You have
already seen how single and double cross bracing can be used to stabilize 
members
that are connected together by relatively flexible joints.
Alternately, rigid plates, 
or shear
panels, can be fastened to the perimeter framework, or connected to each
other 
edge to
edge to form a rigid structure. Or, the joints can be made rigid so
that each 
member
acts as a beam or column that is fixed at each end to its supports.
As you saw 
with
bridges, real buildings are usually stabilized by combinations of these
methods. 
. 

For example, lets add one diagonal brace (represented by 
the blue pinges) to each of the six faces of the cube. As 
anticipated the box is now stable. So triangulating the 
individual faces of the cube stabilizes the entire three 
dimensional structure just like it did for twodimensional 
structures previously (although, as you will see shortly, 
Fig. 196  Bracing the cube 
the equations describing their stability
differ somewhat). 

. 
The
roofs of many houses are pitched upwards to form a peak with slanted
sides. A 
triangular shaped roof truss framework is used to support each end of the
roof. More 
trusses
may positioned inside to support its midsection. The stability
of each truss can be 
analyzed
by treating it as a twodimensional structure with flexible joints. 



Fig. 197  Roof truss designs 
◄ a) A  frame 
b) Kingpost ► 

3 = 3 ( 2 )  3 stable 
(static demonstration models) 
9 = 2 ( 6 )  3 stable 

. 
The
slanted members of the truss, called rafters, experience compressive
stresses due to 
the dead
weight of the roof sheathing and any external loads bearing on it such as
wind, 
snow, or
rain. The horizontal base member of the truss, called the joist,
ties the ends of 
the
rafters together so they do not spread apart due to the load. So
roof joists experience 
tensile
stresses mainly (although they may also have to support a load if
insulation is put 
above
them or if flooring is laid on them). Additional diagonal bracing
like that shown in 
Fig. 197 b) above is often used to support the midpoint of the rafters.

. 
If the
interior space of the roof or attic is to be open, a modification of the
Queenpost truss 
can be
used. However, as seen previously, the Queenpost truss is inherently
unstable. As 
a result
additional bracing may have to be used in the trusses located at both ends
of the 
roof to
stabilize it as shown in Fig. 198 b)
following. 



Fig. 198  Queenpost roof truss 
◄ a) 10 < 2 ( 7 )  3 
b) 15 = 2 ( 9 )  3 ► 
(static demonstration models) 

. 
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Page 118
 Building stability  Threedimensional stability 

