Dynamic load testing of Swiss Bridges.PDF

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Dynamic Load Testing of SwissBridges

Essais ynamiquesespontsSuisses

Dynamische ersuche n Schwerzerrticken

Olivier L. BIJRDET

PhD

IBAP

SwissFederal nstituteof Technology

CH-1015 ausanne

SWITZERLAND

OlivierBurdet,bom n 1960,holdsa

PhDfrom theUniversity of Texasat

Austin. He is in chargeof field testing

for IBAP andalso nvolved nteaching

and esearch ctivities.His research

interests ncludebridgeevaluation nd

mairfienancend designby testing.

Sqmmary

The paperpresentsesultsobtainedusinga simple dynamic estingprocedure requentlyused n Switzerland

for highwaybridges.Typical resultsare shownand tendencies bserved n the dynamicbehaviorof highway

bridgesarepresented. or a deteriorated avement,he dynamic mpactof heavyvehicless shown o be larger

at lower truck speeds.Some correlation s shown between he dynamicpropertiesof the bridge and its

stiffiress,as measuredn a static oad est.

R6sum6

L'articleprdsentees rdsultatsobtenusd I'aided'unemdthoded'essaidynamique implefrdquemment tilisde

pour esponts outiersen Suisse. e comportementypiquedesouwages,ainsique es endances dndrales esponts routiers sont prdsent6s.Dans le cas d'un revCtementd6t6rior6, il est montrd que le coefficient

d'amplificationdynamique stplus grand onque la vitessedepassage esvdhiculesestplus frible. Quelquescorrdlations ontpr6sentdesnile lespropri6t6s tatiques t dynamiques esponts.

Zusammenfassung

Im vorliegenden Bericht werden die Ergebnisse einer einfachen Testprozedur fiir dynamische

Belastungsversuche riisentiert die in der Schweiz oft fiir Briicke verwendet wird. Typische

Versuchsergebnisseund Tendenzen der Arfobahnbriicken werden pr?isentiert. Bei beschiidigter

Fahrbahnfliichest der dynamischeAmplifikationsfaktorbei kleineren Fahrgeschwindigkeiten ft griiBer.

GewisseKonelationenzwischendenstatischen nddynamischenBriickeneigenschaftenerdenaufpzeigt.

StdphaneCORTHAY

Assistant

IBAP

SwissFederal nstinrteof Technolory

CH-1015 ausanne

SWITZERLAND

Stdphane orthay,bom n 1967,holdsa

Civil Engineering egree rom EPFL.

He is involved n staticand dynamic

load estingof bridges.



1. Introduction

Dynamic oadtesting s an importantpart of the acceptancerocess or new bridges n Switzerland SI. As acomplemento static oadtests,dynamic estsyield useful nformationabout he actualbehaviorof the bridgeunder traffic. This information s usually difficult to obtain analytically,becauseof the complexity of theactualstructure.The effect of pavementdeteriorationon the dynamic esponse f the bridge s of particular

importanceor theflumagement f the structure.This informationcanbeeasily and realisticallyobtained roma dynamic est,and hereafterusedby the highwayautlroritieso organizehepavementnaintenance.

Drawing on the largenrxnberof testsperformedby IBAP overthe past wentyyears, he paperdescribeshegeneraldynamiccharacteristics f highwaybridges.Somecorrelation s shownbetweenhe stiffiressobtainedin a static load test andthe measured igenfrequencies.he influenceof the speedof a vehiclecrossing hebridge s discussed.n particular, he effectof an artificialdamagen thepavement n the dynamicbehavior spresented.

2. Dynamic Load Testing

Thepurposeofthe dynamic oad est s to determinehecontrollingparameters fthe dynamicbehaviorof thebridges. The main dynamic characteristicsof the structwe are the fundamentalvibration frequency, hedynamicamplificationfactor and the logarithmicdecrement.Thesepropertiesarc usually not analyzed ndetail in the desrgnphaseof small and middle sizedstructues. Somepararneters, uchas the logmithmicdecrementor the dynamicamplificationfactor, can only be roughly estimatedat the time of the design.However, hesequantitiesarerelativelyeasy o obtainexperimentally, ndcangive valuable nformation orthe exploitationandmainteruulcefthe bridge.

