Full scale testing of old steel truss bridge

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  • Journal of Constructional Steel Research 58 (2002) 843858www.elsevier.com/locate/jcsr

    Full scale testing of old steel truss bridgeA. Azizinamini

    Department of Civil Engineering, University of Nebraska-Lincoln, W348 Nebraska Hall, Lincoln, NE68588-9531, USA

    Received 16 July 2001; received in revised form 17 August 2001; accepted 25 October 2001

    Abstract

    As part of an investigation to comprehend behavior of old steel truss bridges, ultimate loadtests were carried out on a steel truss bridge that was transported to the structural laboratory.The first ultimate load test consisted of testing the bridge in its existing configuration withoutany retrofit. The failure mode was by sudden rupture of a forged diagonal tension member.The mode of failure was brittle in nature and there was no warning. The failed member,together with other forged tension members, was retrofitted and an additional ultimate loadtest was conducted. The retrofitted bridge failed in a more ductile manner. The failure tookplace gradually and there was ample warning before the failure.

    A major conclusion from ultimate load tests was that in inspecting old steel truss bridgesone should pay very close attention to tension members that use forging.

    This paper presents a brief overview of the ultimate load tests conducted. More detailedinformation on the complete scope of the project is presented in Azizinamini et al., Finalreport, STPB-STWD(13), (1997); 479 pp. 2002 Elsevier Science Ltd. All rights reserved.

    Keywords: Bridge; Truss; Retrofit; Testing; Rating

    1. Introduction

    To comprehend behavior of old steel truss bridges, an abandoned steel truss bridge(Rock Creek Bridge) was transferred to the laboratory. Numerous tests were conduc-ted on this bridge before carrying out the ultimate load test. Tests conducted priorto the ultimate load tests included tests to comprehend behavior of connections and

    Tel.: +1-402-472-5106; fax: +1-402-472-6658.E-mail address: aazizi@unl.edu (A. Azizinamini).

    0143-974X/02/$ - see front matter 2002 Elsevier Science Ltd. All rights reserved.PII: S 01 43 -974X(01)00 09 6- 7

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    cyclic load tests. Results of these tests are presented in Ref. [1]. This paper presentsresults of ultimate load tests only.

    Two ultimate load tests were carried out. The first test was conducted beforeretrofitting any elements of the truss. Some of the truss members were retrofittedfollowing the first ultimate load test and the bridge was subjected to a second ultimatetest. The main objective of the retrofit was to increase the load carrying capacity ofthe bridge and prevent the failure of tension members with forged sections.

    2. Description of the bridge transported to laboratory

    The Rock Creek Bridge was constructed in 1920 and abandoned in 1980. Thedeck system, apparently of wood, was removed in 1980 and the bridge was closedto traffic. The bridge is a five-panel, 90 ft long, Pratt-Pony truss bridge with a road-way width of approximately 15.5 ft. (4.72 m). The bridge had suffered some damageto the railing and some of the bottom chord members due to vehicle impact or debriscarried by the creek during flooding. Fig. 1 shows the Rock Creek Bridge beforetransporting to the laboratory.

    The bridge was disassembled at the site by disconnecting the floor cross beams.Fig. 2 shows one of the disassembled trusses being moved by crane. The two trusseswere moved to the laboratory one at a time without disassembly of individual mem-bers. In the transportation process no damage to the trusses was induced. Fig. 3shows the photo of the Rock Creek Bridge after assembly in the structural laboratoryof the University of Nebraska-Lincoln.

    The overall geometry of the bridge is shown in Fig. 4. The cross section and sizes

    Fig. 1. Rock creek bridge on location.

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    Fig. 2. Disassembly of rock creek bridge.

    Fig. 3. Rock creek bridge after assembly in the structures laboratory.

    of the members are shown in Fig. 5. In Fig. 4, each element of the bridge is identifiedwith the designation E1 followed by a number (19). Fig. 5 shows the dimensionsof each element. The vertical posts are built-up members consisting of angle sectionsconnected with laces. The inclined posts and top chords consisted of channel sectionsconnected with laces. Parts of the tension members are joined together using a forg-

  • 846 A. Azizinamini / Journal of Constructional Steel Research 58 (2002) 843858

    Fig. 4. Geometry of the rock creek bridge.

    ing technique. Fig. 6 shows a photo of a tension member with a pin at the end. Theforged area is near the pin.

