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 two-dimensional |
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 two-dimensional 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) |
|
. |
Back
to Knowhere |
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Page 118
- Building stability - Three-dimensional stability |
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