Rigid-jointed Vierendeel girders - Steel Structures.

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.

2 Analysis
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.

3 Connections
Vierendeel girders have rigid joints with full fixity and so the connections must be of the type which prevents rotation or slip of the incoming members, such as welded or friction-grip bolted connections. Welded connections are usually the most efficient and compact although undesirable if the connections are required to be made on site. Normally site splices are bolted for economy. For very large Vierendeel girders delivered and erected piecemeal, fully bolted connections are normally used.

For member and joint efficiency the ends of the verticals are often splayed. This is of advantage in heavily-loaded girders as the high concentrated local stresses are reduced thus avoiding the need for heavy stiffening.

Some typical joint examples are illustrated in Fig. 19.8(b) and (c).

Typical details of Vierendeel girders: (a) typical forms, (b) welded connections, (c) bolted connections
Fig. 19.8 Typical details of Vierendeel girders: (a) typical forms, (b) welded connections,
(c) bolted connections

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