Steel girder bridge field test procedures

Construction and Building Materials 13 Ž1999. 229]239

Steel girder bridge field test procedures Michael G. Barker 1 Uni¨ ersity of Missouri}Columbia, E2509 EBE, Columbia, MO 65211, USA

Abstract Research has shown that in most cases, bridges exhibit capacities higher than analytical load capacity rating predictions. Field testing has become an acceptable means to determine a more accurate estimate of a bridge’s safety capacity. Many factors not considered in the design process contribute to the response of a tested bridge. Several of these, like the actual load distribution and additional system stiffness from curbs and railings, are welcome benefits and can be used to increase weight limits on bridges. However, there are also contributions from bearing restraint forces and unintended composite action that may not be reliable during the service life of the structure. This paper presents systematic field test rating procedures that quantify these contributing factors so that owners may remove unwanted contributors and retain the reliable benefits. The procedure is demonstrated for a three-span steel girder bridge. Q 1999 Elsevier Science Ltd. All rights reserved. Keywords: Field testing; Bridges; Load rating

1. Introduction Many of the United States’ steel girder bridges are posted for restricted truck loading. Most of these posted bridges were designed for considerably smaller live loads than today’s truck traffic. The load restrictions are placed on the bridges by bridge owners using analytical load rating procedures. However, research has shown these procedures tend to underestimate the true stiffness and overestimate the response of steel girder bridges w1x. Analytical rating procedures are based on conservative design assumptions that do not always represent the true bridge behavior. Therefore, steel girder bridges usually exhibit capacities higher than analytical load capacity rating predicts. Testing bridges in the field has demonstrated this additional capacity and bridge field testing has become an acceptable means to determine a more accurate estimate of a bridge’s safe capacity w1x. In most cases, an experimental rating could raise or even remove the bridge’s restricted load posting. The reasons for the

1

Tel.: q1-573-882-2467; fax: q1-573-882-4784.

increase of capacity can be explained by factors that tend to make bridge responses less than those predicted by design and analysis procedures. These factors include: 1. adjustments in as-built parameters, such as dead load; 2. actual impact factor; 3. actual section dimensions; 4. unaccounted system stiffness, such as curbs and railings; 5. actual lateral live load distribution; 6. bearing restraint effects; 7. actual longitudinal live load distribution; and 8. unintended or additional composite action. Field testing measures the response of the structure to load. The response contains the aggregate from all of the above factors. However, some of these factors may be unreliable during the service life of the bridge. For instance, bearing restraint forces, from friction resistance during movement or frozen in place, tend to reduce measured responses in the structure. Bridge owners may want to remove the capacity increase as-

0950-0618r99r$ - see front matter Q 1999 Elsevier Science Ltd. All rights reserved. PII: S 0 9 5 0 - 0 6 1 8 Ž 9 9 . 0 0 0 1 3 - 6

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M.G. Barker r Construction and Building Materials 13 (1999) 229]239

sociated with the bearing restraints given that it may not be dependable at higher load levels over time. Likewise, this may be the case for unintended composite action. Even if a section is built without mechanical shear connectors, it usually acts at least partially composite Žmeasured response will be reduced.. The owner may not be willing to accept a rating based on unintended composite action. Therefore, for field testing to be successful, it is imperative that the unreliable contributions to an experimentally determined load capacity rating be removed. This paper presents a systematic approach to separate and quantify the contributions from the above factors. Therefore, if the owner does not want to consider the benefit of bearing restraint forces, its contribution can be removed by dividing the experimental total rating by a bearing restraint factor to determine a lower acceptable capacity rating. Other undesirable factors can be removed in the same manner. The procedures were developed from a comprehensive field test of a steel girder bridge. The load test was intended to Ži. develop the testing system w2x and standardize field testing procedures w3x; and Žii. determine a safe capacity for the tested bridge w4x. The bridge is a three-span continuous, four-girder structure with favorable characteristics for the research. It is currently posted for restricted loads and a single lane using Missouri allowable stress rating procedures. The testing of the bridge and the field testing rating results are presented. The results show that the bridge can safely be posted well above legal loads for a single-lane structure. The factors that increase the load capacity are determined by the systematic approach and a discussion of the influence of the factors is presented.

