With the cost of building an average 100-m-long and 14-m-wide bridge having increased to R20 000 per square metre, it is important that emerging countries such as South Africa consider how they can preserve the asset lives of bridges.

To avoid the R28-milion replacement cost of a typical bridge, consultancy Zutari associate and Stellenbosch University Professor Pierre van der Spuy says a new way of conducting bridge inspection using a reliability-based methodology can preserve assets that would otherwise need to be strengthened or replaced.

He says it is widely accepted that the replacement, strengthening and rehabilitation cost of bridges needs to be minimised, especially as many bridges are nearing the end of their service lives.

The design life of bridges is typically set at 100 years according to global standards.

Canada, the US, the UK, Denmark and Switzerland are among the countries that have developed guidelines for bridge assessment to preserve asset lives.

In South Africa, structures are currently assessed using the codified traffic loads which is often significantly greater than the actual traffic loading on a bridge. This approach is conservative and leads to costly repair and or strengthening measures.

By assessing a bridge for its remaining service life, a much less conservative solution can be developed.

Van der Spuy says a bridge that is 50 years old should not be assessed as if it should operate for 100 more years, nor as if it is operating at the worst load case, especially in rural areas. He explains that small bridges are often assessed according to the maximum loading, which unnecessarily increases the strengthening cost as a consequence.

Conservatism can be reduced by reducing the target reliability index for existing bridges, the use of advanced calculation procedures such as non-linear finite element analysis, measuring the actual traffic loads on a bridge, testing on in-situ material properties for increased certainty and load testing of a structure as a last port of call.

The reliability index is denoted as a measure of the probability of failure of a structure in a given reference period, taking into account the age of a bridge and actual load, as well as other factors that impact probability of failure.

In South Africa, a 3.5 design value translates to a return period of 50 years for new structures before it will ultimately fail. In Europe, a 3.8 design value translates to a return period of 160 years, bearing in mind that the safety margin for new bridges in Europe is an order of magnitude larger than that for South Africa.

Van der Spuy says that, by accepting a lower reliability index for assessment, it directly reduces the partial factor and leads to a more favourable assessment for an existing bridge.

DRONE INSPECTIONS

Council for Scientific and Industrial Research (CSIR) principal civil engineer Michael Roux, meanwhile, says that a new methodology to inspect bridges, using drones, can often save costs compared with traditional methods of inspection.

The CSIR, together with University of Pretoria, has since 2017 been investigating the viability of using drones for bridge inspections, finding that bridges are typically inspected every five years and often hold safety risks for officials, particularly at sites where bridges are suspended over water or situated in crime hotspots.

The drones used by the entities capture bridge image data to create point cloud models.

The entities determined that drones are best suited to inspect strategic bridges, which are normally classified as strategic by a particular road authority, according to how restricted access to the bridge is, whether it is located over a perennial river or whether it requires an under bridge inspection unit (UBIU) to conduct a principal bridge inspection.

For example, the Bloukrans river bridge, in the Western Cape, is considered a strategic bridge. 

The CSIR found that more hours and money were spent on inspecting bridges using drones in the case of medium and large-sized bridges, however, one instance of drone inspection proved to cost just under R30 000 for a strategic bridge, compared with a cost of more than R70 000 using the traditional UBIU method.

Roux explains that the new methodology of inspection could indeed replace the use of UBIU and capture images with drones to reduce the cost of strategic bridge inspections and ensure the safety of bridge inspectors.

The total cost of using UBIU includes platform fitment and operation, traffic control, the cost of the principal bridge inspector and transport of the unit, which are all eliminated using the new methodology.

Using drones also allows for remote inspections, reduced travel time of officials and the ability to inspect more than one structure a day.

As demand increases to capture images of bridge structures, the cost could be reduced.

The CSIR has also been continuously working to improve the new methodology, including by improving the point cloud model, putting more visible bearings on the drones and fitting the drones with protective cages to enable closer flying to structures.

Roux says it is unlikely that all bridges will be inspected using drones in future, but it can greatly reduce the risk and costs related to strategic bridge inspections for now.

*Van der Spuy and Roux delivered presentations at the Southern African Transport Conference, held from July 10 to 13, in Pretoria.