Dynamic Load Testing of Highway Bridges

RETO CANTIENI

ABSTRÀCT

Between 1958 and 1981 the Swiss FederalLaboratories for !¡laterials Testing and Re-search (E!4PA) perforned dynamic load testson 226 beam and slab-type highway bridges;nost of them were concrete structures. Testprocedures as well as measurement and ilataprocessing techniques are briefly described.As a certãin degree of standardizâtion was¡naintained over the years, it was possibleto draw some general conclusions fro¡n thesunmarized test resulls. In particular, therelationship between fundamental frequencyand maximun span is discussed as well as themeasured dåtnping values. Adalitionally it isshown that a highway bridge exhibits a pro-nounced dynamic response only if (a) itsfundaÍìentaL frequency lies in the same re-gion as one of the two vehicle modes ofconcern and (b) the vehicle speed and pave-ment roughness are tuned to each other sothat Èhe corresponding vibrations of thêvehicle will be excited. As a consequence'it has been proposed to define the alynamicload allowance for highrday briclge trafficloads as ä function of the bridgers funda-mental frequency in the new swiss code.Taking advantage of the rapiil developnent ínthe field of electronics, nerd ilata acquisi-tion and processing methods have been intro-duced at E¡,iPA in the last few years. Thesynchronous measurernent and recording of thetest vehiclers dynanic wheel loads and thebridge response is now possible as well assubsequent digital signal analysis.

Load tests on bridges hâve â l-ong tradition at theswiss Federal Laboratories for Materials Testing andResearch (EMPA). The first pubLications on suchtests stí1l accessible today stem fron the work ofM. Roé, who held a leading position at EMPÀ from1924 until 1949. He described the static and clynamicload tests perforned in ]-922 on the Kettenbrücke(iron chain suspension bridge) in Aarau (1). Hislegendary EI4PA Report 99 (21 gives the proce¿luresand results of 1oäd tests unalertaken between 1923and 1947 on 32 arch and 14 bean-type briélges, in-cluding the farnous structures of R. Maillart. Nêarlyall of these structures, most of the¡n highwaybridges, were subjected to static as well as dynarnicloads -

In 1960 Rösli published some results of dynarnictests on 20 prestressed concrete highway brialgesthat lrere performed by E¡'IPA in the years followingthe Roé era (3). Some of the test reports from thatperiod are still available. They contain the datathat v¡ere used to form the basis for su¡nrnarizingresults of dynanic tests on 226 highr¡ay bridgesperformed by EMPA since 1958 and presented later.

The guestion can be asked, how has it been pos-sible, in a small country like Switzerland' togather a considerable amount of experience in thefield of dynamic load testing of highway bridges. Àconparison of the various code provisions in forcesince 1892 shows that dynamic tests were requireilonly between 1892 and 191-3 and again since 1970.

14r

Therefore' ít has been largety due to the energiesand competence of the EMPA staff thaÈ the majorityof static tests on highway bridgesr which have beenrequired in switzerland since 1892, wère supple-mented by clynamic tests. It has not alYrays been a

sinple task to convince the respective clients(¡nainly Cantonal higlrway ad¡ninistr_ations) of theädvantages of dynamic tests.

There has usually been no reasoir to question theapplicability of the results of static tests. Suchtests, for exanple, allow conparisons of the rnea-sured behavior of the structure with the respectivecalculations. The situation has been somewhat dif-ferent for dynanic tests. The results of thesetests, such as natural frequencies, da¡nping' anddynanic increments (also referred to as impact fac-tor, dynarnic load factor, or dynanic coefficient)coulil neither be compared with calculate¿l values norwith corresponding data from code provisions. Eventhough the state-of-the-art has adlvanced consider-ably during recent yearsr this statement can berevised only slíghtly because

- Computing the frequencies of a sufficient nutn-ber of bridge natural ¡nodes is no longer aproblem today.

- The great number of atternpts undertaken in thelast alecades to predict damping values or cly-namic increments on a theoretical basis havenot led' as far as is known to the author, toany generally adopÈe¿l and easily applicablesolut ions.

- The qualitative and quantitative comparison ofrneasured dynarnic increments with corresponclingcode provisions is generally stí11 not possi-ble. fThe 1979 ontario Highsây aridge DesignCode is an exception (4).1

on the one hand, this is rnainly because the loa¿ls(to which the dynamic incrernents are related) useclin the Load Eests are not the same as those in thecocles. on the other hand, the question can be askedwhether the ¿lynanic effects of rnoving trucks on theresponse of a bridge are really predo¡ninantLy a

function of the average briilge spanr as assu¡ned' forexample, in the present Swiss Code (5).