2. Field test of Bridge R289 Missouri State Bridge R289 was selected to develop the field testing capacity rating procedures due to the desirable characteristics it possesses. It has multiple continuous spans, positive moment region composite and non-composite sections, negative moment region non-composite sections with cover plates, rocker bearings, substantial curbs and railings, and a 133 kN Ž15 ton. single unit truck posting. The Missouri Department of Transportation w5x single unit 133 kN Ž15 ton. posted capacity is based on an allowable stress limit of 0.68 Fy , half way between the inventory level Ž0.55Fy . and the operating level Ž0.75Fy .. The field test results will be compared to the MoDOT posted limit for a single unit vehicle ŽAASHTO H20.. The procedures are equally applicable to other rating levels and other rating vehicles.

Fig. 1. View of Bridge R289 with load truck.

2.1. State Bridge R289

State Bridge R289 is a symmetrical, single lane, straight steel girder bridge with 18.3 m Ž60 ft. exterior spans and a 27.5 m Ž90 ft. interior span ŽFig. 1.. It was built in 1962 and has an east]west alignment. The west interior support is pinned while Missouri Type D rocker bearings support the superstructure at the three remaining supports. There are four W sections spaced at 1.8 m Ž6 ft.. The exterior girders are W690= 125 ŽW27 = 84. with 305 = 16 mm Ž12 = 5r8 in. cover plates welded to the top and bottom flanges over the piers. The interior girders are W690= 125 ŽW27= 84. until the first splice, then a transition to W690= 140 ŽW27 = 94. over the piers and in the interior span. These girders are also cover-plated over the interior supports. All steel material has a 250 MPa Ž36 ksi. yield stress and is relatively free of corrosion. The steel sections are composite with the deck in the positive moment region of the interior span and noncomposite at all other locations. The deck consists of normal weight 28 MPa Ž4000 psi. concrete. The deck is 152 mm Ž6 in. thick and 6.8 m Ž22.25 ft. wide from outside to outside. There are 305 = 305 mm Ž12 = 12 in. concrete curbs with C12= 20.7 railings. 2.2. Analytical rating of Bridge R289

For single unit vehicles, Missouri guidelines post bridges using the H20 truck. From the moment diagrams and rating equations, the critical location was confirmed to be the exterior girder near the centerline of the middle span w32 m Ž105 ft. from east abutmentx. From the MoDOT rating sheets, the posting rating for Bridge R289 is:

M.G. Barker r Construction and Building Materials 13 (1999) 229]239

231

Analytical } H20 } Posting s

Ž 0.68) Fy y sD A . MW L IA ) ) DFA SA

)RVWs 133 kN Ž 15 tons .

ž /

Ž1. where: s steel yield stress s 250 MPa Ž36 ksi.; s analytical design dead load stress s 105 MPa Ž15.2 ksi.; IA s analytical impact factor s 1.23; SA s analytical section modulus with design dimensions s 5621 cm3 Ž343 in 3 .; DFA s analytical distribution factor s 1.09; RVW s rating vehicle weight s 177.9 kN Ž20 tons.; and MW L s analytical wheel line moment for RVW truck s 358 kNm Ž264 kipft.. Fy sD A

Fig. 2. Strain gage layout and stresses at critical section.

2.4. Field test experimental posting When using data obtained from the field testing, the analytical rating stress Eq. Ž1. can be modified for use in posting. The equation becomes: Experimental } H20 } Post s

2.3. Experimental results A total of 24 different diagnostic tests w2x were analyzed in order to obtain the load posting level for Bridge R289 and develop the standardized procedures. These include three different test vehicle weights with eight different transverse locations. The data obtained from a 266.3 kN Ž29.93 ton. vehicle is used in the following rating processes. At the critical location, the analytical wheel line moment for the load truck, M TRK , is equal to 499 kN m Ž368 kip ft., while the H20 rating vehicle analytical wheel line moment, MRVW , is 358 kN m Ž264 kip ft.. The experimental rating uses the ratio of these analytical moments to adjust the experimental response to an equivalent experimental response for the H20 rating vehicle. Fig. 2 shows the gage locations and measured stresses Žstrain. at the critical location. The stresses result from driving the truck at a crawl speed right next to the outside curb over the critical girder. A linear regression applied to the stresses on the section yields a best fit line with a bottom flange stress of 61.6 MPa Ž8.93 ksi.. This best fit estimate represents the worst case loading and maximum experimental stress w sE s 61.6 MPa Ž8.93 ksi.x at the critical 32.2 m Ž105.5 ft. exterior girder section for a 266.3 kN Ž29.93 ton. truck. During controlled speed tests, ranging from crawl speed to 88 kmrh Ž55 mph., the maximum bottom flange stress measured was 25% above that measured during the crawl speed tests at the critical location. Therefore the experimental impact factor is assumed to be IE s 1.25.

