The relationship between historical and contemporary structures
The main hall building for the 1981 International Garden Exhibition at Liverpool by Arups is a fine three pinned trussed steel arch. The ancestry and inspiration for this building can be traced back around a hundred years to buildings like the Galerie de Machines and railway halls in Britain and France of the same period.
In particular the trussed arch structure with tie rods below floor level to resist outward thrusts is common to both the Liverpool Hall and St Pancras Station (1868). With the advent of improved steels and better analytical techniques the structure has become lighter; the joints are more refined; welding on-site and off-site has led to cleaner lines; and polycarbonate and profiled aluminium have replaced the glass as cladding. But nevertheless the kinship is undeniably there.
The composite trusses by Polonceau mentioned earlier use an idea that is widely used today: large cross section timber elements in compression and small cross section (iron or) steel elements in tension.
We could also consider the composite timber and iron roof trusses developed in the early 1800s such as those by Polonceau in France. The rafter (compression and bending elements) in these composite structures might be in timber. Being weaker than wrought iron the cross sectional sizes of these timber elements had to be relatively large. This meant that these elements had good resistance to buckling whereas the tension elements of the truss could be made in thinner, stronger material: wrought iron.
The rationale behind this kind of truss still holds well today. A good example of a recent structure relying on the same principles is the replacement roof at St Luke's Church, Crosby (Saunders/Curtin, 1976) Hardwood compression elements combine with steel tension rods to form the trusses which are in turn covered by timber boarding.
