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Introduction to Structural Loads

A good understanding of how loads work is a must for any shed designer.  Our free article center was formed to help you build, plan, and execute your wood working project as smoothly as possible.


Introduction to Structural Loads
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.

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