^^^^ 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.
On Sat, 22 May 2004 18:09:34 +0000 (UTC), a particular chimpanzee named "Arthur" randomly hit the keyboard and produced:
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.
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 a message he couldn't read for panic news:c8o47i$9uo$ snipped-for-privacy@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?
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
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
Sorry. I wouldn't know if it is a hard wood but it doesn't look like pine.
Apologies for misleading anyone. It was the deflection formula I had in mind but that was not what I wrote.
Thanks Andy
Regards
Bob
The Sheldonian Theatre
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.
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