The dynamic incrernent, as used in the design of a

bridge, is the value of greatest practical interestresulting fron dynarnic toad tests. If straightfor-ward cornparison ôf the tnèasured values with theoret-ical analyses or code provisions is not feasible,there remains the possíbÍlity of conparing the ex-perirnentaL values among each other. This can obvi-ously only be done in a significant way if theprocedures of testing, neasuring, and data process-ing are standardized as nuch as possible. Beingaware of this fact, the decision was made by RösIiand Voellrny in the I950s to standardize the EMPA

dyna¡nÍc load tests on highway bridges. AlthoughEMPA is not the only institution performing suchtests in Switzerland, the number of standard testsperformed by EMPA since that ti¡ne is considerable.

only in the last several years has the clientbeen able to profit from this experience. Now theresults of a current load test are related to theresults of all previously perforned tests. AddiÈion-atly interpretation of the collected test data has

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FIGURE I A two-axle vehicle Ì¡efore passing over a contactthreshold (speed meazurement) and over a 45 mm (l.B in.)thick plank.

led to a proposal for the definition of the dynanicincrement for a future Swiss Loading Code.

STÀNDARD DYNAIIIIC LOAD TESTS ÀS PERFORÈTED SINCE 1958

Test Procedures

The bridges are tèsted dynâmically through passagesof a single, fully loade¿I, tero-axle truck, nhich isprovided by the client. vlith a normal axle spacingof 4.5 m (I5 ft) the gross rdeight of the vehicleusually Iies near the legal limit of 160 kN (35kips). (In general 280 kN or 62 kips is the maxi¡numlegal gross weight lirnit in Switzerland for any tl¡peof vehicle.)

The test vehicle is driven at constant speed,whenever possible, älong the longitudinal bridgeaxis and always in the same direction (Fiqure 1).The tests are begun with a vehicle speed of 5 kn/h(3 ¡nph) which is then increâsed after every passagein steps of 5 to 10 kn/h (3 to 6 mph) up to themaxirnum achievable speed. Tests on the undisturbedpavement are repeated with a plank placed on theroadway in the nain neasurernent cross section. Theplank is approximately 50 mn (2 in.) thick and 300mm (12 in.) wíde.

Data Àcquísition

Deflection is neasured whenever possible at thecharacteristic point of the bridge, which is nor-nally at the nidpoínt of the naxinum span (nainmeasure¡nent cross section). In nany j.nstances de-flection is ¡neasured at additional points of thesuperstructure. This neasurêment is provided by aninvar wire stretchêd between the measurement poíntat the structure and a fixed reference point underthe briilge. fn about 1964 Èhe mechanical vibrationrecorders (vibrographs) that registered the deflec-tion signal on a rotatÍng cylinder \rere replaced byelectronic tneâsurement setups. These usuãIly consistof an inductive dísplacenent transducer, a signala¡nplifier, and an ink recorder.

Transportation Research Record 950

n=20 t=9.76s

þ= t't" t = 2.05 Hz, 8=.0a0

FIGURE 2 Deflection of the midpoint of the Ponte di Campagna Nova,which has one 4.5-m (148-ft) span, under the passage of a 160 kN (35kip) vehicle traveling at 251Ínlh (16 mph) on the undishrrbed pavement(f = fundamental frequency, ô = logarithmic decrement, and 4 =dynamic increment).

Data Processing

Frorn the dynanic deflection signals, registered onpaper stripsr Èhe following ínfornation can usuallybe obtained (Figure 2) !

1. The frequency of one or more modes of thebr iilge ,

2. Danping of the natural vibration dominant infree decay, and

3. The dynamic increment of one or more measure-nent signals as a function of the vehicle speed.

Natural frequency and associated danping can bedetermined by nanual signal analysis in the tinedornain only if the bridge víbrations decay harmoni-calIy after the passage of the vehicle. rf an ac-curate time neasure has been recorded along with thesignal, the natural frequency, Hz, can be estab-lj"shed by counting the number of periods in a givenportion of the decay process:

f=n/t

where

n = nunber of periods, andt = corresponding tine interval in seconds.

Danping, for exanple the logaritht¡íc decrernentô, can be deterrníne¿l fron the sane tirne interval.This requires measurenent of the magnitude of thefirst and last amplitudes having the sa¡ne phase

6 = I/n (Ln) (aO,/An)

where

n = number of periods (n+1 amplitudes), andln = natural logarithm.

The percentage of critical damping, p, is given by

p=(6/2n).I00.

