Joists 3" square !!??

message from Arthur on 22 May 2004
I've just lifted one of the middle boards on a suspended (ground) floor.
The joists are 3" square that are resting on a 'wall' that consists of a
couple of layers of bricks.
The bricks are mortared and this 'wall' runs across close to the board I
lifted
The 'wall' seems to sitting on a footing.

I was expecting to find the joists that support the upper floors which are
6" x 3"
buggers. is this common method?
Or does it look like the builders I bought the house from, bodged it
together.

Thanks for any advice.

Arthur
 
Set Square replied to Arthur on 22 May 2004
3 x 3 sounds unusual - 4 x 2 is more usual - as opposed to (typically) 7 x 2
used on upper floors.

The upper floor joists have to span the *whole* width of the room. The
ground floor joists have intermediate supports, and thus don't need to have
such a large section.

Have a look at http://www.diydoctor.org.uk/projects/load-bearing_walls.htm
which explains it all.
 
Arthur replied to Set Square on 22 May 2004
The page you showed me has these figures

Spacing (distance apart) of joists.

400mm 450mm 600mm
Size of joist
;
38 x 147 mm 2.85 m 2.71 m 2.33 m
;
The 38 x 147 has a cross sectional area of 5586 sq mm
while the 75 x 75 is 5625 sq mm.

At its widest point into the alcove, the room is 3.2m wide
although the length of the joist in the centre of the room
are only 2.3m because of the concrete base provided for a
hearth.

Arthur
"Set Square" <diy@privacy.net> wrote in message
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N. Thornton replied to Arthur on 22 May 2004
yes, but the 3x3 also has a fraction of the strength. 3x3 is not
equivalent to 6 x 1.5 at all. Try the sagulator to see the difference.

3"x3" on 15" or 16" centres is nowhere near the build regs
requirements IIRC.

Having said that, BRs are far above whats needed for the thing to stay
up: the penalties of 3" joists are sagging and bending, noise
transmission, plaster cracking, and the risk of collapse if huge loads
are applied, such as a house full of people, waterbed, etc.

Regards, NT
 
Set Square replied to N. Thornton on 22 May 2004
Don't forget, this is a *downstairs* floor - so there won't be a plaster
ceiling under it (assuming there's no cellar!).

The OP has not given an explicit answer to my earlier questions about
intermediate sleeper walls. In my view, there's a high probablility that
these joists don't span the whole room width - but are supported in the
middle.
 
Arthur replied to Set Square on 22 May 2004
The 'sleeper wall' is what I mentioned at the top although I didn't
know it was called a SW a the time.
To describe it again diagrammatically.

A---------+ +--------------B
| | | |
| E----------------+ |
| |
X=============================Y
| |
C----------F--------------------------------D

A to B = 4m
A to C = 3.2m
E to F = 2.3m
Joists at 400mm centres
All joists 75mm square
X-Y is 2 courses of bricks and the joists are sitting on wooden pad blocks.

The brickwork looks tidy and has gaps in the lower course, for drainage I
presume.

Hope this helps.

Arthur.

"Set Square" <diy@privacy.net> wrote in message
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Coherers replied to Arthur on 23 May 2004
The max clear (unsupported) span is A - X in your diagram - this is the one
to be looking at in the tables.
Assuming your diagram is to scale, you are talking 2 - 2.2 meters for this
(??)
For downstairs floors where there is no ceiling below to support, you are
likely better using the "not more than 0.25KN" table in the regs, which
gives you slightly longer permitted spans e.g.

400mm 450mm 600mm
Size of joist
;
38 x 147 mm 2.98 m 2.87 m 2.51 m
;
As others have said, attempting direct conversions from the figures in the
tables is problematic, but looking at the figures for the 75mm sizes and the
fact you have centres at less than 400 mm, I'd really not be concerned.
Probably been there for nearly 100 years already, so.....
 
Capitol replied to Arthur on 23 May 2004
Is there another sleeper wall either side of the hearth? If so, then the
joists would meet a "deemed to satisfy" interpretation of the building
regulations. If not, then I'd add one to stiffen the floor up a bit.

