Assessment of dynamic loading of bridges Aleš Žnidarič Slovenian National Building and Civil Engineering Institute

Dynamic loading on bridges ratio between maximum (dynamic) and static loadings - DAF traditionally: max DAF of a single vehicle measured at different velocities and loadings problem: combining the extremes of dead load and dynamic effects => very high DAF continuation of work from SAMARIS: theoretical studies (UCD) experiments (ZAG)

Theoretical Studies

Definitions Static Load effect – No interaction between vehicles and bridge is considered. Total Load effect – Interaction between vehicles and bridge is considered. DAF – Dynamic Amplification Factor (for a particular load scenario): ADR – Assessment Dynamic Ratio (for a particular period): There is considerable variation in dynamic amplification between runs of given vehicles at specified speeds and even greater variation when speed, vehicle etc. are changing. Theoretical studies are generally not good at predicting dynamic amplification. Models vary greatly in their complexity. As might be expected, the very simple models are generally the least accurate. Anything short of full 3-dimensional models give significantly different results to simple 2-dimensional models. Articulation is important – articulated tractor/semi-trailers behave quite differently to rigid 2- and 3-axle trucks. Suspension type is important. It is well known that air suspensions are considerably more "road friendly" than steel leaf spring suspensions and should be modelled differently. There are also modelling issues with suspensions and it has been shown that the Coulomb damping usually assumed is a simplification. Road profile is also very important. It has been shown that it is not just road roughness that affects the dynamic amplification in a bridge – the location of the particular bumps that make up the profile is very important and IRI is at best a very crude indication of the level of dynamics that can be expected. This is further complicated that trucks will not follow the exact same track each time they travel over a bridge and will therefore experience a slightly different profile which can have a significant effect. Finally, there is a dearth of knowledge on truck dynamic properties. Some spring stiffnesses, damping coefficients etc. are available for some trucks (and some good truck models exist for particular trucks) but there is little known about the model properties that should be used for the truck population at large.

Distribution of load effect Combining probability of event with bridge response for the event allows for the full, exact distribution of load effect (static or total) to be obtained

Distribution of load effect Examination of the upper tail of the distributions allow for characteristic values & ADR at any return period to be assessed

Influence of return period on ADR Characteristic Value of static and Total Load effect obtained for various return periods trend of ADR with return period to be assessed variability of ADR reduces as return period increases

Multiple road profiles repeated for 100 different road profiles (all of very good/good quality), i.e. IRI<6m/km range of possible ADR reduces as return period increases good representation of a site-specific 1000-year ADR is achieved by using only 1-month of data

Experimental work

BWIM shema Detekcija osi Meritve deformacij Bridge weigh-in-motion measurements were done to collect sufficient information needed for experimental evaluation of the influence lines in both lanes and to statistically evaluate load distribution factors. Meritve deformacij Detekcija osi

Strain transducer

Bridge weigh-in-motion installation

Bridge weigh-in-motion installation

186 m long bridge over 5 spans Slovenia

530m long orthotropic deck bridge Poland

Presentation dd.mm.yyyy

Presentation dd.mm.yyyy

SiWIM – strain signals

DAF and SiWIM updated SiWIM software calculates DAF by comparing measured with “static” signals from 6 tested algorithms the one using FFT low-pass filter was applied: spectrum calculated from hundreds of vehicles only static component left to compare with dynamic signal There is considerable variation in dynamic amplification between runs of given vehicles at specified speeds and even greater variation when speed, vehicle etc. are changing. Theoretical studies are generally not good at predicting dynamic amplification. Models vary greatly in their complexity. As might be expected, the very simple models are generally the least accurate. Anything short of full 3-dimensional models give significantly different results to simple 2-dimensional models. Articulation is important – articulated tractor/semi-trailers behave quite differently to rigid 2- and 3-axle trucks. Suspension type is important. It is well known that air suspensions are considerably more "road friendly" than steel leaf spring suspensions and should be modelled differently. There are also modelling issues with suspensions and it has been shown that the Coulomb damping usually assumed is a simplification. Road profile is also very important. It has been shown that it is not just road roughness that affects the dynamic amplification in a bridge – the location of the particular bumps that make up the profile is very important and IRI is at best a very crude indication of the level of dynamics that can be expected. This is further complicated that trucks will not follow the exact same track each time they travel over a bridge and will therefore experience a slightly different profile which can have a significant effect. Finally, there is a dearth of knowledge on truck dynamic properties. Some spring stiffnesses, damping coefficients etc. are available for some trucks (and some good truck models exist for particular trucks) but there is little known about the model properties that should be used for the truck population at large.

DAF measurements in ARCHES ARCHES experiment: 25-m long simply supported span, susceptible to dynamics 2-month experiments completed (Sep. to Nov. 2006) app. 148 000 vehicles 4 other evaluations Experiment 2 in Slovenia: to assess influence of pavement unevenness on bridge dynamics to get realistic estimate of impact factors of extreme (multiple-presence) load cases to avoid combining the extremes of static loading and dynamic amplification which can yield to over-conservative estimates of traffic loading; this is particularly important when assessing existing bridges to get statistically evaluated impact factors for entire 2-week population of vehicles before and after rehabilitation of the deck bridge over Sava river with very bad pavement and high dynamics bridge WIM electronics is being upgraded for measurements of dynamics Due to the progress in the field of bridge weigh-in-motion measurements it will be for the first time ever possible to obtain and analyse impact factors of random vehicles including the multiple-truck events and thus to confirm theoretical conclusions that were obtained on the limited combinations of pre-defined vehicles.

Dynamic amplification

Dynamic amplification

Dynamic amplification

Dynamic amplification

Dynamic amplification

Dynamic amplification

Dynamic amplification 1 hour = 387 vehicles

Dynamic amplification 1 day = 4120 events

Calculation of DAF CSHM2 workshop, 29.8.2008

Calculation of DAF

DAF Vransko The graph shows impact factors of 23000 vehicles passing the bridge. Preliminary analysis confirms that the theory of combining extremes of loading and of dynamic amplification is not realistic. As the loading is increasing, the impact factors are decreasing.

DAF Vransko – Extreme event

DAF Blagovica We are mostly interested into such multiple-presence events. This is one of the extreme ones, recorded on the first day of measurements when a traffic camera was attached to the SiWIM system.

DAF – 12 m slab The graph shows impact factors of 23000 vehicles passing the bridge. Preliminary analysis confirms that the theory of combining extremes of loading and of dynamic amplification is not realistic. As the loading is increasing, the impact factors are decreasing.

DAF – 8 m slab

DAF – 7.2 m slab (the Netherlands)

Conclusions dynamic amplifications can be very high (>2), but these appear for single and/or lighter vehicles DAF from design codes is too conservative for bridge assessments ARCHES has shown that DAF/ADR is below 1.06 recommendations: DAF = 1.1 or measurements and/or modelling

Дякуємо Вам за увагу!