REINFORCED CONCRETE DAMS Reinforcement in Arch designers have used steel reinforcement in arch clams, where it has seemed desirable to prevent the possible development of cracks, or to give additional security where the uncertainty concerning stresses made tensions seen possible under certain conditions. Cracks may result from changes of temperature when dams are empty are frequently guarded against by using reinforcement, as has been mentioned in the previous articles. The stresses in most of these cases are practically incleter urinate, and the reinforcement is placed according to the judgment of the designer.
These structures are sometimes called reinforced concrete clams in published reports, but are not designed as reinforced structures and are not properly so classed. No fully reinforced arch dams have as yet been constructed, and no dams have been designed in which the stresses have been determined by the use of the theory of the elastic arch.
145. Flat Slab and Buttress Dams.—For dams of moderate height reinforced flat slab and buttress construction has frequently proven economical. In this type of construction the buttresses are usually placed from 12 to 1S feet apart, and the slabs extending be tween buttresses are inclined at an angle of 40° to 4.5° with the ver tical so that the resultant of the normal water pressures passes near the middle of the base of the buttress. Most of the dams of this type in use have been constructed under the patents of the Ambursen Hydraulic Construction Company.
Fig. 79 shows a dam of this type in section through the inclined slab. The loads carried by the slab consist of the normal water pres
sure and the normal component of its own weight. The slab may be designed by the ordinary method for reinforced concrete beams, but the values used for allowable stresses should be very conservative.
The buttress should be made of sufficient width to cause the resultant thrust upon its base to pass approximately through its middle point when fully loaded, and must have sufficient base area to keep the pressure upon the foundation within reasonable limits. Lateral stiffness of the buttresses may be provided by giving them sufficient thickness, and using reinforcement on the sides, or it may be obtained by ties and struts between buttresses.
In many dams of this type, cellular construction is adopted, in which the spaces between buttresses are divided into cells by hori zontal floors, openings through the buttresses providing opportunity to pass under the clam throughout its length. Sometimes vertical walls provide rooms which may be utilized for power house or other purposes.
Slab and buttress dams, like any other masonry dams, require firm foundations. For locations where substantial foundations may be obtained for buttresses on porous material, they possess an advan tage over gravity dams which would be subjected to upward pressure. In these cases it is necessary to provide cut-off walls at the heel of the dam to prevent water passing under and washing out the foundation.