Transactions on the Built Environment vol 35, 1998 WIT Press, ISSN

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1 Truss bridge fatigue evaluation using acoustic strain gauges A.J.P. Halstead', P.W. Szustak* & J.S. O'Connor'' "TVGA Engineering, Surveying, P.C. Elma, New York, USA ''New York State Department of Transportation, Hornell New York, USA dot.state. ny. us Abstract The bridge carrying U.S. Route 15 over the Cowanesque River in Lindley, New York, USA is a single-span steel truss constructed in On this vital trucking link, the traffic is composed of one-third trucks, totaling over 2300 trucks per day. If the bridge were closed for repairs, the subsequent detour would be 51 miles (82 km), causing significant hardship to many communities and businesses. TVGA Engineering, Surveying, P.C. (TVGA) examined options for addressing the inadequate remaining fatigue life of the subject bridge, including- (1) more frequent "100% hands-on" inspections, (2) replacement of rivets with high-strength bolts, and (3) instrumentation of the structure to determine actual stresses in the critical members. After examining the cost-effectiveness of these alternatives, TVGA recommended Option 3 to the New York State Department of Transportation. ^ Instrumentation involved examination of strains in the critical members under traitic loading. Data was collected using magnetically attached acoustic strain gauges, a new technology introduced in 1997, which permits engineers to quickly evaluate strain levels in steel members without specialized training and experience. Data processing was performed at the bridge site to make preliminary determinations of member response while testing continued Following data post-processing and verification, the remaining safe fatigue life was determined to be +32 years. As a result, the bridge was determined to be sate from fatigue problems for its remaining desired service life

2 1 Background 1.1 Structure Description The bridge carrying U.S. Route 15 over the Cowanesque River in the Town of Lindley, New York is a single span riveted steel Camelback Warren Truss with a concrete deck and a concrete wearing surface. The structure has a span length of 170 feet (51.8m), a curb-to-curb width of 28 feet (8.5m) and a skew of 15 degrees. The floor system consists of steel wideflange floorbeams and stringers. The truss members consist of rolled beam tension members, and compression members fabricated from channel sections connected with lacing bars. To support a 6 foot (1.8m) wide sidewalk on the outside of the west truss, the west truss has some heavier members than the east truss. The substructure is constructed of concrete abutments with untreated timber piles. WEST TRUSS FATIGUE CRITICAL MEMBERS: U3-L4 & L4-U5 EAST TRUSS FATIGUE CRITICAL MEMBERS: L2-U3, & U5-L6 Figure 1 - Structure Elevation 1,2 Previous Fatigue Evaluation Results A fatigue analysis on the subject bridge was completed by TVGA Engineering, Surveying, P.C. (TVGA) in June of This study utilized a theoretical analysis to determine the effects of the pre-defined 54 kip (240 kn) fatigue truck, in accordance with the General Procedure outlined in Section 2.1 of the American Association of State Highway and Transportation Officials (AASHTO) Guide Specifications for Fatigue Evaluation of Existing Steel Bridges, 1990 ("the Guide Spec.")- Significant results for the controlling members (shown in Figure 1) are presented in Table 1. All other members were found to have "infinite" remaining fatigue life. 152

3 West meoreticai fatigue Analysis Results M- Location L2-U3, U5-L6 Remaining Safe Fatigue Life years years years 1.3 Inadequate Remaining Fatigue Life Options efcdveness ^dressing truss members with inadequate *» feasibility and cost- Option No. 1, Inspection: This option involves the implementation of more frequent inspections of the fatigue sensitive details in order to assure adequate safety. These "special emphasis" inspections would be performed on an annual basis. This option would not change the computed remaining fatigue life, but would ensure that fatigue crack initiation and propagation would be detected early. Option No. 2, Retrofit of Structure: This option would increase the computed remaining fatigue life, and involves the replacement of the existing rivets at the critical connections with high-strength bolts This procedure has been shown elsewhere to be effective, and would be the most feasible rehabilitation alternative. Option No. 3, Instrumentation: This option would provide a more accurate measure of stress levels and fatigue cycles, and involves strain gauge measurements of members under normal traffic This procedure is in accordance with the so-called "Alternative 1" in Article 2.1 of the "Guide Spec.", and would reflect the actual effects or cyclical truck loadings in combination with the built-in redundancies of the truss bridge system. Therefore, an improved tatigue rating would likely be achieved.,, * cost-benefit analysis was conducted for each option, and included the cost of the traffic delays to businesses and commuters. The cost of Option No 1 was estimated at $300,000 over a twenty-five year period the cost of Option No. 2 was estimated at $250,000, and the cost of Option No. 3 was estimated at $15,000. Therefore, in hopes of 153

