Why is the tower at Pisa leaning?

I’ve been playing around with the foundations on a building and it’s getting a bit fraught – and not for engineering reasons. The superstructure has been designed the project engineer yet the foundations are to be contractor designed, hence designed by a second engineer. I find this odd for many reasons. Firstly, the ground conditions are awkward – nothing that cannot be resolved with a bit of clear thinking, but they need to be designed up to a specification rather than down to a price. For instance, we are having to deal with a large number of high point loads, which do not sit very well with the dirt underneath.

The issue with dividing a project into discrete packages is that each designer resolves the problems in front of them. It may well be that the project engineer has arrived at the most efficient superstructure design possible, but if they are not responsible for the foundations the issues that arise in consequence are, quite naturally, of secondary importance.

And what about the figures to which we are to design these foundations? There is no scope for variation in them because the foundation engineer does not know how they were determined. It is not uncommon to require piling contractors to design their piles to loads supplied by me and I can tell you that the last thing I would do is give them the exact figure. If I have determined a pile load of 215.3kN on a particular pile you can expect a pile design load of 250kN.

BS8004 states that the foundations should have an “adequate factor of safety” and those figures quoted in the geotechnical investigation include a factor of safety of 3 (I asked them). Not at all unusual and arguably the industry standard, but there are occasions when a factor of safety of 2.5 can be used. In this particular instance, rationalisation of the design is hampered by the presumably contractual requirement to adopt a ground bearing pressure of 100 kN/sq m for the gravel. Should there be a problem in the future, a clever barrister (who earn far more than a mere structural engineer) could argue that a design bearing pressure of 99.9 is within the contract and 100.1 is not.

Finally, the foundation is not known at tender stage so the contractor needs to complete the design in order to provide a price for a project that they may not win. I have in the past shared risk by scoping the structural design on the basis that a fee will be forthcoming only if the contractor is successful, but in a competitive tender the contractor is trying to save money and the engineer is trying to achieve a safe design. Was the first person to try this type of contract was Diotisalvi when he was designing the campanile at Pisa?

At face value such piecemeal distribution of responsibilities seems replete with advantages. In my experience that advantage is worn away at the point where different responsibilities rub up against each other. If different people design the steel frame and the foundations, who looks after the holding down bolts.

And what happens when it goes wrong and we all end up in front of the beak? You can be reasonably certain that the same expensive barristers will be arguing that the other people are responsible and not his client, who discharged their duties.

Keith