Truss bridges

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Fig.  some of the trusses that are used in steel bridges
       Truss Girders, lattice girders or open web girders are efficient and
economical structural systems, since the members experience essentially axial
forces and hence the material is fully utilised. Members of the truss girder bridges
can be classified as chord members and web members. Generally, the chord
members resist overall bending moment in the form of direct tension and
compression and web members carry the shear force in the form of direct tension
or compression. Due to their efficiency, truss bridges are built over wide range of
spans. Truss bridges compete against plate girders for shorter spans, against
box girders for medium spans and cable-stayed bridges for long spans.
        For short and medium spans it is economical to use parallel chord trusses
such as Warren truss, Pratt truss, Howe truss, etc. to minimise fabrication and
erection costs. Especially for shorter spans the warren truss is more economical
as it requires less material than either the Pratt or Howe trusses. However, for
longer spans, a greater depth is required at the centre and variable depth trusses
are adopted for economy. In case of truss bridges that are continuous over many
supports, the depth of the truss is usually larger at the supports and smaller at
midspan.
        As far as configuration of trusses is concerned, an even number of bays
should be chosen in Pratt and modified Warren trusses to avoid a central bay
with crossed diagonals. The diagonals should be at an angle between 50o and
60o to the horizontal. Secondary stresses can be avoided by ensuring that the
centroidal axes of all intersecting members meet at a single point, in both vertical
and horizontal planes. However, this is not always possible, for example when
cross girders are deeper than the bottom chord then bracing members can be
attached to only one flange of the chords.
      
General design principles

Optimum depth of truss girder
       The optimum value for span to depth ratio depends on the magnitude of
the live load that has to be carried. The span to depth ratio of a truss girder
bridge producing the greatest economy of material is that which makes the
weight of chord members nearly equal to the weight of web members of truss. It
will be in the region of 10, being greater for road traffic than for rail traffic. IS:
1915-1961, also prescribes same value for highway and railway bridges. As per
bridge rules published by Railway board, the depth should not be greater than
three times width between centres of main girders. The spacing between main
truss depends upon the railway or road way clearances required.

Design of compression chord members
     
  Generally, the effective length for the buckling of compression chord
member in the plane of truss is not same as that for buckling out-of-plane of the
truss i.e. the member is weak in one plane compared to the other. The ideal
compression chord will be one that has a section with radii of gyration such that
the slenderness value is same in both planes. In other words, the member is just
likely to buckle in plane or out of plane. These members should be kept as short
as possible and consideration is given to additional bracing, if economical.
       
The effective length factors for truss members in compression may be
determined by stability analysis. In the absence of detailed analysis one can
follow the recommendations given in respective codes. The depth of the member
needs to be chosen so that the plate dimensions are reasonable. If they are too
thick, the radius of gyration will be smaller than it would be if the same area of
steel is used to form a larger member using thinner plates. The plates should be
as thin as possible without losing too much area when the effective section is
derived and without becoming vulnerable to local buckling.