Box Beam
Introduction
A box girder or tubular beam is a girder that forms a hollow and closed section, unlike a "double T" beam (in or in ), in which its entire section is solid.
Originally built with riveted wrought iron pieces, they have subsequently been manufactured with welded rolled steel elements, extruded aluminum or prestressed concrete.
Compared to a profile, the advantage of a box beam is that it resists twisting better. By having multiple vertical faces, it can also carry more load than a double-I beam of equal height (although it will use more material than a full web beam of equivalent capacity).
The distinction in the name between a box beam and a tubular beam is imprecise. In general, the term "box" beam is especially used if it has a rectangular section. When the girder carries its "contents" "inside" the box, as in the Britannia Bridge, it is called a "tubular" girder, a term also used if the girder has a circular or oval cross section, such as that of the Royal Albert Bridge.
When a large box beam contains more than two side walls, that is, it is composed of several boxes, it is called a "cellular" beam.
Development
Contenido
La base teórica de la viga cajón fue en gran parte obra del ingeniero Sir William Fairbairn"), con la ayuda del matemático Eaton Hodgkinson"), alrededor de 1830. Intentaron diseñar vigas con la mayor eficiencia posible adecuadas para el nuevo material disponible, las planchas de hierro forjado remachadas.
Cellular construction
Most beams are designed to be statically loaded, so that one flange is in compression and the other is in tension. The original Fairbairn steam cranes used a cellular construction for the compression face of their lifting arm, to better resist buckling. This boom was curved and of variable section, and was constructed of riveted wrought iron plates. The concave (lower) face of the arm was formed of three cells, also composed of riveted plates.[1].
When a tubular girder is used to span a bridge span (i.e., loaded at the center rather than at one end, like a crane), the compressive force appears on the top face of the girder and therefore the cells are placed on top. Dynamic forces (moving loads, wind) may also require both sides to be cellular. For example, the preserved section of the Britannia Bridge shows that both the upper and lower wings were of cellular construction, but (according to Fairbairn) cellular construction was adopted in the lower wing, not because of the nature of the forces it had to withstand,[2] but because of its dimensions and the consequent "practical difficulties that would have been encountered if an attempt had been made to achieve the required section area by a massive arrangement".[2]