Question

The suspension bridge at Jarnac is one of the earliest in France. Built in 1827 it predates Roebling’s first suspension bridge by over twenty years. The designer J.P.Quenot visited the Tournon cable bridge by Marc Seguin and traveled to England to see Thomas Telford’s bridges.
The two main supporting cables span
230 feet and sag 20 feet. Droppers
(vertical cables) hang from these main cables and support a 12-foot-wide wooden bridge deck.
The design load (DL + LL) for a bridge of this era was in the order of 60 lb./sq.ft. Given that loading, what is the force on each dropper cable if they are spaced 1 foot apart along the main cable. Each dropper cable supports one end of
a strip of bridge deck (12 ft long and 1 ft wide).

1. What is the uniformly distributed load (UDL) for each of the main cables.

2. Find the total vertical load on each main cable

Answer 1

There are 2 main cables on 2 sides of bridge along the span.

4 dropper cables on both sides support a 12ft*1 ft of bridge deck.

Design load of deck=60 lb/sq ft.

Tension on each cable=((60lb/sqft)*12sqft)/4=180 lb.

Number of dropper cables along one one main cable=(Span of main cable/spacing b/w dropper cable)+1=(230ft/1ft)+1=231 cables.

Total vertical load on each main cable=(number of dropper cables*load on each cable)=(231*180 lb)=41580 lb.

UDL on each main cable=Total vertical load on cable/span of cable=41580 lb/230ft=180.783 lb/ft