The individual elements are connected at nodes; the connections are often assumed to be nominally pinned. The external forces applied to the system and the reactions at the supports are generally applied at the nodes. When all the members and applied forces are in a same plane, the system is a plane or 2D truss. The principal force in each element in a truss is axial tension or compression. Overview of trusses [ top ]Use of trusses in buildings Trusses are used in a broad range of buildings, mainly where there is a requirement for very long spans, such as in airport terminals, aircraft hangers, sports stadia roofs, auditoriums and other leisure buildings. Trusses are also used to carry heavy loads and are sometimes used as transfer structures.
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Load causes global shear and bending which elongates the bottom in tension and shortens the top in compression. The internal reaction to global shear and bending is different in a Vierendeel compared to a beam. By visual inspection we can derive simple formulas for approximate axial and shear forces and bending moments.
Respective stresses are found using formulas for axial, shear and bending stress and superimposing them. Chord tension T and compression C are computed, dividing the respective global moment M by frame depth D distance between centers of chords. Bending of individual struts can be visualized too. The combined effect of these two idealized cases imparls S-snaped deformation and inflection points in both chord and we!
The deformation yields strut bending moments which vary from positive to negative -siong each strut. Top and bottom chords carry each about half the iota! Assuming inflection points at midpoints of chords, the local chord moment M is half the shear V multiplied by half the chord length.
The moment M is maximum at supports where shear is greatest and equal to support reactions. For equilibrium, webs have to balance chord moments at each joint. Their moment equals the difference of adjacent chord moments. One-way girders may be simply supported or continuous over more than two supports.
They may be planar or prismatic with triangular or square profile for improved lateral load resistance. Some highway pedestrian bridges are of the latter type.
A triangular cross-section has added stability, inherent in triangular geometry. It could be integrated with bands of skylights on top of girders. When supports are provided on all sides, Vierendeel frames of two-way or three-way spans are possible options. They require less depth, can carry more load, have less deflection, and resist lateral load as well as gravity load. The two-way option is well suited for orthogonal plans; the three-way option adapts better to plans based on triangles, hexagons, or free-form variations thereof.
Vierendeel truss bridges
What are the characteristics of Vierendeel girder?