The dynanic increnent is defined as

ô = (Aayn - A"¡¿¡)/As¡at . 100

where A¿*r, is the peak value of the bri¿lge responseduring a- passage of the test vehicle and À"¡"¡ is thepeak value of ¿leflection observed under static load-ing with the sa¡ne vehicle. Although the rea¿lout ofÀdvn from a paper strip can be performed in ast?aightforward manner, the deter¡nination of Astatmây be problenatic.

Two basically different nethods have been used.fn earlier years, As¡¿¡ was esti¡nated for everypassage from the sane dynanic trace as the corre-

¡=*re*

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sponding Adv.. with increasing ilynanic responseof a bridge,'the guatíty of this estimate for A"gu¡decreases. Therefore, ín recent years A.¡a¿ has beendeter¡nined fro¡n the traces of three. very slow pas-sages (crawl tests), and the value thus obtainecl issubsequently used in the evaluation of all the pas-sages. Comparative tests ín 1976 .shov¡eil that bothmethods yield the sane value for Astat if the driv-ing lane is marked an¿l accurately naintaíned by thedriver.

There is an additíonal problem in deter¡nining thedynanic increnent, ö. Àside from all the knor¡nquantities such as vehicle propertíes and speecl,pavenent roughness, and bridge properties, the posi-tion of the neasurement point in the cross sectionof the bridge rras foundr under certaín circum-stances, to exert a significant influence on thecalculated value of the dynanic increnent.

this is because the ¿leflection distribution overa bridge cross section under statíc an¿l dynamicLoa¿ling Ís generally not identical. Experience showsthat on the one hand, the deflection (Astat) of astatícally loadecl beam-type bridgè will always varyrnore or less strongly over its cross sectionr de-pending on the shape and stiffness of the section.on the other hand, the vertical motional ampliLudes(responsible for Ad.rn) of the same bridge underdynamic loading will- remain constant over the crosssection, as long as the bridge response consístssolely of longitudinaL bending nodes.

The dynarníc increnent, ô, as a relation betweenÀdvn and À"¿"¡ therefore will depend on the exact 10-cation of the measurernent point in the brídge crosssection. This dependency is al¡nost negligible forbridges with stiff box-shaped cross sections but mayinvalidate results gathered from briclges with wideand flexÍbl-e I-beam cross sections.

RESULTS OF TESTS ON 226 HIGHWAY BRIDGES

In the years from 1958 to 198L the concrete struc-tures section of E'IPA perforned load tests on 356bridges. fn the present contextr the standardizetldynamic part of 226 combined tests on bea¡n and slab-type highway brÍdges is of interest. The rernainingI30 tests concerne¿l tests on other bridge types andpurely static tests. Of the 226 brÍdges, 205 areprestressed concrete, 5 reinforced concrete, l4composite steel and concrete, and 2 prestressedlightweight concrete structures. The number ofspans varies betgreen 1 and 42 erith an average of 4

and a value of peak occurrence of 3. The most commonstructural systens are the continuous beam over norethan one span (72 percent) ând the simply supportedone-span beam (L2 percent). TotsaL lengths andlengths of the maximum span can be characterized asfollows s

ToÈaI Length l.laximurn SpanMÍninun I3.0 n (43 ft) 11.0 rn (36 ft)!,laximun 3,147.5 rn (10'300 ft) 118.8 m (390 ft)I{ean 155.9 n (51r ft) 39.5 m (130 ft)

of the briclges tested, 109 are straight and wíthoutskew,97 are skewed or curved, and 20 are bothskewed and curvecl. Half of the bridges have a box-shaped cross section, 26 percent a multibeatn deck,and 24 percent a solíd or hollow-core slab crosssection. The cross-sectional width varies between4.3 n (I4 ft) ând 30.4 m (100 ft) with a nean valueof. ]-2.9 m (42 ft).

The spring constantr defined as gross weight ofthe test vehicle divided by À"¡¿¡r was found to liebetweên 7 kN/mn (40 kip,/in.) änd 800 kN/mm (4,500kip,/in. ) with a mean value of 173 kN/nn (Ir000

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kip,/in.) and a value of maxímum occurrence of 75kN/rnm (430 kip,/in.).

These data are taken from a computer data bankset up in 1981 in which up to 40 paraneters fromeach of the 226 dynamic load tests are stored.Detailed information on the distributions of theseparameters (test conditions as well as results) isgiven in EMPA Report 211 (6). The ¡nost itnportantfindings derived from these data are presented inthe paragraphs that follow. The nunber of ¡neasureclvalues clisplayed in the different figures does notequal 226 because (a) it was not possible to meeta1I the requirenents of a standåral test as describedpreviously for all tests and (b) the value of con-cern could not alvrays be determined frotn the re-corded signals.

