Overview
ISO 21725-1:2021 - "Simplified design of prestressed concrete bridges - Part 1: I-girder bridges" provides streamlined rules and guidance for the design of road bridges using prestressed concrete I-girder superstructures. The standard defines a simplified design approach (subject to the limitations in clause 6.1) that reduces complexity compared with more elaborate codes while ensuring structural integrity through continuity, redundancy and protective detailing. It covers design documentation, load models, limit states, materials and construction-related requirements specific to prestressed I-girder bridge systems.
Key Topics
- Scope and limitations: permitted use cases, maximum spans, lanes, widths, skew, curvature and other geometric constraints (see clause 6.1 for details).
- Design procedure and documentation: recommended design workflow, calculation reports, geotechnical reports, structural drawings and specifications.
- Limit states format: ultimate limit state (ULS) and serviceability limit state (SLS) verifications, including deflection and durability considerations.
- Actions (loads): dead loads, live loads (design truck/lane loads), dynamic effects, longitudinal forces, earth pressure, wind and seismic loads, thermal forces, jacking and post‑tensioning actions, and specified load combinations.
- Structural systems and layout: superstructure and substructure descriptions, vertical and cross-beam layout, continuity, redundancy and feasibility checks.
- Materials and components: guidance on concrete, aggregates, water, reinforcement, prestressing steel, post‑tensioning anchorages, ducts and admixtures.
- Seismic provisions: seismic hazard classification, soil profiles, site effects and simplified lateral design approaches suitable for I-girder bridges.
- Construction and detailing: bearing seat widths, protection systems and detailing to resist damage and provide alternate load paths.
Applications and Users
This standard is practical for:
- Bridge designers and structural engineers specifying prestressed concrete I-girder solutions for road bridges.
- Consultants and design offices seeking a simplified, consistent design methodology that complements national codes.
- Contractors and fabricators using standardized detailing and material requirements for precast or cast‑in‑place prestressed girders.
- Transport authorities and infrastructure owners who need a streamlined verification approach for routine road bridges within the standard’s stated limitations.
Use cases include rapid design of single- and multi-span road bridges where economy and repeatable detailing are priorities, or as a harmonized baseline when adapting designs across jurisdictions.
Related Standards
ISO 21725-1 is intended to complement national and international bridge design standards and codes of practice (for example, Eurocodes or AASHTO LRFD) and should be used in conjunction with applicable national regulations, geotechnical guidance and material standards.