Live Load Framing Specs
- A load or force can be measured in kilopascals, an equal unit of pressure. Conceptually, loads or forces exerted on a structure can be divided into dead or static loads -- forces that remain constant over time, such as the weight of the house itself -- and live loads -- those forces exerted on the structure in varying amounts. The dead load is the structure of the house; the live load is the contents of the house, or external loads such as wind and snow.
- When calculating the maximum required live strength, it's not terribly difficult to imagine the heaviest possible set of contents in a room. For example; if you may include the weight of heavy objects like a billiards table, grand piano and refrigerator, add the weight of several people. It is more difficult to estimate, and thus specify, the dynamic nature of live load. Imagine a 60-lb. child. Now imagine a 60-lb. child jumping off a chair onto a scale. The force exerted just for an instant can be many times the child's weight. Now, imagine two dozen people dancing on a second story. They can exert an enormous force, which can be magnified even greater if they all come down at once. It is these dynamic spikes in live load that must be carefully planned for.
- Every set of framing plans must be specified or approved by a structural engineer. For common configurations, engineers reference a large body of engineering data such as span matrices for specific types of woods in specific dimensions and configurations. The more structural members, generally the smaller the members can be, as a given load is distributed across multiple supports. For custom or less common configurations, engineers may still reference preexisting engineering data, but make custom calculations to specify adequate strength to withstand all potential loads.
- Framing specifications are not the only considerations in creating adequate strength for live loads. Other factors affect strength. For example; if a floor is sheathed with plywood, gravity will hold the plywood on the floor joists. When they are nailed in place, the nails create tensile strength, making the floor more rigid. If they are glued with construction adhesive and thoroughly nailed down, the floor is even stronger, augmenting the strength specified in the framing alone.
