We've got a 1984-built extension in the house we've just bought. We've noticed that the joists are a bit bouncy. Someone who I won't name has suggested that the joists are 4x2 and are in imminent danger of collapse, and that they ought to be 8x4.
I've just checked. The relevant numbers are:
Span ~4M Spacing 45cm joist size ~50x125 (not 4x2, unless you use a very strange conversion!)
What's the fail load likely to be? We can live with the bounciness...
Andy
From memory when I did my extension (1985!) the limit of an 8 x2 (200 x
50) floor joist was just over 4mYou can do the sums - the rule of thumb is (was?) max deflection allowed is 1/300th of the length under full load.
2x5 in old money.
Don't know, but joists are almost always sized for stiffness rather than strength. Wooden joists will make a fearful creaking noise before they fail completely.
If you wanted to fix the bounciness, replacing the floor with screwed down chipboard/plywood would give you quite a bit of extra stiffness. (The gold-plated solution is to jack the floor up with acroprops before fitting plywood, and replace the ceiling as well.)
Yes - we know it's too bouncy. But it's been suggested it might collapse.
Andy
Well the maximum span for that size of c16 wood is about 2.4m.
I would expect you to hear a few cracks before it broke.
Don't have a party in there.
Copied & pasted;
The timber floor of a property obviously weighs something. This weight consists of the timber joists, the plasterboard ceiling underneath it, the floorboards and even the nails or screws used to fix both of the aforementioned coverings. All of these are known as "the dead load" and the joists themselves must be able to support this dead load without sagging. For a normal property, with chipboard or timber floors and plasterboard ceilings, this dead load is generally taken by architects and planners to be no more than 0.50 Kilo Newtons per square metre. (kN/sq.m)
The weight we place upon a floor by way of bathroom suites, beds, wardrobes etc, is known as the "imposed load". It is again accepted that, for normal household requirements, the imposed load will not exceed 1.5kN/sq.m.
The building regulations tables A1 and A2, list the size of joist necessary to support this weight, over a maximum span. Table A2 uses timbers known as "SC4" which are high strength timbers containing very few, if any, knots. These timbers if you have them in your property, will be stamped C24 by the timber yard that supplied them. They are not common in modern day construction unless specified and we will deal with the more generally used timbers, dealt with in table A1. These timbers are known as SC3 and will have C16 stamped on them.
This table is for a dead load of more than 0.25 but not more than 0.50 and allows for an imposed loading of no more than 1.5 kN/sq.m.
This is an abridged representation of Building regulations Table A1 Spacing (distance apart) of joists. Size of joist 400mm 450mm 600mm
47 x 97 2 x 4 1.92 1.82 1.46 47 x 122 2 x 5 2.55 2.45 2.09 47 x 147 2 x 6 3.06 2.95 2.61 47 x 170 2 x 7 3.53 3.4 2.99 47 x 195 2 x 8 4.04 3.89 3.39 47 x 220 2 x 9 4.55 4.35 3.79
I was given a rough calculator for joists at 16 inch spacing. Divide the span by 2 and add 1 to give the depth of a 2xX joist. Used this for a dipping platform over a pond at a school where I worked. Often populated bt kids but still rigid
Malcolm
Hmm. So at that spacing and span, they are about x2 too shallow. Ouch.
Question still remains: Will this actually break?
Anyone got a copy of Superbeam that could actually answer the question?
Doing some sums would suggest that the beam is unlikely to actually fail, although it falls well short[1] of the requirements expected for bending and deflection. It might bend enough to damage the ceiling below. I would also be very careful about sticking heavy loads on the walls where there is in effect only a single joist carrying it.
You could achieve some extra rigidity with added herringbone struts at
1/3 and 2/3 span. However to actually get the deflection down to something (just!) inside those required by modern standards, you would need to add a second similar sized joist *and* an 8mm steel flitch plate beside each of the existing ones, and bolt them together with M12 bolts. [1] Allowed deflection for that span is 12mm, yours would indicate nearer 40mm!Is this a one-storey or two storey extension?
Are you talking about ground or first floor?
Umm, check sleeper walls - is that the right term?
Yup, and good point. On a ground floor, that is a plausible case for using small timbers. The room might be 4m wide, but if there are a couple of walls under there holding up the floor, it changes the whole picture somewhat.
And the floor gets bouncy because the sleeper walls have subsided.
The depth for 4m span ought to be 9" (225mm) So not surprising they are bouncy. Many beams in buildings are sized OTT to minmise deflection (well over strength for actual breakage.) But I wouldn't put filing cabinets on that floor. You put additional joists between existing if feasible.
First floor (the ground floor is concrete). And there are no intervening walls, I'd have noticed.
Still no fail load figures?
Andy
I've hung most of a Morris 1500 off a single 4x2 over a 10ft span..
The engine would NOT come out, even with the front wheels clear of the ground..
bent a 2 tonne pulley block completely.
Students eh?
Or the cowboys that installed it did not do the beam ends that should anchor into the existing building structure correctly. My parents kitchen extension failed this way after about thirty years. A camera on a long pole showed that the ends nearest to the original boundary wall were cantilevered and relying on friction to keep them in place.
The sleeper walls were after thirty years collections of loose bricks with no mortar or foundations I could see :(
Thirty years of wear and shrinkage led to serious deflections when my full weight was in the wrong place. It is very difficult to reach the faults without taking the entire floor up and my bodge repairs accessed from the doormat hole last only a couple of years.
I'd be interested in any cunning tricks to do a better temporary fix without having to rip their kitchen apart.
Working at a distance of about 6' from a hole 3'x2' underfloor on unlevelled loose dry soil with random amounts of builders rubble. The joists in this case are reasonable 2" x 6" but there isn't a lot of space - about 8" underneath them to get stuff in and out.
or the joists rotted on the contact surface... or the packing material fell out etc,
You are unlikely to get them - there is too much variability. However the load you have did not push the joist into failing in shear at the ends, which would be one indication of failure.
The type of timber makes a difference - so C24 would likely fail later than C16, how well the floor boards are spreading the load etc. and what weakening there is from holes and notches through.
Yes, already posted the results....
I had a bouncy downstairs floor. Took the opportunity to add herringbone strutting at teh mid point when I had the boards up after a flood. Didn't half improve the stiffeness.
Might be possible to glue in stiffeners if it's an upstairs floor.
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