No abstract

The overriding considerations in foundation design are an adequate factor of safety against catastrophic failure, and that total and differential settlements should not affect the functional performance of the structure.2. As a general principle, depending of course on the soils encountered, it is preferable to transfer heavy loads downwards rather than spread them at a higher level by increasing the area of foundation outwards. For the NatWest Tower the loading intensity on the plan area was in excess of 12 t/ft2 (1200 kN/m2) which is far more than London Clay can take.3. In choosing appropriate values of the soils properties, no matter how accurate the results of the site investigation may have been, it has to be remembered that soils are natural materials, subject to considerable variations within quite short distances, and whose engineering properties can change unpredictably after a load has been applied. For these reasons, calculations of settlement, however sophisticated they may claim to be, should be taken with a margin of f 1&25%. In the event, conventional settlement analyses using the Skempton-Bjerrum method based on Terzaghi, differed from finite element analyses by about 5-10%, and by about the same amount from measured values. Some of this last relationship can be explained by differences between theoretical and actual loadings.Professor J. E. Gibson, The City University 6. At the City University we were involved in carrying out tests on the models shown in Figs 17 and 19 of Paper 8673. These tests were to be used to support and verify the design analysis of the main core of the tower. Owing to the complicated Papers published: Proc. Instn Ciu. Engrs, Part 1,1983,74, Aug., 387494. 389 Downloaded by [ RICE UNIVERSITY] on [11/09/16].

There are one or two points of correction and slight elaboration which I would like to make to the text of the Paper. I have been asked questions about equations (7)-(12). These are dimensionally stable provided M is recognized as a unit of mass and that E is recognized as a unit of force. There is, however, a dimensional error in Table 1. The lines for M 1 and 2/pD2 have unfortunately been left in units of weight. The later non-dimensional lines are, of course, correct, because the missing g has been cancelled out on the top and bottom lines, so the lines in which you have probably been most interested are still absolutely correct.107. The values of K,, towards the bottom, have been achieved by using an assumed value for 6 , of 0.07, which was taken at the time to be as good a guess as could be made of a value typical of the sort of structures which we were considering. Mr KingI would like to comment on Fig. 16. While the fundamental pile frequency can be fairly readily calculated the sway loads are more ephemeral. The fundamental sway frequencies are included in the diagram to build up a complete picture. All figures on the outer two lines have been calculated from actual results, but the others, referring to sway, are guesstimates, as is the uncoupled sway frequency in Fig. 15. 109.In 5 85 reference is made to the comparison between the direct determination of the eigenvalues by the QL method and the results of the more elaborate Sturm sequence analysis; Table 2 gives actual figures. The comparison of cols 1 and 2 appeared satisfactory, but at the time we were working fast and did not do a direct comparison of identical cases. The fit is not as good as it might be, particularly on the second mode. The pile mode was not located exactly by the Sturm sequence, hence the query. Since then a colleague, Mr Kent, has done more work (cols 3 and 4) introducing extra degrees of freedom into the centre of the piles. Introducing a horizontal rotational degree of freedom (col. 4) gave a much closer fit. Had we been able to include both 8, and 0, rotations the 'fit' would probably have been exact.110. This points to the danger of oversimplification. While calculations should be simple, if possible, this fact led to a 15% overestimate. If, as in this case, a safe upper bound is to be established, this could be a serious error.

At what stage of the work were the archaeological discoveries made? Presumably there were discussions between the Iraqi Ministry of Construction and the organization responsible for archaeological preservation. It would be of interest to know whether the Engineer was consulted on this matter and, for example, if he was allowed to give consideration to the possible redesigning of the bridge, perhaps by increasing the spans, to avoid destruction of the ancient brickwork. M r P. L. Martin, Rendel, Palmer & TrittonThe foundation design for the bridge was dominated by consideration of the extraordinary scour conditions. Design studies therefore centred on pile capacity under both vertical and lateral load at times of flood. This discussion contribution enlarges on the pile testing which was camed out. 45.The first two pile tests did not confirm either that the permanent piles would have adequate bearing capacity or that settlements under working load would be within acceptable limits. A further test was made. The test had two objectives. The first was to prove the effectiveness of the unorthodox procedure for cleaning the base of the pile shaft. The second was to determine the relationship between end bearing capacity and the standard penetration test data obtained from the site investigations. An extrapolation could then be made to determine base capacity under scour conditions. Ideally this test would have been made on a pile founded at -9.0 m, but, even if skin friction could have been eliminated entirely, it would have been necessary to increase the test load to compensate for the increased overburden pressure (from +5.0 m to river-bed level) over that occurring at maximum scour. Furthermore, a test pile could not by then

The railway loading in this Paper is based on an equivalent loading which was evolved by London Transport in about 1973, and which has now been incorporated, slightly amended, into the loading section of the revised British Standard 153. This bridge is the first railway bridge in the world of this type of span to be designed to this reduced railway loading, applicable to rapid transit systems.67. For impact, the allowance appears to be exactly the same as in the revised BS 153 and the London Transport loading. This impact is less than is allowed on railway bridges elsewhere throughout the world and was based on tests which were carried out on certain London Transport bridges. We reduced our impact loading because our stock is somewhat different from main line stock in that it does not have locomotives generally so far as passenger stock is concerned. In the case of Tyne and Wear (Fig. 3), the axle loading for works vehicles is only about 25% higher than for passenger vehicles. I would like to ask what the operating speeds are and whether any separate calculation was made when determining the impact allowance on static loading.68. From Fig. 7, which shows the general cross-section of the bridge, the deck units appear to be separate troughs carried on the longitudinal stringers. I would like to ask what precautions were made regarding derailment, because it seems to me that if the diesel locomotive became derailed there could be local instability of the deck units, and that is contrary to what is laid down in the new British Standard. Furthermore, I wonder whether there has been as yet any loading test on the bridge or whether there is any plan for a test loading to prove the actual forces in the members.69. Although the works loading is more than the passenger loading, I am surprised that none of the elements is subjected to fatigue loading (4 12) in that fatigue decides the sizes. I wonder if the Authors could say how many passenger trains are expected to cross the bridge each day. In the light of the sort of frequency that we have in London, I would expect that certainly the stringers and some of the shorter loaded members would be determined by fatigue considerations.