Today’s typical analysis for truss bridges used for pedestrian crossings would be
based on:
- AASHTO wind load determined from a pressure of 3.59 kPa (75 psf).
- “Pin” boundary conditions for both bearings at one end and “roller”
boundary conditions for both bearings at the other end.
- Internal member-to-member connections treated as pinned.
- Probably a 3D skeleton analysis, although some engineers still use 2D
analysis of the vertical trusses and for the top and bottom horizontal
trusses and combine the results. For a 3-D model, a true “pin” boundary
condition is a joint that is restrained from translation in all three degrees of
freedom (DOF), but the three rotational DOF’s are released. The “pin”
support acts like a ball joint. A true “roller” boundary condition is the
same as a “pin” with an additional release for translation in the bridge
longitudinal direction.
It may be noted that the term “pin”, meaning “free to rotate” as used in modern
structural analysis, derives from 19th century analyses of trusses that had true physical
pins.