Increasing the Load Capacity of Major Bridges

Increasing the Load Capacity of Major Bridges

Peter G. BUCKLAND Principal Buckland & Taylor Ltd. North Vancouver, Canada pbuckland @b-t .com Peter G. Buckland received his engineering degree from the University of Cambridge in 1960. After five years with a steel fabricator and seven more with consultants, he co-founded Buckland & Taylor Ltd. in 1972.

Darryl D. MATSON Vice-President Buckland & Taylor Ltd. North Vancouver, Canada dmatson @ b-t.com Darryl D. Matson received his B.A.Sc. in 1987 and his M.A.Sc. in 1989, both from the University of British Columbia. He joined Buckland & Taylor Ltd. in 1989 and became Vice-President in 2001.

Summary By the use of examples from the Authors’ experience, various methods are shown of increasing the load capacity of major bridges, while keeping traffic flowing all or most of the time. These methods include refined calculations of loads and load factors, “prestressing” the hangers of a suspension bridge, altering load paths, converting a suspension bridge into a cable-stayed bridge, replacing the suspended structure of a suspension bridge, altering the steel grade of a suspension bridge, and making the roadway deck composite with the main structure.

Keywords: bridge; existing; major; suspension; cable-stayed; load; upgrading.

1. Introduction As traffic loads increase, more lanes of traffic are required, and bridges deteriorate, the bridge engineer is faced with the dilemma of how to increase the capacity of bridges safely and economically. Adding strength to a bridge at the design stage is not normally problematic, but once the bridge is built a small increase in capacity can incur a large cost. The larger the bridge, the more this is so.

A key element is the need to keep traffic flowing during any alterations. This is not just a matter of traffic flow: it also introduces the question of appropriate structural safety when the public is using the bridge during reconstruction, given that bridge failures are disproportionately more common during construction, and tight deadlines for the contractor are a prerequisite.

2. Guidelines Buckland [1] has given guidelines or principles to be applied when increasing the load capacity of suspension bridges. In general, these same guidelines apply to most bridges, and examples of this are given herein. The guidelines can be summarized as follows:

• determine as accurately as possible the true stress condition of the bridge, such as by the survey-and-analysis method, in which a survey of the bridge geometry is compared to a computer model, and any discrepancies are explained and accounted for;

• determine the dead and live loads as accurately as possible;

IABSE, Maintenance and Rehabilitation of Large Infrastructure Projects, Bridges and Tunnels, Copenhagen, Denmark, 2006 May 15-17.

Fig. 1 Seaway International Bridge

Fig. 2 Pelly River Bridge

• establish appropriate load and resistance factors using probabilistic methods, precedence and good sense;

• keep the weight of any new material as light as possible, if necessary discarding old heavy material and substituting lighter material;

• look for ways of altering load paths or otherwise minimizing the amount of strengthening required;

• remember that the work must often be done during short occupancies - constructibility is key to success;

• solve first for gravity loads, but design to enhance, not diminish, the behaviour of the bridge in wind and earthquakes;

• take the opportunity to improve maintainability;

• as a last step, switch on the computer - the analysis can be sophisticated and complicated, but it must serve the design, not drive it.

3. Examples The following examples illustrate some of the guidelines listed in Section 2. Not all of the examples are of large bridges, but those that are not can be considered as prototypes for larger bridges.

3.1 Seaway International Bridge, Cornwall, Canada Following a “health study” by the analysis-and-survey method, it was determined that stiffening trusses of the bridge (Fig.1) would be seriously overloaded by modern traffic loading. The solution was to replace the overstressed chord members of the stiffening trusses with steel members of the same size but of a higher strength steel. Because the stiffness of the system did not

change, the load sharing between the cables and the stiffening trusses also did not change, and the higher strength steel was able to take the applied loads without overstress.

