- Continue the construction
Draw a line through Q in Fig. Sb parallel to da in Fig. 8c. Locate -S" on this line at a hori-
zontal distance of 17 ft (5.2 m) from Q. - Complete the construction
Draw R'S' and db. Test the accuracy of the construction by determining whether these
lines are parallel. - Determine the required length of the cable
Obtain the required length of the cable by scaling the lengths of the segments to Fig. 8Z?.
Thus P'R' = 17.1 ft (5.2 m); R'S' = 18.4 ft (5.6 m); S'Q' = 17.6 ft (5.4 m); and length of
cable= 53.lft (16.2m).
PARABOLIC CABLE TENSION AND LENGTH
A suspension bridge has a span of 960 ft (292.61 m) and a sag of 50 ft (15.2 m). Each ca-
ble carries a load of 1.2 kips per linear foot (kips/lin ft) (17,512.68 N/m) uniformly dis-
tributed along the horizontal. Compute the tension in the cable at midspan and at the sup-
ports, and determine the length of the cable.
Calculation Procedure:- Compute the tension at midspan
A cable carrying a load uniformly distributed along the horizontal assumes the form of a
parabolic arc. In Fig. 9, which shows such a cable having supports at the same level, the
tension at midspan is H= wL^2 /(8d), where H = midspan tension, kips (kN); w = load on a
unit horizontal distance, kips/lin ft (kN/m); L = span, ft (m); d = sag, ft (m). Substituting
yields H= 1.2(960)^2 /[8(50)] = 2765 kips (12,229 kN).
FIGURE 9 Cable supporting load uniformly distributed along horizontal.Unit load = w