Challenges in Modern Bridge Anchoring
Given the many different design approaches possible with bridge construction, there is no simple answer to the question “what are modern challenges in bridge anchoring?” The challenges will vary according to the chosen design. These could take the form of a suspension bridge, a cable-stayed bridge or beam bridge to a cantilever bridge, a truss bridge or an arch bridge.
Whilst the bulk of the engineering challenges are specific to the bridge design, some modern challenges apply to every bridge project. One of these is foundation stability.
Fundamentally, foundation stability must be achieved in a way that aligns with specific soil conditions at the construction site. The approach to stabilising the ground must adequately accommodate the risk of settlement, instability and potential bridge failure.
Different types of bridge design require specific foundation types to ensure the loads are evenly distributed from the superstructure to the ground. This avoids uneven stress distribution and structural issues.
One final reason why the bridge foundation approach is so crucial is in relation to the bridge’s durability and longevity. Only with the right foundation design and construction will the bridge’s stability and functionality be optimised over its design life.
Climate Change and Environmental Impacts
The existing environment and the potential for further environmental impacts are major influences on bridge anchoring design. Water tables, seismic activity and soil erosion need to be accommodated in the foundation design and build too. The choice of anchors, therefore, must ensure the bridge is adequately protected.
A number of incidents in recent years point to climate change increasingly impacting on bridges. Bridge collapses such as the Montana Rail Link bridge in 2023, which experts believe was due to repeatedly high flow levels that increased the rate of scour and degradation on the bridge, highlight how less predictable environmental impacts can be a serious threat to structural integrity and a danger to life.
Focusing specifically on anchor designs for bridges, how does environmental impact affect anchoring systems? Ultimately, it will depend on how well the bridge foundations and structure are designed. The anchoring approach must accommodate the known risks, loads and stresses, and the foundation design and construction must be fit for purpose.
Innovative Solutions in Bridge Anchoring
In bridge designs which require the structure to be anchored to concrete cofferdams, it is common practice for anchoring systems to be cast-in. That usually means the anchors are formed using a bolt which is set into the concrete as it is cast, with a nut or head at the end. Some are also installed with an anchor plate which increases the anchor’s resistance, and they can even feature an embedded plate at the concrete surface.
Whilst this form of bridge anchoring is highly effective and often regarded as the most effective form of anchoring for such applications, it does not always lend itself to the reality of bridge construction projects. This is why a number of innovative solutions are now being deployed in bridge construction projects. So, where cast-in anchors are not ideally suited to the project, it is important to review the alternative bridge anchoring options available – but what innovative solutions exist for bridge anchoring?
Advanced Engineering Practices
Advances in engineering practices mean that performance that would normally only be associated with cast-in anchors can now be achieved using post-installation anchors (where a hole is drilled into the concrete for an anchor to be installed), particularly through the innovative designs developed by EJOT’s anchoring specialists.
Mechanical anchors take many different forms, and are known under many descriptions, including anchor bolts, through-bolts, sleeve anchors, wedge anchors and undercut anchors. Whilst the concept of these fixings may not be new, developments by EJOT in recent years present today’s engineers with products which enable them to approach anchoring for bridge construction in a different way – even to the point where cast-in anchors may not be required at all.
One of the EJOT products that offers new bridge design potential is the LIEBIG Ultraplus. In a project to create a new lifting bridge over the River Yare in the East Anglian town of Great Yarmouth, this mechanical anchor was used instead of cast-in anchors to achieve equivalent performance whilst also being better suited to the complexities of modern construction processes.
The £121m Herring Bridge project required the design team to think ‘outside the box’ to satisfy the structural requirements for a bridge of this type and scale, whilst also accommodating a realistic delivery schedule for the bridge’s key phases. The twin bascule bridge construction was built with cofferdams on both banks of the river to house the machinery and mechanism required to lift, hold and lower each leaf of the bridge, all of which had to be securely anchored to the concrete.
The team was attracted to the potential for using a post-installation anchoring solution to attach the baseplates of the bridge’s operating mechanism. These anchors, however, had to be suitable for the concrete conditions within the cofferdam design and offer a very high load capacity.
Here, LIEBIG Ultraplus anchors were assessed and, after a thorough stress analysis, it was clear that these heavy duty undercut anchors from EJOT would sufficiently accommodate the exceptionally high loads in this project. These anchors are manufactured in high strength carbon steel and have demonstrated reliability in resisting dynamic loads, shock loads and seismic loads.
Stakeholder Engagement and Regulatory Compliance
Key to the success of the anchorage design for any bridge construction, as EJOT UK’s Herring Bridge project demonstrates, is stakeholder engagement from the outset and a shared vision to ensure the approach maintains all the required safety standards, and that regulatory compliance is continually at the front of mind.
To achieve a mechanical anchoring design that will deliver the required performance, a comprehensive geotechnical engineering project must be undertaken to ensure it is compatible with the behaviour of the ground materials. Every mechanical anchor has different performance capabilities and this will determine how many individual anchors will be required to achieve the load distribution goals. For example, when attaching the baseplates for a bridge lifting mechanism, LIEBIG Ultraplus anchors may be more effective than others because of the compression forces they can accommodate in the concrete – a factor that may mean fewer anchors are needed for each baseplate.
Hence why the anchoring approach can have major project management implications. If the target performance can be met for individual baseplates by using fewer anchors, significant cost savings could be achieved as less time will be needed for drilling, installation and post-install testing. And, of course, using fewer anchors means reduced spend on the components.
Ensuring Durability through Life Cycle Analysis
Durability is a major consideration in bridge anchoring design given that the products must not only withstand the often extreme stresses and forces associated with such applications throughout their lifetime, but also resist the effects of the environmental conditions where they are installed.
