Impact of mobile cranes on short span bridges

Gaya, Vishal

Impact of mobile cranes on short span bridges

Thesis / Dissertation

2011

Abstract

he abnormal load management guideline which is currently being used in South Africa is TRH 11. This guideline is based on provisions developed in the 1970s. Design guidelines and traffic loading have changed since then thereby highlighting the need to develop a new abnormal load management system. TRH 11 considers all the abnormal vehicles as one category. However, literature has shown that even though mobile cranes are regarded as abnormal vehicles, they have different suspension system, axle spacing and use different wheels compared to heavy good vehicles. These characteristics help minimise their impact on road structures. This thesis presents a study of the impact of mobile crane on short span bridges. Short span bridges span between 5 and 40m are very common in South Africa. A finite element model representing the interaction of a mobile crane with a bridge has been developed using Adina finite element software. Bridges have been modeled based on the criteria specified by the MOT design code of practice. This is because 70% of bridges in South Africa have been designed using this code. An experimental study to measure the actual dynamic impacts was performed on the Berg River Bridge. The bridge was instrumented with displacement transducers and strain gauges at mid and quarter span respectively. A 36 tonnes mobile crane was used for the field experiment. The impact was measured for different speed scenarios. The acceleration of the bridge under normal traffic loading was also measured. A wooden plank was placed across the lane for one scenario to trigger extensive dynamic vibration and to simulate poor road surface condition. Data collected from the tests were extracted using ME‘Scope Ves and used for comprehensive assessment of the bridge under dynamic loading. Dynamic and static analyses were performed and the impact factor for strain and displacement were calculated. The highest impact factor was 1.16 for a speed of 60 Km/hr. When plotted on the National codes graph, the highest impact falls below the curve representing the South African codes showing that Berg River Bridge is safe under the motion of the mobile crane. Analysis revealed that high vehicle speed and deterioration in road surface contributes to high impact factors. Experimental findings have also shown that the most important impact forces occur at the bridge approach as the mobile crane crosses the joint irregularities that occur between bridge and the abutments. Experimental data were used to validate the finite element model of the Berg River Bridge. The model gave almost similar results to the field experiment. Moment and shear impact factor were obtained from the Finite element model. The model was used to simulate the effects of heavier vehicle weight on the bridge. It was found that as weight increases the impact factor decreases. This decrease is not caused by a decrease in dynamic deflection but an increase of the static deflection.

Keywords

Civil Engineering

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