rehabilitation of long span bridges

8/9/2019 Rehabilitation of Long Span Bridges

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The rehabilitation of long span truss bridge

Tomasz W. SIWOWSKIProfessorRzeszow University of

TechnologyRzeszow, Poland

[email protected]

Tomasz Siwowski, born 1961,received his civil engineering degreefrom the Rzeszow University ofTechnology, Poland. He has

established and worked for PromostConsulting, Poland, before becomingProfessor at the Rzeszow Universityof Technology. One of his main areaof research is related to steel bridgesdesign and assessment.

Summary

The rehabilitation works on the five span continuous Warren type steel truss bridge built in 1961

have been presented in the paper. Because of steel structure deterioration and huge live loadsgrowth during recent years, the bridge has been selected for the comprehensive rehabilitation with amain goal to increase its load carrying capacity. Apart from the strengthening of truss girders (f.e.member sections enlargement, external prestressing, rivet replacement, etc.), the rehabilitation alsoincluded deck grid extension, redecking of deteriorated concrete slab and upgrading of supports.The repair program for existing steel structure contained among others the replacement of twodiagonals deformed by impact of vehicles. The rehabilitation procedure has been presented in the

paper as well as the proof load test results, which proved rehabilitation efficiency.

Keywords:steel truss bridge, rehabilitation, strengthening, external prestressing, redecking,member replacement.

1. Introduction

The bridge over Vistula river in Nagnajow is a key component of the National Road No. 9 in Poland.It has been built in years 1959-1961 as five span continuous Warren type steel truss with RC deckslab (Fig.1). The spans of steel truss are 72+3x90+72 m long. The bridge has been selected for thecomprehensive rehabilitation with a main goal to increase its load carrying capacity up to A class(the highest) according to Polish bridge code, which is similar to the first class load of Eurocode.Apart from strengthening of truss girders, the rehabilitation also included full replacement ofdeteriorated concrete deck, steelwork corrosion protection and upgrading of supports. The repair

program for steel structure comprised also the replacement of two diagonal truss members, whichwere deformed by impact of vehicles. The main rehabilitation works as well as the proof load testresults, which proved rehabilitation efficiency, have been presented in the paper.

Fig. 1: The steel truss bridge in Nagnajow before rehabilitation


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Two truss girders with the height of 9,0 m and the spacing of 8,40 m are built of welded memberswith riveted connections. Upper and lower chords have hollow box section and diagonals have both

box or I sections depending on compression/tension inner force. The main truss girders aretransversally stiffened with portal frames located over each support and horizontal lateral X-bracingin both chords levels. The rolled I and channel profiles are used for lateral bracing. The deck ismade of RC slab with the thickness of 0,18-0,24 m, supported on steel grid of riveted floor beams

with the height of 1,20 m and three welded stringers with the height of 0,90 m. The slab which iscomposite with steel beams is cut every 27,0 m to release temperature stresses. The grade of steel ofthe whole steel structure is 355 MPa and the concrete class of slab was C-30.

The bridge superstructure is supported on five massive piers founded on the caissons whileabutments are founded on Franki piles. On the same supports the superstructure of the adjoiningrailway bridge is rested. The railway bridge is of the same type as the road one with the exceptionof truss members, which are built-up riveted sections instead of welded as for road bridge. Therailway bridge was not rehabilitated and is out of the scope of this paper.

2. Bridge assessment and evaluation

Before the design work has begun, the comprehensive assessment of the bridge state of repair along

with load carrying capacity evaluation (with the proof test) were undertaken in order to find themost relevant methods for bridge rehabilitation. The most severe faults discovered duringassessment were mechanical damages (plastic deformations) due to vehicle collision with bridgediagonals, cracks in one floor beam over a support and loss of member sections due to intensivecorrosion after 50 years of service.

