Discussion. The Design of a High Pressure Pipe Bridge Over the River Sutlej in West Pakistan.

Abstract

The Authors, in introducing their Paper, apologized for certain omissions and errata.On p. 14, 3 38, reference was made to the 1952 scheme. This, of course, should read 1962. Owing to a misunderstanding, Fig. 1 was incorrectly titled. The caption should have been 'Sketch map of West Pakistan'.66. The text explained how annual aerial surveys of the river were taken. These surveys extended over a length of about 20 miles. The Authors had hoped to show a photomosaic prepared from the 1965 survey to illustrate the topography. Unfortunately, this had not come out sufficiently well to project successfully. However, a tracing made from the 1965 dry season configuration was reproduced in Fig. 17 from which the extremely meandering nature of the river could be seen.67. In 1965, the cut-off shown at the western end of the river had formed. As a result, a local steepening of the hydraulic gradient had occurred causing changes in velocity which would affect the erosion rates at the crossing site during the subsequent flood. Such information and similar data to be gleaned from the aerial surveys was very useful to the Irrigation Research Authorities in Lahore who, as described in the text, made annual forecasts, for the next flood, of the likely behaviour of the river at the bridge site.68. Erosion at the crossing site since 1961 was shown in Fig. 18. It could be seen that towards the end of the 1966 floods, some 1200 ft of bridge still remained on the south side of the river. However, during the period from 1961, the largest flow was only 130 000 cusecs, nowhere near the design flow for the bridge crossing. It might well be that due to the combined effects of the Bahakra Dam and the Indus Water Scheme generally, the flow might never reach the design value.69. To conserve space, one or two matters had been omitted from the Paper. One perhaps of interest, was the subject of aerodynamic stability. It was well-known that cylindrical objects in a stream flow could be excited into motion if the damping were sufficiently low and the periodicity of eddy shedding corresponded with the natural frequency of the structure.70. The Authors, of course, were fully aware of this, when the Sutlej Bridge was designed. However, it was anticipated, by analogy with the aerodynamic behaviour of lattice trusses in suspension bridges, that the lattice form of the tubular structure would break the rhythm of eddy shedding, thus removing the cause of excitation even though damping might be very low.71. However, in May 1964, when the structure was not quite complete, the bridge was observed to be making oscillations hardly perceptible to the naked eye, in a very gentle wind. A check with a theodolite showed an amplitude of about l cm at midspan with a frequency of about 1.5 C/S.