Download: ENGINEERING MONOGRAPH No. 6, “STRESS ANALYSIS of CONCRETE PIPE”

ENGINEERING MONOGRAPHS are published in limited editions for the technical staff of the Bureau of Reclamation and interested technical circles in government and private agencies. Their purpose is to record developments, innovations, and progress in the engineering and scientific techniques and practices that are employed in the planning, design, construction, and operation of Reclamation structures and equipment. Copies may be obtained from the Bureau of Reclamation, Denver Federal Center, Denver, Colorado, …

Stress analysis of concrete pipe is in part based on the assumed distribution of earth pressures. Most commonly it has been assumed that the vertical earth load is uniformly distributed over the horizontal width of the pipe and that the lateral load is exerted so that the force diagram is trapezoidal in shape, applied on both sides, and extending the full height of the pipe. Further, it has been assumed that the reaction due to these loads is uniformly distributed over the full width of that portion of the bottom of the pipe in contact with the supporting &ace. That these assumptions are rather arbitrary has long been recognized and efforts have been made to develop assumptions that would agree more closely with tests and with the general fund of knowledge about soil mechanics. In 1930, the late Dr. Anson Marston, director of the Iowa Engineering Experiment Station at Iowa State College, advanced the theory that earth pressures on a rigid conduit and the reactions to those pressures would be exerted in such a manner that the force diagram representing the pressures would have characteristic bulb-like shapes above and below the outline of the pipe. Dr. Marston’s theory was confirmed and extended by an investigation carried out at the Iowa Engineering Experiment Station and reported by IvL G. Spangler of the Station staff. While the design of the concrete pipe for the first section of the Salt Lake Aqueduct was being developed in 1938, W. A. Larsen and H. W. Birkeland, Bureau of Reclamation engineers under the direction of C. P. Vetter, who was in charge of the design, developed an analysis based on the assumption of bulb-like distribution of earth loads and soil reactions. This assumption closely approximates actual conditions as disclosed by tests. Since 1938, a number of other pipe lines have been designed, and the original analysis has been expanded so that various widths of bedding of the pipe may be analyzed. In the original analysis the least work method by summation was used, but in subsequent studies a mathematical solution using integral equations was developed and is described in this monograph STRESS ANALYSIS OF CONCRETE PIPE General Under ordinary conditions a pipe placed under a fill is subjected to loads due to earth pressure around the pipe, the dead load of the pipe itself, and the internal hydrostatic pressure. For convenience of design the hydrostatic pressure is divided into two parts: (a) that part producing a uniform internal pressure, of which the head H is measured from the hydraulic gradient to the top of the inside of the pipe; and (b) the remaining part of the pressure of which the head is measured from the top to the bottom of the inside of the pipe. The uniform internal pressure, part (a), produces only uniform tension around the pipe and is equal to 62.4 Hro, where r. is the inside radius of the pipe in feet, and H is the head in feet measured as stated above. The remaining part of the hydrostatic pressure produces bending, direct, and shear stresses in the pipe. This part is designated as water load and will be discussed in combination with earth load and dead load, Earth Load, Dead Load, and Water Load The assumptions for the distribution of earth pressures are based on results obtained from tests as summarized in Bulletin 112 of the Iowa Engineering Experiment Station. Figure 1 is developed from data in this bulletin and shows the distribution of earth pressures around a rigid pipe placed on compacted bacuill, representing the ordinary condition of a pipe…

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