Structural design is all about calculating loads. The engineering firm Nishkian offers a succinct description: “Loads are commonly understood as forces that cause stresses, deformations, or accelerations. These loads are applied to a structure or its components that cause stress or displacement.” From the general concept of “load” comes two major categories: dead loads and live loads. How do they differ?

DEAD LOADS

In short, the dead load of a structure comprises its completed weight, usually measured in pounds per square foot, before it goes into service. The floors, walls, roof, columns, stairs, permanent equipment and any fixed decor constitute a static load that normally doesn’t change over the life of the building. To quote Nishkian again: “[D]ead loads account for the non-dynamic forces that place continuous and permanent force on a structure.”

Therefore, calculations for the dead load before a building takes on the additional loads from occupancy or use, the weight of its components and the pressures applied in a downward direction to the ground must include the concrete for the foundation system, planned building materials plus any service equipment such as elevators, HVAC units and ductwork, plumbing, fixed manufacturing equipment, etc.

LIVE LOADS

Live loads refer to the dynamic forces from occupancy and intended use. They represent the transient forces that can be moved through the building or act on any particular structural element. Also measured in psf, these loads include the anticipated weight of people, furniture, appliances, automobiles, moveable equipment and the like.

The American Society of Civil Engineers (ASCE) require that minimum live loads in the design should always exceed the maximum anticipated live loads over the course of the building’s life span in case loads are, in fact, surpassed. ASCE guidelines prescribe live loads according to categories of intended occupancy and use per building type.

Because live loads depend on structural strength, knowledge of the exact planned use of the building is critical. The might of the dead load, or lack thereof, often defines how much live load it can handle. Reinforced concrete creates the heaviest dead loads but also supports the most weight with its tremendous compressive strength. Structural steel offers much less of a dead load and provides superior support for live loads in multi-story buildings. Natural and engineered wood rest relatively lightly on the foundation but support less live loads than steel and concrete.

The total dead plus live loads equal the “gravity load” of the structure. But yet more loads act upon buildings, as well.

ENVIRONMENTAL LOADS

Before an edifice can pass code requirements, the structural design must also include other dynamic loads acting on it laterally. These environmental loads can come from snow, wind, soil movement and seismic activity. Naturally, Colorado Front Range codes are light on seismic controls but certainly factor in extreme snow, wind and soil conditions.