Ž 0.68) Fy y sD E .

ž

MRVW ) s E ) IE M TRK

)RVWs 231 kN Ž 26 tons .

/

Ž2. where:

sD E IE M TRK M RVW

sE

s actual dimensions experimental dead load stress; s experimental impact factor; s analytical wheel line test truck moment; s analytical wheel line RVW truck moment; and s experimental maximum stress from truck load at crawl speed.

There is no longer a lateral distribution factor since sE represents the maximum load on the girder. The Ž MRVW rM TRK . term ratios the actual test truck response to an equivalent H20 truck response. The experimental load carrying capacity is 231 kN Ž26 tons. according to Missouri load-posting provisions. The experimental rating is 73% higher than the analytical posting rating for a single-lane bridge. The reasons for the increase can be explained by the factors discussed previously that tend to make bridge responses less than those predicted by design and analysis procedures.

3. Systematic field test rating equation Although the experimental rating is 73% higher than

M.G. Barker r Construction and Building Materials 13 (1999) 229]239

232

the analytical rating for the single-lane structure, bridge owners may be interested in how much some or all of the above factors contribute to this apparent additional strength. For instance, the owner may not want to include the contribution from unintended composite action or bearing restraint effects, assuming that these are not reliable at high load levels. Although Eq. Ž2. yields the experimental load capacity rating, it contains contributions from the factors as an aggregate sum. To develop the field test rating equation, Eq. Ž2. is slightly modified to transform the live load stress to a moment:

does not differ from Eq. Ž2.. Dividing Eq. Ž4. by the analytical rating Eq. Ž1. defines the factors that contribute to the apparent addition capacity:

Exp } Post s Ana } Post

)

ž

)

s

ž

MRVW MT ) ) IE M TRK SE

ML E DFA ) ) ME DFE

Ž3.

MT s experimental total moment; and SE s experimental section modulus. The resulting capacity is the same as Eq. Ž2.. MT is simply the moment associated with the stress sE and a section modulus SE . This step is necessary to move to the next stage. Eq. Ž3. is further modified to examine the additional capacity the experimental results indicate over the analytical posting: Experimental } H20 } Post Ž 0.68) Fy y sD E . MRVW M M M ) E ) LE ) T M TRK ML E DFE ME SE ) SAADIM ADIM SA

)RVWs 231 kN Ž 26 tons .

ML E DFE SAADIM

IA IE ME MT ML E ME DFA DFE MW L U ML E MRVW DFE MTRK

) DFE

IM SAD A SA

Ž4.

where: ME

/ž

/

0 Ž5.

The contribution from each factor is broken down as follows: 0.68U Fy y sD E 0.68U Fy y sD A

where:

IE )

ž

S AADIM SE ) ADIM SA SA



MW L MW MRVW ) DFE M TRK

/ž /

)RVWs 231 kN Ž 26 tons .

s

/ž /ž /

ž /ž /

Experimental } H20 } Post Ž 0.68) Fy y sD E .

0.68 Fy y sD E I ME ) A ) 0.68 Fy y sD A IE MT

s experimental elastic moment with bearing restraint effects removed; s experimental elastic moment adjusted for longitudinal distribution; s experimental lateral distribution factor; and s analytical section modulus with actual measured dimensions.

Again, the additional factors cancel so that the result

SE ADIM SA

contribution adjustmentrcorrection of the design dead load versus the actual dead load computed with as built dimensions; contribution for the impact factor; contribution for bearing restraint force effects; contribution for longitudinal distribution of moment; contribution for the lateral distribution factor, contribution from additional system stiffness, i.e., curbs, railings, etc.; contribution for actual section dimensions for section modulus calculations; and contribution for unintentional or additional composite action.