4 determining inai ine siruciure actually nas an adequate remaining sare Transactions the Built Environment vol 35, 1998 WIT Press, ISSN fatigue life, Option No. 3 was recommended and adopted. 2 Procedures "Alternative 1" of Article 2.1 of the "Guide Spec." provides for the calculation of the effective stress range to be used in the fatigue evaluation "through field measurements while the bridge is under normal traffic." This approach takes into account the actual wheel load distribution, the actual end conditions of the truss members, and the multiple load paths that are inherent in such a complex structure. Actual impact is included as each truck passes over the bridge, and is related to the speed and load configuration of each individual vehicle. The load distribution of trucks observed during the field testing was assumed to be representative of "normal" conditions. Truck weight and classification data was collected during the field testing by "weigh-inmotion" sensors on Route 15 located approximately miles (16-24 km) north of the bridge. Comparing this with data collected at other times during the year, this assumption appears to be valid. The load distribution observed with instrumentation was also assumed to be "normal" for the time from original construction to present day. This assumption is incorporated into the "Guide Spec." procedures and equations, and is inherently conservative. Strain time-history data was collected using strain gauges placed on the flanges of the critical members. The gauges used were Acoustic Strain Gauges (ASG) manufactured by SonicForce, LLC. The ASG is a magnetically attached gauge which utilizes a non-contact ultrasonic technology to measure applied axial strain in structural steel members. This technology, introduced in 1997, permits engineers to quickly ascertain strain levels without specialized training or experience, and requires no paint removal. These gauges were connected to a computer, and strain time-history data was collected in a generally continuous manner for the critical members. Measurements were taken at L2-U3 and U3-L4 of the east truss, and at U3-L4 of the west truss. Gauges were placed at locations which had no section loss. Strain gauges were not placed on members L4-U5 and U5- L6 of the east truss, or on member L4-U5 of the west truss, as these results can be inferred due to the symmetry of the truss system. A sample time-history for one of the gauges is shown in Figure 3. This presents recorded microstrain (^strain) versus time, and clearly shows the effects of a number of trucks, with both small and large magnitude cycles.. Cycles of fatigue loading were calculated from the recorded strain 154

5 n F (ASTM) Ana s 77-* T-? "" aeveiopea by s. D. Downing and * r"* ^ciety of Testing and Materials Prices for Cycle Counting in Fatigue ! Wmaxkf*\ " p*»*w #, W\^L_ 360 N\lj,^NM^ JW '^Ha^w VTRJB ^ ^ :05:55 Approx. 34 ust am = 1 ksi (6.9 MPa) Elapsed Time Figure 3 - Sample Strain Time History Data (Member L2-U3 of the Truss) 3 Fatigue Loading Evaluation 3.1 Cycle Count Histograms Cycle count histograms were generated via Sonic Ware (SonicForce's software package) using the rainflow algorithm proposed by Downing detj^'%^' ^.s algorithm analyzes the strain time-histories determines the range of individual strain cycles (peak maximum to peak minimum), and places the strain cycles into the appropriate "intervals" depending on the range of the cycle. The cycle count histograms represent the total count of strain cycles in each interval for a grveftime period A sample cycle count histogram is presented in Figure 4 A 20 kip (89 kn) gross vehicle weight was assumed to be the minimum to cause fatigue damage per Fisher [2]. For this structure it was determined that, due to the variability of axle configurations and position of trucks relative to the curbline, a 20 kip (89 kn) vehicle could be assumed to induce approximately 30 ^strain in the critical members therefore, as shown in Figure 4, the histograms are presented using a m immum x-axis value (or threshold) of 3 0 ^strain 155

