COLUMN HEAD failure is emerging as the most likely cause of last Sunday's fatal terminal collapse at Paris Charles de Gaulle Airport, structural engineers said this week.
Engineers said that from initial inspections it seemed likely that collapse of the 32m wide concrete, steel and glass roof was caused by failure in the concrete edge supports rather than of the roof structure itself.
Investigations will now almost certainly examine remedial strengthening work carried out in 2002 on 20 of the column heads supporting the roof. This was needed after cracking was noticed in the column heads following erection of the first vault sections (NCE 19 September 2002).
The collapse happened at 7am on Sunday morning. Four people were killed and hundreds injured when a 24m wide section of the roof covering the airport's newly opened Terminal 2E crashed down almost without warning onto an internal concourse.
The landmark building opened just 11 months ago. It was designed an in-house architectural and engineering team at operator Aeroports de Paris (ADP).
The terminal is a precast reinforced concrete vault mounted on columns, enclosing a two-storey portal frame concourse structure (see box, page 6). Vault and concourse are structurally independent.
The collapsed section of roof consisted of six precast rings each made up of three sections stitched together with insitu concrete. It was one of only two sections broken into by escalator connections from the main airport building.
Early indications suggest that collapse was most likely triggered by failure either of concrete in the column heads supporting the vault roof, or in the bolts connecting silicone bearings to the vault edge and column heads.
It is known that operator ADP and contractor GTM opted to wrap the top 300mm of 20 columns with carbon fibre external reinforcement in 2002 after cracks were discovered soon after the roof was positioned.
This was designed to help contain unexpectedly high bursting forces and prevent the column shoulders shearing off.
It was eventually decided to wrap all of the 170 columns as a precaution.
Suggestion by structural engineers this week that failure in this edge support system is to blame for the collapse is based on photographic evidence showing the collapsed structure with vault shell largely intact.
'The rings making up the vault shell have largely maintained their shape, even though it has dropped to the ground, ' said Atkins head of structural design Mike Otlet.
'If the collapse had started in the vault you would expect to see the rings in pieces.'
Had they failed, the precast vault elements would have mainly fallen inward, with the springing points of the vault left resting on the column heads or falling inside the columns.
Otlet's view was backed by ICE vice president, Babtie director Gordon Masterton and WSP group technical coordinator Stuart Alexander.
'French investigators should take a close look at the means of support, ' said Masterton.
'Eye witness reports suggest that the shell was deforming for about an hour before the collapse which suggests the edges were spreading, ' he added. Passengers saw longitudinal cracks open up in the crown of the vault, prompting evacuation of the terminal.
Once one the column heads had failed, loads would have passed to adjacent columns via longitudinal vault edge beams.
'If there was a systemic problem with the column head, design or construction you would see progressive failure taking place pretty rapidly, ' Otlet cautioned.
WSP's Alexander also said that design of the column heads was a likely cause for concern:
The vault edges bear onto the outer corner of the columns.
'You'd normally place loads directly over the centre of the columns, ' he warned pointing out that the eccentric loading would have aggravated stress regimes normally found in reinforced concrete pier structures.
Under sustained loading concrete performs to only 70% of its design strength, Alexander said. Meanwhile, if point loads are too large concrete is susceptible to splitting.
It is thought that the column heads were subject to considerable movement from the large vault roof, which may have contributed to fatigue-related failure. The roof itself was designed to cope with 80mm of lateral movement due to wind and snow loading, and with 20mm of thermal movement.
'That would contribute to considerable rocking of the vault back and forth on the column heads, ' said Alexander.
'They would be small movements, but relentless.'