4. Standard test plans Frederick w4x and McDaniel w3x present standardized procedures for inspecting, instrumenting, testing, and load rating steel girder bridges through field testing. Six different test plans are offered depending on the factors to be determined. The test plans vary in level of effort and expected results for load rating bridges with

M.G. Barker r Construction and Building Materials 13 (1999) 229]239

experimental test results. Table 1 illustrates the test plans. For instance, if the owner wants to remove unintended composite action from the experimental rating, Plans III, IV, V or VI must be used. If the owner also wants to remove bearing restraint effects, Plans V or VI must be used. Test Plan VI is not typically necessary since the owner may not be interested in separating the longitudinal distribution and additional system stiffness contributions. However, Test Plan VI quantifies each factor and will be used to demonstrate the procedures for Bridge R289.

233

2. determine experimental impact factor IE ; 3. calculate the experimental distribution factor DFE ; 4. determine the bearing restraint forces and moments MB R ; 5. remove axial stress from critical section stress profile; 6. calculate total measured moments MT ; 7. calculate the elastic moment ME at the critical cross section; 8. determine the experimental section modulus at the critical section; and 9. calculate the elastic longitudinal adjustment moment at critical section ML E .

5. Application of field Test Plan VI rating Test Plan VI is a comprehensive plan that will fully describe the behavior of the bridge. However, the instrumentation requirements have still been minimized for economy. Therefore, some of the parameters, that could be accurately determined with more instrumentation, are presumed to follow design assumptions. These design assumptions are only used when the effects have been shown to be minimal w2,4x. The Test Plan VI instrumentation for Bridge R289 is shown in Fig. 3. The exterior girder is critical at the interior midspan so it is instrumented ŽSection 2. to determine experimental moment. The bearing restraint gages ŽSection 1. are single gages on the bottom flange located 152 mm Ž6 in. from the supports. The three other girders at the interior midspan also have a single gage on the bottom flange. The negative moment pier sections were placed 0.92 m Ž3 ft. from the centerline of the bearing to avoid interference with the bearings and diaphragms. Even though 95 strain gages were placed on the bridge, only the load test stress measurements from the 18 gages of the test plan will be used for this demonstration. The other gages were used to develop the testing rating procedures w2x. The general procedure to quantify the contributing factors is: 1. inspect the bridge to determine actual dimensions, properties, and dead load sE ;

5.1. Actual dimensions and section properties The exterior girder design and actual section properties are shown in Table 2 w2x. From the dimensions and a prismatic analysis, the dead load stress at the critical section is 95 MPa Ž13.77 ksi.. 5.2. Experimental impact factor During controlled speed tests, ranging from crawl speed to 88 kmrh Ž55 mph., the maximum bottom flange stress measured at the critical section was 25% above that measured during the crawl speed tests. Therefore, an experimental impact factor of IE s 1.25 is assumed, which is greater than the design impact of IA s 1.23. At other sections, the experimental impact results were slightly less. 5.3. Experimental lateral distribution factor An estimate of the experimental wheel line Žfactor of 2. lateral distribution factor can be determined from the measured bottom flange stresses across the transverse critical bridge section: DFE s

2) Ž si ) SA i . CriticalGirder qÝ Ž si ) SA i .

Ž6.

Table 1 Test plan factors Factor

Plan I

Plan II

Plan III

Plan IV

Plan V

Plan VI

Impact factor Experimental dead load Actual dimensions Unintended or additional composite Lateral distribution Bearing restraint Longitudinal distribution Unaccounted system stiffness

Yes Yes Yes No No No No No

Yes Yes Yes Yes No No No No

Yes Yes Yes No Yes No No No

Yes Yes Yes Yes Yes No No No

Yes Yes Yes Yes Yes Yes No No

Yes Yes Yes Yes Yes Yes Yes Yes

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M.G. Barker r Construction and Building Materials 13 (1999) 229]239

Fig. 3. Field Test Plan VI instrumentation. Table 2 Bridge R289 exterior girder section properties Property

South exterior properties

SA SAADIM ADIM SSteel dsteel ADIM ISteel AADIM Steel Haunch tslab

5621 cm3 Ž343 in3 . 5981 cm3 Ž365 in3 . 3605 cm3 Ž220 in3 . 67.9 cm Ž26.74 in. 122414 cm4 Ž2941 in4 . 167.5 cm2 Ž25.97 in2 . 8.5 cm Ž3.36 in. 15.2 cm Ž6 in.