3.2 Pelly River Bridge at Faro, Yukon, Canada The Pelly River Bridge at Faro (Fig.2) was required to carry heavier loads in order to make transport from a nearby mine more economical. Some main truss members required strengthening, which was not too difficult, but conventional engineering would have also required that all the “stringers” (longitudinal beams supporting the open grating steel deck) would also need to be strengthened. It would have been almost impossible to strengthen the existing stringers, and


Fig. 3 Port Mann Bridge, general view

Fig. 4 Cross-section, Port Mann Bridge Approaches showing (above) the original cross-section, and (below) the cross-section with deck extensions supported on small beams and brackets, with lateral bracing and diaphragms to resist torsion.

replacing them would be equally difficult as a heavy truck using the entire bridge width crossed the bridge every 15 minutes 24 hours per day.

The solution was to invoke the new section of the Canadian Bridge Code “Evaluation of Existing Bridges” (then in draft) that sets load factors based on a number of criteria, including how well the loads are known, and the consequences of a failure should it occur. In this case the loads were very well known, because it was almost impossible to overfill an ore truck. Thus, failure of a compression chord of the truss, for example, would be sudden and catastrophic, and would demand a large load factor. However, failure of a stringer by bending would be ductile, its load would be shared to its adjacent stringers, and failure would not be catastrophic; therefore, it demanded a lower load factor, and expensive strengthening was avoided.

3.3 Port Mann Bridge, Vancouver, Canada The Port Mann Bridge (Fig. 3), opened to four lanes of traffic in 1963, carries the 8,000 km long Trans-Canada Highway over the Fraser River. By the twenty-first century traffic had increased to the point that another lane was required. The fifth lane was installed by widening the concrete deck of the approach viaducts, and by removing access walkways on the arch spans, and trimming off outstanding flanges of the arch ribs to make more room (and reinforcing them). The extra capacity was obtained on the approaches by adding lateral bracing to the bottom flanges of the steel girders to resist eccentric loading by torsion instead of by bending (Fig 4). Extra capacity was gained on the arch spans by using current limit states design rules, including the evaluation load factors described in 3.2. Because dead load is dominant and has a low load factor, there was capacity to take the extra factored live load.


Fig. 6 Rock Creek Canyon Bridge

Fig. 5 Hagwilget Bridge

3.4 Hagwilget Bridge, BC, Canada This small, but spectacular, single-lane bridge (Fig. 5) was designed in 1928 to carry single 13.5 t trucks. It was required to increase its capacity to two 55 t trucks. This was accomplished, without strengthening, by several methods [2]. First, it was determined by the survey-and-analysis method that the cables, towers and anchorages would be able to support the increased load. The cables were unwrapped in places where damage was most expected.

Fortunately, although they appeared to be ungalvanized, they were in excellent condition. The two main problems were: uneven loads in the hangers, and the inability of the stiffening trusses to take the increased loads. The former was solved by adjusting hanger lengths, the latter by two methods. The first was to make the open grating deck composite with the top chords of the stiffening trusses, which reduced stresses in both the top chords and the lateral bracing system. This was made easier by the need to replace the deck in any case. The second was to further adjust the hangers so that the trusses were given an upward, hogging moment under dead load. When the traffic load is applied, the trusses first return to zero stress, and then are stressed in sagging moment. The bending moment diagram is complex, and bi-modal along the span, but suitable “tuning” the technique worked.

The important feature of the renovation is that the load capacity of the bridge was significantly increased without strengthening.

3.5 Rock Creek Canyon Bridge, BC, Canada

Rock Creek Canyon Bridge (Fig. 6) is a two-lane steel deck-truss bridge, 286 m long over a 90 m deep canyon, with a concrete deck supported by steel stringers spanning between transverse floorbeams. It was designed to AASHTO HS 20 loading. In the early 1990’s it was required to widen the bridge to 10 m between curbs and increase the capacity to CS-600 loading, while keeping at least one

lane open to traffic. To minimize the amount of strengthening required to carry a thicker, wider deck with heavier (and more eccentric) traffic, the new deck was made composite with the main trusses. This added capacity to the top chords, and increased the moment arm to reduce demand on the bottom chords.

The deck replacement project and upgrading (including seismic improvement and painting), was completed in 1992 - on time, under budget, and for about half the cost of a new bridge.


Fig. 8 Lions’ Gate Bridge, general elevation

Fig. 7 Elevation, Belgo Bridge: (a) original suspension bridge, (b) bridge party supported by suspension system, and partly by cable-stayed system, (c) renovated bridge in cable-stayed configuration.

