early in 1970. Each tube section consists of a 3/8-inch thick steel 
shell,the interior of which is covered with a 27-inch thick lining of 
reinforced concrete. Each steel shell is stiffened transversely by 
8-inch structural steel tees spaced at 6-foot centers. The steel 
shells were fabricated at South San Francisco on shipways, launched, 
and towed to a nearby wharf for concreting while afloat. Concrete 
placement sequence was in four stages so as to keep each tube section 
trim in the seawater and avoid high stresses in the shell. The first 
stage was the floor; second, the straight central wall; third, the 
curved side walls; and fourth, the crown. Unit weight of the concrete 
was critical; at 145 pcf the complete tube section would float satis- 
factorily, but at 150 pcef it would sink (Western Construction, 1967). 
After being concreted, the weight of a tube section ranged between 
9,000 and 9,500 tons; these weights were a few hundred tons less than 
needed for sinking. A decrease of 1% in density of seawater would 
affect the buoyancy as much as 100 tons for a structure of this dis- 
placement. The typical tube section contained 4,200 cubic yards of 
concrete which had been transferred from the Sus mixers by means of 
10-cubic yard buckets operated by gantry cranes.—’ The cranes, which 
were 70 feet high and equipped with 100-foot booms, moved along a 15- 
foot gage track. Each completed tube section was equipped with temporary 
water tight bulkheads. After a section was lowered about 130 feet into 
a trench on the floor of the Bay and coupled to the preceding section, 
the seawater trapped between the adjoining bulkheads was pumped out. 
Removal of the water in|the joint between two sections created sufficient 
hydrostatic pressure at the outboard end of the lowered section to cause 
the joint gasket to compress longitudinally and form a water tight seal. 
Since 1946 there has been an extensive application of prestressing 
and post-tensioning techniques in the area of concrete structures. In- 
cluded are pipes, piles, long-span beams and slabs, bridges, piers, 
walls, roofs, and offshore platforms; additionally, there are many mis- 
cellaneous items (e. g., electric light poles and railroad ties). 
Modern methods of fabricating precast prestressed concrete structural 
members have assured the production of much larger units than would 
otherwise have been possible; prestressed pavements, aircraft runways, 
tanks, tunnels, folded plate roofs, and thin shell structures are 
examples. Nevertheless, prestressed concrete is a material requiring 
specialized knowledge. The structural design must be entrusted to a 
specialist in this area of structural engineering, the production of 
precast units requires unusually high standards of workmanship, and the 
assembly and erection necessitates considerably more technical detail 
than is the case with conventional reinforced concrete. 
The largest group of marine structures consisting of prestressed 
concrete are wharves (Gerwick, 1959). 
oS ee as 
x3// 
Information on concrete production for BART is contained in Part 
Four of this report. 
