## Canada Builds |

BUILDINGS

Angle of Incidence

Ashrae Window Test

Doors

Exposed Bldg Face

Fire Protection

Fire Walls

Floors

Footings

Foundations

Height Area and Use

Means of Egress

Mezzanines

Newton's Cannon

Roofs

Thermal Massing

Vegetation

Walls

Ventilation

ENERGY

Solar Energy

Solar PV Panels

Geothermal Energy

Cost of Energy

RESOURCES

Measuring Height

Size a Radiator

Size a Wood Beam

Size a Steel Beam

Deflection Calculations

TANGENTS

The Area of a Circle

Stonehenge

World Population

Bouncing Ball

The Number One

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Angle of Incidence

Ashrae Window Test

Doors

Exposed Bldg Face

Fire Protection

Fire Walls

Floors

Footings

Foundations

Height Area and Use

Means of Egress

Mezzanines

Newton's Cannon

Roofs

Thermal Massing

Vegetation

Walls

Ventilation

ENERGY

Solar Energy

Solar PV Panels

Geothermal Energy

Cost of Energy

RESOURCES

Measuring Height

Size a Radiator

Size a Wood Beam

Size a Steel Beam

Deflection Calculations

TANGENTS

The Area of a Circle

Stonehenge

World Population

Bouncing Ball

The Number One

Calculate the maximum span for a built up beam consisting of 4-2x8 wood
sections. The actual size of the 2x8’s is 1-7/8” x 8” which is larger than
the 2x8’s considered in the OBC Part 9 tables which are 1 ½” x 7 ¼”. The
beams actual size is used to calculate the section modulus (S) and the
species of the wood (Douglas fir) is used to determine the extreme fibre
bending stress (fb). This information can now be used to calculate the beams
maximum load carrying capacity.

Figure 1 Section through
a built up wood beam

Wood type is Douglas fir. Use extreme fibre bending stress from tables
and multiply by repetitive use factor (Cr) of 1.15 to obtain a design value
for extreme fibre bending stress
[1]

Figure 2 Compare Old and New lumber

The size of an old piece of lumber may be larger than the sizes that are
indicated in the current building code which are based on newer sizes. In
the example above, compare an old 2x8 with a new one. In common language, we
call both of these things a 2x8 and this refers to its nominal dimension.
When doing structural calculations, the actual size needs to be considered.
In this case, use the actual dimensions of the 2x8 pieces shown above to
calculate the Section Modulus of each shape.

The Section Modulus for the old 2x8 is larger than one might have
thought. Take a 75 lb design load and see how each joist performs under
similar conditions

Figure3
Compare spanning capabilities of old an new floor joist

[1] From Canada Wood Council publication on
design values for visually graded lumber. Listed values show 1620
psi for select structural Douglas fir and 1380 psi for no. 1 and
better. Repetitive use value (Cr) of 1.15 is used for beams
consisting of 3 plies or more