Fundamentaf Frequenc ies

Figure 3 shows the distribution of fundanental fre-quencies measured on 224 bridges. The result of anattenpt to establish a relation between the funda-mental frequency of a briõlge and the length of itsmaximurn span is shoren in Figure 4. The scatter of

1234 56 789 t0 >105

Fundomenlol Frequency f IHzJ

FIGURE 3 Distribution of the fundamentalfrequencies, f, measured on 224 bridges.

Moximum Spon L I fl J

40 80 120 160 ?OO 240 280

+oe=41+os

dt= I o.8t Hz

0r020304050607a8090Moximum Spon L [mJ

FIGURE 4 Fundamental frequency, f, as a functionof the maximum span, L, for 224 bridges (o¡ = standarddeviation).

the neasured values around a curve determinedthrough nonlinear regression is consiilerable. Thisis not surprising in view of the J.arge varíations ingeo¡netry and stiffness ôf the bridges tested. Toachieve a snal-ler scatter, the following lirnitatíonswere introduced:

- Eli¡ninate cantílevered structures'

10

II

Ed

"z

I44

- Limit the horizontal radius of curvature of thelongitudinal bridge axis to >900 n (3,000 ft),

- Limit Èhe skew to <15 deg., -- Li¡nit the spring õnstant, k, to 70 kN/nm < k

kip/in.),- Elininate results that were not obtained fron

measurements on the maximum span.

The regression function cal,culated from the remain-ing 100 values is almost identical to the functionshown in Figure 4 but the scatter has di¡¡inishedfron o¡ = 10.81 Hz to of = !0.62 Hz. As will- be shownIater, the dynamic increnent will probably be de-fined in a futurê Svriss Code on bridge loading as afunction of the bridge's fundamental frequency.Therefore, the previously ¡nentioned equation toestimate this frequency from the maximum bridge spannay be of sone practical value in the early designstages.

Danpinq

The distribution of the logarithnic decrement, ôrdeter¡nined fron the free decay process of 198 con-crete bridges is shown in Figure 5. Bêcause theð-values âre scattered over a consideråb1e range,an attempt r.¡as made to separate the bridges ¡riÈhrelatively strong damping from those with relativelyweak damping.

0" .01 .05 .09 t3 .17 .21 > 22

Loqorithmic Decrement ô

FIGURE 5 Distribution of the logarithmicdecrement, ô, measured on l9B concretebridges (minimum, 0.019; mean, 0.082;maximum,0,360),

considering the superstructurê of a bridge as avibrating bo¿ly, overall damping can be separatedinto internal or structural danping and external orsysten danping. Thus, structural damping is ¿lue toenergy clissipation during all kinds of vibrations ofthe superstructurei and system damping is due toenergy dissipation during relativê movernents betweensuperstructures and substructures and during allkinds of vibrations of the substructure elements.Because only eoncrete bridges were taken into ac-count, material damping was not considered. Infor-nation was not available on the influence of vibra-tional anplitude on clamping¡ therefore, the dampingrelation between this amplitude and the maximun spancould not be taken into account. As r+ill be seenIater, thís influence does not seen to be sig-nificant when cornpared with the factors actuallyunder consideration.

Concerning structural dampíng it was founil thatbridges responding predominantly in a longitudínaInode of flexure have on the average a smaller loga-rithmic decrement than bridges that respond Ín a

Transportation Research Record 950

superposition of noales of longitudinal flexure,torsion, and transverse flexure. Straight structureswith narrow, closed cross sections showed a neanô = 0.063 (p = f percent) ¡ and curved or skevredbridges with wide cross sections showed a mean ô =0.087 (P = 1.38 Percent).

The El,lPA data bank does not contain the necessaryinformation to investigate system da¡nping in detail.For example the type of bearing constructions andthe relative stiffness of piers are often not givenin the test reports. An ãnalysis to confirn theresults of Green (]) showed that the total lêngth ofa bridge indicates the danping to be expected. Longbridges with a total length of ¡nore than I25 n (410ft) showed a mean ô = 0.056 (p = 0.89 percent),and short bridges erith a total length of less than75 m (246 ft) a mean ð = 0.098 (p = 1.56 percent).Àlthough the total length of a bridge surely influ-ences the previously mentioneal parameters of struc-tural damping, it presumably also reflects influ-ences of system damping. The long structures havean average of 0.26 supports per 100 n (33 ft) ofbridge length, the short ones a corresponding valueof 0.60.

