I love the idea of glazing the whole of the St Pancras station roof (CTRL supplement NCE 28 June). We were not able to take such bold steps when the station was last refurbished some 20 years ago and it will be an improvement on the dark shed we have now.
But I trust engineers and architects understand that the roof structure is a tied arch using the floor as the tie. The picture shows a large number of significant longitudinal openings in the floor which will sever these ties.
The text also suggests that the whole of the undercroft area will be opened out to form a ticket hall and other facilities.
This will remove the support to the floor!
The present floor is, as your article correctly says, a suspended structure supported at close centres on columns. It is a grid of riveted girders supporting buckle plates designed to create a very stiff construction to support the railway tracks. These were converted to the present concrete slab form around 1979 to spread loads and reduce maintenance.
I imagine it will be quite easy to contain the sidethrust of the roof arches into the new concrete box if this extends for the whole area under the station roof. But the first of the openings for the new escalators has been started within the blue painted hoardings in the carriage road between platforms four and five. I do hope they know what they're doing!
John Rastall (M), john. rastall@tesco. net