The largest application of precast, pretensioned concrete construct- 
ion in the world is exemplified by the twin-span Greater New Orleans 
causeway which extends north from metropolitan New Orleans for 24 miles 
across freshwater Lake Ponchartrain. This structure is the longest 
highway bridge in the world (Palmer, 1957). The first span was begun 
in 1955 and completed in 1956, at a cost of $30,000,000 using a simpli- 
fied design and assembly-line methods for fabricating the precast pre- 
tensioned parts of the bridge. The second and parallel span was opened 
in 1969. Except for two bascule spans and three humps for passage of 
water-borne traffic, there are no vertial or horizontal curves throughout 
its entire length. The following facts pertain to the first of the two 
spans (VanBuren, 1957). Production facilities for the bridge decks in- 
volved three pretensioning benches each of which was 490 feet long by 43 
feet wide and capable of producing a line of eight bridge sections at 
one time. By casting sections on one of the three benches everyday, a 
daily production schedule of eight slabs or 448 feet of bridge deck was 
established. Nearly 5,000 hollow, centrifugally cast, prestressed, 
steam-cured cylindrical piles were made. Each pile has a wall thickness 
of 4 inches, an outside diameter of 54 inches, and average length of 88 
feet. The bridge consists of 2,215 identical bents each of which com- 
promises four precast elements; specifically, two piles each weighing 
30 tons, one cap weighing 30 tons, and one prestressed slab weighing 
185 tons. Each slab is 56 feet long by 33 feet wide. Precast caps 
were possible because the piles were hollow; solid piles would have 
necessitated cast-in-place caps. The compressive strength of the con- 
crete constituting the bridge deck slabs was 3,000 psi before prestress- 
ing wires were released at a 36-hour age. All components were precast 
in a specially built yard. Erection reached a peak of 1/2 mile per week. 
A significant floating concrete structure erected between World Wars 
I and II is the Lake Washington Floating Bridge situated at Seattle 
and completed in 1940. The bridge includes 25 cellular flat-bottom rein- 
forced concrete pontoons, each 59 feet wide and varying from 117 to 378 
feet in length (Andrew, 1940). Outside walls and bottom slabs are 8 
inches thick and inner compartmental wall are 6 inches thick. Drafts of 
the pontoons range from 7 to 14 feet. Two graving docks were constructed 
on the Puget Sound waterfront were a tidal range of 10 to 17 feet afforded 
suitable means of launching the units; they were towed to Lake Washington 
through the existent ship canal and locks. The bridge is located where 
the freshwater lake depth varies between 100 and 220 feet. The assembly 
of pontoons is stabilized by means of steel cables attached to anchors 
set in the floor of the lake. Ocean-going ships pass through a 200- 
foot wide opening created whenever a special pontoon section is retracted 
longitudinally. The water level in the lake fluctuates a maximum of 3 
feet and is controlled by the locks. The pontoons are not subjected to 
freezing. Pontoons are connected by means of shear keys (3 x 3 x 1 foot) 
which fit into recesses and the connections are supplemented by 54 bolts 
(3-1/4-inch diameter) distributed around the outer margins of adjacent 
pontoons (Andrew, 1941). Water content of the 6-inch-slump concrete was 
