longitudinal axis.  That is, the forces are distributed axially and the members do not

experience bending moments.  A structure fulfilling these requirements is said to be

statically determinate.  


Flexible joints in early iron or steel truss bridges were created by providing a hole, or eye,

on each end of a strut.  This allowed the members to be pinned together with a hinge pin

that acted like a door hinge.  This is the function of the pinge connector used in the



a) truss bridge

 b) polymorf model

Fig. 171 - Gusset plate joint

Fig. 170 - Pinned joint

of a truss bridge


Polymorf system.  The pinge joint closely mimics the hinging action of metal pinned

joints when it is used to build static demonstration models of structures for load testing.

Iron and steel bridge designers eventually gained enough experience in building trusses

that the expensive pin joints were eliminated in favor of cheaper riveted, bolted, or

welded gusset plate joints (Fig. 171).  However these structures are still analyzed as though

their joints are pinned, which is approximately true.


Like a simply supported beam, a truss bridge is supported by a fixed hinge joint at one

end of its span and a rolling node at the other.  This prevents the joints from experiencing

a bending moment and to allow for expansion and contraction of the structure due to

different loading conditions and temperature variations.


Warren truss bridge


Another common early bridge design was the Warren truss bridge patented by James

Warren and Willoughby Monzoni in 1848.  The diagonal braces of this design point both

towards and away from the midpoint of the bridge.   Thus they experience both tension

and compression stresses as a load, such as a vehicle, crosses from one end to the other.



Fig. 172 - Warren truss bridge



◄  7 = 2 ( 5 ) - 3

therefore stable

(static demonstration models)

click image to enlarge


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Page 107 - Building stability - Warren truss bridge

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