After detailed FEM study (Fig.2)the actual load carrying capacity of the bridge was established onthe D level of Polish bridge code (20 metric tons vehicle). The most critical elements were deckgrid beams (stringers) and some riveted connections, both in truss girders and deck beams joints.Moreover several diagonals and sections of both chords revealed the lack of carrying capacity whenthe required level of A class service load had been considered in calculations.

The rehabilitation program included repair/replacement of both impact deformed members.

Additionally, in case of tension member it was necessary to strengthen it. The general scheme fortension members strengthening in the rehabilitation program assumed section enlargement bywelding cover plates on existing webs and flanges. It was not possible for deformed tensionmember. The assessment on heat straightening of the member in question revealed it wasimpossible. Therefore the decision was taken to replace the damaged member with a new one, withre-dimensioned cross section to carry increased traffic loads.

The compression member in question had got sufficient load carrying capacity (if undamaged) tocomply with new requirements. Before the decision was taken to replace or to repair/leave it, astudy had been performed to answer following questions:

what is the ultimate load carrying capacity of deformed member;is it possible to leave the deformed member without repair in the bridge after rehabilitation (i.e.

assuming new loads);is it technically feasible to repair the deformed member.

Fig. 2: FEM models used for analysis: bridge truss and deformed member


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In order to answer these questions a comprehensive FEM study on the deformed member has beenperformed including detailed damage inspection and inventory, analysis of load carrying capacity indeformed state and analysis of redistribution of forces in truss girder due to damage of the member(Fig.2). Taking into account the FEM analysis results it was quite obvious, that the deformedcompression member had to be replaced [1].

The assessment of actual bridge condition and evaluation of load carrying capacity let to establishedthe scope of required rehabilitation works, which were necessary to fulfill road administrationexpectations. The scope of works (limited in this paper to bridge superstructure) included:

repair, strengthening and replacement of relevant members of truss girders;strengthening of riveted connections;repair and strengthening of deck grid steel beams along with RC deck slab replacement;execution of new bridge deck equipment.

The rehabilitation works mentioned above have been described in the following chapters of thepaper.

3. The rehabilitation works description

3.1

Truss members strengthening

Two direct strengthening methods of truss members have been employed in the rehabilitation:member section enlargement with additional welded plates (passive strengthening for upper chordand compression members) and external prestressing (active strengthening for tension members).The scheme of member section enlargement as well as strengthening works execution are showedon Fig.3. It was done with simultaneous members relieving after RC deck slab demolition in orderto ensure old and new added parts cooperation in carrying dead load of the deck after rehabilitation.Using member influence lines the special sequence of old concrete slab demolition was elaboratedto obtain the expected level and of members relieving.

Fig. 3: Scheme of member section enlargement and view of strengthening execution

Fig. 4: External prestressing of tension members


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Tension diagonals were strengthened by external prestressing with high strength rods (Fig.4). Thiswork was undertaken at the final stage of rehabilitation, after execution of the new RC slab togetherwith full deck equipment. Active prestressing enabled the full utilisation of the member capacityalso taking into account the immediate and rheological losses of prestressing force. The rods of 26mm and 36 mm diameter were applied along with system nuts and anchorage plates (Fig.4). Thetailor made steel anchorages were located under the deck slab (active) and on the upper chord

(passive). The prestresing begun in the middle of the both truss girders going simultaneouslytowards both ends of the bridge. The rods were protected against corrosion with special HDPE

pipes and additionally against vandalism with steel tubes (lower parts of rods till 2 m above decklevel). Active anchor blocks were equipped with easy removable caps with elastic stuff in order toadjust prestressing force during bridge operation.