3.6 Belgo Log Conveyor Bridge, QC, Canada. This small suspension bridge, with a main span of 183 m (600 ft.) carries only logs on a conveyor. Using the survey-and-analysis technique, it was found that for a variety of reasons [3], some members of the tower were overstressed by a factor of up to 2.75.

The bridge was immediately secured by applying a permanent upward force at the centre of the main span, which relieved the stresses in the most affected members by about 10%. They were still overstressed by any known code or theory, but they were at least 10% short of failure.

The long-term solution was to erect a new tower near, but not at, mid-span, and progressively turn the bridge into a cable-stayed configuration (Fig. 7). An interesting challenge was that the bridge had to survive a Québec summer and winter, with a temperature variation of about 70oC (125oF) in the half-completed state shown in Fig 6b. A suspension bridge rises and falls with changes of temperature far more than a cable-stayed bridge does, so on a cold day the suspension structure wanted to lift the new cable-stayed components, which it did not have the capacity to support; and on a hot day the suspension structure wanted to drop, inducing high shears at the end of the cable-stayed structure. Designing the structure to accommodate these effects added considerable interest.

So far as is known this was the first full conversion of a suspension bridge to a cable-stayed bridge, but this prototype has shown that with care the technique can be applied to larger spans.

Following this success, plans were prepared to convert the Lions’ Gate Bridge in Vancouver, with a main span of 472 m, from a three-lane suspension bridge to a four- (or five- or six-) lane cable-stayed bridge. In the end, it was decided to keep the Lions’ Gate Bridge as a three-lane structure, but the analysis had shown that the scheme was viable for a larger suspension bridge.

3.7 Lions’ Gate Bridge, Vancouver, Canada

3.7.1 The Lions’ Gate Approach Viaduct The Lions’ Gate Bridge in Vancouver (Fig. 8) consists of a three span suspension bridge and a 670 m long approach viaduct on the north end. The bridge was opened to traffic in 1938. The approach viaduct had a 178 mm thick reinforced concrete deck supported


Fig. 9 Lions’ Gate Viaduct, original cross-section

Fig. 10 Lions’ Gate north viaduct, present cross-section

Fig. 12 Lions’ Gate Bridge, open to daytime traffic while partially completed.

on transverse crossbeams that rested on two steel plate girders, as shown in Fig. 9. However, the deck had only 22 mm of concrete cover over the top reinforcing steel, and no wearing surface, and it succumbed to the effects of de-icing salts that were applied after 1955.

By 1972, corrosion of reinforcing steel was spalling the concrete, and 1,000 pot-holes were being repaired each year. The problem was how to replace the three-lane deck while maintaining traffic of 60,000 vehicles per day. A temporary bridge over the work area would be too heavy, have unacceptable grades, and provide very restricted access.

In 1975, the concrete deck of the Lions’ Gate Approach Viaduct was replaced by a wider, lighter, steel orthotropic deck during six-hour night-time closures of the bridge.

The solution adopted in 1975 was to close the bridge entirely to traffic from 23:30 to 06:00, remove a 12 m transverse strip of concrete deck, complete with the crossbeams supporting it, and replace it with a wider, but lighter, orthotropic steel

deck (Fig 10), ready for traffic in the morning. The traffic lanes were widened, as were the sidewalks, a barrier was placed between traffic and pedestrians, drains were installed, and 38 mm of epoxy asphalt was applied. The new deck is composite with the trusses for live load, which adds considerably to the ultimate capacity. The technique was a great success, and has since been adopted on other bridges, including George Washington, Golden Gate, Throgs Neck, and Champlain Bridges.

Lessons learned, which were invaluable for the major work on the suspended spans to come later, included the need for a high level of planning and management by both engineer and contractor, and a design that was tolerant of errors: there was no time for correction in the middle of the night. The contractor was required to have stand-by equipment in case of breakdown. Fifty-five panels were installed, and the bridge opened on time every morning except for an hour’s delay on the first and last pieces – the most difficult.

