There are three basic types of hinge designs. One type, illustrated in Fig. 7.30a, develops a hinge by cutting through all but the back flange. The front flange is connected with a slotted plate known as a friction plate. When the post is struck, the friction plate separates from the slotted bolt holes as the back flange bends. This type of hinge creates a maintenance problem, since the post is destroyed and must be replaced after each impact. It is also more difficult to predict the resistance of the hinge, which is dependent upon the post size and depth of cut.
Another type (Fig. 7.30b) utilizes a rear hinge plate. This plate is similar to the friction plate but does not have slotted bolt holes. With this type of hinge, the sign support is completely cut in two pieces, with the hinge plate bolted on the back and the friction plate on the front. When impacted, the friction plate releases through the slotted bolt holes and the hinge plate bends back. Maintenance after impact is simplified, since the hinge plate can be removed and the upper and lower support pieces reused with a new hinge plate. Proper operation of the friction plate design is dependent upon proper bolt size and torque. If the bolts are too small, or not torqued sufficiently, wind loads will cause the friction plate to become loose and the top of the sign to fall back. If the bolts are too large or torqued too much, the support will not separate properly upon impact [45].
The third hinge type (Fig. 7.30c) utilizes a rear hinge plate and a front hinge plate with a weakened section. When impacted, the section fractures through the plane of the holes, thus permitting the back hinge plate to bend. This design has an advantage over the friction-hinge plate design while remaining easy to repair. The advantage is that the torquing requirements on the friction plate are not critical for proper operation. The front hinge plate in Fig. 7.30c is weakened by drilling holes so that only 33 percent of the plate material remains. Figure 7.31 shows commercially available frangible hinge plates available from Transpo Industries. The three hinge systems presented in Fig. 7.30 are unidirectional and should not be used in areas requiring bidirectional performance. Only the Transpo hinge system offers bidirectional performance.