Figure 6 shows the ô-distributions for twoclasses of bridges, which were formed by combiningthe paraneters of structural dånping and the totalIength: (a) Iong, straight bridges with narrow,closed cross sections and (b) short, curved and/orskewed bridges with wide cross sections. These twoclasses seem to have well separated mean values anddistributions of overäll da¡nping.

01 05 .09 13 .,7 .2, >.22

Logorilhmic Decrement à

FIGURE 6 Distributions of the o-values for trvoclasses of concrete bridges.

ic Increments

Where a falsifying influence of the locatíon of therneasurernent point in the body cross section was tobe expected, these values erere elinìinated beforeÈrying to interpret the measured dyna¡nic increments,0. When the measurement point lies outside theregion of direct influence of the test vehicle, theresulting O i{ill be too high to a .Lesser or greater

fl Long, straight, narrow bridgssw¡th clogd c.os stion:21 values {min., O.030; mean,0.0¿E; max..0.079).

I short. cur""d and/or d(ewd,w¡de brilgos: 33 values (min,,

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u =4-A

FIGURE 7 Definition of the coefficient, a, describing thelocation of the measurement point in the búdge cross sectionrelative to the direct influence region of the vehicle,

Mæn Spon.L, IflJ40 80 120 460 200 240 280

0 t0 20 30 4050 60 70 8090lleon Spon L, ImJ

FIGURE B Dynamic increments, @, for passageswithout a plank as a function of the mean spanL. for 73 concretebridges (a < 0.8). Thesolidline indicates the provision in the present Srdssfüde (ä).

extent dependíng on the transverse bending stiffnessof the bridge. Therefore, only data corresponding¡6 c 1 0.8 (see Figure 7 for the definition o¡ c)were retained.

First the relationship between Èhe dynarnic íncre-¡nent and the nean span of the bridge was investi-gated. It cân easily be seen from Figure I thatthis reLationship does not correspond to the func-tion inplied from the present Swiss Code (5). Eachof the measured 4-values displayecl in Figure Irepresents the peak value of the élynanic incrementsestablished from a standard test with the vehiclepassing over the undisturbêd pavement. As mentíonedearlier the conparison between rneasured values andcode provisions can be qualitative only.

Then Ít was found that the fundarnental frequencyis an inportant paraneter Ínfluencing the responseof a bridge to the passage of a test vehicle, Ageneral explanation for this observation is as fol-1ows. The bridge as well as the vehicle are rnechan-ical ¡¡ass/spring,/darnper systetns whose dynanic be-havior is cleter¡nined by natural modes; thê bridgeresponse will therefore be influenced by the rela-tíonship between the frequencies of these nodes.Stated differently, one of the conilítions to befulfilled for a distinct bridge response is a match-ing of the frequencies of dynamic wheel loads andthe natural node of a briclge.

The natural nodes of the EMPA test vehícles werêpresurnably scattered over a relatively narrow fre-quency band. Thê vehicles were all fully loadedtwo-axle tip trucks with leaf springs. Under normalconditions of pavement roughness, the dynarnic wheelloads of such vehicles occur naínly in two frequency

145

ranges: (a) in the range of the bo¿ly bounce fre-quency betv¡een -2 Hz and *5 Hz and (b) in the rangeof the wheel hop frequency bet\reen -I0 Hz and -15 Hz.The body bounce ¡node of a vehícle is excited by rel-aÈively long waves, and the wheel hop node by reta-tively short waves of the roadway unevenness. De-pending on the vehicle speeil, an unevenness of acertain length may be effective ín both ranges. Asan exanple of special interest, a plank excites bodybounce vibrations for low vehicle speeds and almostpure wheel hop vibrations for speeds above 40 krn/h(25 ¡nph) .

Figure 9 shor¡s the maxirnum dynanic increnents,ô, of test series on the undisturbed pavenent as afunction of the bridge's fundarnental frequency. Thecurve encompassing the moasured values shows a clearpeak in the region of.2.5 Eo 4 Hz (í.e., in theregion of the vehicle's body bounce frequency). Thestate¡nent that the shortwave amplitudes of normalpavements are too smal1 to excite the vehicler swheel hop mode significantly is based on one singlemeasurement value in the corresponaling frequencyr ange.

Fro¡n the equivalent diagran for thè test serieswith a plank lying on the roadway (Figure l0) it canbe seen that a first resonance peak lies in therange of I.5 to 3 Hz and a second peak at frequen-cies above 7 Hz. Taking into account that (a) aplank represents a large anplitude of excitationco¡npared with the anplítudes ôf roughness of a usuaLpavement and that (b) the natural frequencies of anonlinear system, such as a leaf-sprung truck, de-crease with increasing anplitude of excitation, thenthe two observecl peaks lie in the range of the bodybounce and the wheel hop frequencies of the vehi-cles, rèspectively.

