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Vierendeel girders

Trusses can have a rectangular configuration, avoiding the potential intrusion of diagonal members, if the joints are rigid.

Because deep trusses are particularly efficient for long spans they are sometimes designed to be a full storey-height deep, the top boom being within the upper floor zone and the lower floor zone incorporating the bottom boom. However because of fenestration, some or all of the diagonal truss members may be undesirable. If they are omitted the resulting "truss" is effectively a series of rectangles and if the joints were pinned it would collapse. However, if the shear forces are accommodated by making the joints between members stiff enough the structural integrity is retained.

Vierendeel girder

Such trusses are called Vierendeel girders, and the members are subject to bending.

In this case individual members become subject to bending moments and shear force in addition to direct tension or compression. These types of structure are called Vierendeel after the professor whose experimental work still forms the basis of their design.

The joints may be heavy, but the absence of diagonals makes this form suitable for storey-height construction.

Using standard computer programs, the analysis is not difficult, but the resulting joints are often very heavy in appearance. However the system does allow full storey-height construction without obstruction to openings. Clearly in this situation the verticals would be at column positions. It is common to see this type of truss in the walkways, for instance at airports. This is because the height available relative to the span reduces the boom forces and moments and eases the problem of forming the joints.

Trusses, particularly Vierendeel girders, can be used vertically (to resist horizontal loads).

While the descriptions so far have concentrated on the truss spanning horizontally, it is by no means necessary and often (particularly with Vierendeels) the truss spans vertically.

 

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