Regards
Capitol
 
Set Square replied to Arthur on 23 May 2004
OK - but you haven't specified the most important dimension - which is A-X
(or B-Y)

This is the largest *unsupported* joist span.

You need to find out what span is ok for 75x75 joists at 400 centres, and
compare it with this measurement. My guess is that it's ok.

I still can't understand why they used square section joists - since 100x50
would use less wood but have a greater load bearing capacity!

I'm nor sure whether you've ever told us what the joists are made of.
Forgive me if you have. Are they the usual softwood - or are they hardwood.
I just wonder whether they used 3x3 oak for some reason. If so, these would
be a lot stronger than pine.
 
Arthur replied to Set Square on 23 May 2004
Sorry. I wouldn't know if it is a hard wood but it doesn't look like pine.
 
Lurch replied to N. Thornton on 22 May 2004
Although, the OP has said that they have been in for 12 years, and they're
downstairs. Not that I'm disagreeing with you though, just a 'in this instance'
type observation.
 
Hugo Nebula replied to Arthur on 23 May 2004
On Sat, 22 May 2004 18:09:34 +0000 (UTC), a particular chimpanzee
The strength of a joist (or any other element or beam, etc) is
proportional to the depth squared. It's deflection is proportional to
the depth cubed. So your 75 x 75 joist is equivalent to a 50 x 97
sized section for strength, but more springy.

Although if the joists are ca. 100 years old, they're probably
seasoned timber as opposed to the quick-grown kiln-dried balsa wood
that passes for joists these days.
 
Set Square replied to Arthur on 22 May 2004
Yes, but are you sure that - in the widest part - the joists go from side to
side in a *single* span? With downstairs floors, it's quite usual to have
one or two little support walls under the floor, supporting the centre
section(s) of the joists.

Incidentally, the bending stiffness isn't determined *just* by
cross-sectional area - the actual shape matters. If you think about it, a 6
x 2 joist will provide a lot more bending stiffness in a vertical plane when
its 6" dimension is vertical than it will if you lie it on its side. A 3 x 3
won't be as stiff as a 6 x 1.5 even though the cross-sectional area is the
same.
 
Bob Minchin replied to Set Square on 22 May 2004
The stiffness or resistance to deflection of a beam is proportional to the
thickness and proportional to the cube of the depth.

Regards

Bob
 
Andy Wade replied to Bob Minchin on 23 May 2004
Square, not cube. The section modulus, Z, of a uniform rectangular beam is
given by Z = b*d^2/6 where b is the breadth of the beam and d its depth.

But the deflection, for a given beam and loading is proportional to the cube
of the span.
 
Bob Minchin replied to Andy Wade on 23 May 2004
Apologies for misleading anyone. It was the
deflection formula I had in mind but that was not what I wrote.

Thanks Andy

Regards

Bob
 
N. Thornton replied to Arthur on 22 May 2004
a sleeper wall?

You didnt tell us how old the house is, or when these timbers went in,
or how long these 3x3s are, or what spacing they have between them. If
its a new build, its not OK. Even if its an oldie, its borderline, and
you'd need to watch your loading. More info.

Regards, NT
 
Arthur replied to N. Thornton on 22 May 2004
Yep.
Old mid-terrace, circa pre-1910.
The 75 by 75's look very old like the they have been there a long time;
not like they were installed by the slag arse builders I bought it from 12
years ago.
The bricks used are like the bricks the house is built from.
And they have 15" (400mm) centres.

At its widest point into the alcove, the room is 3.2m wide
although the length of the 4 joists in the centre of the room
are only 2.3m because of the concrete base provided for a
hearth.

