9. FAILURE BEHAVIOUR AND SERVICEABILITY

9.1 Failure Behaviour

The TRL tests confirmed that arches strengthened by the Archtec method ‘failed in gradual but ductile manner’ (20,21). Strengthened arch barrels crack similarly to unstrengthened barrels and the first signs of cracking occur at similar proportions of their ultimate failure loads. Typical results are summarised in Table 9.1. Peak load capacity is achieved in both the unstrengthened and strengthened arches at similar deflections. Like reinforced concrete, the introduction of reinforcement increases overall ductility of the strengthened arches and severe overloading results in progressive cracking and distortion and not sudden collapse.

Table 9.1 Development of Barrel Cracking with Load

The main observations are:

1. Progressive development of cracks and hinges occur for both the unstrengthened arch and the strengthened arch.
    
2. The first significant cracks were observed at approximately 1/3 the failure load for the unstrengthened arch and at just under 1/4 the failure load for the strengthened case.
   
3. A failure mechanism occurs in the unstrengthened arch immediately after hinges 3 and 4 are formed, whereas in the strengthened case the arch barrel continues to carry load; more warning is given of an imminent failure mechanism.

Figure 9.1 contains a series of photographs taken during the second Archtec test indicating the high degree of ductility exhibited prior to final collapse. In each case the arch remained stable under at least its self weight and further load/displacement had to be applied by the jack to cause further distress.

In addition, it should be noted that design in accordance with the relevant standards(5) results in an ultimate capacity of the arch well in excess of double the maximum envisaged service load.

Figure 9.1 Failure of strengthened arch observed in the second Archtec test at TRL
(Note that between each stage the arch remains stable under a minimum of its own self weight and further load/displacement had to be applied via the jack to progress the failure)

9.2 Serviceability

No clear definition of serviceability exists for masonry arches(22). Deflections and cracking behaviour is normally used to define a serviceability limit state. However, in arches these quantities are generally small and very difficult to detect under expected service loads and they cannot be calculated by conventional structural analysis. However, results from monotonic and cyclic load tests have been used to derive masonry stress limits in terms of a limiting factor of the ultimate capacity below which permanent damage does not occur from repeated loading.

Based on work done by TRL in the 1980’s, the Highways Agency(5) (United Kingdom) assessment standards for arches are based on serviceability being maintained provided applied loads do not exceed half the ultimate capacity. Cyclic loading on bridge piers has been investigated by British Rail Research(23) and some progress made in linking fatigue of brickwork with a serviceability limit state. It was concluded that, for dry brickwork, if applied loads do not exceed half the ultimate capacity an infinite number of load cycles could be sustained. However, for saturated brickwork lower load levels are required.

Both observations of monotonic loading and cyclic loading have lead to the recommendation of a 50% rule and are in effect stress limit based. The current Archtec design method, based on the Highways Agency(5) 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, the Finite/Discrete Element 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, see Figure 9.2. Corresponding structural analysis of the test arches predicts compressive stresses in the Archtec strengthened arch are lower than the unstrengthened arch under the same loading, see Figure 9.3. For example, at the maximum service load (refer to Appendix B in the earlier Gifford report(1)), the maximum compressive stress in the masonry at the load line is reduced from 1.3 MN/m2 to 1.1 MN/m2; a reduction of 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 strengthened bridges has a beneficial effect on serviceability.

It is understood that some clients are concerned that specific deteriorated conditions in arch barrels, such as loose bricks, could be exacerbated by strengthening. The risk here is that debris falling from a bridge would represent an unacceptable hazard. Arguably, an example of an arch barrel in a weakened condition that could develop loose bricks as a result of partial ring separation was one of the series of LINK arches tested(11), test ref. 2 in Table 9.2. Displacement results for this test are included in Figure 9.2. They 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.

Figure 9.2 Test Load versus Displacement in Barrel at Load Position

Figure 9.3 Predicted Principal Compressive Stresses in Masonry
Under line load, close to extrados at hinge 1 position

Bridge owners and experts in the field recognise the desirability of further research with respect to the serviceability limit state in arch bridges and it is understood that the CSS and Bridge Owners Forum, amongst others, are currently looking in to this. However, at the current time no specific guidance or criteria exist with respect to explicit evaluation of the serviceability state in arches.

Table 9.2 Comparison of Ultimate Failure Loads in the TRL Tests

To provide increased confidence that the serviceability of a bridge would not be compromised by Archtec strengthening additional checks can be introduced into the design process with the Client’s agreement. It has been proposed that the following additional serviceability criteria are included in the design process for Network Rail bridges;

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 will be checked to ensure they do not exceed those in the unstrengthened bridge under existing live loading. Strains would need to be 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 the limiting criteria, which is beyond the scope of a single organisation.