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 |
|
. |
Back
to Knowhere |
 |
Page 107
- Building stability - Warren truss bridge |
 |
|