Chap. XXIII.

CENTERING.

1399

Perronet's system of forming Centres was to omit the tie-beam, and from his elegant workon bridge-building a few examples are selected: this engineer imagined that when the

CENTRE, 76 PEBT SPAN.

Fig. 2328.

timbers were prolonged they found a portion of their support in the opposite sides of thearch ; a Centre so contrived may be regarded as composed of a number of trusses or framedvoussoirs, and are subject, as he found, to great disarrangement, if not loaded very carefully.

Fig. 2329.

CENTRE, 64 FEET SPAN.

The centres used by Labelye for theconstruction of the arches of WestminsterBridge w f ere an improvement upon thoseof Perronet, and calculated to bear moreequally the weight that was placed uponthem, see fig. 429. p. 424. The centresfor Blackfriars Bridge were contrived partlyupon Labelye’s and partly upon Perronet’sprinciples, but they were supported upon in-clined timbers, which greatly assisted thewedging up and gradual lowering of themafter the last voussoir was placed; see fig.

432. p. 428.

Thatof Cravant , situated on the River Yonne ,had a span of 64 feet and a rise of J : therewere five trusses or sets of principals, placed 5 feet 6 inches from middle to middle ; eachwas composed of three triangles, the larger timbers of which were from 16 to 19 feet inlength and 10 inches square; the pendant binding-pieces were nearly 8 feet in length, andof the same scantling. The thickness of the arch at the key was 4 feet 3 inches; the

4 u 4

Fig. 2330.