431. It might happen that circumstances would not make it convenient
to obtain points of support below the bridge on which to erect the
struts. In such a case, if suitable positions for ties can be obtained,
a bridge of the form represented in Fig. 60 may be used.
A D is a horizontal rod of pine 40" × 0"·5 × 0"·5; it is trisected in
the points B and C, from which points the ties B E and C F are secured
to the upper parts of the framework. A D is then supported in the
points B and C, which may therefore be regarded as fixed points. Hence,
for the reasons we have already explained, the strength of the bridge
should be increased nearly threefold. Remembering that the bridge
has two beams we know it would require about 70 lbs. or 80 lbs. to
produce fracture without the ties, and therefore we might expect that
over 200 lbs. would be necessary when the beams were supported by the
ties. I perform the experiment, and you see the bridge yields when the
load reaches 194 lbs.: this is somewhat less than the amount we had
calculated; the reason being, I think, that one of the clamps slipped
before fracture.
[Illustration: FIG. 60.]