It is interesting to note that a situation similar to the arch roof tie found in St Pancras Station and described in John Rastall's letter arose in the design of the Sydney Opera House.
The dead weight of the Opera House roof had to be much greater than that of the St Pancras roof. However, the problem of providing tie resistance against the thrust of the roof over areas of concrete floor pierced with openings was identical.
Design calculations showed that steel ties were impractical.
Instead, prestressed ties, cast free to slide over the auditorium floor and stressed before the arch pedestal units, were used.
This principle partly resolved the problem but where openings would have fouled straight concrete ties, the geometry was generated as trusses and frames in prestressed concrete avoiding these obstructions.
Perhaps John Rastall's fears could be allayed by passing this information on to the engineers and architects for the present refurbishment of St Pancras.
Michael Johns (M), 16 Winton Crescent, Croxley Green, Richmansworth, Herts, WD3 3QX