ABSTRACT
As a result of the aging bridge population and increased volumes of traffic, Canadian municipalities have been faced with the thorny issue of bridge upgrading. Many bridges require improved pedestrian facilities or safer traffic barriers. Others are suffering from increased axle loads and require deck improvements. Some bridges are functionally inadequate and need widening.

Some of these structures are in good condition, others have deteriorated decks on sound substructure. Few are candidates for complete replacement. Under current budget constraints, ways to utilize the residual life expectancy needed to be found, which required creative solutions to be developed.

The author's firm has undertaken many such projects and describe cases of innovative designs that proved constructible, reduced maintenance, and were highly cost-effective. None were 'traditional' solutions to bridge engineering problems. Examples include:

• Concrete deck overlays to strengthen timber deck bridges,
• Deck slab overlays for timber superstructure replacement,
• Supplementing existing stringers with new members,
• Precast removable shear-connected slabs for timber superstructure replacement,
• Provision of a separate pedestrian structure to permit deck widening and safety improvements,
• Lightweight short-span pedestrian trusses,
• Strengthening weakened abutments by backfilling, and
• Traffic barrier upgrades for aging bridge superstructures.

The paper describes the situation prior to the improvement and examines the alternative solutions. The advantages gained by the improvement and the method of using the residual life expectancy of the structure are described.

INTRODUCTION
A typical bridge inspection program conducted on behalf of a municipality in Western Canada will reveal bridges in various states of repair and functional adequacy. Generally, only the newest structures will have clean bill of health.

Faced with budgetary constraints, the owner of the inventory will only want to replace bridges which are either severely deteriorated or functionally inadequate. As a result, the majority of the inventory will be candidates for maintenance, upgrading, or safety improvements. It is important therefore to be able to identify the nature and extent of the work that is necessary for cost-effective repair and upgrading.

The objective is to maximize the potential of the portions of the bridge that have a usable residual life expectancy. In some cases, the residual life can be enhanced by the improvements. Where life expectancy is threatened by overloading, for instance, adding supplementary members can increase the useful life of a component. To maximize the value of the owner's investment, it is also important to avoid expenditure on items which will be discarded later.

A number of creative ways of extending the life and improving the safety of existing municipal bridges have been developed. The descriptions of several cases follow.

CONCRETE DECK OVERLAYS
Short span timber bridges are in widespread use. Their condition can vary from structurally sound to very weak. The worst examples are typically deficient resulting from either overloading or rot. The best timber bridges are usually creosote-treated, have a sufficient number of piles per bent, and adequately-sized sawn stringers. Often, however, the laminated timber decks are working loose and the asphalt surfacing needs frequent repairs.

Bridges in this category which are functionally adequate can be dramatically improved by the addition of a reinforced concrete overlay of about 150 mm thickness (Figure 1). Often consisting of several 4.2 m spans of longitudinally laminated timber, lateral load distribution and live-load capacity are much improved by the stiffer deck. Most importantly, the laminated timber decks are strengthened and protected from moisture ingress. The elimination of asphalt maintenance is a significant additional benefit.

The installation of concrete overlays is straightforward. Firstly, the asphalt surface is milled off, followed by placement of reinforcing bars. These will typically be 10M epoxy-coated bars at 180 mm centres each way. A grid of galvanized nails are driven into the timber deck to shear-connect the overlay. The deck concrete is then placed and finished (Figure 7). With modern deck concretes, traffic can potentially reuse the bridge within a few days (typically as soon as 25 MPa compressive strength is attained).

This type of improvement dramatically reduces the observed traffic noise beneath the span. This results from lower live-load deformations in the timber deck. The reduced attrition between components, and improved protection against moisture, ensures that the residual life expectancy of the existing timber bridge is maximized.

Concrete deck overlays can often be installed for less than 10% of the bridge replacement value. Reinforced concrete overlays should be seriously considered for short-span timber bridges in good condition.

DECK OVERSLABBING
Other timber bridges comprise a series of 7.6 m spans consisting of transverse laminated timber decking supported by sawn stringers. Modern tridem axles typically overload the stringers and strengthening is necessary if load restriction is to be avoided.

Initially, a concrete overlay will be investigated. This option will not be appropriate if the improved load distribution is insufficient for design purposes. Also, if the stringers are significantly checked as a result of overloading, their load capacity may be suspect.

As a result, alternative strengthening measures are needed. Where the support bents have adequate strength, a full-depth slab spanning between bents can be built on top of the existing superstructure. Essentially, the existing bridge is being used as permanent formwork and falsework. Using this approach, a longitudinally spanning concrete slab of 300 mm minimum thickness is constructed on top of the existing timber deck. Reinforcement to meet structural requirements is detailed, assuming no contribution from the existing timber which remains in place. The stringers are only required to support the concrete deck at the pier and abutment locations (Figures 2 and 3).

