3. POP BOTTLE BRIDGE

Pop Bottle Bridge, which had recently been transferred from the BRB Residuary Body to Lincolnshire County Council, was selected for these tests (details of the bridge are contained in Appendix A). Its construction and previous use make it an ideal representative of British arch bridge stock and the disused and dismantled railway permit easy access for test instrumentation.

An Archtec design, including a full geometric survey had already been prepared to strengthen Pop Bottle Bridge to a live load rating of 40/44 tonnes. Details of the Archtec strengthening are contained in Appendix B.

Pop Bottle Bridge (National Grid reference TF 439 223) is a skewed two-span brick masonry arch bridge. Each span is approximately 5.0m measured in the skew direction and rise at their crowns 2.3m. The barrel is built from three rings of brick with bricks laid to the English or Helicoidal Method and has a skew angle of 25°. The overall barrel thickness is 355mm. The central pier is 800mm wide and approximately 2.1m high. Figure 2.1 shows an elevation and the road above.

Part of side elevation	Road above
Figure 2.1 Pop Bottle Bridge

The bridge carries the B1359, a single two-way carriageway approximately 7.7 m wide, over a disused railway. The carriageway has a grass verge on one side and a footway on the other. The permitted traffic speed limit is 40 mph. There is clear access to the underside although a fence and trees prevent through passage.

Using modified MEXE and mechanism analysis the live load rating of the bridge was originally calculated to be 13 tonnes(2). Later, a Special Assessment was undertaken by Gifford(3) using the DE technique and the rating increased to 40/44 tonnes. However, the presence of four transverse cracks warranted the recommendation that an Archtec strengthening scheme be developed. These four transverse cracks at approximately quarter span locations are shown in Figure 2.2 together with the Archtec bridge survey contours. Cracks are shown in red.

Red lines mark crack positions in the barrel	Plan – Showing carriageway
Figure 2.2 Archtec Survey Contours

The transverse cracks at the quarter points of the arches were of some concern and suggest the beginning of hinging failure at the quarter points due to live loading effects, possibly as a result of frequent heavy longitudinal braking. Archtec strengthening would provide additional capacity to achieve the required load rating and provide stitching across the hinge locations.