 on 600mm spacing.

[PeterW]

Mine are 225x95 at 600mm centres for a 4.5m span - this was designed by DWB in Hull.

 

The strongbacks are 38x95 from the design and they are nailed on. I will probably go back and make these even stronger with multiple coach screws before we board up !

[jamiehamy]

We have a section at 4.5m at 600mm centres.  As per above, not as solid as I want, but no discernable sag. 

[ProDave]

@readiescards  Are there any internal partition walls above supported by this sagging floor?

 

That might be quite a difference. My long spans are just supporting the floorboards of the identical sized rooms above with no walls sitting on the joists.

[Simplysimon]

  On 20/01/2018 at 22:52, PeterW said:

Backing block is missing out of the trimmer where it meets the joists either side. 

 

Looks like the joists may have been installed back to front - usually there is a solid backer for the trimmer to sit against in the posijoist where the hanger goes. Without it, the trimmer will twist - doesn’t look to be double thickness either - and the centre posijoists will move. 

 

You could take the flooring up in front of the chimney section upstairs and do two things 

- pack behind the trimmer with a solid timber the full depth of the ceiling void 

- concrete screw through the trimmer to the chimney blocks behind. 

 

This would stop the trimmer twisting and give it some lateral strength. 

 

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i'd advise against packing to chimney, there should be a 50mm gap around it, hence the trimming.

 

  On 21/01/2018 at 11:08, ProDave said:

Are there any internal partition walls above supported by this sagging floor?

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yes, from picture showing uneven clearance under door. 

 

@readiescards, presuming joists are as detailed from architect? fitted as per spec by fitter? do you have drawings showing detail for fitting which specifies trimmer.

 

  On 21/01/2018 at 09:09, Oz07 said:

Is the sag static or moving depending on load? If it's a permanent sag could you not lift the underlay and carpet and latex floor?

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floor bouncing so not ideal, not a solution to the problem which could be quite difficult to do as partitions up, doors hung. don't suppose you fancy a couple of decorative columns and a beam below?

[Simplysimon]

switching between the two pictures, the first looks as though the lhs of the trimmer is lower than the top of the joist, equally spaced, though in the second it is flush, may just be a trick of the light, either that or it's been moved up which would mean that the  lower chord woudn't be resting on it.

 

[PeterW]

  On 21/01/2018 at 12:25, Simplysimon said:

i'd advise against packing to chimney, there should be a 50mm gap around it, hence the trimming.

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I agree however packing behind those two hangers may be the only option to stop it twisting - you could do this with timber or even a split block but attaching that into a gap isn't going to be easy !

 

If that is a Wolf System or similar joist system floor then the trimmer should be the depth of the posijoist and the hangers should also be the same depth as the trimmers (if they are Simpson or Cullen) otherwise you will get the lower chord not being supported properly.