Fig. 4. Bearing forces and moments.

where:

si s bottom flange stress for girder i; and SA i s either actual dimension or nominal design section modulus for girder i.

divided by two due to the cover plate as discussed in w2x:

BearingForce s

Ž s bt fw o y s bofn e . ) A b f 2

5.4. Bearing restraint forces and moments

where in Fig. 4:

The bearing restraint forces, moments and sign convention is illustrated in Fig. 4. The bearing restraint force calculations have been derived in Imhoff w2x. In general, a bearing force at an abutment can be estimated by w6x:

s bt fw o s stress on left side of bearing; and

BearingForce s A b f ) s b f

Ž7.

where: A b f s area of the bottom flange at the bearing; and s b f s measured stress on the bottom flange at the bearing. The bearing restraint force at a pier is slightly more difficult and requires gages on both sides of the bearing. The net stress between the two stresses represents the stress caused by the bearing restraint. The force is

Ž8.

s bofn e s stress on right side of bearing. The bearing restraint moment at each support is calculated by multiplying the bearing restraint force by the depth of the neutral axis d N A Žmeasured or design.: MB R s BearingForce) d N A

Ž9.

The bearing moment is applied as an external moment at the support. Moment distribution techniques can be used to distribute the external moment to each side of the support ŽFig. 4.. For Bridge R289, and assuming Žfor approximation. constant moment of inertia in all spans, the distribution can be calculated with the following equation:

M.G. Barker r Construction and Building Materials 13 (1999) 229]239

Distribution s

1 Critical } Span Li 1 Ý Li

ž /

where: Ž 10 .

A Comp A Steel

where L i is the span length on each side of pier. Thus, a distribution factor of 0.4 was used for the interior span side of the pier for Bridge R289. Once the bearing restraint moments for both of the bearings on each side of the critical longitudinal section have been calculated, a straight line interpolation between the two values ŽFig. 4. can be used to determine the bearing restraint moment at the critical section in question. For a critical section at the centerline:

A Conc

ž /

} Section s MBCritical R

2 MBPiera1 q MBPiera R R 2

Ž 11 .

where: } Section MBCritical R

MBPiera1 R 2 MBPiera R

s bearing restraint moment at the critical cross section; s bearing restraint moment at the first pier location; and s bearing restraint moment at the second pier location.

5.5. Remo¨ e axial stress from section profile The moment sections have three gages along the depth. From these, a strain profile can be determined using linear regression w2x. The best-fit line will have the following form:

s wasy

1 Intrcpt dq Slope Slope

Ž 12 .

N

Thus, at a section, the stress profile ŽFig. 2. with the bearing axial force removed is: 1 d Intrcpt ) q y saxial Slope 2 Slope

ssy

Ž 15 .

5.6. Total measured moments The experimental total moment in a section can be divided into three components: bending about the steel neutral axis, bending about the concrete neutral axis, and a couple representing the interaction composite action as shown in Fig. 5 w6x. The total moment is calculated using the stress Eq. Ž15. as follows: MT s MU q ML q Na

Ž 16 .

ADIM ML s Ž s 0 y sC G . Ssteel

Ž 17 .

MU s

Ž Econc Iconc . ADIM . Ž Esteel Isteel

ML

ADIM Ž dsteel Nas sC G AADIM y dC G . q Hnch q steel

ž

Ž 18 .

ADIM dslab 2

/

where:

s stress in girder including axial bearing restraint stress; Intrcpt s neutral axis from bottom flange; Slope s slope of the stress profile; and d s depth from bottom flange. wa

The net bearing force Žaxial force. at a moment section is found through summation of bearing forces on an appropriate free body. The axial stress from the bearing force can be removed as follows w2x:

saxial s

s equivalent steel composite area; s nominal or measured area of the steel section; s nominal or measured area of effective concrete; and s ratio of steel and concrete modulus of elasticities.

