Your shed design must take
building loads into account--the weight of the shed itself,
what goes in it, and the elements outside, such as wind and
snow.
A framing system is like a network of streams along which
structural loads flow; from the roof, through the building
and foundation, and ultimately into the ground. Like
water, loads tend to take the path of least resistance to
the lowest point, in accordance with the law of gravity.
Just as water won't run uphill, structural loads from a
rafter won't jump out into the attic to appear in the middle
of the floor. They flow down the rafter to the support
wall framing below.
Load Paths
Loads must be carried from the top of the structure to the
foundation without interruption. If there is a break
in the system, for example, if you fail to install an
adequate header over a door, weight from above will cause
deflection on top of the door frame, and the door will
stick. Even minor structural weaknesses can pop nails
in drywall or siding, bind windows, crack trim, and create
other problems in your shed.
Load-bearing and non-load bearing elements
Major structural elements such as floors, posts, columns,
roof framing, and all exterior walls are load-bearing.
They carry the weight of the shed structure and everything
in it (such as people) or on it (such as shingles).
Many interior walls are not load-bearing. These
partition walls simply divide up space. You can cut
through a partition to create a doorway, or remove the wall
all together. But wherever you cut into a bearing
wall, you need to account for the load it carriers by
installed a structural header.
Types of Loads
Structural loads consist of dead loads, such as the weight
of the building materials and mechanical equipment, and live
loads, such as the weight of people, furnishings, stored
materials, and snow on the roof. Another type of load,
shear load, is the force the building encounters when the
wind gusts, the earth quakes, or the foundation shifts
because of evens like soil washout. A point load is
the downward force exerted by a single heavy thing inside or
on the top of the structure, such as a fireplace, hot tub,
or water heater. Lastly, there is the spread load, the
outward force on walls cause by the downward and outward
force of rafters, usually because of heavy snow pressing
down on the roof.
The architect's or engineer's job is to anticipate all
conditions that could reasonably be expected at the site.
In residential design, potential loads and stresses are
typically provided by local building codes. For
example, floor systems are generally designed to support 40
pounds of live load per square foot. Because wood is
resilient, the frame can absorb the extra strain as you move
around or concentrate heavy furniture in one room.
In a wood frame there is bound to be some movement,
particularly on the floor where joists span from one support
to another. Bu the standard limit of movement, called
deflection, is 1/360 the length of the span. That
means if the floor joists were 360 inches long, they would
have to be strong enough to deflect no more than 1 inch when
loaded with people and furniture.
Although even a small shed has building loads, 2x4s
generally are more than enough to handle them. In a
large barn, however, your plans will have to conform to the
guidelines on approved span tables and be checked by the
local building department. Inspectors will check both
the size and spacing of the girders, joists, and rafters.
Span Tables
The allowable spans for joists, rafters, girders, and other
load-bearing elements of a building frame are all subject to
local building codes. Codes specify the loads that
framing member must bear in each location. For
example, 40 lbs per square foot for floor joists in living
space. They also set deflection limits, given as span
in inches (L) over a given number. For example, and
L/360 limit for joist means a 10 foot joist can bend a
maximum of 120 inches divided by 360, or 1/3 inch, under the
load.
Reading a Span Table
Span tables are organized by wood species and grade because
they have different strengths. Different sizes for
each grade are given different maximum span lengths in feet
and inches for the most common on-center spacings. For
example, looking at the table below, if you wanted to span
13 feet with southern pine 2x8s, you would need to use at
least No. 1 grade at 16 inches on center. If No. 1
were unavailable, you would have the choice of using thicker
lumber (such as 2x10s), or spacing No 2 grade 2x8s at 12
inches on center.
|
Floor Joist Span Ratings |
|
Strength: For 40 psi live
load 10 psf dead load |
|
Deflection: Limited in span in
inches divided by 360 for live load only |
|
Species |
Grade |
2x8 |
| |
|
Spacing on Center |
|
Spruce/pin/fir/(southern) |
|
12" |
16" |
19.2" |
24" |
|
Select Structural |
15' |
13'7" |
12'10" |
11'11" |
|
No. 1 and better |
14'8" |
13'4" |
12'7" |
11'8" |
|
No. 1 |
14'5" |
13'1" |
12'4" |
11'0" |
|
No. 2 |
14'2" |
12'9" |
11'8" |
10'5" |
|
No. 3 |
11'3" |
19'9" |
8'11" |
8' |
Some codes use
two tables. One gives the design values for each grade
of wood-measurements of fiber strength in bending (Fb) and a
ratio of stress to strain called modulus of elasticity, or
the E-value. Another gives span lengths according to
these vales. You will first determine which size
lumber will work for your span. Looking at the first
table below, if you had a span of 14 feet and 6 inches,
you'd need to use at least 2x10s--ones made from a wood with
a minimum E-value of 1.2 (spaced at 16 inches on center) and
an Fb of 1,036. The second table below shows the
design value for one type of wood (hemlock/fir) for this
thickness. By matching the E-values and Fb ratings,
you find that you can use No. 2 hemlock/fir (or any better
grade).
|
Floor Joist Span Ratings |
|
With L/360 deflection limits.
For 40 psf live load |
|
Joist |
On Center Spacing |
E-Value (in Million PSI) |
| |
Spacing on Center |
|
Size |
|
0.8 |
1.0 |
1.2 |
1.4 |
|
2x6 |
12" |
8'6" |
9'2" |
9'9" |
10'3" |
|
16" |
7'9" |
8'4" |
8'10" |
9'4" |
|
24" |
7'3" |
7'3" |
7'6" |
8'9" |
|
2x8 |
12" |
11'3" |
12'1" |
12'10" |
13'6" |
|
16" |
10'2" |
11'0" |
11'8" |
12'3" |
|
24" |
11'8" |
9'7" |
10'2" |
10'9" |
|
2x10 |
12" |
14'4" |
15'5" |
16'5" |
17'3" |
|
16" |
13' |
14' |
14'11" |
15'8" |
|
24" |
11'4" |
12'3" |
13'0" |
13'8" |
|
Fb |
12" |
718 |
833 |
941 |
1043 |
|
16" |
790 |
917 |
1036 |
1148 |
|
24" |
905 |
1050 |
1186 |
1314 |
|
Design Values for Joists and Rafters |
|
Strength: For 40 psi live
load 10 psf dead load |
|
Deflection: Limited in span in
inches divided by 360 for live load only |
|
Species & Grade |
Size |
Design Value in
Bending (Fb) |
E-Value (in million
PSI) |
|
Hemlock/Fir |
2x10 |
Normal |
Snow Loading |
|
Select Structural |
1,700 |
2,035 |
1.6 |
|
No. 1 and better |
1,330 |
1,525 |
1.5 |
|
No. 1 |
1,200 |
1,380 |
1.5 |
|
No. 2 |
1,075 |
1,235 |
1.3 |
|
No. 3 |
635 |
725 |
1.2 |
It is possible
for do it yourselfers to determine lumber sizes and spans.
But a final determination is best left to an architect or
engineer unless you use code-approved plans that pass muster
at the local building department.
Please make sure you check out our
shed
plans in our shed plans package before you leave our
site and see if they meet your needs!
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