    One of the main problems with old steel truss bridges is the narrow width(generally 16 ft, [4.88 m]), which does not allow passage of wide trucks. If theexisting bridge foundation allows, one possible retrofit scheme could be replacingthe floor beams with wider and stronger beams. For the Rock Creek Bridge theexisting floor beams were badly damaged. Therefore, these floor beams were replacedwith wider and stronger beams. The new floor beams were 20 ft (6.1.m) in length,versus the 15.5 ft (4.72 m) used in the original bridge. The intention was to investi-gate the effect of widening the bridge, to the extent possible.

    3. Rating of the bridge

    Rating of the bridge was conducted using the commercial program BARS (BridgeAnalysis and Rating System) [2]. This program is used to rate various bridge typesby a majority of state transportation agencies in the US. Results of the rating areusually reported as service load carrying capacity of the bridge in terms of a truckthat has three axles with distances between the front and middle axles being fixedat 14 ft and the distance between the middle and rear axles ranging from 14 to 30ft. This three-axle truck is referred to as AASHTO HS-20 truck load [3]. The safeservice load carrying capacity (inventory rating) is a reflection of the capacity of themost critical member within the bridge. In the rating analysis it was assumed thatyield strength of steel members was 36 ksi. The lowest and highest yield strengthsobtained from material tests conducted on various samples taken from the bridgemembers were 38 and 49.4 ksi.

    In rating the bridge, it was assumed that the dead load of the floor system waszero, simulating the existing condition of the bridge. Further, the rating was carried

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    Fig. 5. Cross sections and size of the members in rock creek bridge as identified in Fig. 4.

    out for the widened bridge. The most critical member of the bridge was the diagonalbracing in the third panel of the truss (El 9 in Fig. 4). The rating factor based onAASHTO HS 20 truck which weighs 72 kips was 0.655. This implies that, underservice loads (inventory rating), the largest truck allowed to pass over the bridgewould weigh 47.2 kips (0.65572 kips).

  • 848 A. Azizinamini / Journal of Constructional Steel Research 58 (2002) 843858

    Fig. 6. Typical tension member with pin and forged section.

    4. Ultimate load test # 1

    Two ultimate load tests were conducted on the widened bridge. The first ultimateload test was conducted on the widened bridge without any retrofit. The bridge wasloaded until failure. This test is referred to as ultimate load test #1. Following theconclusion of this test the bridge was strengthened. The bridge was then loaded againto failure. This test is referred to as ultimate load test #2.

    5. Test setup

    From the rating analysis, the tension bracing member in the third panel (El 9 inFig. 4) was identified as the most critical. The loading configuration was selectedsuch that the most critical member would fail first. Fig. 7 shows the wheel locationsof an AASHTO HS20 truck on the bridge that would produce the highest force in

    Fig. 7. HS 20 truck location for maximum force in truss bracing.

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    Fig. 8. Schematic of loading used in ultimate load tests # 1 and #2.

    the critical member as the truck crosses the bridge. Reactions due to wheel load oneach floor beam were then calculated. In doing so, it was assumed that each wheelrests on an imaginary spreader beam spanning between the floor cross beams andbeing simply supported. The reactions R1 and R2 produced on the second and thirdfloor cross beams are identified in Fig. 8. The ratio of R2/R1 was found to be 0.72.

    For the test setup, a combination of hydraulic rams and spreader beams was usedto simulate truck reactions on the cross beams. Fig. 8 shows the spreader beamsused between the floor cross beams as well as the locations of the hydraulic ramsidentified by the symbol X. As seen in Fig. 8, the truck was assumed to be as closeas possible to one of the trusses (referred to as the south truss). With this loadingconfiguration, failure would be restrained to the south truss.

    Fig. 9 shows the photo of the bridge and test setup before start of ultimate load test.

    Fig. 9. Loading setup for ultimate load test # 1.

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    6. Results of ultimate load test # 1

    The load was increased monotonically until failure occurred. The resulting loaddeflection curve is shown in Fig. 10. The deflection reported in Fig. 10 was measuredat the center of cross beam number 3 shown in Fig. 8. Bridge failure occurred atan applied load of 28.3 kips per ram. The total applied load at the time of failurewas 113.2 kips. As mentioned earlier, the heaviest truck that would be allowed topass over the bridge would be 47.2 kips.

    The failure was very brittle and occurred in one of the tension braces in the thirdpanel (El 9 in Fig. 4). As indicated in Fig. 5, El 9 consisted of two square bars.Both bars were fractured. The fracture took place near the pin where the straightportion of the brace was connected to the pin using a forging technique. The fracturewas directly over the forged area. This behavior demands that in strengthening orinspecting old steel truss bridges special attention should be given to members withforged sections.

