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Loading - the short course
Building codes have developed loading requirements for
different types of materials based on established standards in the construction industry. Loads include both live and dead loads and may also include wind and snow loads if local building codes so deem.
The Wood Truss Council defines loads to be the "forces or other actions that arise on structural systems from the weight of all permanent construction, occupants and their possessions, environmental effects,
differential settlement and restrained dimensional changes."
Live loads are produced during construction and during the life of the structure. Live loads are established for both floors and
roofs. Construction workers and their materials are considered roof live loads during the construction process.
Ongoing maintenance workers and their materials are examples of live loads during the
structures's life. Floor live loads are incurred on the floor system during occupancy and use.
Dead loads are the weights of the various structural members and objects that are permanently attached to a
structure. Dead loads can be broken down into two types - building dead loads and collateral dead loads. A building dead load consists of the actual building system. This will include the framing
materials (floor or roof) and covering materials such as decking, felt, and shingles . Collateral loads include the weight of permanent materials other than the actual building system.
These items may include drywall, electrical systems, sprinklers, and the like.
One interesting property of wood is its ability to withstand greater loads (stress) for short periods of time. To apply
this property to loading, most building codes allow modifications of allowable stress based on the time (duration) the load exists.
Another property of lumber that effects loading is it's propensity to sag under the influence of applied weight or outside forces. This is known as deflection.
i.e. L/240 or L/360. The length of the rafter or joist, in inches, is divided by 240 or 360 to determine allowable deflection in inches.
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