115. It is customary in all large cities to utilize the space under sidewalks for storage and other purposes. Indeed in many cases engines and boilers used for driving machinery or hoisting elevators are placed beneath the sidewalk. Sidewalk vaults, as such spaces are termed, necessitate a wall at the curb line to retain the roadway in place and to hold up the sidewalk.

116. The New York building law requires, "In buildings where the space under the sidewalk is utilized, a sufficient stone or brick wall shall be built to retain the roadway of the street, and the side end or party walls of such building shall extend under the sidewalk of sufficient thickness to such wall. The roofs of all vaults shall be of incombustible material. Openings in the roofs of vaults for the admission of coal or light, shall be covered with lens lights in iron frames, or with iron covers having a rough surface, and rabbeted flush with the sidewalk. Where areas are covered over, iron, or iron and glass combined, stone, or some other incombustible material shall be used, and sufficient strength in such covering shall be provided to insure safety to persons walking on the same, and to carry the loads which may be placed thereon. Open areas shall be properly protected with suitable railings."

117. A method of construction in use where partition walls can be placed under the sidewalk vaults is shown in Fig. 46. The partition walls a are placed about every 10 feet, and are usually 12 inches thick. The outer or street wall is shown at b. It is built in the form of an arch of hard brick laid in cement mortar. These arches are usually 16 inches or two bricks thick, and the "rise," or height of the arch above the springing line, is one-sixth of the span.

Vault Walls And Vaults 48

Fig. 46.

118. When it is desired to have the vault unobstructed by partitions, each sidewalk slab may be supported by an I beam column, as shown in Fig. 47. In this method of construction, the iron columns a support the outer ends of the I beams b, which carry the stone flag or concrete pavement, shown at c. The space between the I beams is filled by the fireproof blocks d. The curb stone is shown at e; when bluestone or granite flags are used, they are usually carried out to the gutter line and project over the curb stone, or, if sufficiently thick, the curb stone is omitted. The wall between the columns is indicated at /, and g is the concrete base on which the columns rest.

Vault Walls And Vaults 49

Fig. 47.

Vault Walls And Vaults 50

Fig. 48.

119. Another method of construction shown in Fig. 48, is used in large cities, where the vault requires light from above. At a is shown the area vault covered with thick glass in iron frames, known as patent or vault lights; b is the longitudinal I beam resting on the iron columns c, and bolted to the sidewalk girders d; e shows the flag or concrete sidewalk; f, the wall at the curb line carrying the ends of the sidewalk beams. This wall is shown in the illustration as built of stone, but brick is often used; both stone and brick should always be laid in cement mortar. At g is shown the floor of the vault, usually made of 3 inches of concrete, covered with 1 inch of Portland cement mortar, made in the proportion of 1 of cement to 1 of sand; h shows the footings under the sidewalk wall and under the iron column carrying the area I beam, and j shows the foundation wall of the building.

120. Sidewalk vaults are either arched over with brick or hollow tile; the top of the arch is leveled off with sand, cinders, or concrete, preferably the latter, and the sidewalk is laid on this; or a sidewalk made of large stone flags may form the roof of the vault. The best stone for this purpose is North River bluestone flag, though any compact limestone will answer the purpose. Granite is also used, but wears so smooth as to be objectionable in winter. The joints of the stone are closely fitted, and often rebated; then they are caulked with oakum, which is forced in, to within about 2 inches of the top, and the remaining space is filled with hot asphalt or asphaltic mastic. The joints will require cleaning out, and refilling, every few years, if they are to be kept water-tight.

If brick or terra-cotta arches are used, and a brick pavement is laid on them in sand, the top of the arch should be coated with hot asphalt.

121. Vaults are sometimes extended out beyond the curb line, under the street. This arrangement does not differ from that shown in Fig. 48, except that another line of columns and girders is placed under the curb line.

Sidewalk vaults are sometimes carried 25 feet below curb lines, and have two stories under the sidewalk, but the construction does not differ materially from that already given. In nearly all the large cities, special permits are required for such kinds of vaults.