Sunday, 17 May 2015

Bearing replacement (2)

The bearings that allow Hammersmith Flyover to expand and contract are situated underneath each of the 15 columns or piers rather than on the top - which not only means that the piers move as well, it also means that the bearings are in a 2m deep pit below the pavement and makes replacing them a very difficult operation.

Construction of a pier pit in 1960
To replace the bearings, it is therefore necessary to firstly lift the pier to remove the old ones. This is easier said than done. Hammersmith Flyover is over 50 years old and the piers are not designed to be lifted. There is a danger that the columns will crack, the pit floor will fail or that the pier will topple over setting off a domino effect and causing the whole bridge to tip over.

So, before any lifting could take place it was necessary to strengthen the base of the pit, by drilling steel rods into the concrete slab, stress more steel bars around and through the base of the pier to prevent it "exploding" apart and also to demolish then rebuilt the top of the pit wall (see below) to allow lateral jacks to be installed to prevent the pier toppling.  All of these operations have taken a year of preparation.

Pier pit wall strengthening

The new bearings are attached to the underside of the piers through large steel plates bolted and grouted to the underneath. As mentioned in an earlier blog, the carrier plates must be installed to a very small tolerance. The two plates have to parallel to less than 1mm. However, "if it can be measured, it can be improved". Using precise Leica theodolites and industrial grade corner cube reflectors, measurements of 0.2mm are achievable. Setting the carrier plates to a tighter tolerance than necessary, maximizes the amount of accuracy that may be lost due to fabrication tolerances.

Installation of the carrier plates
Supplementary jacks at each corner of the pier help to keep the pier level and steady whilst the precise work of setting the carrier plates is undertaken.

Working in tight areas means that Automatic Target Recognition (ATR) is critical. The distances measured are shorter than the minimum normal focusing range of approximately 1.5m. Similarly, ATR does not need ambient light and can be used in dark conditions.

The next step is to pour grout into the gap between the carrier plates and the pier base to ensure full load bearing...

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