WOODWORKING QUIZ for 29 June 04

[Ralph Jones]

Good Evening Friends,
When choosing floor joists from 2x timbers or boards, how do you determine the deflection if those timbers? For live load only.

Respectfully,

[Ralph Jones]

Good Evening Friends,
Well, it looks like I through you a curve on this one aye?

To figure the deflection of any timber on it's edge and supported at each end, is really simple and easy to do.

Simply convert the length of the timber to inches and divide by 240 live load pounds of weight.

Example; a 2 x 8 timber 10' long would convert to 120" / 240 = 0.5" deflection, which is an acceptable deflection for one board/timber.

Thank you for your support.

Respectfully,

[echnidna]

We use a more scientific approach in oz Ralph.

we look up the span tables developed by the CSIRO which also takes into account different timber species.

[Ralph Jones]

Hello Bob,
We have similar charts here also but I thought that the individual timber/board deflection would make a good quiz as some times it is necessary when installing joists if a water bed is going to rest on the floor.

A water bed normally is dead weight until some one lays or moves on it, as then it displaces and becomes live weight.

Respectfully,

[bitingmidge]

Yep Ralph, but same as yesterday. The deflection for a given span will vary depending on the strength of the timber!


Cheers,

P

[journeyman Mick]

Ralph,
most of your framing would probably be softwood (I've worked with carpenters from overseas before who were appalled by our hardwood framing, if you're not used to it it's pretty hard to hand nail). Just to give you an idea of the strength range consider the following, where most softwoods would be in the range of F4 to F8 (and occasionally F11).

For a joist span of 14'
unseasoned timber:
F4 - F5 2 x 10
F7 - F8 2 x 9
F11 - F14 2 x 8
F17 2 x 7

You would need to know what size joists to use (by consulting the tables) before you could use your formula. Your formula would give the same answer for a bit of 2 x 1 on edge also and this doesn't make sense. :confused:

Mick

[Ralph Jones]

Good Evening Friends,
You fellows are correct as I was looking at the wrong column when I arrived to my answer. Dividing the length of the timber into inches but, my error was dividing the inches. It should have been 360 instead of 240.

Example a 2 x 8 10' should have been 120/360 = 0.333" deflection.

Respectfully,

[journeyman Mick]

Ralph,
I still can't see how your formula works as it will give the same answer no matter what the cross section of the timber. :confused:

Mick