It can also be seen from Figures 9 and I0 thatthe dynamic increments do not follow a one- or t¡ro-peaked resonance curve. Instead they are scatteredbelow the peaks. Unfortunately the infor¡nationavailable is not sufficient to determine the reasons

.t

0,t 2345678910Fundomenlol Frequency f IHzì

FIGURB 9 Dynamic increments, S, for passageswithout a plank as afunction of the fundamentalfrequency, f, for 73 concrete bridges (a < 0.8).

300

0.1 2345678910Fundomentol Frequency f IHzJ

FIGURE 10 Dynamic increments, @, for passageswith a plank a¡ a function of the fundamentalfrequency, f, for 69 concrete bridges (a < 0.8).

èe

€Ie

*¿Eo

ò

r00

:QL80

E

;Et.'

Ò

àe

.ô-

6 200E

;5 't00

Õ

l4

1"""r-. \"op ^ \--

o uA o- o^oo' ',s.åYe

" %' iq o

r-46

for this scatter in detail. Nevertheless' an atternPtwas made to evaLuate the influence of three param-eters available in the EMPA data bank (6). It \tasfound that darnping does not appear to exert an i¡n-portant influence on the dynamic increment and thatstraight' beân-type bridges respond only slightlymore strongly than more complicated structures. Theinvestigation also showed that the influence ofpavement roughness on dynaníc increments for pas-sages srithout a plank was greater for Èhose bridgeswith rnediun or poor pave¡nents than for those withsnooth pavements.

Hence, the interpretation of the EMPA test re-sults can be sum¡narized as follows: A híghway bridgeexhibits a pronounced dynamic response only if (a)its funda¡nental frequency lies in the aa¡ne region asone of t-he tvro vehicle ¡nodes of concern and (b) thevehicLe speed and pavènent roughness are tuned toeach other so the corresponding vibrations of thevehicle will be excited.

rhe solicl curves indicated in Figures 9 and l0were proposed to be integrated gualitatively intothe nen version of the swiss code on High\day BridgeLoadings. The curve shown in Figure 9 is si¡nilar tothe curve used in the 1979 Ontario Highway aridgeDesign Code (4). Hordeverr these tl¡o curves vteredeveloped independently ancl were based on two dif-ferent sets of experinental data.

CURRENT EMPA TESTING METHODS

Scheduled Tests vrith a single vehicle

To reduce variations in the load paraneters further'several inprovements have been introduced in recentyears. Unfortunately EMPA has not been able toobtain its own test vehicle. It is now possiblethowever' to make use of the same Army vehicle forall the tests. Payload' tiies, and tire pressureare always the sane' so it can be assu¡ned that thedynamic properties of the vehicle remain approxi-nately constant.

In addition the driving lane is marked with rub-ber cones or paint (Figure 1) and the vehicle speedis controLled with a special test wheel (FigureII). Thus it is possible to maintain constant speeal

FIGURE 11 With the help of a test wheel, the speed can be

accurately measured and controlled during the entirepaEsage.

Transportation Research Record 950

FIGURE 12 Servohydraulic vibration generator (SCHENCK).

within t0.5 kn/h (0.3 nph) fron 2 km/h (1.2 mph)up to 100 km/h (62 mph). Furthernore, additionalpassages are carried out in the range of criticalspeecl, where the bridge response reaches íts peakvaIue. In this range' detected by on-Iine prepro-cessing of the measurement signals, the speed incre-rnent, Àv, ís thus reduced to I to 3 km/h (0.6 to1.9 nph).

ExciÈation with a Servohvdraulic Vibration Generator

The natural frequencies of a bridge can be deter-mineil very precisely through swept-sine excitationwith a vibration generator. In 19'17 EMPA purchasedthe servohydraulic actuator shown in Fígure 12. Thewhole system is ¡nobile and produces a sinusoidalforce vrith a peak value of 5 kN (1.1 kip) in thefrequency range between 2.3 Hz an¿l 20 Hz. For fre-quencies below 2.3 Hz the force clecreases vrith thesquare of the frequency because the piston stroke islinited to t 50 mn (t 2 in.). The nost i¡nportantadvantage of such a vibration generator is the highfrequency resolution that can be achieved. If asuitable function generator is used to proalucê thedrive signal, this resolution nay be as high as0.001 Hz. After looking for the natural frequencíesfirst in a relatively quick sweep, these can laterbe evaluated in greater detail.

