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SPECIAL CONSIDERATIONS Sewers and Culverts Sewers and culverts are designed to carry the design flow and provide sufficient strength to support imposed loads at an economical cost. Hydraulic design requires determination of system type, determination of design flow, selection of pipe size, and determination of flow velocity. Design for safe supporting strength requires determination of earth and live load, selection of bedding, determination of load factor, application of factor of safety, and selection of pipe strength. Design information is presented in the "Concrete Pipe Design Manual", American Concrete Pipe Association; WPCF and ASCE "Manual of Practice on Design and Construction of Sanitary and Storm Sewers"; EPA "Manual of Practice on Sewer System Evaluation, Rehabilitation, and New Construction"; and Design Aids listed on pages 28 through 30 of this booklet. Hydraulic Coefficients of Flow Flow through sewers and culverts is primarily dependent upon the size and slope of the line and the roughness of the pipe wall. By determining these factors in hydraulic equations, engineers are able to design a sewer or culvert to carry the proposed flow. The most important single factor affecting the hydraulic design of sewers or culverts is the roughness of the pipe wall and; therefore, the selection of the friction factor is of great importance. Roughness has a retarding effect and reduces the velocity of flow in the pipe. Friction effects are commonly evaluated by means of the friction factor "n", used in the Manning equation for flow in open channels. Increased friction losses are indicated by high values of "n". Therefore, a pipe with an interior surface which results in a minimum of frictional resistance is necessary for hydraulic efficiency. It has been definitely established by authoritative tests that the roughness coefficient of concrete pipe is equal to or better hydraulically than other pipe materials. Laboratory results indicated the only differences were between smooth wall and rough wall pipes. Rough wall or corrugated pipe have relatively high "n" values which are approximately 2.5 to 3 times those of smooth wall pipe. Test programs have established values of the roughness coefficient for concrete pipe from 0.009 to 0.011. An evaluation of "n" values for different pipe materials is given in the Design Data No. 14 listed on page 29. Recommended values for the roughness coefficient "n" for concrete pipe in different types of pipe systems are as follows: Sanitary Sewers: "n" values of 0.012 to 0.013 are used to account for the possibility of slime or grease build-up and minor head losses due to obstructions such as fittings and manholes. Storm Sewers: "n" values of 0.010 to 0.012 are used to account for foreign debris and minor obstructions in storm sewers. Culverts: "n" values of 0.010 to 0.012 are used to provide for reasonable margin of safety. Industrial Wastes Certain liquid wastes from industrial processes may have various objectionable effects on materials of construction used for sewers, pumping stations, treatment plan equipment and structures, and sewage treatment processes. Obviously such wastes should not be allowed in the sewer except after such pretreatment as will prevent objectionable conditions. The practice is widely established of enforcing regulations against discharge to the sewer of improper wastes. Generally the only waste that directly damages concrete pipe is acid. A common requirement in sewer pollution control regulations is that the pH of a waste discharged to a sewer shall not be lower (more strongly acid) than 5.5. Authorities state that concrete pipe can carry liquid with a pH as low as 4 without harm. Highly alkaline sewage has no adverse effect on concrete pipe. Sewer acid conditions are common to industries using pickling or other acids in plant processes. Such places should be required to neutralize the acid wastes or a protective lining should be provided on the carrier pipe. Such linings should be as recommended by the pipe manufacturer. Manufacturers in most areas can supply concrete pipe with epoxy coatings. Concrete pipe in any well-designed sanitary sewer system will withstand the action of domestic sewage. In sanitary sewers carrying industrial wastes, the dilution of any concentrated acid waste is in general sufficient enough to raise the pH of the mixture so that it will not affect the concrete pipe. Hydrogen Sulfide in Sewers Much has been said and numerous studies have been made relating to the presence of hydrogen sulfide in sewers. The great attention given to hydrogen sulfide is because of its highly objectionable odor, its toxicity, interference with treatment plant operation, and its ability when oxidized to cause corrosion of various materials used in sewer construction. Hydrogen sulfide in amounts sufficient to cause damage is not common in sewers, and even when it is found it is present for only part of the time. Its formation is possible only when certain factors which influence its formation exist, such as conditions which permit sewage to become septic with no or small amount of dissolved oxygen. In the northern half of the United States, sewer failures from sulfide corrosion are almost unknown except in a few cases where damage resulted from discharges from force mains. Sewage temperatures are lower than in more southerly climates, so significant amounts of sulfide are rarely formed in the gravity sewers. Sulfide control is now a well-developed technology. Engineers who are faced with sulfide problems may now apply control procedures which will overcome sulfide producing tendencies in existing systems and provide proper design to minimize problems in the future. Sources of information for sulfide control are provided in the ACPA’s publication "Design Manual, Sulfide and Corrosion Prediction and Control." Load Carrying Capacity The strength of a reinforced concrete pipe is stated in terms of D-load which is the load in pounds per lineal foot per foot of internal diameter. The strength test requirements under the three-edge-bearing method are classified according to the D-load that produces a 0.01 in. crack and the D-load that produces the ultimate load. ASTM or equivalent specifications for reinforced concrete circular, arch, or elliptical pipe have design tables of different strength classifications that give the diameter, wall thickness, compressive strength of the concrete, and the amount of reinforcement required. When concrete pipe is subjected to external loading, resisting stresses induced in the pipe wall are flexural, axial, and diagonal tension. Tensile stresses are developed in the wall on the inside at the crown and invert and on the outside at the springline. Concurrently, compressive stresses are developed in the walls opposite to the tensile stresses. The reinforcing of a concrete pipe basically consists of the placement of steel reinforcement in those zones of the pipe wall where tension stresses exist. Reinforcement in the pipe wall where compression stresses exist is not required but it is used in various methods of reinforcing for ease of placement. When the load carrying capacity of the pipe is controlled by diagonal tension stresses, a common method of resisting these stresses is in the form of stirrups placed radially within the pipe wall at the crown and invert zones of the pipe. Structurally, stirrups resist radial tension and diagonal shear stresses in the concrete pipe wall. Care should be taken when using elliptical, quadrant, or stirrup reinforcement to properly mark the top and bottom of pipe as designated in the appropriate ASTM specifications. Projects
occasionally present unique conditions that require modified and/or
special pipe design. Anticipated loads, stresses and size requirements
exceeding those accommodated by design tables are examples where
modified or special design is warranted and permitted in the ASTM
specifications. Special design of any specific D-load for large
quantities of a pipe size may also be a worthwhile economic
consideration. Public domain software (made available through funding
by the American Concrete Pipe Association and the Federal Highway
Association) is now available. The user friendly programs provide
structural analysis for circular pipe, elliptical pipe and box
culverts. All analysis is in accordance with AASHTO Section 17.4. |Home| |About ICPA| |Meet Our Staff| |Join ICPA|
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