structure. The structure should sustain the combination of vertical and lateral loading that produces the maximum shear and bending moments in the structure.
Manholes are necessary in a power distribution system to permit the installation, removal, splicing, and rearrangement of the cables. A manhole is merely a subterranean vault or masonry chamber of sufficient size to permit proper manipulation of the cables. Arranged on the sides of the vault are devices that support the cables.
The location of manholes is determined largely by the layout of the base that is to be supplied with power. Whenever a branch or lateral extends from the main subway, there must be a manhole; and there must be manholes at intersections of subways. In general, cables are not made in lengths exceeding 400 to 600 feet; and as it is necessary to locate splices in manholes, the distance between manholes cannot exceed these values. Furthermore, it is not advisable to pull in long lengths of cable because the mechanical strain on the conductors and sheath may then become too great during the pulling-in process. It is recommended that manholes be located not more than 500 feet apart. The lines should preferably be run straight between manholes.
Manholes are made in many shapes and sizes to meet the ideas of the designer and to satisfy local conditions. An example of a typical manhole is shown in figure 4-39. If there are obstacles at the point where a manhole is to be located, the form of the manhole must be modified to avoid them. The form approximating an ellipse (fig. 4-40) is used so that the cables will not be abruptly bent in turning them around in the manhole. When the rectangular type of manhole is used (fig. 4-41), care must be taken not to bend the cables too sharply.
The size of a manhole will vary with the number of cables to be accommodated; but, in any case, there must be sufficient room to work in the manhole. A 5- by 7-foot manhole is probably as large as will be required in isolated plant work, while a 3- by 4-foot manhole is about as small as should be used. When transformers are located in a manhole, the size should be increased to allow for working space around the transformer and for ventilation. About 2 or 3 feet of volume should be allowed per kilovoltampere of transformer rating.
Manholes are built of either brick or concrete or of both of these materials. When many manholes are to be built of one size and there are no subterranean obstructions, concrete is usually the cheapest and the best material. But when only a few are to be constructed or when there are many obstructions, a manhole with a concrete bottom, brick sides, and a concrete top is probably the best. Such a manhole can be constructed without having to wait for concrete to set before you can remove the forms. There is a growing use of precast concrete manholes that are shipped directly to the project site.
A manhole with brick walls is built by first pouring the concrete floor and then building up the brick walls thereon. If the manhole is large, the roof can be either of steel-reinforced concrete or of brick set between rails. Probably for installations in which only a few manholes are to be built, the brick-between-rails method is the best. For a small manhole, no masonry roof is necessary, as the cast-steel manhole cover forms the roof.
Cement mortar for building brick manholes or for conduit construction can be made by mixing together 1 part of cement, 3 parts of sand, and about 1/3 part of water, all by volume.
A concrete manhole is built by first pouring the concrete floor and then erecting the form for the sides. In a self-supporting soil, the sides ofthe hole constitute the form for the outside of the manhole. If the soil is not self-supporting, there must be an outer form of rough planks (plywood), which is usually left in the ground. Steel reinforcing, such as old rails, must be placed in the concrete top of a large manhole. All reinforcing steel should be completely encased in concrete to prevent corrosion.
Manhole covers should always be made of cast steel and covers should be round so that they cannot drop into the hole accidentally.
So-called watertight covers are seldom used now, as it is not feasible to make a satisfactory watertight cover at reasonable expense. A cover should not be fastened down because if it is and accumulated gas in a manhole explodes, the vault and cover will be shattered. A ventilated cover should be used to allow the escape of gas. The newer types have ventilating slots over approximately 50 percent of their area. Dirt and water will get into the hole, but the dirt can be cleaned out and the water will drain out and no harm will result. If ventilation is not provided, an explosion of gas may occur and do great damage.
When feasible, a sewer connection should lead from the bottom of every manhole. The mouth of the trap should be protected by a strainer made ofContinue Reading