Structural steel and reinforced concrete are used to support buildings. Architects design beam structures to resist various stresses, including tension and compression. The size of the beam, materials used, and weight of objects placed on it affect beam deflection. Cantilevers and vibration also affect design.
Many buildings use structural steel framing to support the weight of the building, its furnishings, and all the people who work or visit it. Other materials may 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 various stresses that occur from building weight, people or vehicles, and the possible effects of wind or snow. Stresses cause beam deflection, which is the bending or twisting of 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 separates the beam, which steel can resist well, but concrete cannot. The rebar is placed within reinforced concrete structures to resist tensile forces.
Compression is a force that pushes from both ends of a beam towards the center. Any vertical wall or beam is subjected to compressive stress from the weight of the building above it. Concrete is very good at resisting compressive forces and steel a little less so 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 positioned at 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 slab cast on a steel beam structure can have low deflection, because the weight of the slab is distributed or evenly distributed across the surface of the beam. The vertical beams of the walls must be designed to support the weight, called load, of the steel and concrete floor to avoid any deflection of the walls.
A beam can deflect more if a large weight is placed farthest away from where the beam is supported or attached to the building. This type of loading is very important to beam deflection calculations and may require additional support walls or beams below the maximum deflection point. Beams supported only at one end are also carefully analyzed for beam deflection.
Any structure that is supported on one end is called a cantilever and is commonly used for balconies, walkways, and cantilevered roof designs. The cantilever must be carefully designed to support the maximum expected loads of objects or people as well as a generous safety factor. Cables extending to the floor above or support posts below the cantilevered section can be added to support additional loads, but may affect the aesthetics or visual appeal of the cantilevered design.
Another design issue is vibration, which is a form of beam deflection. Earthquakes, winds, and movement of people or vehicles can cause 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 quantities, but larger vibrations can damage walls or furnishings, or even lead to the destruction of the building. This can occur differently depending on where the beams are supported and should be included in the design of steel or concrete structures.
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