Ž 19 .

where:

s

235

BearingForce A Comp

A Comp s A Steel q

A Conc n

MT

s0 sC G

s total experimental moment without bearing restraint axial stress removed; s stress at bottom flange ŽEq. Ž15., ds 0.; s stress at steel centroid ŽEq. Ž15., ds dC G .;

AADIM s area of steel girder using measured dimensteel sions;

Ž 13 .

Ž 14.

Fig. 5. Total experimental moment.

M.G. Barker r Construction and Building Materials 13 (1999) 229]239

236

ADIM dsteel s depth of steel girder using measured dimensions; ADIM s depth of deck slab using measured dimendslab sions; dC G s depth of steel centroid from bottom flange using measured dimensions; Hnch s depth of haunch using measured dimensions; Econc s modulus of elasticity of concrete; Esteel s modulus of elasticity of steel; Iconc s moment of inertia of concrete slab; and ADIM Isteel s moment of inertia of steel using measured dimensions.

5.7. Elastic measured moments At each moment section, the elastic moments with axial and bearing moments removed can be calculated: M E s MT y M B R

Ž 20 .

Fig. 6. Analytical and experimental statical moment diagram.

where:

Ž 21.

s statical moment for the analytical load truck moment; s statical moment for the experimental data; s analysis load truck moment at left, maximum and right Ž i s 1, 2, 3. sections; and s percentage of length to point of maximum moment.

Ž 22 .

The following equation determines the longitudinal adjustment moment at the critical section:

STATA 5.8. Experimental section modulus STATE The experimental section modulus SE is the experimental moment of inertia divided by the intercept of the stress equation w2x: I Exp s y Ž MT . )Slope SE s

I Exp Intrcpt

MC

a

ML E s

5.9. Longitudinal adjustment moment The elastic longitudinal adjustment moment represents the elastic moment that should be at the critical section if the experimental longitudinal distribution acted like that used in the analytical rating. The adjustment is accomplished by forcing equal statical moment behavior w4x. The procedure is as follows. Use the experimental elastic moments for three sections to construct an experimental interior span moment diagram as shown in to Fig. 6. Produce an analysis interior span moment diagram using the load truck. It can be a wheel line or full truck analysis, the magnitude is not important since it’s the ratio that is important. Then the statical moments can be calculated with the following equations: STATA s MC2 y Ž 1 y a . ) MC1 y Ž a . ) MC3

Ž 23 .

STATE s ME2 y Ž 1 y a . ) ME1 y Ž a . ) ME3

Ž 24 .

STATE ) MC2 STATA

Ž 25 .

5.10. Summary of calculations The above calculations can be easily executed in a computer spreadsheet w2x. The moment, section property and distribution results are input into Eq. Ž5. to separate and quantify the factors that contribute to the experimental capacity rating.

6. Results of field Test Plan VI rating The calculations with the experimental data are too numerous to show here. Frederick w4x presents examples of the calculations for several scenarios. However, Table 3 summarizes the results for the contributing factors for six sections: the end span positive moment region w7.3 m Ž24 ft.x, over the first pier w18.3 m Ž60 ft.x, and centerline of the interior span w32 m Ž105

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237

Table 3 Bridge R289 Test Plan VI results Section

7.3 m Exterior

7.3 m Interior

18.3 m Exterior

18.3 m Interior

32 m Exterior

32 m Interior

Analytical posting

153.0 kN Ž17.2 tons. 363.9 kN Ž40.9 tons. 1.162

185.9 kN Ž21.0 tons. 528.4 kN Ž59.4 tons. 1.242

169.9 kN Ž19.1 tons. 813.1 kN Ž91.4 tons. 1.981

224.2 kN Ž25.2 tons. 1354.0 kN Ž152 tons. 2.282

133.4 kN Ž15.0 tons. 231.3 kN Ž26.0 tons. 1.159

186.8 kN Ž21.0 tons. 378.1 kN Ž42.5 tons. 1.364

1.016 1.033

1.016 1.033

1.000 1.022

1.000 1.025

0.984 1.064

0.984 1.087

1.120

1.280

1.224

1.098

1.045

1.204

1.186 1.054 1.059 1.313

1.238 1.009 1.043 1.301

1.125 1.082 1.139 1.392

1.436 1.022 1.248 1.286

1.264 1.038 1.007 1.042

1.216 1.012 0.928 1.008

2.37

2.83

4.78

6.05

1.73

2.02

Experimental posting Experimental dead load stress Impact factor Measured section dimensions Unaccounted system stiffness Lateral load distribution Bearing restraint effects Longitudinal distribution Unintended or additional composite action Product totals

