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Steel Designers' Manual - 6th Edition (2003)
used to support medium- to large-span bridges over land during erection: see
Fig. 19.7(d).
19.9 Rigid-jointed Vierendeel girders
19.9.1 Use of Vierendeel girders
Vierendeel girders, unlike trusses or lattice girders, are rigidly-jointed open-web
girders having only vertical members between the top and bottom chords. The
chords are normally parallel or near parallel; some typical forms are shown in
Fig. 19.8(a).
The elements in Vierendeel girders are subjected to bending, axial and shear
stress, unlike conventional trusses with diagonal web members where the members
are primarily designed for axial loads. Vierendeel girders are usually more expensive
than conventional trusses and their use is limited to instances where diagonal
web members are either obtrusive or undesirable. Vierendeel girders in bridges are
rare; they are more commonly used in buildings where access for circulation or a
large number of services is required within the depth of the girder.
The economic proportions and span lengths are similar to those of the parallel
chord trusses already discussed in section 19.2.
19.9.2 Analysis
Rigid-jointed Vierendeel girders 557
Vierendeel girders are statically indeterminate structures but various manual
methods of analysis have been developed. The statically determinate method
assumes pin joints at the mid-points of the verticals and chords of each panel. The
method, however, is only suitable for girders with parallel chords of constant stiffness
and when the loads are applied at the node points. Various modified moment
distribution methods have been developed for the analysis of Vierendeel girders
which allow for inclined chords, chords of different stiffness in the panels and
member widening at the node positions.
The use of computers offers the most accurate and efficient way of analysing
Vierendeel girders, particularly those with inclined chords, chords of varying stiffness
and when the loading is not applied at the node positions. A further advantage
of computer analysis is that joint rotations and deflections are easily calculated.
Plastic theory may be applied to the design of Vierendeel girders in a similar way
to its application to other rigid frames such as portal frames. Failure of the structure,
as a whole, generally results from local failure of a small number of its members
to form a mechanism. Once the failure mode is established the chords and vertical
are designed against failure. Computer programs are available for the plastic analysis
of plane frameworks including Vierendeel arrangements.