Overview
This lecture explains the construction, engineering principles, and design considerations of modern flyovers, focusing on structural stability, post-tensioning, and various interchange types.
Structure and Load Transfer in Flyovers
- High-tension steel tendons in conduits are used for structural integrity but can corrode, compromising safety.
- Flyovers use pre-fabricated box girders that interlock for efficient assembly.
- The load from vehicles and the flyover is transferred through the bearings, piers, pile cap, and finally to piles reaching a hard ground strata.
- Stable foundations require piles to reach hard strata; otherwise, stability is at risk.
- Pile caps connect multiple piles, supporting the construction of piers and box girders.
Box Girder Construction and Assembly
- Box girders are fabricated with steel reinforcement bars, concrete pouring, and formwork.
- Segmental launching gantries assemble box girders between piers in a repetitive cycle.
- Epoxy resin and reinforcement bars ensure tight connections between segments.
- A deliberate gap is left during assembly to allow for the final segment insertion, filled later with reinforcement and concrete.
Post-Tensioning and Its Importance
- Steel tendons are inserted and tensioned through the box girders to enhance load capacity and prevent cracking.
- Post-tensioned concrete keeps the entire girder in compression, minimizing tensile stress and cracks.
- Hydraulic jacks and special wedges keep cables tensioned permanently, even after the jack is removed.
Types of Flyover Interchanges
- Three-way interchanges are used where one road meets another, minimizing land use and lane weaving.
- Cloverleaf interchanges allow all movements without signals but require more space and can cause weaving issues.
- Diamond interchanges suit moderate traffic and minimize land use but may have delays from traffic signals.
- Partial Cloverleaf interchanges combine features to reduce space and weaving problems.
Additional Engineering Features
- Post-tensioning cables can be bonded (grouted) or unbonded (greased in conduits).
- Bonded cables resist corrosion better but need more cables for the same effect.
- Expansion joints accommodate temperature changes, allowing the deck to move without cracking.
- Bearings allow the road deck to rest and move with expansion joints.
Key Terms & Definitions
- Box Girders — Hollow, rectangular structural elements used in bridge construction.
- Pile Foundation — Deep foundation transferring loads to hard strata below ground.
- Post-Tensioning — Applying tension to steel tendons after concrete hardens to improve strength.
- Expansion Joint — A gap allowing movement in long structures due to temperature variation.
- Bearing — Component that supports the road deck and allows controlled movement.
Action Items / Next Steps
- Study types of flyover interchanges and their advantages/disadvantages.
- Review how post-tensioning increases concrete durability.
- Observe real-world flyover structures for expansion joints and bearing details.