What Is Beam Deflection?

Many buildings use a steel structural framework to support the weight of the building, its furnishings, and all the people who work or visit there. Other materials can be used to support buildings, including reinforced concrete, which is concrete with an internal steel frame called rebar. Architects design the beam structure using calculations for the various stresses that occur from the building weight, people or vehicles, and any possible effects of wind or snow. Stresses cause beam deflection, which is the bending or twisting of the structural members due to building loads, movement of people, or changing weather.

Structural beams can be exposed to different types of stress. Tension is a force that pulls the beam apart, which steel can resist well, but concrete cannot. Rebar is placed inside reinforced concrete structures to resist tension forces.

Compression is a force that pushes from both ends of a beam toward the middle. Any vertical wall or beam is under compression stress from the weight of the building above it. Concrete is very good at resisting compression forces, and steel somewhat less because it can bend. This is why structural steel is produced in a shape that looks like a capital letter "I," called an I-beam. These are designed with two steel plates placed 90 degrees to a main beam, and running the full length of it, to prevent twisting or bending.

The amount of beam deflection depends on the size of the beam, the materials used, and the weight and position of any object placed on it. A concrete floor poured on a steel beam structure may have little deflection, because the weight of the floor is distributed, or spread out evenly over the entire beam surface. Vertical wall beams must be designed to support the weight, called the load, of the steel and the concrete floor to prevent any deflection in the walls.

A beam can deflect more if a large weight is placed at a point furthest away from where the beam is supported or attached to the building. This type of load is very important for beam deflection calculations, and can require additional beams or supporting walls below the maximum deflection point. Beams only supported on one end are also carefully analyzed for beam deflection.

Any structure supported on one end is called a cantilever, and is commonly used for balconies, walkways, and overhanging roof designs. The cantilever must be carefully designed to support the maximum expected loads from objects or people as well as a generous safety factor. Cables extending to the floor above or support posts under the cantilevered section can be added to support additional loads, but can affect the aesthetics or visual appeal of the overhanging design.

Another design concern is vibration, which is a form of beam deflection. Earthquakes, winds, and movement of people or vehicles can cause the structural steel or concrete to vibrate. Vibration is a repeated movement of the beam in a back and forth motion. It may be acceptable in small amounts, but larger vibrations can damage walls or furnishings, or even lead to destruction of the building. This may occur differently depending on where the beams are supported, and must be included in design of steel or concrete structures.