I¡npulse-fype Excitation

Determination of the natural frequencies or in factall nodal paraneters of a structure through inpulse-type excitation is com¡non in the investigation ofnachines, for exarnple, tool ¡nachines. The structureto be analyzed is struck with single hammer bloets

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and the input force and system responsê (accelera-tion) are ¡neasured to yield the infornation neces-sary for the nodal analysis. In contrast to aserept-síne excitation as described in the previousparagraph with a Long duration -input time signal anda very narrow-bantl frequency spectrun, an inpulse-type excitation provides a tine signal of very shortduration with a corresponilingly broad frequencyspectrum.

The advantages of the impulse-type excitation areobvious: (a) exciting rdith a portable hammer is ¡nuchIess expensive than exciting with a cornplete servo-hydraulic systen, and (b) one hamrner blow excitesalI nodes si¡nultaneously whereas sweeping throughthe frequency band of concern nay be tine consuming.

Disadvantages have to be considered when applyingirnpulse-tyÞe excitatíon to highway bridges. Due torelatively strong damping, the arnplitudes of thebridge response may not be significant for a tirnelong enough to aIlow the ¿leternination of a spectrumwith a reasonable frequency resolution. rhis fre-quency resolution (i.e., the distance bet¡¡een twolines in the spectrum) equals one time window(length of the signal in seconds). Consiclering anean logarithnic decrement for bridges of 0.082(p = f.¡ percent, see Figure 5), and a correspondingfundamental frequency of 3 Hz, the initiaL amplitudewill be damped out to 50 percent after 2.8 sec andto 1 percent after 18.7 sec. Thus, assuming aninitial signal-to-noise ratio of at least 40 dB forsuch a free decay process and transforrning one 20sec time window, a frequency resolution of 0.05 Hzcan be achieved. This resolution is consiclerablyIower than that resulting from a swept-sine excita-tion test.

CURRENT DÀTA ACQUISITION AND PROCESSING IT1ETITODS

Digital Signal Analysis

With neasurement signals recorde¿l on paper stríps,data processing is possible in the tirne donain onlyand has to be perforned manually with the help ofpencil and ru1er. Several problems occurring withthis kincl of data processing have bèen mentionedearlier. To solve these problems and above all toallow digital data processing ând signal analysis inthe frequency ilomain, a Pulse-Code-ModuLation (PCM)system was purchased by EIqPA in 1977. Analog signalsfrorn up to 32 transducer/amplifier units are iligi-tizecl by this systen with l2-bit resolution and thenrecorded in digital forrn with a tape unit. Uponplayback, the signals can be transferred directly indigital form to the disk of a conputer. With thehelp of corresponding softvrare, standard time domaínanalysis can then be performed si¡nultaneously foralJ. sígnals.

FrequencÍes of the bridgers natural rnodes aredeternined by transforning the time signals intofrequencies (i.e., by calculating povrer spectra,Figure 13). This task is perforned by a Fast FÕurierTransform (¡'FT) analyzer, Nicol-et 6604, directlylínked with the computer. Because the FFT analyzeris able to treat two ti¡ne signals si¡¡ultaneously, ítis possible to investigate the phase and ânplituderelationships between two signals as a function offrequency. Hence, the shapes of all modes contribut-ing to the signals can be deternined.

Measurement of Dvnanic l{heel Loa¿ls

Until sufficient know].edge of the test vehicle'sdynaníc wheel loads is acquired, no ¿letailed inter-pretation of the bridge response r¿ilI be possible.The first successful attempts to solve the problemof cont'inuously rneasuring dynamic forces between

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FREQUENCY tHzlNote: I9G 02: Deflection at the midpoitrt of the largest span.Horizontaì axis: Frequency 0.50 Hz, 400 lines, logaithmicsale. Vertical uis: Mem-squared displacement densiÇ,full rcale = 0.1 mm2lEz (diviie by 64i to convert ro sq.in./Hz), logarithmic scale,

FIGURE 13 Power spectral density (PSD) obtained fromthe free decay of a three-span prestressed concrete bridge,32 m,4L m, and 3ó m (105 ft, 135 ft, and 120 ft). Thefrequencies of the first three modes are indicated.

tires and the riding surface were made in thè UnitealStâtes and Gernany (8r9).

The dynanic wheel loacls of a highway vehicle canbe measurecl dírectly or indirectly. with the directnethod the normal wheel hubs are replaced by spe-cially designed and instru¡nentecl neasurement hubs(force transducers). This methoil is very accuratebut also very expensive (!q). vrith the indirectnethod, the tire is used as a neasurement spring(i.e., the deformation of the tire is rneasurèd in-stead of the force). The necessary calibrationcurve of the force vèrsus defornation relationshipmust then be determined ín an additional stâtictest. Conparative tests shov¡ed that vertical tireCleflection reflects the actual wheel toad with suf-ficient accuracy (11).

