Martin Brown GE19 Bridge Displacement Director, Health, Safety and Environment London Rail

Outline Some Background GE 19 and the incident Why it occurred Lessons for Design, Construction and Constructability and Site monitoring Some conclusions and suggested next steps

East London Railway Extension The East London Line Extension, connects the old London Underground East London Line (running from New Cross to Shoreditch, including Marc Brunels Thames Tunnel), part of the existing suburban line to Crystal Palace and Croydon, and some new Infrastructure. Phase 2 is a proposal to link to Clapham Junction GE19

Bridge GE19 Bridge set at 1:30 gradient, falling from west to east Situated near the Broadgate Tower, near to the Liverpool Street Station throat

Bridge GE19 : Plan View Set at 60 degrees to the Great Eastern Main Line, which is a six track Lying almost due West to East

GE19 in numbers Spans 84 metres, with 8 metres deep truss Made from steel weighing 812 tonnes Bridge decked out with 417 Omnia planks, each weighing approximately 100k Approximately 500 tonnes of concrete was cast as counterweight (45% of total length) pre-launch, at the East end The bridge sits on four bearings

What Occurred Launch successfully completed during a possession weekend 3 – 5 th May 2008, landing within 1 mm of design Between 5 th and 21 st May preparation and lowering work was undertaken Mid-May it was realised the bridge had moved, and needed to be plan jacked approximately 38 mm (East to West), against lateral restraints Thermal expansion was about 20mm during day With the plan jack completed, final lowering was scheduled to be completed on 29 th – 30 th May On 28 th May, at 19.15, Bridge dropped 200mm at East End onto the permanent bearing

Bearings Layout At launch, the base of the bridge was 650mm above the East bearing, and 400mm above the West The bridge sat on steel plates on top of the final bearings The bearing set up, provides constrained movement as shown. The West end is fixed The bearing have transport restraints The bearings have smooth finish stainless steel contact surfaces Movement is fixed Lateral movement North-South Longtitudinal movement East-West Free movement

500 tonne Hydraulic Jack, with security collars Top cap of abutment Permanent bearing Lower Member of Bridge GE19 Temporary spacer plates Temporary PTFE Covered plates 3 degree temporary wedge Note the inclination of the bridge has been enhanced for clarity

Some load assumptions The load on the eastern end bearings was about 450 tonnes With the slope about 3.5 degrees, simple assumptions give a resolved horizontal down slope load of about 18 tonnes Again with some assumptions, the breakout friction at the PTFE plate would be 3% to 4%, giving a stable position over some time Cooling temperature change could lead to movement of 2 to 3 mm, converting static to dynamic levels Dynamic friction would be 1% or less The transit bolts would have provided maybe 2 tonnes each However once movement began, the spacers would begin to be squeezed out, and the transit bolts bent. This is what we think happened

Omnia Planks Note this plank shows signs of damage from the bridge displacement Note the lacing in both directions to secure the planks in position, fitted post incident

Omnia planks issues The Omnia planks were held in place by contraband tape, with a filler material in the gaps between planks Shear pins were fitted between the planks, to take up the loads when concreted The planks were not laced, following a value engineering and pre-launch review. There was no secondary means of preventing the planks dropping onto the live railway When the bridge was displaced the planks were subject to a significant movement in some places; 5 fell onto the track, and several others were dislodged The method was based on previous successful installations

Lessons from the Displacement of the Bridge Design of the Temporary Works Lack of explicit standard or work instruction for placement of the PTFE Reduced focus, once launch was complete Design of bridge did not fully encompass constructability of all elements Failure to understand the high consequence/ low probability risks involved

High load sliding elastomeric bearings have been specified for concrete box girders. Preformed fabric is bonded to a steel plate that is recessed to receive a PTFE sliding surface. The top is stainless steel on a steel backing plate. Anchorage to the upper and lower concrete surfaces is with end welded studs. Under these conditions, the following controls are recommended: The preformed fabric should be 2.0 in. thick. The average bearing pressure on the preformed fabric due to dead load and live load without impact should not exceed 2,000 psi. The average bearing pressure on the PTFE surface should not exceed 3,000 psi. Top and bottom surfaces should be level. Concrete or epoxy grout should be cast against the in-place bearing. Bridge Design Manual December 2001 © by Texas Department of Transportation (512)

Conclusions These circumstances were (as far as we can tell) unique If our analysis is right, the potential for failure had not been fully considered before, why was this ? Once explained, everyone felt the risks were obvious. If our analysis is wrong what did happen ? The management of the design, in parcels of work, may have assisted in the lack of design integration Greater use should be made to what if analysis where high potential risk exists, especially where there can be further consequential risk

Next Steps If we are right, we need to change the guidance on how, where (and when) PTFE plates are used in structures For those who sit behind the desk planning and designing, we need to ensure production of the information for those on the tools to act correctly, this is an educational issue We need to reinforce an approach that questions design at a fundamental level, looking at the what if. This is also an educational issue We need to understand more how Human Factors impact on undertaking fundamental and proactive analysis, design checking and site inspection, this may be a research issue

Questions to : Full Report can be found at : networkandservices/networkandservices/eastlondonrailway/2111.aspx or Report.pdf