132 



their construction the value of the San Francisco piers has been substantially en- 

 hanced by the progressive rise in construction costs, a tendency which is seldom taken 

 into account by advocates of more temporary construction. 



REINFORCED CONCRETE CYLINDER COLUMNS 



This type includes various forms of reinforced concrete column supports, bearing 

 either directly on hard substrata or on pile foundations. It has been used in the con- 

 struction of fourteen piers on the San Francisco waterfront. While the details of 

 design vary somewhat in the different structures, the cylinders used in Pier 28, con- 

 structed in 1912 and 1913, are typical of this type of construction. 



Pier 28 is 150 feet in width and 677 feet in length. It is supported on 452 re- 

 inforced concrete cylinders which are spaced approximately 15 feet apart in both 

 directions. The cylinders are 3^2 to 4 feet in diameter and are supported on bases 

 from 5 feet to 7 feet in diameter, which rest on a compact bed of sand, clay and gravel 

 at a depth of from 40 to 70 feet below low water. 



The cylinders and bases were constructed in cylindrical open steel caissons about 

 8 feet in diameter which were sunk to the required depth by dredging out inside with 

 orange-peel buckets and driving with a pile driver as the excavated material was 

 removed. After the caissons were sealed by driving into the hard bottom the water 

 was pumped out and the bottom was excavated by hand to an even bearing. The 

 cylinder forms were built up with 2-inch stave lumber hooped with steel bands, and 

 the reinforcement consisted of ^-g-in. square bars spirally hooped with No. wire. 



The concrete in the cylinders was mixed in the proportion of one part of cement 

 to six parts of aggregate. The aggregate consisted of several sizes of sand and crushed 

 rock so as to give a graded mixture. The concrete was poured through a jointed pipe 

 and was tamped and spaded so as to fill the 3-inch spaces between the reinforcing 

 steel and the forms. After the concrete had set not less than 24 hours the caissons 

 were pulled, the wood stave forms being left permanently in place. 



The cylinder type of construction has proved satisfactorily where the work was 

 properly done. The structure is fire-proof and permits of the construction of a fire- 

 proof shed which is not feasible in this locality on a timber substructure. It is rigid 

 and permanent in character and the maintenance is reduced to a minimum. The most 

 serious disadvantage is the difficulty of securing thoroughly satisfactory work, par- 

 ticularly on the outside surface of the cylinders and at the joints. Unless the mortar 

 is well spaded to the outside so as to afford the necessary protection for the reinforcing 

 steel there is pitting of the surface, particularly between tide levels. In pouring the 

 cylinders the work is carried up to the underside of the deck structure in one operation 

 and completed after the placing of the deck forms. In pouring these long cylinders 

 laitance is apt to collect at the top and unless this is remo\'ed down to solid concrete 

 before the next section is poured, a plane of weakness results. A considerable number 

 of cylinders have been repaired due to these defects Init the fact that large numbers 

 constructed from 9 to 16 years ago are in first class condition proves that satisfactory 

 construction of this type is entirely feasible. 



Reinforced Concrete C.\issons 



The Harbor Board has developed a method of constructing concrete piers in 

 which pre-cast reinforced concrete caissons are placed and filled solid with concrete. 

 The caissons thus ser\-e as watertight forms for the concrete, and afterwards become 

 the permanent outer shells of the piers. 



