Interpreting deflection numbers in a 1st floor joist design.

[epsilonGreedy]

I have got a quote and design for the first floor with metal open web joists (Wolf) and wonder if these deflection criteria equate to a good solid floor with little bounce?

 

Quote

Floor Load Set = Domestic + Partitions - EC5
Floor dead + imposed load = TC=1500+350+210N, BC=200N 

 

The longest clear span is 5.45 meters.

 

Without prompting from me the designer spec'ed 400mm centres and 256 deep joists. The wood chord width is 97mm on the longest joists.

 

The reason for this post is that previously a forum member suggested specifying a design deflection limit more demanding than the industry standard. However in the past few years the UK floor design industry has switch to a European technical standard so I do not know how to specify a better floor when requesting a quote.

 

The designer of the first quote has responded by saying "We can increase the thickness of the timber on the longest spans where you might feel the most bounce. Although this would increase the cost it would not be significant".

[Moonshine]

umm i don't think those are deflection criteria, they look like the loading.

 

From my limited knowledge what i understand is that deflection limit should be 0.003 times the span, with a maximum deflection of 14mm (with strutting), 12mm (without strutting). http://www.newbuildinspections.com/wp-content/uploads/2016/06/LABC-Warranty-Technical-Manual-V8.1.pdf  page 32

 

So for that max span of you are into the 14mm/12mm maximum. 

 

There is an interactive span calculator for MiTek metal web joists which may be useful to you, as it calculates deflection for different joist constructions

 

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

 

For 5500mm the PS-10 posi at a 253mm depth @ 400 C's (97mm width) its calculating a 12.52mm deflection.

Edited February 14, 2020 by Moonshine

[Vijay]

Is the defection the max it will move bang smack at the centre of the span?

[ProDave]

I am surprised at the 253mm depth. For a 5 metres span we have 300mm depth, and it is a pretty solid floor with little bounce.

 

Don't make the mistake we did however.  One half of the house only has a 4 metre span.  Most of the joists run through so are over sized (sized for the 5m span)  But where they are interupted by the stairwell the engineer specified thinner top and bottom chords.  The result is our bedroom floor has noticeable bounce on  one half.  Had I anticipated this, I would have insisted on the same size jousts throughout.

[Oz07]

Also if blockwork 300mm works guage. Just a thought 

[Russell griffiths]

I have i joists for my roof, the designer said it was all within the standard deflection, I asked for it to be improved and less deflection. 

He asked why and looked baffled, I said I wanted it better, it took two different meetings to get my point across and get the deflection rate down to 6-7mm over 5m. 

 

Bear in mind this is a roof not your floor, so take this just as reference. 

[Dreadnaught]

5 minutes ago, Russell griffiths said:

I asked for it to be improved and less deflection.

5 minutes ago, Russell griffiths said:

it took two different meetings to get my point across

 

I wonder why did you want a stiffer roof, will you be regularly walking on it?

 

Edited February 15, 2020 by Dreadnaught

[PeterW]

It’s also probably worth getting the whole floor validated as I’ve done a couple with Wolf system joists and whilst the standard beam calcs are correct, once you glue the board to the top chord you reduce deflection further. 
 

Worth bearing in mind that if you don’t secure the end bearing points properly you will get more deflection as the bottom chord moves - this needs to be tightly restrained. 

[Russell griffiths]

1 minute ago, Dreadnaught said:

 

I wonder why did you want a stiffer roof, will you be regularly walking on it?

 

No I’m just a nutter. 

 

why did I just take out the three windows I had fitted and re, do them. 

 

[Vijay]

10 minutes ago, Russell griffiths said:

I have i joists for my roof, the designer said it was all within the standard deflection, I asked for it to be improved and less deflection. 

He asked why and looked baffled, I said I wanted it better, it took two different meetings to get my point across and get the deflection rate down to 6-7mm over 5m. 

 

Bear in mind this is a roof not your floor, so take this just as reference. 