Therefore, ín 1977 EIrPA acquíred an opto-eIec-tronic system that neasures the distance between thevehiclers axle and the riiling surface (i.e., thevertical tire deformatíon) vrith the aid of an in-frared bea¡n reflected on the pavement (Figure 14).The nain parts of the measurenent setup are an in-frared e¡nitter, a reception camerar and an elec-tronic control unit. The vertically oriented in-frared enitters are locate¿l next to the tires; thereceivers are slanted at approxirnately 45 degrees.The infrared light with a wavelength of 930 nn isfocused by an objective so as to produce a spot on

FIGURE 14 Two opto-electronic systems for meazuring thedynamic wheel loads mounted on the rear axle of a test vehicle.

DETBUEL BRIDGEINSTR. , U6Ø?TEST NO 2ø65 3.1ø HZ

FREE DECAY

148

the pavement surface of apProxÍmâte1y 15 nm (0.6in.) dianeter. The essential component of thecamera, which observes the position of the liqhtspot, is a two-diÍìensional photodetector.

Electric currents that give ã measure for theposition of the center of the light spot are inducealin the photodetector. these currents are transfornedto voltages that can be stored on magnètic tape bythe PCl,!-data-acquisition system' which is installedeÍther in the clriverrs cabin or neår the bridgebeinq tested. Signal transfer from the rnoving vehi-cte to the stationary PcM-system is performed by a

2.45 clf,z telemetry Iink. Hence, measurenent signalssterì¡ning from vehicle and bridge are acquired andrecordeal synchronously. Figure 15 shows an exanple

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Note: Horizontal uis: Frequency 0-100 Hz,400 lines, logaúthmic scale.

Vertical axis: Mem-squred dynamic wheel load density, full scale = 103

kN2/Hz (divide by 20 to convert to q. kip/Hz), logarithmic sale.

FIGURE 15 Power spectral density (PSD) of a dynamic wheelload signal,

of the power spectral density ôf a neasureil dynanicwheel load. The four-channel dynaníc wheel loacl¡neasurenent system was extensively used withÍn thefranework of a research program but has not yet beenintroduced to the standard dynanic tests on highwaybr idges.

Transportation Research Record 950

REFERENCES

1. ¡.t. noé. versuche und Erfahrungen an in derSchweiz ausgeführten Stahlbauten I922-I945.Technische Kommission des verbancles Schweizerí-scher Brückenbau- und Stahlhochbâu-Unterneh-mungen, zurich, 1951.

2. M. Roð. Versuche und Erfahrungen an ausgeführ-ten Eisenbetonbauwerken in der Schweiz 1924-L937. EMPA Report 99, 1937, ltith supplenentsfor 1939, 1940' 1943, 1945' and 1947.

3. A. Rösli. Ueber das dynamische Verhalten vonvorgespannten Brücken' 6. IABSE-CongresstStockholn, 1960' preli¡ninary report.

4. ontario Highway Bridge Desígn code' 1979.ontario Ministry of Transportation and Cornmuni-cations, Downsvieht, ontârio, canada, Jan. 1979.

5. Sch\deizerischer Ingenieur- und Architekten-Verein. Normen für clie Belastungsannahmen, dieInbetriebnahne und die Ueberwachung der Bauten,Nr. 160, Zurich' tlay 1, 1970.

6. R. Cantieni. Dynamic Load Tests on HighwayBridges in Swítzerland--60 Years Experience ofEMPA. EMPA-Report 211, 1983.

7. R. Green. Dynamic Response of Bridge Super-structures, ontario observations. SupplementaryReport SR-275. Proc.' Symposium on DynamicBehaviour of Bri¿lges' Transport and Road Re-search Laboratory' Crowthorne, U.K.' 1977.

8. A. Mühlfeld. Entwícklung eines hochfrequênz-technischen verfahrens für Reifen- ul¡d Schwin-gungsmessungenr Diss.' TH Braunschweiq, 1949.

9. Dynamic Pavenent Loads of Heavy Highv¿ay Vehi-c1es, NCHRP Report I05. TRB, National ResearchCouncil, washingtonr D.c., 1970.

I0. P. sweatman. The Dynaníc Loading Performanceof Heavy Vehicles Suspensíons. Proc., Austra-lian Roâd Rêsearch Board' vol. 9, 1978.

11. v. Gersbach et a1. vêrgleich von Verfahren zurMessung von Radlastschwankungen, Autonobil-Èechn. zeitschrift, vol. 80' 1978.

Publication of this paper sponsored by Committee on Dynamics and Field Test-

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