Bridge Safety Assessment

RDS are experts at performing bridge assessments. We have extensive expertise in performing bridge safety assessments using probabilistic methods and extending the service life of infrastructure where safe to do so. Prof O’Connor has used Reliability Theory to justify the retention of many road and railway bridges Internationally that would othersise have to be replaced or repaired.
Prof. Eugene O’Brien is a world-renowned researcher in the area of bridge traffic loading, and calculating accurate loading from bridge Weigh-in-Motion (WIM). Data recorded at the site may be used to identify vehicles and calculate the corresponding load effects on the bridge.
RDS have also worked extensively in bridge health monitoring and can design a monitoring installation to suit a particular concern to give an indication of the condition of the bridge.

Relevant Projects: Swedish Rail, Eurocode Calibration Studies, Railway Bridge Assessments

Bridge Traffic Loading

As safety depends on applied load as well as resistance, a probabilistic assessment of traffic load is an important element in bridge safety assessment – most bridges are never subject to the full code design loading. Assessment of traffic load usually involves an analysis of vehicle weight data (see Weigh-in-Motion).

New bridges are designed for a notional load model. However the actual vehicles crossing a bridge are generally much lighter so bridges are often safer than the code-loading model would suggest. Permit vehicles need to be separated from standard non-permit vehicles. Permit vehicles are better controlled so lower factors of safety can be justified. RDS can identify the vehicles likely to have a permit from the axle configuration which is available in Weigh-in-Motion data.

Relevant Projects: T&E Project, Swedish Rail, Eurocode Calibration Studies

For short-span bridges , safety is governed by extreme examples of individual heavy vehicles.

For long-span bridges , safety is governed by extreme combinations of moderately heavy vehicles, i.e., by traffic congestion events.
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Short-Span Bridge Traffic Loading

  • The weights and dimensions of permit vehicles are specified by the permit issuing authority and may not be factored when assessing a bridge. Non-permit vehicles are much more challenging and require a statistical analysis for accurate evaluation.
  • For short-span bridges, the critical non-permit loading event generally involves very few vehicles. It may consist of just one or two vehicles, either meeting or overtaking one another on the bridge. The governing case depends on the transverse stiffness of the bridge (which determines the extent to which load is shared between the girders).
  • In short-span bridges, the critical case is for free-flowing traffic where bridge dynamics [provide hyperlink to 3.2.4] applies. The characteristic maximum vehicle weight can be found for a specified return period from a statistical analysis of vehicle weights.
  • However, it is more common to use an influence line to calculate the load effects (bending moments or shear forces) due to each passing vehicle and hence to find the characteristic maximum load effect. A commonly assumed return period for bridge assessment is 75 years. A database of Weigh-in-Motion (WIM) data can be used to identify vehicles at the site and calculate the corresponding load effects on the bridge.
  • This can be done directly using the WIM data but there is rarely enough data and a modelling process known as Monte Carlo simulation is usually required.

Relevant Projects: T&E Project, Swedish Rail, Eurocode Calibration Studies

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Long-Span Bridge

Long-Span Bridge Traffic Loading

For longer spans, congested traffic conditions govern where many vehicles are closely spaced on the bridge and no significant dynamics applies. Most Weigh-in-Motion [provide hyperlink to 3.3] (WIM) technologies only operate in free flowing traffic conditions so information on both weights and inter-vehicle gaps are not available for congested conditions.

A conservative solution is to ‘collapse’ the free-flowing WIM data by reducing inter-vehicle (back-axle to front-axle of following vehicle) gaps to a minimum. For example, in the derivation of the Eurocode, a minimum axle-to-axle gap of 5m was assumed.

Relevant Projects: T&E Project, Eurocode Calibration Studies​

Bridge Health Monitoring

When there are safety concerns with bridges, they can be kept in service by installing a monitoring system to keep track of accelerations, rotations, strains, etc. – indicators of its condition. Structural Health Monitoring is challenging as not all sensors will respond to particular types of damage. RDS staff have worked extensively in bridge health monitoring and can design a monitoring installation to suit a particular issue of concern.

Relevant Project:  Swedish Rail
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Bridge Health1
Bridge Health2
BridgeDyn1
BridgeDyn2

Bridge Dynamics and Vehicle/Bridge Dynamic Interaction

Some bridges have particular problems with dynamics, in which case particular investigations may be necessary. For others, the Eurocode allowance for vehicle/bridge dynamic interaction tends to be conservative and evaluating it more accurately can sometimes be a way of demonstrating that the bridge is safe due to that conservatism.

RDS staff have access to a range of dynamic bridge and vehicle models and the skills to do probabilistic studies in order to incorporate dynamic issues into a study of bridge safety. They also have experience of direct measurement of dynamic amplification factors (see figure) which overcomes many of the shortcomings of numerical models.

Relevant Project:  Swedish Rail, SETO