The generalmethodology sed n the dynamic oadtestof a bridgewill be presented singasanexamplehe

bridge shown in figure 1. The Riddes-Leytronbridge is a cable-stayedstructgrewith a slender declqsupporting wo lanesof traffic and a sidewalk.The bridge was built from 1991 o 1992 arfi tested n thesummer f 1992.

i -j :,==::i:::::.::-:i ::-:i

i ii 11.39m i+ - - - >

b) CrossSection) Side ViewFigure l: Geometryofthe Riddes-Leyhon bridge

3. Methodology

Dynamic oadtesting s performedby exciting hevibrationofthe bridgeandby measwing ts propertiesafterthe excitationhasceased.Severalmettrods re available or theexcitationofthe bridge, n particular:eccentricrotatingnrrsses, mpactof a heavyweightandpassage f a loaded ruck. This lastmethod s oftenpreferredfor the dynamic oad testingof bridgesbecauset gives,alongwith reasonably ccurate aluesof the above



mentionedquantities,a good of the effect of the actual raffic on the structure.By varying the

speedof the truck onthe bridge, he full rangeof traffic speeds anbe investigated. urthermore,his method

is easily mplementedwhile someof the other onesnecessitatemore complicatednstallationprocedures. he

measurements re taken and recorded by a dynamic data acquisitionsystemwith integratedFast-Fourier

Transform FFT) Myzer, allowing an mmediate nterpretation fthe resultsduring he test.

The trucks used or the dynamicexcitationof the bridge are usually3-

axle tnrcks, as shown n Figure2, with a total weight of250 kN (total

ilnmsof 25 mefic tons), navelingon ttrc bnidgeat severalspeeds.The

effect of a deterioration fthe pavements simulatedby the infoduction

QU""r= 190 kN Qt olt= 60 kN of a nornmlized plank on the path of the tnrck. This induces a strong

impact when the tnrcks passesat mid-span, that representsthe effect of

a pothole in the pavement, or the inegularity of the surfrce caused by

packed snow. Figure 3 shows the location of the insfurrents and the

resul8 for the dynamic testing of the Riddes-kyhon bridge" Absolute

displacement sensorsare used for the measuremerils, fid therefore only

components with a relatively high frequency (larger than 0.2I{z) are

recorded, as shown in Figure 3a (vertical dispacement at mid-span as a

i -z.oo i -3.80m l-r.zo il-t+----_------.*tr-=.--.-*-rr

function of the truck position).The static nfluence ine of the truck passlngon the bridge is then added o

obtainthe completedynamic nfluence ineshown n figure3b.

t---f.iil

tffiJ--->

Qtot4= 50kN t 1 kN

Figure 2: Typical truck used ndynamic oad esting

a)

b)

Static deformation

dstat

Dynamicdefonnation

d = < D ddyn stat

4.

a) Relativeverticaldeflectionat mid-span aused y thepassage f the ruck

b) Dynamic nfluence ine ofthe passage fthe truck. Determination fthe dynamicamplification actor.

Figure 3: Description ofthe dynamic load test

Results

Because lot of infornration s gatheredn the courseof dynamic oad esting, heresultsare usuallypresented

graphically.First, as shown n figure3b, the dynamicnfluence ine ofthe bridge subjectedo thepassage f a

truck is drawnfor all travel speeds,with and without plank. This allows a simplevisualdetermination f the

dynamic amplification factor O. The natural frequencyof the bridge is obtained rom accelerationspectra

performedby the FFT analyzer. The logarithmicdecrement s obtained rom the decayof the bridge free

i\,ft r"i .,,.r",rV

ii,



( ' oscillations,after the truck has left the bridge, or at least when it is frr enough from ttre instnnrents

(figme3a).

