4. TEST ARRANGEMENTS

4.1 Overview

Pop Bottle Bridge was subjected to Supplementary Load tests both before and after strengthening with the Archtec system.

The tests were carried out in accordance with the guiding philosophy set out in BA 57/94 – Load Testing for Bridge Assessments.

The bridge and strengthening anchors were instrumented, as indicated in Figure 4.1 to monitor the following key parameters;

1. Vertical displacements at quarter points using LVDTs.
    
2. Strains around the intrados of the arch barrel over 500mm gauge length using VW gauges with extensions.
   
3. Displacement transducers at locations of specific cracks around the intrados.
    
4. Electrical Resistance strain gauges attached to the strengthening anchors.

The bridge was closed to general traffic on the occasion of each test and the loading comprised two, ballasted, two axle lorries (18 tonne gross weight, 11.5 tonne on rear axle).

The first test, on the unstrengthened bridge, was carried out on the 12 January 2004 and the second test, following completion of the strengthening, on the 1 March 2004.

Invitations to witness the tests were extended to interested parties and a number of third parties were represented at the second test.

Figure 4.1 Instrumentation

4.2 Instrumentation

The instrumentation outlined above was installed on two longitudinal sections through the bridge as indicated in Figure 4.3. The instrumented sections were aligned under the centre of each of the two loaded lanes. Full details of instrumentation arrangements are contained on the drawing included in Appendix C and illustrated in Figures 4.2.

VW with 500mm extension

Displacement transducers at specific crack locations

Figure 4.3 Plan of Bridge showing Instrumented Longitudinal Sections

The instrumentation that was installed is summarised in Table 4.1.

Table 4.1 Summary of Instrumentation

Each part of the instrumentation is described in more detail in the following sections.

4.2.1 Displacement Measurements

Live load vertical displacements at each 1/4 span position were measured on two longitudinal sections for each span (eight gauges in total). Wire operated LVDTs were used with a potential reading accuracy of 0.001mm. However, after reviewing the data recorded during the first test, noting the very small displacements, and the likelihood of accidental movement of the support poles, these measurements were abandoned for the second test.

4.2.2 Intrados Strain Measurement

Eighteen VW strain gauges with extensions to provide a gauge length of 0.5m were set out on the intrados of each arch on each instrumented section as indicated in Figure 4.1. All 72 gauges were read for each live load position to build up a picture of intrados macro strains (average strains across bricks and mortar joints).

4.2.3 Crack Monitoring

As explained in Section 3.0 four notable transverse cracks exist across the full width of the intrados of the bridge and displacement transducers were installed across each of these cracks close to both the instrumented sections.

4.2.4 Anchor Instrumentation

ERS (Electrical resistance strain) gauges were attached to eight of the retrofitted anchors on or close to the instrumented sections. Fourteen gauges in pairs were positioned on each anchor at seven locations along the anchors so that one pair of gauges were at the centre of the anchor closest to the intrados and the remaining pairs at the centres of the three brick rings, see Figure 4.1. Gauges were mounted in pairs so that some redundancy existed should gauges be damaged during installation.

Although the primary role of the instrumented anchors was to measure anchor strains during the load test the wiring to these gauges has been left in place and they could potentially be monitored again in the future.

4.3 Loading

The loads were applied using two ballasted two axle lorries (18 tonne gross weight, similar to vehicle reference RE as defined in Annex D of BD 21/01). Only the rear axle, with a nominal weight of 11.5 tonnes, was of interest in each case. The front axles were far enough away to have no significant effect. Each axle load in the two axle vehicle is statically determinant and, as the axle loads were not varied in the tests they were measured before each test on a Department of Transport weigh bridge.

Trucks side by side – LC7	Positioning of 11.5 tonne axle
Figure 4.3 Application Of Test Loads Using Ballasted Lorries

The lorries were positioned in a variety of locations along the two instrumented lines, including cases where both vehicles were maintained effectively in line and others with only one vehicle on the bridge to provide data in relation to transverse distribution. In total 28 load cases were applied to both the unstrengthened and strengthened bridge.

Photographs of typical load case arrangements are shown in Figure 4.3. Full details of all Test Load positions are provided in Appendix D.

This report focuses on the six most extreme load cases where the largest measured displacements and strains were recorded. These cases are those with two vehicles positioned side by side with the 11.5 tonne axles lined up in turn with span quarter points, (ie North span first quarter, mid span, second quarter and ditto South span) see Figures 4.3 and 4.4 for representative cases.