ft.x for both the exterior and interior girders. If a factor is greater than 1.0, the experimental response increases the rating above that predicted by analytical methods. The opposite is true if the factor is less than 1.0. The factors will be discussed individually for all the sections. 6.1. Contribution from section properties factor Table 2 presents measured and nominal section properties for the exterior girder. The measured dimensions of the steel sections were slightly larger than design values. This leads to an increase in the section modulus as shown with the actual section factor. The experimental ratings are approximately 4% Žaverage. higher due to the increased section. At the critical section, the extra steel section increased the rating 8.7%. 6.2. Contribution from impact factor The measured impact factor had little affect on the rating. At the critical section, it actually lowered the rating 1.6% in comparison to analytical provisions.

6.4. Contribution from dead load adjustmentr correction factor

The dead load adjustmentrcorrection is clearly more of a correction for this bridge. Some of the analytical dead load stresses were found to be in error. The factor ranges widely and is not an indication of the benefits of field testing. This adjustment could be made and the posting changed by simply re-doing the calculation with measured or corrected stress inputs, therefore, it is not discussed here any further. However, barring mistakes or drastically different measured dimensions, this factor should be very near 1.0 since both are based on an analysis. 6.5. Contribution from bearing restraint effect factor

The bearing restraint effects increased the load capacity an average of 3.6%. The effect was fairly consistent and minimal. If further tests demonstrate that the effects are this trivial, it may be easier to assume a nominal reduction and not expend the time and effort to measure and calculate the effects.

6.3. Contribution from longitudinal distribution factor 6.6. Contribution from lateral distribution factor The longitudinal distribution factor shows that the pier regions did not act as stiff as the design analysis predicted. The average pier factor is 1.19 while the interior span centerline average factor is 0.97. More of the statical moment is going to the positive moment region and less to the pier region than predicted by the analysis.

As expected, the experimental lateral distribution characteristics had a large effect on the experimental rating. The average benefit was a 24% increase. For the critical section, the analytical rating was increased 21.6% just due to the lateral distribution.

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6.7. Contribution from unintended or additional composite action factor The unintended or additional composite action factor varied depending on the construction. The noncomposite sections definitely showed considerable partial composite action. The capacity at these sections increased an average of 32.3% due to the horizontal shear developed from the friction forces between the steel girder and the concrete deck. This friction, however, may not continue at high loads or over time. The owner can remove the effect simply by dividing the total experimental rating at these sections by the factor. Doing so would not affect this bridge since the reduced ratings at the non-composite sections still do not control. However, if a non-composite section does control, removing the benefit would significantly reduce the experimental rating. Since the critical section is composite, the additional composite factor should be and is small, increasing the rating a mere 4.2%. This could be attributed to slightly larger concrete area or stronger concrete, along with other minimal factors. 6.8. Contribution from additional system stiffness factor The contribution from additional system stiffness is easy to visualize, yet hard to pinpoint. This benefit is realized because components such as the curbs and railings are taking some of the load. The factors vary between 1.04 and 1.28. Theoretically the factor equation makes sense. The analytical wheel line moment is compared to an equivalent experimental wheel line moment Ždistribution factor is removed to get to a wheel line.. Since the experimental moment has been adjusted to match the longitudinal distribution of the analytical moment, this factor should be very similar to comparing the statical moments. The experimental statical moment is that from only the girders, therefore, the difference in the statical moments should be the amount taken by components other than the girders. Imhoff w2x compared the statical moment differences for many truck runs and compared the result with the additional system factor with some success. However, further work needs to be done to get better agreement. 6.9. Resulting field test posting load for Bridge R289 The critical interior span centerline experimental total rating is 231 kN Ž26 tons. for a single lane prior to any adjustments. This represents a 73% increase over a single lane analytical rating of 133 kN Ž15 tons.. Dividing 231 kN Ž26 tons. by the bearing restraint factor of 1.038 to remove the unreliable benefit, yields a reduced

