I refer to the letter from John Rastall (NCE last week) regarding the Channel Tunnel Rail Link works at St Pancras Station and confirm that the original Barlow trainshed roof is indeed a tied arch structure.
Ironwork arches at approximately 9m centres are tied by the elevated train deck comprising a grillage of wrought iron rivetted girders, supported on cast iron columns at 4.5m centres.
The primary girders run across the station width and every alternate line has additional strengthening plates to take the tie forces.
Investigations and computer modelling indicated that the roof structure, columns and foundations have the capacity to take the future loads, but that the horizontal girders and infill buckle plates of the train/ platform deck do not.
We therefore decided to construct a new 400mm thick reinforced concrete slab above the original girders to form the train and platform structure.
This transmits loads directly into the columns, while providing a stiff horizontal diaphragm plate.
At the springing points each side of the arches a continuous width of the new deck solidly ties the ends of the tie girders and arches. On the western side the horizontal tie forces are transmitted via a series of 'bridges' - the solid bits of the concrete diaphram after we have 'cut out' the light wells.
Around a fifth of the cast iron undercroft columns will be removed, mostly to allow creation of the light wells. Those remaining will continue to support the platform structure above.
Mr Rastall can rest assured that his affection for St Pancras is shared by the engineers and architects of RLE now giving the station new life.
Paul Baldwin (M), senior construction engineer, Rail Link Engineering