The correct anchor specification for a bridge construction project will be determined after a life cycle analysis is conducted. There are, however, different types of metals to choose from which can offer higher levels of resistance to corrosion compared to more standard options, and these are widely used in applications where moisture levels are high, or where the local atmospheric conditions are particularly aggressive.
The continued effectiveness of the bridge anchoring design is checked through periodic structural health monitoring.
Anchorage Design and Construction Techniques
The anchorage design for bridges can take numerous forms, depending on the ground conditions, foundation design, and type of bridge being built, amongst numerous other factors. The construction techniques being applied and build process will also determine which types of anchors are most suited to the bridge project.
For example, does the project allow for anchors to be cast into the concrete foundations given the design and installation of cofferdams and the demands of the overall project timescale? Whilst cast-in anchors are usually the preferred approach in projects where extremely high loads must be accommodated, they are not always feasible due to other construction priorities given the additional design time and time needed for the concrete to cure.
Design Considerations for Bridge Anchoring
Would the shorter curing time offered by post-installation chemical anchors or resin anchors be suitable? This may not be the ideal solution either, particularly if the project is located in an area where temperatures are low as this will extend the curing time needed for chemical anchors.
And this approach may be complicated further by the availability of skilled installers – installing chemical anchors requires a very specific skillset which may mean having to wait longer for suitably qualified contractors to arrive on site. This could have implications for the cost-effectiveness of the anchoring approach overall.
So, could mechanical anchors provide a solution where chemical anchors and cast-in anchors are not feasible? That will depend on numerous factors but, in principle, yes, providing the anchor can demonstrate that it meets the performance criteria including being able to accommodate wind loads and hydrodynamic forces.
In particular, the LIEBIG Ultraplus heavy duty undercut anchor can be used in a way that enables high loads to be accommodated reliably in line with internationally recognised technical approvals, including an ETA (European Technical Assessment) and compliance with the ACI (American Concrete Institute). This type of anchor was used to attach the heavy lifting equipment to the concrete cofferdams in the Herring Bridge project in Great Yarmouth, providing an alternative to cast-in anchors.
Material Selection for Optimal Performance
One of the main questions that arises when considering the most appropriate anchoring design is what materials enhance corrosion resistance for anchors?
The anchor’s material selection will chiefly depend on where you are building, the characteristics of the local environment, the climate and the material we are fastening into. Only by considering these three factors is it possible to recommend an anchor that will offer the required corrosion resistance.
Corrosion can take different forms in different environments, so in order to manage the risk we have to take account of a number of variables. These include levels of humidity, temperature, salt, industrial pollution and the potential for galvanic corrosion – this type of corrosion can occur when two dissimilar metals are in contact.
Given that many bridges are constructed in close proximity to water and are often used to carry vehicles which are continually emitting exhaust gases, mechanical anchors used for bridges will usually need to be specified to resist the effects of aggressive atmospheric conditions. That will mean the bridge anchors have to be manufactured in high quality zinc coated carbon steel or stainless steel.
Sustainability and Environmental Considerations
How does the anchoring design of a bridge impact the project’s overall sustainability? Firstly, it can influence the construction process, which could impact on how well optimised on-site processes are and the amount of waste produced. However, the long term performance and durability of the anchors will be a major sustainability factor, affecting the lifespan of the bridge and the frequency of maintenance – planned or unplanned – throughout the structure’s service life.
If a mechanical anchor is not capable of withstanding the environmental impacts in the bridge’s location, it is likely to corrode and require premature replacement. That process in itself carries a high carbon cost given the need for plant, equipment, personnel and materials to be transported to site and the financial implications could be significant too. If the bridge’s operation is interrupted, that could impact on local people and regular bridge users, causing unnecessary disruption to everyday lives.
Hence why the materials used in the manufacture of the mechanical anchor must be closely scrutinised when designing a bridge anchorage. Choose the correct type and, providing they are installed correctly and not exposed to unforeseen conditions, they could exceed their notional design life and contribute to the long term sustainability of the bridge.
Reducing Environmental Impacts
One of the most important ways to reduce environmental impacts on a bridge’s anchors is to ensure the material specification used for their manufacture is sufficient for the conditions in which they are to be installed. The anchors must also be installed in accordance with the manufacturer’s guidance as even the slightest deviation from the recommended anchor installation process could compromise its long term performance and public safety.
If a mechanical anchor needs to be replaced unexpectedly due to corrosion, it will need to be removed and replaced. That process could mean the surrounding foundation structure is damaged, leaving holes in the concrete surface or crumbling on the surface. This will result in dust and debris being released.
It is also important to consider the risks associated with the anchor removal process, starting with a thorough risk assessment. In particular, dust generated by the mechanical action of drilling and grinding can contaminate the air with fine, hazardous particles, exposing workers to health risks that could lead to respiratory diseases in future.
Sustainability in Material Selection and Construction
Maximising sustainability throughout the bridge development process is a priority from the initial concept, detailed design and construction through to maintenance and operation once handed over. Construction techniques and material selection are two important factors affecting the project’s sustainability, and this is why the anchorage design requires careful consideration.
Firstly, the anchors used for bridge construction must offer optimum longevity through durability to help minimise maintenance requirements. A sustainable bridge is, after all, one that stands the test of time well. Hence why when using post-installation anchors, stainless steel or high quality zinc coated carbon steel products must be used.
Secondly, the anchor selection can make the construction process more efficient, which could result in a shorter build schedule, reduced waste and less intensive resource use. For example, the use of LIEBIG Ultraplus anchors in the Herring Bridge project helped to streamline the cofferdam construction because it meant that the construction team did not have to factor in the locations of cast-in anchors, saving valuable time in the process.
