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


(demonstration models)



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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Page 103 - Building stability - Queenpost truss bridge

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