FrequencyHzJ

Cross ection

g- Eoxgilderwihcor$ntdepfrt

: Boxgiderwithvariable" detrt

- shbrr Open multipbgir&rs)

I Compoc*te

e Arch

s- Cable+tayed

-CantieniFo.v dh,

60 80

Spanm]

Figure4: Naturalfiequencyasa fimctionof bridgespan

Figure 4 shows he natural requencyas a finrctionof bridgespan or approximatelyninetyb'ridges estedby

IBAP. The regressionine wasproposedby Cantieni n [5, 6], basedon over200 dynamic oadtests of Swiss

bridges. A good correlationhas beenfound betweenhis dataand datagatheredby IBAP. Figure5 shows

histograms ftlre dynamicanplification fictor for thebridges estedover thepast wentyyears.Somebridges

arenot included n theSaphsbecause f inconpletemeasurementsn the earlyyearsof dynamic esting.The

Swisscode or bridgesspecifiesa constantdynamicamplification rctor for bridgesof 1.8. t is interesting o

note that a significantnunrberof bridges ndicatedhigh€rvalues.Most bridges hd a smalldaryiry, withlogarithmicdecrements maller han0.1.

12

[%of bridges]

25.3?2.7 Z1.g

18.7

5.3 6.7

1.0 1.2 1_5 1.8 2. 1to 1 2 to 1.5 to 1.8 to 2.1 to 2-5 > 2.5

Dynamic mplificationactoro

Histograms f the dynamicproperties

41.7

21.419.0

6.0 6.02.4 3.6

0.00 0.05 0.10 0-15 0.20 0.25to 0.05 to 0.10 b 0.15 o 0.20 to 0.25 to 0.30 > 0.30

Logarithmicdeqement 6

14024mm0

C,

50

30

2g

1 0

Figure5:

Figure6 shows he maximummeasured ertical accelerationand displacemed nducedat mid-spanof the

Riddes-leyfon bridge by a passrqgruck, as a firnction of the truck speed. n the case of smoth riding

surface, he responseof the bridge is small and increaseswith increasiqgspeed,vrhile in the case of a

deterioratedsurfrce, the responses strongand the influenceof tlre truck speed s less clear. It has been



1. 5

0.5

2.5

. -- j3 - \-i-]:- Wth plank

rl'n,J10 ml

\Wrthoutplank

20 30 40 50

Truckspeed kmihl

2-5 r-------

i"

Im/szl^ i m a rZ i -

i

i

1 5 i ' -

i

40 50

Figure 6: Dynamic responseofthe bridge as a tmction ofthe truck speed

observed in other structures that lower truck speedscan induce larger displacements (and accelerations) than

higher speeds. One possible explanation is that a truck traveling at a lower speed will cause two distinct

shocks as its two rear axles successively hit the plank. At low truck speeds, the two shocks occur

approximately one second apart" which is close to the natural period ofmost bridges. At higher speeds, he two

shocks are very close to one another, and remote from the natural frequency of the bridge

Human beings are especially sensitive to vibrations, particularly to accelerations. Some codes [2] give limiting

values of acceleration as a firnction of the fundamental frequency- In practice, ttrese limits ars never reached if

the riding surface is smootlr, but they can be exceeded in the case of a deteriorated pavement. The results of

the dynamic load tests give therefore useful information on the sensitivity of the bridge to deterioration, and

can be used in defining the maintenanceprogram for the pavement.

