Frequently  Asked  Questions

Back to Bridges     Back to Home Page


What is in this page?  Not very much at present, but more will be added.  The questions in this page are ones that have been sent to Brantacan most often.  They are not always easy to answer.


What makes a bridge hold up?

This is either easy to answer or difficult to answer, depending on the kind of answer you are seeking.

The easy answer is -

A     The designers designed it to hold up under the likely conditions

       at the site, and they got it right.

B     The construction procedure was designed correctly, and the

       construction process was adequately supervised and executed.

       Construction includes the preparation of all the parts and the

       materials from which they are made.

C     The maintenance schedule was well planned and correctly

       carried out.

This all seems obvious, but there have been famous examples in which A, B or C was not done right, resulting in problems, or even collapses.

An apparently different answer for A concerns energy.  The lowest possible energy state of a bridge occurs when it is lying on the bottom of the river, or of the valley, having collapsed.  One job of the designer is to ensure that between this condition and the actual state of the bridge as built, there are energy states so high that they will never be reached under any foreseeable loads.  This has to be the case both for the whole bridge and for every individual part that isn't redundant.

This can be symbolized by a simple graph, showing the energy of a system versus the force applied.

The graph above applies to any linear, elastic structure.  But what happens if we apply more force?

The difference here is that when a certain critical condition is reached, something breaks or buckles and the structure collapses, and the energy plummets down on one of the vertical lines.   This happened to the first Quebec bridge during construction.  In the case of the first Tay bridge, the structure actually got built, but the effect of strong winds had been greatly underestimated, and the construction had not been performed adequately.

The red line has been drawn as a possible limit to the load, giving a safety factor.  The structure would be designed so that no foreseeable loading could make the energy go past the red line.  The curve is symmetrical in this example.  In the case of the Tay bridge, the wind could have blown from either side, with the same disastrous result.

For actual live loads, such as traffic, the loading would seem to be always be in the same direction.  This is not necessarily true of every individual member.  It is even possible that the stress in a member could change from compressive to tensile as a load moved, or a bending or a torsion could change direction.  In steady flight, the wings of an aircraft are always pushed upwards, but on the ground, the wings hang down.


What is the best kind of bridge?

What is the strongest kind of bridge?

If you need build across a deep valley, a simple crossing could be made by building an embankment.  If you do it properly, it will be very strong.  This is, of course, seldom practical, as there is usually a river, a road or a railway in the valley.

So we have to build a bridge.  The type of bridge will depend on the length of the longest gap that has to be cleared by a single span, on the type and volume of traffic, on the type of ground, on environmental considerations, on the available budget, and on the relative cost of different designs.

The relative costs of different designs does not necessarily remain the same under a change of scale.  If you scaled the Forth railway bridge down by a factor of ten in length, you would have the same number of parts, but vast numbers of them would be ridiculous complications.  If you look at a very short truss, all the members will almost certainly be very simple struts and ties, but if you look at a really large truss bridge, many of the members will structures in their own right, such as trusses, or tubes or box girders.

The great variety of bridges tells us that there is more to the art of bridge building than span and load.  Technical and aesthetic change, for example, must be taken into account.  Travel north from London on the M1 motorway and you travel in time.  Look at the Forth railway bridge and the Quebec bridge.  Why has this type of bridge not been repeated on that grand scale?  Was it because railways had reached their peak of expansion?  Was it because there were no more wide and deep gaps for them to cross?  Was it because better and cheaper types of bridges were created?




===Back to Bridges     Back to Home Page