Assessing Load Capacity and Strengthening Methods for Aging Concrete Bridge Under Heavy Traffic

Monson Engineering undertook the inspection, load assessment and strengthening design, of a concrete deck bridge that had suffered damage. The bridge has a 6m clear span crossing a narrow river, and it is the only means of access to a couple of properties within a rural setting.

The original bridge consisted of a series of narrow precast concrete planks forming the deck, spanning across the width of the bridge onto two longitudinal steel beams. The road consisted of a thin wearing course applied directly over the precast planks, without any waterproofing membrane.

The main steel longitudinal beams span onto substantial concrete abutments on each side.

Cracking, spalling, and degradation of the precast concrete planks forming the deck, appeared to have occurred over the years. However, a more recent use of heavy construction lorries had damaged the edge of the precast planks significantly. As a consequence, It was decided that the deck was not feasible to support heavy vehicle loading.

Evaluating Strengthening Options

Once the condition of the concrete deck was established, the next step was to consider the potential strengthening techniques required to improve the load capacity.

As the bridge was in use, and needed to remain operational due to construction traffic and being the only access to the properties, consideration had to be given to the speed of any repairs in-situ. Earlier options to cast a new RC deck onto the existing precast planks were discarded due to the time needed for the concrete to achieve it's design strength, rendering the bride our of operation for several weeks.

A decision was made to replace the precast concrete deck planks, with galvanized steel beams, to be bolted onto the existing longitudinal steel beams.

Generally the condition of the main longitudinal beams carrying the deck weight was adequate, except for the top flange. It had been found that the top flanges had high levels of corrosion, understandably, due to the entrapment of water and moisture between the flange and the underside of the concrete deck. Epoxy repairs to the top flange in-situ was a real option; however, to minimize environmental factors related to working above a watercourse, and again to reduce construction time, it was decided to install new longitudinal beams to the side of the existing, adding further strengthening to the overall bridge deck.

View of damaged precast concrete deck

Vehicle Load Assessment

A load assessment of the existing longitudinal beams and the new bridge deck had to be undertaken.

Traditionally the load assessment for a bridge would be carried out according to BS 5400-2:1978 (Steel, concrete and composite bridges: Specification for Loads), and other design manuals, such as BD 37/01 (Design manual for roads and bridges). However, with the introduction of the Eurocodes, many of these codes of practice and manuals were withdrawn.

The first point of call for assessing the load onto a bridge is currently BS EN 1991-2:2000 (Traffic loads on bridges). Here, typical axle loads are given for different Load Models. However, these relate to "traffic for which a very heavy industrial international traffic is expected, representing a large part of the total traffic of heavy vehicles". Some small reduction factors are given for highways and motorways; however, no further guidance is given in the code for smaller roads that will invariably have smaller traffic flows. The code may be therefore, impractical to implement for projects other than motorway bridges.

Engineers wishing to undertake the load assessment of a bridge can currently refer to CS 454 (Assessment of highway bridges and structures). This document encompasses former design guides BD 21/01, BA 16/97 and BD 37/01. Bridges with a span of up to 20m are assessed typically for vehicle axle weights; whereas spans greater than 20m would be generally assessed under a UDL plus a knife edge load, similar to the previous BS 5400-2 requirements. In any case, the effects of road surface category and traffic flow category are taken into account.

Vehicle axle-loads for bridge design

The bridge has to be strong enough to support up to 32-tonnes 4-axle lorries. This category generally covers heavy construction lorries, fire engines and bin lorries, necessary to access the properties. The relevant axle weights for this vehicle were taken from CS 454 as shown below:

Axle weights for a 32 Tonnes 4-axle rigid vehicle

Undertaking the bridge deck repairs

The removal of the original precast concrete planks, and their replacement with new galvanized deck beams, was able to be carried out within a short timeframe. The installation of new primary beams eliminated the need to carry out extensive repairs to the existing beams.

Construction lorries were able to use the bridge immediately.

Temporary working platform over river, and installation of new galvanized steel deck

Photographs showing finalized deck, prior to installation of tarmac wearing course

Moving Forward with Solutions for Aging Bridges

Confronting the challenges posed by aging bridges that endure heavy vehicular loading is crucial. Thorough load assessments and suitable strengthening methods are essential for their survival.

The longevity and safety of our transportation infrastructure depend on timely evaluations and innovative solutions tailored to meet specific structural needs. Structural Engineers play a vital role in enhancing the load capacity of these critical structures.

By proactively addressing the issues facing aging bridges, we can ensure the continued safe transport of goods and people, fostering connectivity across regions. Any small bridge can be critical. Sharing knowledge about effective load assessment and strengthening techniques contributes to building a more resilient infrastructure. Whichever method is employed to strengthen the bridge, each method uniquely helps preserve our older bridges for years to come.