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Forces of habit


I would like to expand on a possible cause of 'column head failure', identified in your 27 May Paris airport report as the likely cause of the vault collapsing.

The chief architect apparently thought of the arch as a 'tunnel on columns'.

It seems he was misled into thinking that the feet of the arch would exert inward force on the columns. This could explain the design detail whereby the vault edges bear into a rebate on the outer corner of the column heads.

Unlike a tunnel, the vault will exert outward force on the columns despite the inward slope of the lower part of the vault. The vertical face of the rebate would not act as a restraint.

This would explain the 'unexpected bursting forces' which caused the column shoulders to shear during construction. The carbon fibre bandages applied to the tops of the columns as a remedial measure would not have contributed significantly to the shear resistance of the outer shoulders.

I wonder if the architect's concept caused the designers to inadvertently misinterpret the direction of the horizontal thrust on the columns calculated by the inevitable computer analysis?

Bob Johnson, Chaddleworth, Newbury, bob.

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