Design and Erection of Multi-Span Steel Arch Bridges

The main beam truss of a multi-span highway steel bridge can be constructed as a continuous beam or a discontinuous beam. The continuous beam is easy to erect and dismantle, but requires that the foundation of each intermediate bridge foot must be stable, and that the top of each intermediate bridge foot must be kept within a reasonable height difference range (due to the inelastic deflection caused by the pin hole gap, the force transmission has new characteristics), and no accidental sinking should occur during use. If the above conditions are not met or the intermediate bridge foot foundation adopts a floating support, it is advisable to use a discontinuous beam.

Steel arch bridges, renowned for their aesthetic appeal and structural efficiency, are widely used for long-span crossings over valleys, rivers, or urban areas. Multi-span configurations further extend their applicability to complex terrains, requiring meticulous structural design and innovative erection techniques. This article outlines key considerations in the design and construction of multi-span steel arch bridges.

Structural Design Principles

1. Load Analysis and Configuration
   Multi-span steel arch bridges typically adopt continuous or discontinuous spans depending on site constraints. Designers must evaluate dead loads, live loads (vehicular/pedestrian), wind, seismic forces, and thermal effects. Finite element analysis (FEA) is critical to simulate stress distribution, buckling risks, and dynamic responses. The arch rib geometry—whether parabolic, circular, or catenary—is optimized to balance thrust forces and minimize bending moments.
2. Arch Rib and Bracing Systems
   The primary arch ribs are fabricated from high-strength steel plates or box sections, designed to resist axial compression and bending. Lateral stability is ensured via transverse bracing (K-braces or X-braces) and spandrel columns. For multi-span bridges, intermediate piers are strategically positioned to redistribute loads and reduce individual span deflections.
3. Deck-Arch Interaction
   The deck may be suspended from the arch via hangers (tied-arch) or supported on spandrel columns (through-arch). In multi-span designs, continuous decks enhance load distribution but require expansion joints to accommodate thermal movement. Composite steel-concrete decks are increasingly used to improve stiffness and durability.
4. Foundation and Abutments
   Massive thrust forces from the arch necessitate robust foundations, often employing drilled shafts or pile groups. Abutments and piers must resist horizontal thrust while accommodating potential ground settlement.

Erection Techniques

1. Temporary Supports and Cantilever Methods
   For river or valley crossings, temporary towers or falsework may support arch segments during assembly. Alternatively, the cantilever method involves incrementally extending symmetric arch segments from both abutments, stabilized by temporary cables until closure.
2. Segment Lifting and Swivel Erection
   Prefabricated steel arch segments are lifted into place using cranes or gantries. In constrained environments, swivel erection—rotating preassembled arch sections into position—minimizes disruption. Precision in welding and bolting is vital to ensure alignment and structural integrity.
3. Stress Control and Closure
   Real-time monitoring systems track deformations and stresses during erection. Closure segments are installed under controlled temperatures to mitigate thermal expansion mismatches. Post-tensioning tendons may be added to optimize load-bearing capacity.
4. Multi-Span Coordination
   In multi-span bridges, sequential erection requires balancing construction loads across spans. Temporary piers or tie-back systems prevent excessive deflection in adjacent spans during assembly.

Challenges and Innovations

Multi-span steel arch bridges face challenges such as cumulative deflection, differential settlement, and wind-induced vibrations. Advanced solutions include self-anchored suspension systems, tuned mass dampers, and smart sensors for health monitoring. Modular construction and 3D modeling (BIM) further enhance precision and efficiency.

The successful delivery of multi-span steel arch bridges hinges on harmonizing aesthetic vision with engineering rigor. By leveraging advanced materials, computational tools, and innovative erection strategies, engineers can achieve durable, cost-effective solutions that blend seamlessly into their environments. Continuous research into lightweight alloys and automation promises to redefine the future of long-span bridge engineering.