 

I've had the same sort of issue, my room attic trusses, the floor in some places had a deflection amount of 19mm. I was told it was much better than industry standard and not to worry about it at all 

[Mr Punter]

As I have suggested previously, if you specify that you require maximum 8mm or 0.002 x span (whichever is least) deflection there will be no issues with bouncy floors and the cost difference is fairly small.

 

The standard you will be offered is 12mm or 0.003 x span which is crappy.

[Vijay]

17 hours ago, Vijay said:

Is the defection the max it will move bang smack at the centre of the span?

 

Anyone confirm this?

 

5 hours ago, Mr Punter said:

As I have suggested previously, if you specify that you require maximum 8mm or 0.002 x span (whichever is least) deflection there will be no issues with bouncy floors and the cost difference is fairly small.

 

The standard you will be offered is 12mm or 0.003 x span which is crappy.

 

Is only 4mm the difference between a floor that you would feel bounce and one you wouldn't?

[epsilonGreedy]

19 hours ago, Vijay said:

Is the defection the max it will move bang smack at the centre of the span?

 

 

The floor designer explained the new European standard is max permanent deflection under load including an extra allowance for material creep over a period of time. My understanding is that reduced bounce, which we householders use to assess how solid a floor is, comes as a by product of reducing static load deflection.

Edited February 15, 2020 by epsilonGreedy
Revised explanation of material creep.

[epsilonGreedy]

17 hours ago, ProDave said:

I am surprised at the 253mm depth. For a 5 metres span we have 300mm depth, and it is a pretty solid floor with little bounce.

 

 

This is worrying.

 

I first got a design and quote 2 years ago while buying the plot. At that time the longest clear span was 4.0m. That design used the PosiJoist size where the joists are 22.5cm high. Since then we removed the classic central entrance hall and created a max clear span of 5.4m, so when the new design came through I though good that has bumped the floor into the next 25cm joist height category.

 

Loosing 2" of ceiling height would be an issue if I requested 30cm high joists.

 

Could you confirm your joist centres? Mine are specified at 400mm throughout. Also I assume your joists are open metal web joists? 

[Jeremy Harris]

Bounce is just a term for deflection under a dynamic applied load.   It needs to be below the level where it will cause annoyance, and depending on the room, it can be annoying at a pretty small value.  If there's furniture near the centre of the room, for example, even a few mm of movement as someone walks across the floor can result in noise, as one leg and then another loses support from the deflection, making it rock. 

 

Our first floor is made from 253mm deep Posijoists, on 400mm centres, with 18mm OSB flooring, then a bonded layer of 12mm bamboo, with a maximum unsupported span of 4.2m.  That still has a small amount of bounce, enough to make a chest at the end of the bed make a small amount of noise.

[Visti]

2 hours ago, Vijay said:

 

Anyone confirm this?

 

If it is a simply supported beam (at each end) and uniformally loaded (weight distributed evenly across it's length), then yes the centre will be the point of max deflection. 

 

If it is a cantilever, it'll be the unsupported end that'll experience the most.

 

If you have stud walls or a large weight focused at one point, then the max deflection will depend on the result of all the bending moments. 

Edited February 15, 2020 by Visti

[PeterW]

54 minutes ago, epsilonGreedy said:

 

This is worrying.

 

I first got a design and quote 2 years ago while buying the plot. At that time the longest clear span was 4.0m. That design used the PosiJoist size where the joists are 22.5cm high. Since then we removed the classic central entrance hall and created a max clear span of 5.4m, so when the new design came through I though good that has bumped the floor into the next 25cm joist height category.

 

Loosing 2" of ceiling height would be an issue if I requested 30cm high joists.

 

Could you confirm your joist centres? Mine are specified at 400mm throughout. Also I assume your joists are open metal web joists? 

253mm is fine - See below 

 

5700

PS-10

253

400

122

11.97

 

That is a total span inc end bearing of 100mm each plus your span and it’s a 253 at 400 centres with 122mm chords and less than 12mm deflection - make sure you put the strongbacks in, and glue down the egger boards and that is not going to move. 