3.7.2 The Lions’ Gate Bridge suspended spans.

The Lions’ Gate Bridge (spans 187-472-187m) was opened in 1938 as a two-lane bridge, but by the 1960’s it was carrying three lanes of dense traffic. Eventually, in 2000 and 2001 the entire suspended structure was replaced (Fig. 11) during 10-hour night-time closures (and a few weekends)


Fig. 12 Lions’ Gate Bridge, new deck piece being lifted

without interruption of daytime commuter traffic (Fig. 12). The project has been well reported elsewhere [4, 5, 6, 7], and exemplifies all of the principles presented earlier in this paper.

• The survey-and-analysis method was developed on this bridge. It showed that, in addition to settlement of one tower, and an increase in dead load over the years, both of which were known, the main cables had stretched, which was both unknown and unexpected. Also, the cables had slipped slightly through the cable bent saddles. Thus, the stresses were accurately determined, and in some cases found to be unacceptably high.

• Early concern about traffic loading prompted a study of long-span bridge loading, the first of its kind, based on a probabilistic approach, that later influenced both the American AASHTO-LRFD and Canadian bridge design codes.

• Project-specific load and resistance factors were derived, based, in part, on variability of loads found during the study.

• In order to save weight, while making the bridge almost 50% wider and adding paving where none had been before, the stiffening trusses were replaced with new, lighter, tubular members, and the orthotropic steel deck was made to act compositely with the trusses as well as with the stiffening troughs and floorbeams.

• To relieve high bending stresses in the north cable-bent caused by cable stretch and slippage, instead of attempting to strengthen, the cable-bent legs were changed from “fixed” to “pinned” at their bases.

• A method of erection was fundamental to the replacement scheme and was worked out in considerable detail during the design, so that bidders could understand what would be needed for replacement during 10-hour closures.

• While improving the traffic capacity of the bridge, the aerodynamic stability and seismic performance were both enhanced at little extra costs.

• Although the bridge is 47% wider, the amount of steel to be painted is only about half what it had been, and almost entirely under the roadway deck, protected from salt spray.

• Some pioneering computer programs were written and used for this project, but the fundamental principles of the renovation design came from creative thought.

4. Conclusions Examples have been given of the use of the guidelines outlined in Section 2 to increase the capacity of seven bridges of different types and using various methods. In all cases, traffic was kept running except for one lane closures on Seaway, Port Mann and Rock Creek Bridges, and short full night-time closures of Hagwilget and Lions’ Gate Bridges.


5. References [1] BUCKLAND, P.G., “Increasing the load capacity of suspension bridges”, Journal of Bridge

Engineering, Amer. Soc. Civ. Eng., Vol. 8, No. 5. 2003, pp 288-296.

[2] BUCKLAND, P.G., MEDILEK, G.C. and MATSON, D.D., “Hagwilget suspension bridge: increasing capacity without strengthening”, Proc. Bridge Management 3, Guildford, UK, 106. E & FN Spon, London, UK, 1996, pp 867-874.

[3] BUCKLAND, P.G. and MORGENSTERN, B.D., “Conversion of a suspension bridge into a cable-stayed bridge”, Canadian Journal of Civil Engineering, Vol. 18, No. 2, 1991, pp 273-281

[4] BUCKLAND, P.G., “The Lions’ Gate Bridge – investigation”, Canadian Journal of Civil Engineering, Vol. 8, No. 2, 1981, pp 241–256.

[5] BUCKLAND, P.G., “ The Lions’ Gate Bridge – renovation”, Canadian Journal of Civil Engineering, Vol. 8, No. 4, 1981, pp 484-508.

[6] BUCKLAND, P.G., “Lions’ Gate: contributions to suspension bridge engineering”, The Structural Engineer, I. Struct. E., 81, No. 10, 2003, pp 26-30.

[7] BUCKLAND, P.G. & MATSON, D.D., “The reconstructed Lions’ Gate suspension bridge, Vancouver”, Bridge Engineering, Proc. Inst. Civil Eng., BE3, 2003, pp125-133.


Increasing the Load Capacity of Major Bridges

Documents

Transcript of Increasing the Load Capacity of Major Bridges