"Ian Stirling" wrote in message
...
Suz wrote:
We want to specify heavier than normal joists in the extension. What
size
should we ask for?

Why?
What's the span between supports, and what's the load you want to put
on
it?

If you tell him, don't believe his answer.

Doubling the width, or doubling the frequency of joists doubles the
stiffness of the floor.
Doubling the height of the joists quadruples the stiffness.

Maximum deflection is inversely proportional to the Moment of Inertia,
I. For a beam of rectangular cross section, I =b*d^3/12. So double
the width, double the stiffness, like the man said, but double the depth
of the beam, and it's 8 times as stiff.


--
Kevin Poole
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