5. Comparison with the Results of Static Load Testing

Because dynamic loading testing is usually performed on b'ridges that have also been subjected to static load

testing, comparisons can be made between the behavior of a bridge under static and dynamic loading. Clearly,

the two behaviors are related, in that the bridge stiffiress, or spring constant fr appears in both the static load

test and in the components of the natural frequency. Figure 7 shows the fi.urdamental requency of each of the

NaturaltrequencyHz]I r - - - - - - - - - - . -

i

'r-Tf

i

: -TTTT

i

i rrTri

L'.. Tf .ii,-n-' E i

, T - - f f T r| _ t . - T - _

I I"ETT;F--.Tf,irE.-.--:.:Trr

,.r- I;ffi#"ry-j-rj' *q+j {+ir=:ffi "*: ,?G dtri-r*=

'---'---

I i--'f,.i- L-j -r--i= *

L ff i f f i'E r

i i Ei r ' f f - * =

i-.r-

"'

tus

t i--

i

0.s--

E t

'rr

E-q

-rT-rr-rr'

'-elFEx:-rrrr -rr

n l 1 r - - - - - f l

0 1 0 2 0 3 0

Truckspeed km/hl

7

6

5

4

3

2

1

0

i

i_*__ i

0 i00 2w 300

Stiffnessk kN/mm]

Natural frequencyof the bridgeasa functionofthe bridge stiffiressigure 7:



6.

tested bridges as a function of its stiffiress obtained from a static load test. As expected there is on average an

increase in natural frequency with an increase in stiffiress. However, the scatter is rather large, especially for

concrete bridges, as their nuNS strongly depends on the construction method and cross section. A similar

scatter can be observed when looking at the conelation between the dynamic amplification factor and the

bridge span or natural frequency. While some tendenciescan be observed, he scatter of the data makes the

derivation of a simple approximate formula impractical.

Conclusions

The paper presents resutts obtained from simple dynamic load tests of bridges. These tests are frequently

performed in Switzerland as part of the acceptanceprocessof new bridges. The main results are the natural

frequency of the bridge, the dynamic amplification factor and the logarithmic decrement.

By using a truck to induce vibrations of the bridge, it is possible to simulate the effects of pavement

deterioration. Bridges that are especially sensitive to pavement deterioration are identified, and this information

can then be used in establishing of the maintenanceprogzrm of the structure. Truck speed is shown as having

a great importance on the dynamic response of the bridge, especially in the case of a deteriorated riding

surface.

Some correlation is shown between the span and the natural frequency of a bridge, bufi a considerable scatter is

observed. [n a similar lnanner, there is some correlation between the bridge stiffiress observed in a static load

test and the dynamic properties ofthe bridge.

REFERENCES

t ll HASSAN M., BURDET O. and FAVRE R., Interpretation of 200 Load Tests of Swis Bridges. IABSE

Colloquium:Remaining tructuralCapacity,Copenhagen,enmark,March 1993.

L2l NORME DIN 4150 (Teil2), Erschiitterungen m Bauwesen,Einwirkungen auf Menschen n Gebriuden,

September 975.

t 3l BURDET O., LoadTesting ndMonitoringof Swiss ridges,CEBInformations ulletinn" 219,August 993.

t 4l BURDET O., CORTHAY 5., Static and Qtnamic Load Testing of SwissBridges, International Bridge

Conference,Warsarv,Poland,1994.

t 5l CANTIENI, R. , DynamicLoad Testingof Highway Bridges, ABSE Proceedings, -75l84, Ziirich" August

1984.

t 6l CANTIENI, R, DynamicBehsviorof HighwayBridgesUnder he Passage f Heavy Vehicles,Reportno.220,

EMPA, Dtibendo{ Switzerland, 992.

171 FAVRE R., Risqttede ddformationsrrdversiblesdaw lesponts, JourndeOFFJGPC:Maintenance es ponts:

rdsultats ctuelsde a recherche, tirich,Switzerland,March 1993.

t 8l SIA 169, Recommandation,SwissStandard),Recommandationour la maintenance esouvrages egdniecivil

(Recommendationor the maintenance f Civil Engineering tructures).Soci6t6 suissedes ingdnieurset des

architectes.urich,Switzerland,987.

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Dynamic load testing of Swiss Bridges.PDF