Futuristic bridge support system integrated with an older concrete bridge.

Bridge Over Troubled Waters: Ensuring the Safety of Aging Concrete Spans

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Rhea Morgan in Tech & Innovation December 2025 • 5 min read.

"Innovative techniques and recalculation models offer hope for aging prestressed concrete bridges with minimal reinforcement, preventing costly replacements."


For civil engineers, evaluating the load-bearing capacity of existing prestressed concrete bridges has become a routine task. However, significant changes in standardization have made it increasingly difficult to verify the shear resistance of these bridges using current regulations during structural assessments. This often leads to the need for costly reinforcement measures or even complete bridge replacements.

A new assessment model has been developed at TU Wien (Vienna University of Technology) to address this challenge, offering a more accurate representation of structural behavior. Following successful application on single-span beams, the model is now being tested on multi-span prestressed concrete bridges, focusing on the intermediate supports. Eight experiments were conducted to verify the model assumptions. The experimental setup and test specimens were designed to replicate the stress conditions of real bridges at a 1:2 scale, featuring a large negative moment combined with decreasing shear force.

This research systematically investigates the influence of various factors, including the degree of prestressing, cross-sectional shape, amount of transverse reinforcement, and shear slenderness. The results were evaluated using Austrian recalculation guidelines and a parameter study, highlighting the potential of the "flexural-shear crack" (FSC) model.

The Challenge: Aging Bridges and Evolving Standards

Futuristic bridge support system integrated with an older concrete bridge.

Bridge design standards evolve, incorporating the latest knowledge and technological advancements. However, these changes can create problems for older bridges, which may no longer meet the updated requirements. This is especially critical for civil engineering structures because of the enormous economic implications of bridge replacements.

The introduction of the Eurocode series [1-3] for European construction standards has highlighted this dilemma in assessing the load-bearing capacity of prestressed concrete bridges. Germany [4] and Austria [5] have introduced recalculation guidelines for bridges to give engineers flexibility in evaluations. Despite these guidelines, shear resistance verification often remains problematic [6-8], leading to expensive upgrades or replacements. Many bridges built in the 1960s and 1970s have proven their functionality through years of service, highlighting the need for more precise evaluation models.

  • Economic Impact: Bridge replacements have significant economic consequences due to their high cost.
  • Standard Evolution: Evolving standards can render older bridges non-compliant, necessitating reassessment.
  • Recalculation Guidelines: Guidelines provide engineers with flexibility in assessing existing bridges.
  • Shear Resistance: Shear resistance verification often remains problematic, leading to expensive upgrades or replacements.
To tackle this problem, TU Wien developed a new assessment concept within the "Traffic Infrastructure Research" initiative of the Austrian Research Promotion Agency (FFG). This concept realistically depicts shear force behavior [9, 10] and includes different shear force models: the FSC model (flexural-shear crack), the ST model (shear-tension), and main tensile stress verifications. This assessment concept has already been used in engineering practice to evaluate shear force capacity at the end support of prestressed concrete bridges [11]. Unlike current regulations [1, 12], this model acknowledges the substantial contribution of concrete to shear force transfer, aligning with other recently developed approaches [13-15]. Verification of these models using existing test databases [16] is limited because most tests are performed on single-span beams with point loads (Image 1a). Multi-span bridge structures, in contrast, are mainly subject to distributed loads, and shear forces at the intermediate support are combined with large negative moments (Image 1b).

Looking Ahead: Sustainable Solutions for Bridge Infrastructure

Innovative models like the FSC offer a promising path forward for ensuring the safety and extending the lifespan of existing prestressed concrete bridges. By providing a more accurate assessment of shear capacity, these models can help prevent unnecessary and costly replacements, contributing to a more sustainable approach to infrastructure management. Further research and application of these advanced techniques are essential to address the challenges posed by aging bridge infrastructure.

About this Article -

This article was crafted using a human-AI hybrid and collaborative approach. AI assisted our team with initial drafting, research insights, identifying key questions, and image generation. Our human editors guided topic selection, defined the angle, structured the content, ensured factual accuracy and relevance, refined the tone, and conducted thorough editing to deliver helpful, high-quality information.See our About page for more information.

Everything You Need To Know

1

What specific challenges do civil engineers face when assessing the load-bearing capacity of aging prestressed concrete bridges?

Civil engineers encounter difficulties due to evolving design standards, such as the introduction of the Eurocode series. These changes make it hard to verify the shear resistance of older prestressed concrete bridges using current regulations. This often results in costly reinforcement measures or complete bridge replacements. Moreover, many bridges built in the mid-20th century are reaching the end of their intended lifespan, further amplifying the need for precise assessment methods.

2

How does the new assessment model developed at TU Wien address the problems with shear resistance verification?

The new assessment model developed at TU Wien offers a more accurate representation of structural behavior. It realistically depicts shear force behavior and incorporates different shear force models, including the "flexural-shear crack" (FSC) model, the ST model (shear-tension), and main tensile stress verifications. Unlike current regulations, this model acknowledges the significant contribution of concrete to shear force transfer, thus providing a more precise evaluation of shear capacity and potentially preventing unnecessary replacements.

3

What is the significance of the FSC model, and how does it contribute to the assessment of shear capacity?

The "flexural-shear crack" (FSC) model is a key component of the new assessment concept. The FSC model is one of the shear force models implemented. It provides a more accurate assessment of shear capacity in prestressed concrete bridges. The results were evaluated using Austrian recalculation guidelines and a parameter study, highlighting the potential of the FSC model. This model acknowledges the significant contribution of concrete to shear force transfer, enabling a more realistic evaluation of shear resistance and potentially reducing the need for costly bridge upgrades or replacements.

4

What are the economic and practical implications of the problems with shear resistance in aging bridges?

The problems with shear resistance have significant economic and practical implications. Bridge replacements are extremely expensive, creating substantial economic consequences. If shear resistance verification fails, it often leads to costly reinforcement or even complete replacement of the bridges. The necessity of these costly interventions stems from the inability of the existing assessment models to fully capture the shear behavior in the existing bridges, which also may lead to traffic delays or other inconveniences.

5

How do innovative assessment models like the FSC model contribute to a sustainable approach to bridge infrastructure management?

Innovative models such as the FSC model contribute to a more sustainable approach by providing a more accurate assessment of shear capacity, which helps extend the lifespan of existing prestressed concrete bridges. By preventing unnecessary and costly replacements, these models reduce the economic and environmental impact associated with bridge construction and demolition. The use of advanced recalculation models is essential for ensuring the safety and longevity of aging bridge infrastructure, contributing to the effective and sustainable management of resources.

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