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Journal of Constructional Steel Research 58 (2002) 843–858 www.elsevier.com/locate/jcsr Full scale testing of old steel truss bridge A. Azizinamini Department of Civil Engineering, University of Nebraska-Lincoln, W348 Nebraska Hall, Lincoln, NE 68588-9531, 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 load tests 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 without any 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 load test was conducted. The retrofitted bridge failed in a more ductile manner. The failure took place gradually and there was ample warning before the failure. A major conclusion from ultimate load tests was that in inspecting old steel truss bridges one should pay very close attention to tension members that use forging. This paper presents a brief overview of the ultimate load tests conducted. More detailed information on the complete scope of the project is presented in Azizinamini et al., Final report, 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 prior to 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: [email protected] (A. Azizinamini). 0143-974X/02/$ - see front matter 2002 Elsevier Science Ltd. All rights reserved. PII:S0143-974X(01)00096-7

Full scale testing of old steel truss bridge

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Page 1: Full scale testing of old steel truss bridge

Journal of Constructional Steel Research 58 (2002) 843–858www.elsevier.com/locate/jcsr

Full scale testing of old steel truss bridge

A. 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: [email protected] (A. Azizinamini).

0143-974X/02/$ - see front matter 2002 Elsevier Science Ltd. All rights reserved.PII: S0143 -974X(01)00096-7

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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 (1–9). 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-

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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 (0.655×72 kips).

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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 load–deflection 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.

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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 ultimate load carrying capacity is the objective.The retrofitting of compression members would require attaching additional platesto portions of the top chords cross section to increase the moment of inertia for out-of-plane buckling.

Due to time and resource limitations it was decided to retrofit only the tensionmembers and achieve the objective of eliminating brittle failure associated with forg-ing of tension members. This was accomplished by adding additional members tothe bridge parallel to members 31, 1 and 2 such that failure of these members wouldbe avoided. The details of these additional members are described in the follow-ing sections.

9. Strengthening methods

9.1. Bottom chord members

The bottom chord members in the first and second panels (members 1 and 2 inFig. 11) are composed of two bars (see Fig. 5) with a total gross area of 4.5 in2.The allowable tensile capacity of these members is more than the 32.3 kips load perram required to fail the top chord members. However, the bottom chord membersare strengthened to achieve the retrofitting goal regarding ductility (recall that bottomchord members were forged). This is accomplished by using a 1 in. U-bolt connectionto a 1.5 in. diameter rod in-line with the current two bar elements. Figs. 12 and 13show a photograph of the assembled system. The gross area of the rod is 1.77 in2.

Fig. 12. Details of retrofitting scheme for bottom chord tension members with gorged sections. Noticethe u bar, end plate and rod assembly.

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Fig. 13. Details of retrofitting scheme for bottom chord tension members with gorged sections. Noticethe u bar raping around the pin.

Assuming an Fy of 40 ksi, this U-bolt bar system adds 70 kips load capacity to themember. The tensile capacity for these members then exceeds that required by a32.3 kips load per ram by a factor of 1.87.

9.2. Truss bracing member

For the tension truss bracing members, the current two bar members were removedand replaced by a single member. The replacement member has a rectangular crosssection of 3.5×0.5 in. Figs. 14 and 15 show a photograph of the replacement member.

Fig. 14. Details of the retrofitting scheme for tension brace members. Notice the flat bar welded totapered u plate.

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Fig. 15. Details of the retrofitting scheme for tension brace members. Notice the slot preventing com-pressive force built-up.

The connection to the pin is achieved by welding the rectangular bar to a taperedU section of steel. The pin resides in a ‘slotted’ opening. By having the connectionslotted, the member provides no compressive resistance but slides under com-pression. Under tension it becomes engaged and acts as a tension member. Thedimensions and size of the U section are determined based on the magnitude of localstresses and the space available at the installed location. Assuming Fy of 45 ksi, thisnew system will raise the tensile strength of the member to 78.75 kips, far beyondthe capacity needed for a load of 32.3 kips per ram.

