Lecture Notes

Special Problem

The Problem: floor/ceiling joists are 2x6's for a 15-foot span

example of a joist system: 1, 2

span table

standard lumber sizes:

Nominal Size Actual Size

2 X 4 1 ^1/2" X 3 ^1/2"

2 X 6 1 ^1/2" X 5 ^1/2"

2 X 8 1 ^1/2" X 7 ^1/4"

2 X 10 1 ^1/2" X 9 ^1/4"

2 X 12 1 ^1/2" X 11 ^1/4"

The Solution: ???

Beam Configuration Moment of
Inertia (in⁴) Section Modulus (in³)

top bottom

2 X 6 20.8 7.6

2 X 8 52.7 14.1

2 X 10 107.2 22.6

two 2 X 6 41.6 15.1

2 X 6 and 2 X 8 72.0 22.2 18.0

2 X 6 and 2 X 10 137.9 35.1 25.9

Mathcad file

Nominal Size	Actual Size
2 X 4	1 ^1/2" X 3 ^1/2"
2 X 6	1 ^1/2" X 5 ^1/2"
2 X 8	1 ^1/2" X 7 ^1/4"
2 X 10	1 ^1/2" X 9 ^1/4"
2 X 12	1 ^1/2" X 11 ^1/4"

Beam Configuration	Moment of Inertia (in⁴)	Section Modulus (in³)
		top	bottom
2 X 6	20.8	7.6
2 X 8	52.7	14.1
2 X 10	107.2	22.6
two 2 X 6	41.6	15.1
2 X 6 and 2 X 8	72.0	22.2	18.0
2 X 6 and 2 X 10	137.9	35.1	25.9
Mathcad file