Info

0 50 100 150 200

Total Load (kN]

Strengthened, Archtcc tesl t -LIn-strengthened tu y bonded |

0 50 100 150 200

Total Load (kN]

Strengthened, Archtcc tesl t -LIn-strengthened tu y bonded |

Figure 10. Comparison of maximum predicted compressive stresses between strengthened and unstrengthened bonded rings.

Both observations of monotonic loading and cyclic loading have led to the recommendation of a 50% rule and are in effect stress limit based. The current Archtec design method, which uses load factors based on the UK Highways Agency standards, embraces the serviceability limit state implicitly within the load and material factors used at the ultimate limit state. Whilst this method is consistent with current practice, FDEM analysis used in the design of Archtec strengthening enables the behaviour of the arch under serviceability loading to be investigated in ways never before possible.

Comparison of results from the unstrengthened and Archtec tests show that under identical loads, displacements are very similar. Corresponding structural analysis of the test arches predicts compressive stresses in the Archtec strengthened arch that are lower than the unstrengthened arch under the same loading. For example, using the bridge proportions of the Archtec tests and UK highway 40/44 tonne vehicle axle loading, under the maximum service load the maximum compressive stress in the masonry barrel was reduced by approximately 15%. The reduction in stress is due to the fact that the strengthening introduces bending capacity into the arch barrel, which can therefore resist the applied loading at the critical points more effectively. Hence, on the basis that serviceability can be defined by a stress limit, the reduction of stress levels in the masonry in Archtec strengthened bridges has a beneficial effect on serviceability.

Other aspects of bridge serviceability might be concerned with specific deteriorated conditions in arch barrels, such as loose bricks and ring separation. The risk here is that debris falling from a bridge would represent an unacceptable hazard. An example of an arch barrel in a weakened condition that could develop loose bricks as a result of partial ring separation has been tested and used in comparison with Archtec. Displacement results show that Archtec strengthening significantly increases the stiffness of the ring separated barrel restoring it to that of the fully bonded case (as-built condition). The implication is that strains in the intrados have been reduced and the risk of bricks loosening is thereby also reduced. Provided an arch is maintained in reasonable condition the risk of bricks loosening should be reduced compared to an unstrengthened arch. There is also no reason to doubt that similar trends in behaviour will occur if the inner ring itself is in a deteriorated condition.

Bridge owners and experts in the field recognise the desirability of further research with respect to the serviceability limit state and phenomena such as masonry fatigue. However, at the current time no specific guidance or criteria exist with respect to explicit evaluation of the serviceability state in arches.

To provide increased confidence that the serviceability of a bridge is being improved by Archtec strengthening additional checks have been introduced into the design process. As a precautionary measure in the absence of other guidance, the following additional serviceability criteria are included in the design process:

1 Either check that stresses under the required live loading do not exceed those in the unstrengthened bridge under existing live loading, or alternatively check that stresses in the strengthened bridge are below an agreed serviceability limit state value.

2 To be sure that existing defects are not made worse, or for that matter introduced into arch barrels by Archtec strengthening, strains along the intrados under the required live loading are checked to ensure they do not exceed those in the unstrength-ened bridge under existing live loading. Strains are calculated over a reasonable length so that an estimate of radial joint cracking, critical to loosening of bricks, is included.

These criteria are considered very conservative and stresses and strains beyond these limits may be quite safe and have no adverse serviceability effects. However, further fundamental research is required to establish appropriate limiting criteria.

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