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in engineering a structural element, usually in the form of a girder, which is primarily subjected to bending.
Beams are used extensively in construction and in mechanical engineering in the structures of buildings, bridges, trestles, transport means, machinery, machine tools, and so forth. Beams are manufactured basically out of reinforced concrete, metal, and wood. Depending upon the number of supports and the nature of the support constraints, beams may be differentiated as single-span, multi-span, cantilever, with fixed ends, simple, continuous, and others. According to the shape of its cross section, a beam may be classified as rectangular, T-shaped, I-shaped, box-shaped, and so forth. The most efficient beam cross sections (for bearing capacity as well as for material expenditure)— for example, the I-beams and the box beams—are characterized by a concentration of material at the upper and the lower edges of the cross section, where the maximum normal bending stresses are acting. Rectangular sections are expedient in beams of relatively large height and small width.
Beams may have sections that are constant or variable in their dimensions; a beam with variable cross section permits a decrease in its mass. According to their purpose, beams may be classified as primary (longitudinal beams, which cover a span between supports) and auxiliary (transverse beams, which cover distances between other beams). A system of longitudinal and transverse beams is called a grid of beams.
Reinforced concrete beams are manufactured as monolithic or precast. Monolithic beams are designed, in most cases, as multispan continuous beams. They usually have a rectangular or a T-shaped section; the latter are frequently encountered in ribbed construction (where a monolithic beam is rigidly connected with a slab) and more rarely, in the form of independent beams. Precast reinforced concrete is widely used for single-span beams with various sections, such as rectangular, T-shaped, I-shaped, hollow, and II-shaped. Precast multispan continuous beams are composed of several elements, joined together during the installation process. Prestressed reinforced concrete beams have become widespread.
Metallic beams are utilized, for the most part, for heavy loads. The most effective metallic beams are those with I-shaped (rolled or composite) and box-shaped (composite) sections. Composite beams may have a practically limitless height and bearing capacity.
Wooden beams usually serve to cover short spans and occur in the form of single-span and simple structures. They are made of boards, joists, and logs. In order to increase the bearing capacity of these structures, composite sections are employed, using dowels, pegs, or adhesives.
Calculation of the strength, rigidity, and stability of a beam is usually carried out according to the laws of the strength of materials. Beams are calculated according to loads—that is, dead load (from their own masses and the masses of the structures resting upon them) and live or work load. The determination of support reactions, bending moments, lateral forces, and deflections in statically determinate beams is accomplished analytically or graphically on the basis of the equations of equilibrium. Statically nondeterminate continuous beams are usually calculated with the aid of trinomial equations (equations of three moments) when rigid supports are to be used and equations with five terms when there is elastic displacement. In order to calculate beams which are to lie on a yielding foundation (for example, soil), foundation calculation models are utilized. Selection of the beam section is done basically according to the bending moment (normal stresses) that the beam will have to undergo. Besides this, the section is tested for the action of lateral forces (tangential stresses) and principal stresses. In special cases, beams are calculated for stability. The determination of the tangential stresses in beams was first proposed by the Russian engineer D. I. Zhuravskii.
L. V. KASAB’IAN
(or ray), a concept in geometric optics (light ray) and geometric acoustics (sound ray) that designates the line of propagation of the energy flux emitted in a certain direction by a point source of light or sound. In a homogeneous medium a ray is a straight line. A ray in a medium with smoothly varying optical or acoustic properties is curved, and its curvature is proportional to the gradient of the index of refraction of the medium. Upon passing through a boundary that separates two mediums having different indexes of refraction, a ray is refracted according to Snell’s law of refraction. The term “beam” (“ray”) is also used to designate a narrow beam of particles, such as an electron beam or cathode ray.
a linear, usually horizontal supporting member in a building or some other structure. The beams connect, sometimes by hinge joint, the vertical members and serve as supports for other horizontal members and slabs in floors, ceilings, and roofs. Beams are made of metal, reinforced concrete, or wood. They may be of lattice or solid construction (with a rectangular, T, double-T, or some other cross section).
A structural member that is fabricated from metal, reinforced or prestressed concrete, wood, fiber-reinforced plastic, or other construction materials and that resists loads perpendicular to its longitudinal axis. Its length is usually much larger than its depth or width. Usually beams are of symmetric cross section; they are designed to bend in this plane of symmetry, which is also the plane of their greatest strength and stiffness. This plane coincides with the plane of the applied loads. Beams are used as primary load-carrying members in bridges and buildings.
ii. An invisible path produced by radio signals. An aircraft can follow the beam when approaching or going away from a navigational fix. A radar detects an aircraft when it is illuminated by its beam.
iii. A long, heavy, metallic or wooden member in any structure to withstand any bending and I-beam shearing loads.
iv. The direction extending from the side of an airplane at right angles to the plane of symmetry.
v. The breadth at its maximum width of an airplane fuselage, hull, or vessel.