Pratt truss bridge


One widely used design was the Pratt truss, created by Thomas and Caleb Pratt in 1844. 

In this design the diagonal struts point toward the midpoint of the bridge, in the opposite

direction of the Howe truss.


Fig. 168 - Pratt truss bridge



◄  13 = 2 ( 8 ) - 3

therefore stable

(static demonstration models)

click image to enlarge


As shown below this arrangement subjects these struts to tensile stresses (since the blue

highlighted rubber bands are stretched).  This permits the diagonal struts to be made

thinner and lighter from iron or steel resulting in a more efficient structure.




Fig. 169 - Load induced

stresses in the Pratt truss

(training aid model)

click image to enlarge



Although Pratt trusses could be built with wooden upper chords, vertical posts, and end

posts, which are subjected to compressive stresses, they were mainly all-iron or all-steel

structures.  One advantage of an all metal truss bridge design is that the internal forces

its structural members are exposed to can be statically determined because the struts can

be pinned together to form flexible joints.  This allowed engineers of the day to more

reliably predict how their designs would perform under load than earlier designs, which

had fixed joints and therefore were indeterminate.

Recall that when a structure is in equilibrium it is stable and will not move or deform

significantly when subjected to an outside force as long as the sum of the forces acting on

and in it are equal to zero.  This requires that the sum of all of the stressing and reacting

forces acting on each of the joints of the structure are equal to zero.  Thus three basic

structural conditions must be present for a static analysis to be done on a truss design:

     1)  the structure must be inherently stable, that is, its stability is due to the triangular

          arrangement of its members;

     2)  the joints must be flexible; and

     3)  the structure must be in a state of equilibrium and the sum of all forces must be zero.

This allows the external load to be dissipated throughout the structure so that the internal

stresses and reactionary forces within each member are aligned parallel with the member's

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Page 106 - Building stability - Pratt truss bridge

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