Polymorf - Knowhere - Three-dimensional stability

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)

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