3.2 Members replacement

3.2.1 Tension member

Post tensioning steel bars were selected to relieve tension member and stabilize the truss girderduring the replacement procedure. The tension force in the bars was strictly controlled duringtightening. Bars were prestressed up to the value of axial force in the member in question resulting

from dead load. It was assumed that the new RC slab had already been made, all cover plates oftruss members had been welded, and the crane necessary to perform the replacement operation wasstanding on the bridge deck. Because the connection of new member was designed with HSFG

bolts, some unexpected slip and relaxation was assumed. Additional safety margin of 50% axialforce was applied due to uncertainty of current state of forces in stage under construction. It wasassumed that temporary structural monitoring system would be deployed during the replacementand post tension force would be adjusted if necessary. The prestressing of the replaced member wasrealized with four high strength M-24 Macalloy bars and special anchor blocks fixed to the upperand lower chord of the truss (Fig.5). The deformedmember was replaced with new one of thesimilar shape with webs and flanges re-dimensioned to larger thickness, providing necessaryenlargement of member cross section.

3.2.2 Compression member

Replacing compression member was much more complicated procedure than replacing tensionmember. The difficulties was mainly caused (but not only) by a necessity of relieving compressionmember and stabilizing the truss girder geometry during the replacement procedure. Three differentapproaches were considered:

replacing the member without relieving;erection of temporary stiffening structure for relieving member and stabilizing its nodes;

Fig. 5: Tension member replacement


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relieving the member by means of the temporary support and imposed load.

The first approach was analyzed and discarded due to excessive internal forces in truss members inquestion. While the internal forces were close to the ultimate load carrying capacities of trussmembers, the process of cutting the compression member would lead to uncontrolled dynamicimpact to the structure. This would certainly have increased forces in members as compared to the

state of its static equilibrium without compression member. The second option required design andmanufacturing of special stabilizing structure, which would act as temporary reinforcement andstiffening of truss girder. This option was discarded because of complicated execution and need for

purpose-built structure for only one use. Finally the last option was chosen to build a temporarysupport under truss girder (Fig.6). Hydraulic jack was used to push the truss node up to the level,when the axial force in compression member was completely reduced and thus the member wasrelieved. After relieving the deformed member was removed and replaced with new element. Thewhole procedure of compression member replacement has been described in detail in [1].

3.3 Deck strengthening and slab replacement

After old deck slab demolition the existing floor beams were strengthened with steel plates bolted tolower flanges with HSFG bolts. The special epoxy glue was also used to create the additional

bonding between old and new steel elements. The deck grid strengthening procedure comprisedalso the assembly of three additional stringers in order to relieve two existing ones. The tailor madesections of new welded stringers were bolted to existing floor beams with HSFG bolts. On the topflanges of floor beams and stringers shear studs were installed to connect the new RC slab incomposite action with steel grid. When the new steel grid had been executed the concrete slab was

monolithically cast on the formwork rested on the grid. Special sequence of slab concreting wasestablished to reduce rheological effects (creep, shrinkage). Finally the insulation layer and twopavement courses were placed on the slab following anticorrosion works undertaken on the wholesteelwork. The new steel sidewalk brackets were also mounted out of truss girders with the

prefabricated concrete slabs laying on them.

3.4 Strengthening of riveted connections

Strengthening of relevant riveted joints was executed by replacing rivets with HSFG bolts withadditional use of epoxy glue. This hybrid bolted/bonded connections were extensively tested inPoland and proved to be the best method for riveted joints strengthening [2]. The existing rivetedconnections of floor beams were partially dismantled (in turn: web, bottom flange and top flange).Due to very dense grid scheme of deck beams no additional supports were needed during

dismantling. The procedure of execution of hybrid bolted/bonded connections was as follows:removing old rivets in relevant part of joint;

Fig. 6: Compression member replacement


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geometric inventory of existing holes pattern to produce precise new lap plates;sandblasting of both surfaces;epoxy glue placement on clean surfaces;assembly of lap plates in the joint along with temporary bolts mounting;final HSFG bolt tightening after glue hardening.

Strengthening of truss girder joints was also executed with use of the same technology as for thedeck grid with additional enlargement of gusset plates together with supplementary bolts

installation (Fig.7). During joint strengthening the existing connection was partially dismantled byremoving of 50% rivets. The remaining rivets ensured the relevant load carrying capacity requiredfor dead and technological load on the bridge. The geometrical adjustment of new lap plate wasthen done following surface cleaning, glue placing, bolt installation and tightening. In a similar waythe second part of truss connection was strengthened. After the whole joint had been tightened two

parts of additional lap plate were welded together. In case of tension members lower part of lapplate was welded also to upper part of the member to ensure tension force transfer from member tojoint.