    7. Retrofitting of the bridge trusses

    Failure of the Rock Creek Bridge during ultimate load test #1 was a brittle failure.The goal of retrofitting was to increase the level of ductility as well as to increasethe strength of the bridge. The brittle failure in ultimate load test #1 was attributedto the forging process used in tension members. Therefore, the strategy was to ident-ify the critical tension members and, through retrofitting, eliminate the possibility offailure of tension members with forged sections. Sections 8 and 9 describe the pro-cess to identify the critical tension members and the strengthening method used foreach member.

    Fig. 10. Displacement, center of cross beam, ultimate load test # 1.

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    8. Identifying critical members

    The overall strength of the bridge is controlled by members requiring the lowestapplied load in order to reach their ultimate strength. Such members would be thefirst members to fail in the course of loading the bridge. Strengthening these memberswould result in an increase of the maximum load that can be applied on the bridge.In order to identify the most critical members, a three-dimensional linear model ofthe bridge was analyzed using the SAP90 [4] program. The loads were applied inthe form of concentrated loads that corresponded to the loading configuration usedin the ultimate load testing, assuming an arbitrary value of 10 m kips per ram. Thisresulted in obtaining the internal forces (P1) in the truss members when the bridgeis loaded with 10 kips per ram. The load per ram required to fail a specific member(Pu) can then be expressed as

    Pu 10(P2 /P1),where P2 is the ultimate strength of that specific member. As mentioned above,members with the lowest Pu are the most critical members.

    Results of the analysis for the tension members are summarized in Table 1. Mem-

    Table 1Calculations for retrofitting tension members

    Member numbera P1 Member Area (in2) Fy (ksi) P2, Member 10(P2/P1)forceb (kips) capacity

    (kips)

    1 36.2 4.5 33 148.5 41.02 36.2 4.5 33 148.5 41.03 30.8 6.75 33 222.8 72.34 13.7 4.5 33 148.5 108.45 13.7 4.5 33 148.5 108.46 15.8 4.5 33 148.5 94.07 15.8 4.5 33 148.5 94.08 13.5 6.75 33 222.8 1659 6.4 4.5 33 148.5 23210 6.4 4.5 33 148.5 23221 14.2 5.2 33 171.6 120.825 5.8 5.2 33 171.6 295.829 12.5 3.75 33 123.8 99.031 18.6 1.53 33 50.5 27.232 19.1 3.75 33 123.8 64.833 5.1 3.75 33 123.8 242.735 7.65 1.53 33 50.5 66.036 7.87 3.75 33 123.8 157.3

    a See Fig. 7 for member numbering system.b Force P1 was calculated using three-dimensional analysis of the bridge, same loading configuration

    used in test, and 10 kips load per ram.

  • 852 A. Azizinamini / Journal of Constructional Steel Research 58 (2002) 843858

    ber numbers indicated in the first column of Table 1 are identified in Fig. 11. Theforce P2, shown in Table 1, is simply the product of the cross sectional area andassumed yield strength values. Table 1 shows that member #31 is the most criticalmember. During ultimate load Test #1, member #31 failed at a load level correspond-ing to 28.3 kips per ram, which is very close to 27.2 kips per ram shown in Table1. The next tension members governing the maximum load per ram are members#1 and 2, as indicated in Table 1. These members also incorporated forging. Themaximum load per ram that could be applied before reaching yield capacity of mem-bers #1 and 2 is 41 kips.

    Results of three-dimensional analysis are provided for compression members inTable 2. In this table, the theoretical critical buckling stress, Fcr, is calculated fromequation E2-1 of Ref. [5] without factor of safety. This equation is similar to thatused by AASHTO Standard Specifications for Bridge Design. Equation E2-1 of theAISC manual without the factor of safety is as follows:

    Fcr [1 (Kl /r)2 /2C2c]Fy,where K is the effective length factor, l the length, r the radius of gyration, Cc

    (2p2E /Fy)1/2, E the modulus of elasticity and Fy is the yield strength.As indicated in Table 2, the effective length of the top chord members of the

    truss for in-plane buckling was assumed equal to the distance between panel points.However, for out-of-plane buckling, the effective length was taken as the entirelength of the top chord. With these assumptions it was found that out-of-plane buck-ling is the governing case. Table 2 shows that, with respect to compression members,

    Fig. 11. Member numbering system used in Tables 1 and 2 in calculations for retrofitting the bridge.

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    members 11 and 12 (see Fig. 11) are the most critical. The load required to failthese members is 32.3 kips per ram.

    These calculations indicate that retrofitting of compression members would benecessary if significant increase in ul...

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