Arthur

"N. Thornton" <bigcat@meeow.co.uk> wrote in message
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N. Thornton replied to Arthur on 23 May 2004
Ah. Well as long as you dont load it real heavily it shouldnt go
anywhere. Wood bends a lot before it breaks. Those old houses dont
normally meet modern build regs, and this certainly doesnt. It would
be twice as strong on a new build today. Nothing to worry about,

Regards, NT
 
Arthur replied to Arthur on 23 May 2004
Thanks, everyone.
Taking on board all of your advice, I think the floor flat and is in good
condition.
The 75 x75 joists look as old as the house. Perhaps there was a public
policy
at the time of conserving timber or the original builders 90+ years ago got
a good
deal on the 3" sq stuff and cut their cloth accordingly.
Perhaps even the joiner and the brickie on that day pulled a fast one and
cut up the 6" by 3"'s
then slipped down the back jigger with half a dozen lengths of 6 by 3.

At the same time I've seen that the joists in the hall are 75 x
50mm..perhaps
there is a clue in that.

Thanks again.

Arthur.

"Arthur" <tallysdad@btopenworld.com> wrote in message
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BigWallop replied to Arthur on 22 May 2004
Is the whole joist 3'' square ? Or is it set 3'' into the brickwork and you
only see 3'' sticking above the supporting wall ? If the joists are only three
inches square, then get the builder back in to replace them all with something a
lot more robust than that.
 
BigWallop replied to BigWallop on 22 May 2004
So it's a Barrat Homes property ? They have a clause somewhere in the small
print on the deeds. It says something like "If you have any more than X amount
of people in your property at the same time, they do not take responsability for
the soundness of the flooring structure". So get your title deeds out and have
a read through them.
 
Arthur replied to BigWallop on 22 May 2004
Sorry. Should have mentioned it is a very old mid-terraced...circa
pre-1910.
Unless you are being flippant at the expense of Messuir barret :)

Arthur

"BigWallop" <spamguard@_spam_guard.com> wrote in message
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responsability for
 
BigWallop replied to Arthur on 22 May 2004
No. Not at all flipant. The Barrat Homes deal used to have a clause in the
deeds that says they don't accept responsibilty for the weight you put on the
floors in the house they sold you.

I was shown this by a lawyer years ago. I couldn't believe my eyes.
 
nightjar replied to BigWallop on 23 May 2004
I've been to more than one party where the loading almost certainly exceeded
the normal domestic design levels. In one case, the floor was moving so much
as people danced that I quickly made my way to the nearest solid wall and I
kept to the wall as I made my way out of the room.

Colin Bignell
 
Peter Taylor replied to BigWallop on 22 May 2004
BigWallop wrote

The section size needed for the joists depends upon the load they support, what
timber they are, their span and how far apart they are. If you can post this
information I can tell you if they are adequate or not. Is the floor firm and
solid or does it deflect badly when you jump on it? Is it level or sagging?

Peter
 
Arthur replied to Peter Taylor on 22 May 2004
They are on 15" centres, There is a little flexing which is why I lifted the
board.
The floor looks flat enough.

Arthur

"Peter Taylor" <petertaylor@DELETETOMAILMEclara.co.uk> wrote in message
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Michael Mcneil replied to Arthur on 23 May 2004
"Arthur" <tallysdad@btopenworld.com> replied to a message he couldn't
read for panic
news:c8o47i$9uo$1@titan.btinternet.com

Top posted too. Tut tut!

I do wish people would pay a little more attention to what they are
doing on here. I know I am no better than I should be but...

Anyway Peter, good answer. I saw the OU programme last night about the
early days of The Philosophical Society. The programme made mention of
the ceiling in one of the colleges at Oxford, designed I think by
Christopher Wren. -I wish I'd taped it. (Sheldonian???)

They gave the maths for the design in which the herring-boned joists are
supported only at the edges of the room as an algebraic formula. (There
are trusses above it but they were unnecessary -shades of things to come
eh? What!) Do you happen to know it? If I could remember the name of the
theatre or gallery I could find it on the net.

Any suggestions anyone?
 
M Junk replied to Michael Mcneil on 23 May 2004
<snip>
The
Sheldonian Theatre
www.sheldon.ox.ac.uk
apparrently Wren's first building????, has a 70ft 'dome' without supporting
pillars
 

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