|Don's Home Home - Garden||Contact|
Overview | Terms | Span Table | Values for Douglas Fir | Material dead weight | Links
Of course it depends on how you place things. Placing heavy objects at the ends will not put as much stress on the joists as weight in the middle.
TERMS Live loads Loads that move or change Furniture, appliance, pelople, wind, snow etc. Dead loads loads that are constant The weight of the structure Construction materials, plumbing, etc. Values for residential buildings Floors live load = 30-40 lbs/sf (psf) dead load = 10 lbs/sf Ceiling live + dead load = 30 lbs/sf Since it is possible to use the attic for storage, the live load of the attic floor is set at 20 psf according to code.Strength and stiffness are the main criteria in determining load capacity. If a floor is strong enough to avoid breaking but not stiff enough, it may flex under load causing a plaster ceiling below to crack.
Maximum deflection limits are set by building codes. They are expressed as a fraction; clear span in inches (L) or (ℓ) over a given number. For example: a floor joist appropriately selected to span 10 feet with an L/360 limit will deflect no more than 120"/360 = 1/3 inch under maximum design loads.
Examples of live load values are: Living room floors L/360 & 40 psf Bedrooms and habitable attic floors L/360 & 30 psf Attic no storage 10 psf Attic floors with limited storage L/240 & 20 psf.E value or modulus of elasticity of the individual elements. E is a ratio that relates the amount a given load causes a material to deform. A material with a higher E value is stiffer. For example: No.2 grade eastern white pine has an E value of 1,100,000 and No.2 hem-fir has an E value of 1,300,000. Hem-fir is a stiffer material.
The local lumber company said a 50 year old home here in N. California probably used No 2 Douglas Fir.
He said it's possible the grade is stamped on the lumber.
No. 1 Douglas Fir has an E value 1,700,000 psi
No. 2 Douglas Fir has an E value 1,600,000 psi
See Allowable Property Values | West Coast Lumber Inspection Bureau
Fb value or extreme fiber stress in bending. Loads cause beams, joists and rafters to bend. As a beam bends the outermost (extreme) fibers are compressed along the top edge. And at the same time, fibers stretch along the bottom edge. The outermost (extreme) wood fibers on the top and bottom surfaces are stressed more than those fibers in the middle. An Fb value indicates design strength for those extreme fibers. The higher the Fb the stronger the wood.
No. 1 Douglas Fir has an Fb value value 1,300 psi
Lumber grade. A higher grade of a given species has a higher strength rating (Fb) and often has a higher stiffness value (E) too.
Species of wood. All species are not created equal. For example southern pine is much stronger and stiffer than spruce.
Other values for Douglas Fir No. 2 2x6 16" on center per 2015 Span Table at AWC Ceiling joists 10 psf live load L/240 17-8 20 psf live load L/240 14-1 (flex for 12' is 0.6 in) 30 psf live load L/360: 10-9 span 40 psf live load L/360: 9-9 span 50 psf live load L/360: 9-1 span Simplified Span Table at West Coast Lumber Inspection Bureau No. 2 Douglas Fir - Ceiling Joists - 10 PSF Dead Load / 20 PSF Live Load / Drywall Ceiling / Limited Attic Storage / 16" on center
Stack of books 3" x 6" x 1' 43 lbs/sf Links: Material Strength - Forces - Structures 2015 Span Tables2 | American Wood Council (AWC) Span Tables for Joists and Rafters | American Wood Council (AWC) | American Forest & Paper Association (AF&PA) Wood beam calculator | Cornell Understanding Loads and Using Span Tables | Building and Construction Technology | UMass Amherst Calculating Loads on Headers and Beams | Building and Construction Technology | UMass Amherst American Forest and Paper Association Home How to Calculate Load Bearing Beams | Hunker Southern Pine Allowable Load Tables