This type of repair can be highly cost-effective where the following apply:

• The existing timber structure is in sound overall condition and free of rot, but superstructure strength is inadequate.
• Supporting bents have sufficient strength to resist the new concrete deck. Where there is no sign of deterioration in the substructure, there will usually be adequate capacity.
• The existing bridge width is adequate for traffic requirements and the approach alignments are good.

With this type of structural upgrading it is normal to replace the parapets. Either concrete barriers or steel railings can be attached to the new deck slab. If necessary, lightweight cantilevered walkways can be readily installed outside of the parapets. The finished superstructure has the appearance of a brand new bridge and will require minimal maintenance (Figs. 8 and 9).

A major advantage of this upgrading method is the speed of implementation. After removal of asphalt, the concrete deck and parapets are installed. Then after the approaches are regraded, traffic can reuse the bridge. Typically, only a short road closure is required. For critical bridges, lane by lane construction is possible.

SUPPLEMENTARY STRINGERS
An alternative to concrete overslabbing is to install additional stringers between existing. This option is available where it is appropriate to allow existing stringers to remain in service. The requirements for this technique are:

• The existing bridge is in good overall condition but the stringer capacity is inadequate. No rot is evident.
• The level of maintenance required by the asphalt surface is acceptable.
• Preferably, the bridge consists of one or two spans.
• Clear space existing between stringers for the supplementary stringers to occupy.
• A suitable method for stringer installation can be developed.
• The bridge deck width, approach road alignments, and parapets are adequate.

The technique can be enhanced by installing a new concrete overlay and/or new bridge parapets where necessary to reduce maintenance or improve user safety.

This strengthening method is cost-effective in that the entire existing structure is reused. Either timber or steel stringers can be employed (Figures 10 and 11). New timber will need to be treated unless only limited duration service is required from the strengthened structure. Steel stringers can be protected by painting, galvanizing, or by using weathering steel.

The main constructibility issue to be solved is installation of the stringers. Longer than the clear distance between caps, to facilitate installation they need to be of slightly less depth than the existing stringers. Placing the stringers is best achieved by excavating behind an abutment and installing them from one end (Figure 12). Raising stringers between supports is a less straightforward alternative method.

The relative stiffness of the new and existing stringers will determine the contribution to live load resistance. As a result, the stringer size needs to be determined by the design requirements. After installation, the stringers are wedged up tightly against the laminated timber deck.

Although not as dramatic an improvement as with concrete overlays, there is still a considerable reduction in observed traffic noise for bridges strengthened using supplementary stringers. This is indicative of lower deformations and imposed stresses under live loading. As a result, a considerable enhancement is useful life of the strengthened superstructure can be expected.

PRECAST REMOVABLE SHEAR-CONNECTED SLABS
In situations where the existing timber is deteriorated and an increase in deck elevation cannot be tolerated, it may prove necessary to replace the timber superstructure. Where the substructure is in sound condition and the bridge is functionally adequate, new precast concrete slab units can be placed on top of the existing substructure and shear connected together.

Where the substructure consists of essentially permanent materials such as concrete piers, the new superstructure is also considered permanent. Where the substructure comprises timber bents, the superstructure is expected to outlast the substructure. As a result, the superstructure elements should be removable so that the substructure can be eventually replaced in a straightforward manner.

This can be achieved by using precast reinforced concrete panels, spanning longitudinally, spliced together by field-welded shear connectors. A 4 m span will require panels 300 mm thick by about 2 m wide. The shear connectors are welded at the top and are protected by 75 mm of asphalt surfacing (Figure 13). The panels are secured by dowels to the timber caps.

To recover the panels, the asphalt is removed and the welds gouged out. The panels are lifted off the dowels and stored. The panels can be reused on site or at an alternative location.

SEPARATE PEDESTRIAN WALKWAY
In some situations, pedestrian safety considerations dictate that a facility outside of the roadway barrier be provided. It is unlikely that the existing timber bridge will have sufficient width to incorporate an internal pedestrian walkway. Two methods have been developed to permit the roadway to occupy the full width of the timber deck while safely accommodating pedestrian traffic.

For many short span bridges, pedestrian decks can be cantilevered beyond the existing deck edges (Figure 4). This is generally difficult to achieve using cast-in-place concrete. Although precast panels could be employed, they would require temporary support while the cast-in-place deck is constructed. The simplest method is to attach lightweight galvanized steel cantilever beams infilled with steel safety decking (Figure 14). Where a concrete deck surface is preferred, steel metal decking can be used with concrete topping. Pedestrian railing posts are attached to the ends of the cantilever beams.

