Quick Scan model municipal concrete slab bridges

With additional research to the alpha reduction factor and shear force transmission in slab bridges

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In this thesis, existing concrete municipal slab bridges are regarded. Many of these bridges were designed and built in the 1960s and 1970s. These bridges are still in service, and subjected to higher and more frequent loads than at the time of design. This leads to uncertainties in the safety of these bridges. Many municipalities become increasingly aware of these uncertainties and want to verify their bridges according to current standards. Especially the shear assessment is critical, since some former codes do not contain shear checks.

Municipalities usually do not have sufficient financial resources to investigate and recalculate all of their bridges. A Quick Scan model is a cheap and fast tool to do structural checks, or to classify a number of bridges for structural safety. A literature study and Finite Element Modelling research was performed in order to make the Quick Scan more accurate. In general, existing municipal bridges are not exposed to the same heavy traffic as governmental highways. Nevertheless, municipal bridges were calculated with the same load models as governmental bridges. For the assessment of existing municipal bridges research was done to the possibility of reducing the design loads of Load Model 1 (LM1) from the Eurocode. This reduction can be done with the α- factor on variable loads. The possible reasons for this reduction is threefold.

Firstly, municipal roads are significantly less exposed to heavy traffic compared to governmental highways. Therefore the chance of occurrence of the governing vehicle according to LM1 is decreasing. This assumption was supported by Weight-in-Motion measurements on a municipal road in Rotterdam. With the reliability index (β) for existing bridges in Consequence Class 2, this lead to a governing axle load of 225 kN, instead of the 300kN from LM1. The second reason for the reduction of the design load is the fact that municipal bridges usually have relatively small spans (5-20m). LM1 is designed to simulate a fully loaded bridge with a span of at least 20m. For small spans it can be useful to calculate with real occurring traffic, since axle distance is more important than axle loads only. A third (small) reduction is the fact that an existing bridge has a shorter reference period than a new bridge. This leads to smaller chances of the occurrence of the governing vehicle. New load models for small span municipal bridges were designed taking into account these reductions. The governing vehicle for small span bridges is a 5-axle vehicle with axle loads of 137,5-165 kN and axle distances of 140-175mm. Occurring shear forces due to these loads are higher than due to a long vehicle with higher loads, mora axles and greater axle distances.

The new governing vehicle was used to determine the reduction factor α. This was determined by a comparative research in the Finite Element Modelling program RFEM. The resulting shear stresses and flexural stresses from the different load models were compared. For spans up to 11m the loads from LM1 can be reduced by an α factor 0,8. This factor increases linearly to a factor 1,0 for a 20m span.

Besides the differences in loads and dimensions, municipal bridges also differ from governmental bridges in lay-out. In general, the distance from the carriageway to the edge of the slab is larger for municipal bridges due to the presence of a footpath or bicycle lane separated by a kerb. Bridges in 10 different municipalities were examined on lay-out. Roughly 58% of the municipal bridges in the Netherlands have significant edge distance (>1,2m) This leads to a difference in the force transmission, since the high axle loads from LM1 cannot occur near the edge of the slab. The influence of this edge distance for different spans was investigated with RFEM. Also, the variable and permanent loads were investigated separately to give insight in the resulting shear stress due to different loads. Distinction was made between slabs in cracked and uncracked state. Cracking has significant influence on the transverse force transmission of the axle loads. The transverse force transmission is influenced by the ratio between longitudinal and transverse stiffness, which was conservatively chosen as 1/3. Also, distinction was made between in-situ casted slabs and prefab slabs. The self-weight of in-situ casted slabs leads to a constant shear force on the support. A prefab slab was treated like a theoretical slab, where the self-weight leads to peak forces near the edge due to torsional moments near the edge.

The critical edge distance was determined for different spans. For a cracked in-situ casted slab with an edge distance >1,5m, the middle of the slab is always governing in shear. An edge distance <0,7 leads to the edge of the support being governing in shear.

Research by Eva Lantsoght to shear force in reinforced slabs under concentrated loads close to the supports was used for rules and assumptions based on experiments. These rules were used to get a better understanding of the results in RFEM, and to develop the Quick Scan Model.

The Quick Scan model uses findings from literature research combined with findings from finite element modelling. The output is a Unity Check which can function as real structural shear check if the concrete compressive strength and steel yield strength are known. When only dimensions are known, the Quick Scan model can function as classification for a series of bridges in a municipality. The Quick Scan model was tested by a series of case studies, which did not yet lead to the verification of the model.

Results from the Quick Scan model were compared to results from the FEM research. Occurring shear stresses in the Quick Scan model are conservative with a maximum error of 11%. With the possibility of lowering the reliability index β to 2,5 (‘disapproval’ level) and taking into account the possible error due to several uncertainties, the upper boundary of the Unity Check was found as 1,4. Bridges with a Unity Check: 1 < UC < 1,4 according to the Quick Scan model need further assessment or material testing in order to fulfil the requirements.

Extensive research was done to loads according to the Eurocode (LM1) on slab bridges and the force transmission of these loads. The capacity of existing concrete slabs was investigated relatively brief. More research to the capacity side of existing concrete bridges is expected to be beneficial.