MtnDon
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# Posted: 4 Nov 2011 03:17pm
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The AWC calculator has been mentioned in a few threads, but I don't see reference to it in this section.
There are two versions of the calculator:
Maximum Span Calculator for Wood Joists & Rafters
and, the Span Options Calculator for Wood Joists & Rafters
When one is open the other may be selected from an internal link.
To get good use out of these it helps to be aware of a few usually accepted as good practice things about deflections and loads.
* Living room floors deflection should be kept to a maximum of L/360
* Living room floors Live Load is usually calculated at a value of 40 psf. LL is the load of all furniture, equipment, people... things that can be easily moved in and out.
* Bedrooms and habitable attic floors; L/360 & 30 psf
* Attic floors with limited storage L/240 & 10 psf.
* Rafters usually are calculated with a maximum deflection of L/240
* Dead Loads (DL) are normally set to 10 psf. DL is the weight of the materials used. Steeply pitched roofs use more material so it may be advisable to increase the DL. DL should likely be increased appropriately when planning on the use of much natural rock or heavier than usual building materials.
The deflection notation method of L/360 means that if we take a span of 20 feet, L = 240 in inches. So, 240 / 360 = 0.666 inches. That means that with an average LL of 40 psf, evenly distributed, a twenty foot joist could be expected to be 0.666" lower in the center of the span than at the end supports.
L/360 is the maximum deflection for installing ceramic tile floors.
IMO, it is not a good idea to push the limits on rafter and joist sizing. If a certain size joists just squeaks by it may be best to go up a size or use closer spacing of the joists.
If a plan has been properly laid out ahead of construction and it is known that a large cast iron wood stove is to be placed in a certain area it may be a good idea to double up joists or to install extra joists with reduced spacing. That's a planning detail that could save headaches down the road.
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