6 Approx. 34 strain = 1 ksi (6.9 MPa) Strain Range (^strain) Figure 4 - Sample Cycle Count Histogram (Member U3-L4 of the West Truss) 3.2 Effective Stress Ranges From the cycle count histograms, it is possible, using Miner's Law, to determine an "effective stress range." Miner's Law combines the frequency and amplitude of the recorded stress cycles to obtain the expected mean stress range caused by a truck crossing the bridge. Miner's Law is defined in Article 2.1 of the "Guide Spec." as where S, is the calculated effective stress range, /). is the fraction of recorded stress cycles occurring within a given interval, and S% ^ the midwidth value of the interval. The effective stress ranges for the critical members were calculated using Miner's Law with the generated stress histograms. Effective stress ranges for the critical members are presented in Table 2 assuming a minimum threshold value of 30 ^strain. 156

7 stress ranges by Miner's Law Truss West Loaition L2-U3, U5-L6--' Effective;Str ess Range 1.8 ksi (12..4 MPa) 2.0 ksi (13. 8 MPa) 1.9 ksi (13. IMPa) Note that to this point, all stress calculations and measured results have been based on the fact that the strain gauges were placed in locations of no sec,on loss. Thus the effective stress range presented above for the critical members should be factored by A^ where A. represents the gross area of the member, and A,, is the &t area (induing section loss and all nvet or bolt holes). The factored stress ranges for the critca members are presented in Table 3. Table 3 - Effective Stress Ranges Accounting for Section Loss Truss West LOCJition L2-U3, U5-L6 Eff, ectivf; Stre< ;s Range 2.5 ksi (17.2 MPa) 2,.6 ksi (17.9 MPa) 2. 6ksi (17.9 MPa) 3.3 Remaining Fatigue Life The remaining safe fatigue life was calculated for the critical members? -'jive ***** sh in Table 3, in accordance with Section 3 of the Guide Spec. The critical members were checked for infinite remaining fatigue life as specified in Article 3.1 of the "Guide Spec " where R is the reliability factor, S, is the calculated effective stress range and & is the limiting stress range for infinite fatigue life as defined in Article 3.3. None of the critical members passed this check and the remaining safe fatigue life was computed based on the formulation given in Article 3.2 of the "Guide Spec " (2) 157

8 A IV «(3) where 1^ is the remaining fatigue life in years, /is 1.0 for calculating safe life, K is the detail constant from Article 3.3 of the "Guide Spec.", Tg is the estimated lifetime average daily truck volume in the outer lane, C is the stress cycles per truck passage, and a is the present age of the bridge in years. The calculated values of remaining safe fatigue life of the critical members using Equation (3) are presented in Table 4. Table 4 - Fatigue Analysis Results Truss West Location L2-U3, U5-L6 Remaining Safe Fatigue Life Theoretical Instrumentation years years years years years years 4 Conclusions This paper summarizes the results of a fatigue evaluation performed on the "fatigue critical" members of a riveted steel truss bridge constructed in The evaluation was conducted using both "theoretical" and "instrumentation" analyses. The "theoretical" analysis yielded a safe remaining fatigue life of -50 years, an extremely conservative result based on a computed effective stress range using assumed truck weights and an analytical structural model. This analytical model ignored the beneficial effects of multiple load paths in the structure, member end effects, and actual weight and wheel load distribution of the truck traffic. The "instrumentation" analysis utilized acoustic strain gauges to obtain a more accurate determination of stress levels in the critical members under actual truck traffic loading. Stress cycle histograms were generated from the recorded traffic data using a rainflow algorithm. Using Miner's Law, an effective stress range was calculated based on the actual stresses recorded in the members due to normal truck traffic. Results of the "instrumentation" analysis demonstrated that the structure's remaining safe fatigue life of 32 years exceeds its remaining desired service life of 25 years. 158

9 " "* engineering evaluation was References nl * i VP ^' ^^' *"^ Counting Algorithms, International Journal of Fatigue, Butterworth & Co., Ltd., pp ^ ^ Stee, [3] J. W. Fisher, B. T. Yen, D. Wang & J. E. Mann, Fatigue ana- Fracture Evaluation for Rating Riveted Bridges, National 159