4. Monitoring and proof test

4.1 Monitoring during members replacement

A temporary structural monitoring system was deployed to assist in operation of replacement ofdeformed compression member. The system consisted of:a)

displacement transducers completed with geodesy survey;b)

strain gauges in key structural members;c)

pressure sensor in hydraulic jack.

The continuous monitoring of displacements was set-up in 5 points of each truss girder. Strainmeasurements were performed in 3 members, equipped with 4 strain gauges each. The new memberwas controlled after its installation, during lowering the jack on the temporary support. The value offorce in hydraulic jack was controlled by monitoring the value of oil pressure. The displacement ofthe jack was measured with displacement transducer. The structural monitoring system allowed forcontinuous control of replacement operation and on-site decision making. Comparison of recordedand expected values before and after operation allowed to check the safety of replacement operationand to verify whether the bridge structure has been repaired properly. More detailed description ofmonitoring during members replacement has been presented in [1].

Replacing a member in steel truss girder was a complicated and dangerous task, requiring constantmonitoring during operation. In order to relieve the replaced member a very precise estimation offorce in the member had to be made. A detailed and well calibrated FEM model of the wholestructure was very useful to simulate and predict strains and displacements in all steps of thereplacement operation. The possibility to get immediately measured values of monitoreddisplacements and strains was the key factor for successful and precise decision making such asadjusting jack force, releasing post tensioning bars or finally cutting the deformed member.According to monitoring results it can be stated that the executed replacement operation retainedoriginal geometry of the bridge and existing force distribution.

Fig. 7: Procedure of execution of hybrid bolted/bonded connection


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4.2 Proof test

The proof test carried out after bridge rehabilitation had two main goals: to check the strengtheningefficiency of the applied methods by a comparison of the same parameters (displacements, strains,accelerations, etc.), measured before and after strengthening works and thus to prove the increasedload carrying capacity of the bridge. As a test load three heavy trucks with the weight of 30 tons

each were used in various placement configuration according to the influence line of measuredparameter. In the tables below these comparisons have been showed, in turns truss girdersdisplacements (the middle of chosen span), strains in truss members and dynamic coefficient,calculated as the ratio of average dynamic and static strains in the members in question.

Table 1: Comparison of truss girders displacement

Truss girder

of first span

Elastic displacement

after strengthening

[mm]

Elastic displacement

before strengthening

[mm]

Displacement

ratio

No. 1 13,3 16,4 0,81

No. 2 13,4 15,9 0,84

According to comparison of displacements in the middle of truss girders (table 1) the globalstiffness of strengthened bridge increased about 18%. It is mainly due to the section enlargement ofmany truss members (upper chord, diagonals), riveted joints strengthening and also the strongercomposite action of rehabilitated deck. In table 2 the average strain reduction ratios for main trussmembers have been presented. The average strengthening level is between 13% and 45% dependingon a member. The highest level was obtained for floor beams, the smallest one for tension diagonalswith section enlargement. Strains in diagonals strengthened with external prestressing wereobviously the same, because prestressing force was not measured during proof test and the crosssection of member left almost unchanged.

Table 2: Strain reduction ratio for truss members

No. Member Strain reduction ratioScope Avarage

1 Lower chord 0,74-1,07 0,87

2 Upper chord 0,26-1,20 0,68

3 Compression members 0,30-0,91 0,67

4 Tension members with welded plates 0,68-1,08 0,875 Tension members with external prestressing 0,77-1,27 0,99

6 Cross-beams 0,10-0,85 0,45

7 Existing stringers 0,56-0,71 0,65

Table 2: Dynamic coefficients measured in main members of the bridge

No.Velocity

[km/h]