rating of 222 kN Ž25 tons.. Since the section is already composite, this 222 kN Ž25 ton. capacity could safely be used as the single lane posting capacity of the structure, a 67% increase over the current posting. Since the legal load is 178 kN Ž20 tons., the posting can simply be removed and the bridge can easily carry a single lane of legal trucks. Part of the 133 kN Ž15 ton. restricted analytical posting includes reducing the two-lane structure wroadway width 6.1 m Ž20 ft.x to a single lane. To remove this restriction, superposition of stresses from separate transverse positions can be used to determine the experimental posting. Although the calculations are not shown here, superposition was used w7x for the critical exterior girder truck position w s E s 61.6 MPa Ž8.93 ksi.x and a truck position on the near edge of the adjacent lane w s E s 12.4 MPa Ž1.80 ksi.x. The resulting experimental posting is 193 kN Ž21.6 tons.. Even after removing the bearing restraint effects, the bridge could be opened for two lanes of traffic and the load posting removed. 6.10. True benefit from field testing Bridge R289 To truly examine the benefits reaped from the field test. The 222 kN Ž25 ton. limit should be divided by the dead load adjustmentrcorrection factor of 1.159 since this is an analytical correction. The resulting rating capacity is 192 kN Ž21.6 tons. for a 44% increase over the current analytical capacity rating.

7. Summary Field testing is a valuable means of evaluating existing bridges. Field testing provides the engineer with valuable knowledge of system response, load distribution, actual section properties, bearing restraint effects, and dynamic impact for the tested bridge. This information allows the rating engineer to reduce the inherent conservatism of current analytical rating methods and safely post the bridge for higher truck loads. However, the contributions from all the components acting in a load resisting system are complicated. The factors tending to increase the load capacity need to be separated and quantified to Ži. remove the unwanted contributions and Žii. confirm the origin of the useable benefit. This paper presents standardized procedures for load rating steel girder bridges through field testing. A systematic approach is presented to separate and quantify the contributions from various factors that affect the experimental rating capacity of the structure. Therefore, if the owner does not want to consider the benefit

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of bearing restraint forces, its contribution can be removed by dividing the experimental total rating by a bearing restraint factor for a lower acceptable capacity rating. Other factors can be removed in a similar manner. The testing system and standardize field testing procedures were developed from a series of diagnostic tests on a three-span steel girder bridge. The rating equation procedures presented in the paper have been optimized for time effort and cost. The results indicate that the current analytical 133 kN Ž15 ton. single unit weight limit can be removed for the single lane bridge. The field test results show a posting capacity of 222 kN Ž25 tons. after the questionable benefits from bearing restraint forces are removed. This is a 67% increase in the posting load over current analytical rating procedures.

Acknowledgements The authors gratefully acknowledge the Missouri Department of Transportation for support of this work.

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The opinions offered herein represent those of the author and not necessarily those of MoDOT. References w1x Lichtenstein AG. Bridge rating through nondestructive testing. Final Draft, NCHRP Project 12]28Ž13.A. Washington, DC: Transportation Research Board, National Research Council, 1993. w2x Imhoff CM. Testing and modeling of bridge R-289. Master’s Thesis. Columbia, MO: University of Missouri-Columbia, 1998. w3x McDaniel WT. Field test of bridge R-289 by utilizing University of Missouri Bridge field test unit. Master’s Thesis. Columbia, MO: University of Missouri-Columbia, 1998. w4x Frederick TL. Experimental load rating of an existing slab on steel girder bridge. Master’s Thesis. Columbia, MO: University of Missouri-Columbia, 1998. w5x MoDOT. Rating manual. Missouri Department of Transportation, Bridge Division, 1990. w6x Bakht B. Observed behavior of a new medium span slab-ongirder bridge. Structural Research Report SPR-88-01. Downsview, Ontario: Research and Development Branch, Ministry of Transportation, 1988. w7x Barker MG, Imhoff CM, Frederick TL, McDaniel WT. Field testing and load rating steel girder bridges. Draft Report 97-3, MCHRP. Missouri Highway and Transportation Department, 1998.