Joist span calculations?

Question

Can anyone point me, or explain to me the method/formulas for calculating maximum spans for lumber. I can find the span tables for sawn lumber but I'm looking to use engineered lumber. Also, more than anything else, I just want to find out how to do these calculations myself.

Answer 1

Herte is a simple way of doing it.

Let:

L = the span of the  joist in feet

s = spacing of the joists in feet

d = the depth of joist in inches

b = the width of joist in inches

w = unit load in lbs per square foot or floor area suppopted by the joist

W = unit uniformly distributed load on the beam or joist in lbs/ft

M = bending moment in inch lbs

V = shear force in lbs

S = allowable bending stress in lbs per square inch

v = allowable longitudinal shear stress in lbs per square inch

The unit load per square foot of floor area is the design load (40 psf to 60 psf depending on the location plus the unit weight per square foot of the flooring)

1. W = ws

2. M =(( WL^2)/8)12

3. S = 6M/(bd^2) = 6x12(WL^2)/(8bd^2)

4. L = ((8S b d^2)/(72W))^0.5 = ((S b d^2)/(9W))^0.5

5. V = WL/2

6. v = 1.5V/(b d) = 1.5WL/(2 b d)

7. L = 2 v b d/(1.5W) = (4/3) v b d/W

Whichever is shorter value of L obtained from Eq. 4 and 7 is the maximum allowable span. Note the S and v depend on the type of lumber. The allowable stresses are normally provided in tables of lumber tests conducted by the Forest Products Laboratories.

Answer 2

If you have not had any courses in structural analysis, trying to explain how to analyze and design these forces within this forum is impossible.

You need or consult with a professional engineer or the manufacturer of the engineered lumber.