Dynamic coefficient

Lower chord Upper chordTension

diagonal

Compression

diagonalFloor beam Stringer

1 101.03

(1.05)

1.04

(1.01)

1.04

(1.04)

1.02

(1.03)

1.01

(1.03)

1.00

(1.00)

2 201.01

(1.02)1.03

(1.03)1.01

(1.03)1.02

(1.03)1.02

(1.05)1.02

(1.04)

3 301.01

(1.04)

1.01

(1.04)

1.03

(1.04)

1.00

(1.02)

1.01

(1.02)

1.00

(1.00)

4 401.03

(1.04)

1.03

(1.08)

1.02

(1.03)

1.03

(1.05)

1.04

(1.07)

1.00

(1.00)

5 10 + impact

1.25

(1.54)

1.39

(1.57)

1.63

(1.84)

1.54

(1.55)

1.25

(1.29)

1.02

(1.07)Remarks: the values measured before rehabilitation are given in brackets


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Dynamic behavior of the bridge after rehabilitation could be evaluated by dynamic coefficientscomparison, measured before and after rehabilitation. The dynamic coefficients were calculated onthe base of strain measurement in several bridge members when loading by heavy truck passing the

bridge with relevant velocity. In table 3 the dynamic coefficients have been presented. The changeof average dynamic coefficient for the bridge is rather small, only 3%. The biggest reduction (about5%) was measured for lower chord, the smallestfor compression members (about 1,3%). Thedynamic test revealed, that dynamic behavior of the bridge did not change so much due torehabilitation and strengthening works. The lesson learned is, that the chosen methods of truss

bridge strengthening do not cause significant improvement of dynamic characteristics and thus donot considerably decrease a fatigue danger.

5. Conclusions

The proof test results revealed the high effectiveness of all strengthening and repair methods usedon the bridge. Its load carrying capacity and stiffness were considerably enhanced and the dynamic

behaviour did not change. The actual level of stresses under proof load (which was comparable todesign load of A class) was not large, only 15% ofRefor both chords and 20% ofRefor diagonals(Reyield strength of existing steel). The global safety factors estimated on the strain measurement

basis were quite high and amount about 5 for chords, 8 for diagonals and 10 for steel deck members.All these values revealed, that rehabilitation works carried out on the bridge allowed to gainexpected level of strengthening and reliability. The present load carrying capacity fulfilsrequirements of the road administration for bridges located on international road network in Poland.

It seemed that the bridge would serve the next 50 years without the need of extensive and costlymaintenance works. But it was wrong supposition. A year after rehabilitated bridge was open totraffic the heavy truck hit the portal end post of the truss girder causing extensive deformation ofsteelwork. Several riveted joints were also influenced by this struck. The detailed inspection justafter accident discovered several cracks under the gusset plates of riveted joints. Some fatiguecalculations based on the European methodology proved that the remaining fatigue life of the

bridge is very short [3]. Therefore it was decided to implement structural health monitoring systemwhich ensures the required level of bridge safety. The lesson learnt from the case of Nagnajow

bridge is that even very efficient rehabilitation methods used for old steel riveted bridges do notrestore the required level of safety without structural health monitoring. Therefore SHMimplementation should be one of the most important part in rehabilitation strategy for such kind of

bridges.

6. References

[1] SIWOWSKI T., TOWSKI P.,Replacing members in steel truss bridge a case study,

The proceedings of 12thInternational Conference on Structural Faults & Repair-2008,Engineering Technics Press, Edinburgh, June 2008.

[2] AGODAM., The reinforcing of steel bridges with heterogeneous joint and ropeconstruction applications, The proceedings of 5thInternational Conference on Structural

Faults & Repair-1993, Engineering Technics Press, Edinburgh, June 1993.[3]

KULPA M., SIWOWSKI T., Fatigue life assessment of steel truss bridge after failure,The proceedings of XXVI Scientific Conference on Structural Failures -2013, Miedzyzdroje,May 2013 (in press).

rehabilitation of long span bridges

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