10. Ultimate load test #2

After retrofitting the bridge, it was loaded again to failure. The same load setupused for ultimate load test #1 was used for the second ultimate load test. The appliedload was increased incrementally. The failure of the bridge in this test occurred ata load of approximately 38.5 kips per ram. The failure of the bridge was governedby out-of-plane buckling of the top chord member of the second panel as shown inFigs. 16 and 17. The resulting load–deflection curve is shown in Fig. 18. At the finalstages of the test, three of the vertical posts failed in bending and the second panelcross beam experienced web buckling.

At first glance it may appear that buckling of the compression members is associa-ted with a brittle mode of failure. However, as shown in Fig. 18, the load–deflectionrelationship of the bridge exhibited an adequate level of non-linearity prior to failureand there was ample warning. This is mainly due to the fact that when the floor

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Fig. 16. Failure of top chord compression member during ultimate load test # 2.

Fig. 17. Rock creek bridge, after conclusion of ultimate load test # 2.

beams are loaded it causes the vertical member to rotate. These rotations then causethe top compression members to move out of plane. This subjects the top com-pression members to secondary stresses. These second order stresses cause the com-pression members to behave as beam columns rather than compression memberssubjected to pure axial loads. The mode of failure for columns subjected to bothaxial load and bending moment could exhibit some level of warning before com-plete collapse.

The test results obtained from ultimate load test #2 do indicate an increase inductility and strength. The failure mode was gradual and ductile, which is contraryto the brittle failure observed in ultimate load test #1. In contrary to ultimate load

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Fig. 18. Displacement, center of cross beam, ultimate load test # 2.

test #1, there was ample warning before failure. The attained ultimate load was 1.36times that achieved in ultimate load test #1.

11. Summary and conclusions

This paper presents a summary of ultimate load tests carried out on a steel trussbridge that was transported to the structural laboratory of the University of Nebraska-Lincoln. The original width of the bridge was 15.5 ft. The floor beams were replacedand wider beams (20 ft.) were used. Two ultimate load tests were conducted. Fromthe first test it was observed that rupture of forged tension member produces a verybrittle mode of failure without any warning. The failed member, together with otherforged members of the truss, was retrofitted and an additional ultimate load test wascarried out. The paper describes possible retrofitting procedures for tension membersthat have forged sections. By retrofitting the truss, the failure was forced to takeplace in the top chords of the truss, which are compression elements of the truss.The compression elements of the truss act as beam–columns. Therefore, they aresubjected to out-of-plane bending from the onset of load application. As a result,the mode of failure is more ductile than rupture of forged tension members andample warning exists before failure of the bridge.

In addition to the tests described in this paper, the bridge was subjected to othertests including cyclic tests. These tests are described in detail in Ref. [1].

One of the conclusions drawn from the ultimate load tests conducted is that wheninspecting old steel truss bridges one must pay a great deal of attention to tensionmembers that have forged sections. A good retrofitting strategy is to eliminate anypossibility of failure of forged tension members.

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References

[1] Azizinamini A, Mehrabi A, Lotfi H, Mans P. Evaluation and retrofitting of historic steel truss bridges.Final report, STPB-STWD(13), University of Nebraska-Lincoln, February 1997; 479 pp.

[2] Bridge Rating and Analysis Software (BARS). American Association of State Highway and Transpor-tation Officials.

[3] Standard Specifications for Highway Bridges. American Association of State Highway and Transpor-tation Officials (AASHTO).

[4] SAP90. In: Wilson EL, Habibullah, editors. A series of computer programs for the finite elementanalysis of structures. Berkeley (CA): Computers and structures, Inc; 1992.

[5] Manual of steel construction, allowable stress design. 9th ed. Published by American Institute of SteelConstruction (AISC).