[Visti]

253mm is standard height for pozi joists, and the majority are 600mm centres.

 

Also, joists longer than 4m are possible, as you can see in some of the early SE calcs for our frame. Comes at the cost of a lot of deflection though as bending moments get worse the longer the beam.

 

We had those PZ2 joists rotated 90 degrees to shorten them down to 3.6m for more robustness.

 

[Jeremy Harris]

Just double checked our drawings.  We have 253mm deep Posijoists with a 400mm spacing over a span of 4,286mm. 

 

It's a pretty stiff floor, and got a great deal stiffer when I bonded 12mm thick bamboo flooring to the 18mm T&G boards.  It does still have very slight movement, though.  If you walk across the bedroom floor the oak chest we have at the end of our bed tends to make a noise as the floor deflects slightly.  TBH, it's not really a problem, and I've found that sticking some felt pads under the corners of the chest has pretty much stopped the noise.

 

Some bounce/dynamic deflection is inevitable with any suspended floor, the issue is really what is personally acceptable.  In general, I think our timber first floor is pretty good, and a fair bit better than the deflection in a friend's fairly newly built house that has a beam and block floor.  The movement in that is surprisingly high, more than I would have expected, TBH.

[Mr Punter]

5 hours ago, PeterW said:

253mm is fine - See below 

 

5700

PS-10

253

400

122

11.97

 

That is a total span inc end bearing of 100mm each plus your span and it’s a 253 at 400 centres with 122mm chords and less than 12mm deflection - make sure you put the strongbacks in, and glue down the egger boards and that is not going to move.

 

12mm is acceptable for the housebashers but if you are a self builder, get a quote for less than 8mm deflection if you want a really solid feel to the floor.

[Gone West]

On 14/02/2020 at 14:26, epsilonGreedy said:

The longest clear span is 5.45 meters.

 

Without prompting from me the designer spec'ed 400mm centres and 256 deep joists. The wood chord width is 97mm on the longest joists.

 

Our longest span is 5.9m and for that area we have 254mm deep metal web joists with chord width of 122mm at 400mm centres. This is covered with 22mm Caberdek and there is no noticeable deflection.

[ProDave]

I have just checked and our joists are about 280mm overall depth.  they are all on 600mm centres.  Most of the joists are about 11 metres long. they don't span 11 metres of course but run end to end of the building with intermediate support from the wall either side of the stairwell.  The only difference in the "downrated" ones that only span the 4 metre gap is the timber used for the top and bottom chords is thinner.

 

Generally the floor is pretty solid but as I noted on that bit with the downrated joists there is some noticable bounce so had I anticipated that, I would have specified the same joist size throughout.

[epsilonGreedy]

2 hours ago, PeterStarck said:

Our longest span is 5.9m and for that area we have 254mm deep metal web joists with chord width of 122mm at 400mm centres. This is covered with 22mm Caberdek and there is no noticeable deflection.

 

 

This is informative and encouraging. I only have one 5.4m joist span specified with a 122 chord, this is the joist that runs past an internal chimney breast and which also takes the load of shorter joists that fill in around the chimney. The remainder (x 10)  of the 5.4m span joists have 97mm chords. The 4m span joists that form the floor of the master bedroom have 72mm chords.

 

Collating all the advice and experience in this thread I will phone the floor designer on Monday and ask him how much 122mm chords on all > 3.5m spans will add to the floor cost. 

[epsilonGreedy]

11 hours ago, Mr Punter said:

12mm is acceptable for the housebashers but if you are a self builder, get a quote for less than 8mm deflection if you want a really solid feel to the floor.

 

 

Thanks I might stir this into the conversation with the floor designer on Monday.

 

Presumably deflection numbers are calculated with an industry standard reference load? 

[Moonshine]

1 minute ago, epsilonGreedy said:

 

Thanks I might stir this into the conversation with the floor designer on Monday.

 

Presumably deflection numbers are calculated with an industry standard reference load? 

 

Yes, and those were the loadings in your first post