Queenpost truss bridge |
. |
One
design solution to this problem was to cut off the apex of the A-frame
trusses. This |
created
a trapezoidal shaped truss with a top horizontal member that was connected
to the |
deck
beam by two vertical struts. The design provided two support points
for the beam |
thereby
significantly reducing the bending moment of a deck load. Thus the Queenpost |
. |
 |
 |
 |
. |
a) Unbraced truss |
b) Truss instability |
c) Braced truss |
. |
8 < 2 ( 6 ) - 3 |
Fig. 159 - Queenpost truss |
9 = 2 ( 6 ) - 3 |
unstable |
(demonstration models) |
stable |
|
. |
could
span greater distances without the added height. However,
the Queenpost truss |
structure is inherently unstable. At least one additional bracing
strut is required to stabilize |
it.
Additional braces can be added to increase its stiffness. But this
will make the structure |
statically indeterminate requiring a more complex analysis than simple
statics provides. |
. |
|
|
 |
◄ Fig. 160 - Reinforced |
Queenpost bridge. |
(model with flexible joints) |
Fig. 161 - Railroad bridge ► |
(with fixed joints) |
click image to enlarge |
|
. |
In
addition to vertical loads, a truss bridge must be braced to stop the
trusses from tipping |
over due
to their own weight, the weight of the load, or sideways, lateral loads
imposed on |
them by
winds or the rocking motion of earthquakes. Portal bracing is used for
this. |
. |
|
◄ Fig. 162 - Sideways loading |
|
of a truss bridge |
|
portal bracing ► |
(demonstration models) |
|
|
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to Knowhere |
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Page 103
- Building stability - Queenpost truss bridge |
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