Since my last letter to NCE (NCE 29 November) regarding the bolting together of the external structural column panels, I have read the full report of the US Federal Emergency Management Agency, and your various articles since, including the latest (NCE 5 September).
You refer to the need for 'hardening of facades', 'increasing ductility of the building frame especially its connections', and 'to modify connection design to give more capacity for energy absorption' as desirable improvements.
During construction of the World Trade Center, external columns were pre-assembled into sets of three in two or three storey height units, with panel top and bottom joints staggered, presumably to give some continuity up the faþade.
Photographs of the collapse showed that these were fixed together with four and sometimes six bolts through the end plates, and that these bolts failed, allowing the panels to separate. This is similar to Ronan Point, where the panel connections parted.
My intuition would have been to make these end connections equal in strength to the column elements they connected, as a matter of common engineering prudence. If so, resistance to the initial impact of the aircraft would have been very different indeed - possibly enough to have prevented penetration and to have shed most of the aircraft to the ground.
Should some responsible body now simulate the effect to establish whether this simple prudence would have avoided the disaster?
KR Rollinson, Rollinson Consultants, Gloucestershire GL7 5NY