An alternate approach is to provide an entirely separate pedestrian bridge, generally alongside the upgraded timber bridge. To avoid instream work, these typically clear-span from bank to bank. Through-section bridges work best as their construction depth below finished grade is very shallow (Figure 15). Spans of up to 36 m are used with a steel truss on each side of the walkway. The trusses can be spliced on site from 12 m segment lengths.

Walkway widths will typically be 2 m. Usually, concrete topping on metal decking is placed between the truss bottom chords. For a 36 m span, the trusses will be about 1.6 m deep. These bridges can be detailed to be readily removed and reused elsewhere should the strengthened timber bridge be replaced at a future date.

SHORT-SPAN PEDESTRIAN TRUSSES
Short span steel pedestrian bridges can be used as a safety facility alongside a road bridge, or as a replacement for a deteriorated timber structure. A typical span would be about 8 m. These bridges are appropriate for use in public parks, on golf courses, or for trails crossing small watercourses.

These bridges can be designed using lightweight pieces which are easily fabricated and transported to site for assembly. Although timber decking can be used, concrete topping on metal deck is of similar cost and is more durable.

An economical way to design these bridges is to use Vierendeel trusses spanning longitudinally as railings with flooring spanning between the bottom chords (Figure 16). Galvanized steel is usually more economical than aluminum. These designs are typically equivalent to the cost of a replacement timber structure or less. The short-span steel trusses require no maintenance and are much more durable.

ABUTMENT BACKFILLING
An inexpensive repair for some abutment structures is to install either partial or complete backfilling. Existing timber abutments frequently exhibit signs of overloading. Timber lagging can bulge, and supporting members lean forwards under pressure from the retained fill.

One four-span bridge we inspected was in generally acceptable condition, but the abutments were showing signs of distress. Strengthening schemes were investigated but were fairly costly. The owner wanted to minimize costs as the bridge was fairly old. The 17 m long bridge spanned a slow-moving slough and had excess hydraulic capacity.

Accordingly, the two end spans were backfilled with quarry tailings, leaving only the upper portion of the timber abutments exposed (Figure 5). The backfill was sloped towards the two centre spans which provide the hydraulic opening for the slough.

This inexpensive repair was completed quickly by the municipal maintenance crews. It eliminated our concern about abutment stability and ongoing maintenance to the aging structure.

TRAFFIC BARRIER IMPROVEMENTS
Many existing short span bridges are found to be structurally adequate. This is often the case when the bridges are made from precast or cast-in-place concrete.

Municipal bridge inventories often contain a series of short span creek crossings, up to about 10 m span, consisting of precast concrete elements on cast-in-place abutments. Commonly, these bridges are functionally adequate. They typically carry two-lane rural roads with deck widths of about 8 m.

We have seen numerous such bridges which are functioning well and yet have glaring safety deficiencies. Typically, the parapets are weak and terminate abruptly at each abutment, immediately adjacent to the paved roadway.

We needed to develop a cost-effective means of improving the traffic barriers which could be installed with the minimum of inconvenience to road users. We favoured the use of galvanized steel parapets; however, the existing precast bridge components were not suitable for attachment of the post baseplates.

Although deck strengthening was feasible, the superstructures did not need a structural overlay.
We found that it was less costly and much less disruptive to install new galvanized steel box beams immediately outside of the bridge deck to support the parapet posts (Figure 6). The box beams were attached to the bridge abutments with new drilled-in anchor bolts. This parapet concept is entirely prefabricated and can be rapidly installed at a reasonable cost.

To provide safe terminations, the rail ends were fabricated with direct attachments for standard W beam guardrail. The guardrails were flared away from traffic and terminated safely to minimize the hazard to bridge users (Figure 17).

CONCLUSIONS
The emphasis on value that is commonly applied to new municipal infrastructure, can also be applied to upgrading of existing bridge inventories. Many bridge owners are trying to extend the life of their structures using new approaches to bridge rehabilitation.

We have outlined a number of cost-effective ways of strengthening existing timber bridges or replacing superstructures. We have also shown how inexpensive safety improvements can be made by providing separate pedestrian facilities, and by upgrading traffic barriers. These and other similar methods should be considered when faced with extending the useful life of an existing bridge.

ACKNOWLEDGMENTS
The authors acknowledge the bridge owners whose projects formed the basis of this paper:

City of Kelowna, B.C.; Ron Westlake, P.Eng.
City of Burnaby, B.C.; Barry Davis, P.Eng.
Township of Langley, B.C., Wayne Randell, P.Eng.