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ISO 28842:2013 — Simplified Design of Reinforced Concrete Bridges
Guidelines for structural design, load analysis and construction of RC bridges | Civil engineering guide
Introduction to ISO 28842
ISO 28842:2013 provides simplified design guidelines for reinforced concrete bridges, covering the full range from conceptual design through detailed structural engineering. Developed by ISO/TC 71/SC 5, this standard addresses the need for practical, codified design procedures for medium-span RC bridges commonly used in road and highway infrastructure. It covers girder bridges, slab bridges, and frame-type bridges with spans typically ranging from 5 m to 40 m.
This standard fills a critical gap between overly simplified empirical methods and complex, time-consuming finite element analysis. It provides design engineers with validated procedures that balance accuracy with practical efficiency.
Structural Systems and Loads
Types of Superstructures and Design Loads
The standard covers several superstructure types: solid slabs (simply supported and continuous), T-beam and box girder bridges, and slab-on-girder systems. Design loads include dead loads (self-weight, superimposed dead loads), live loads (design truck/tandem/lane loads per local or international standards), longitudinal forces (braking, acceleration), earth pressure, wind loads, earthquake inertial forces, and thermal forces. Detailed load combinations are provided for both ultimate and serviceability limit states.
| Structural Element | Design Criteria | Key Checks | Typical Reinforcement |
|---|---|---|---|
| Deck slab | Flexure + shear + fatigue | Moment capacity, punching shear | #13-#19 @ 150-300 mm |
| Girder/beam | Flexure + shear + torsion + deflection | Strength, crack control, fatigue | #16-#36, stirrups #10-#13 |
| Pier/column | Axial + flexure + shear (biaxial) | Slenderness, P-delta, confinement | #19-#36, ties #10-#13 @ 100-300 mm |
| Foundation | Bearing + settlement + sliding | Overturning, bearing pressure | #13-#25 @ 200-400 mm |
| Abutment | Earth pressure + vertical + lateral | Sliding, overturning, eccentricity | #16-#25 vertical, #13-#16 horizontal |
For seismic design, the standard specifies minimum lateral force levels based on seismic zone and soil type. In high seismic zones (PGA > 0.3g), special detailing requirements including closer stirrup spacing and additional confinement reinforcement are mandatory.
Engineering Design Insights
Substructure Design Considerations
The substructure design section provides detailed procedures for columns, piers, abutments, and foundations. A key insight is the treatment of column slenderness: for columns with kl/r > 22 (where k is the effective length factor, l is unsupported length, and r is radius of gyration), second-order effects must be considered using the moment magnification method. The standard provides simplified equations for calculating magnified moments without requiring full second-order analysis.
Foundation design covers spread footings, pile caps, and foundation mats. The bearing capacity verification uses allowable stress design with safety factors of 3.0 for bearing capacity and 1.5 for sliding resistance. For pile groups, the standard provides simplified group efficiency equations considering pile spacing and soil type.
One of the most practical features is the standardized bearing design section for elastomeric bearings. The standard provides design equations for bearing pad thickness, plan dimensions, and anchorage requirements based on compressive stress, shear strain, and rotation limits.
Construction and Detailing
The standard specifies minimum reinforcement requirements including crack control provisions (maximum bar spacing based on crack width limits of 0.3 mm for exposure class 1 and 0.15 mm for exposure class 2). Development length and lap splice requirements follow simplified equations derived from fundamental bond mechanics, with modifications for epoxy-coated bars, bar size, and concrete strength.
Practical Bridge Design Example
A practical application of ISO 28842 involved the design of a 25 m span reinforced concrete T-beam bridge for a rural highway in Southeast Asia. The bridge comprised five T-beams at 2.0 m spacing, a 200 mm thick deck slab, and two-lane traffic width of 8.0 m. The design used the standard’s simplified procedures: dead load of 24 kN/m³ for reinforced concrete, superimposed dead load of 2.0 kN/m² for wearing surface, and live load equivalent to HL-93 truck loading.
The substructure design followed the standard’s simplified column provisions: 1.2 m diameter reinforced concrete columns with 16 bars of 25 mm diameter longitudinal reinforcement and 10 mm stirrups at 150 mm spacing. The foundation used 2.5 m square spread footings at 2.0 m depth below ground, with allowable bearing pressure of 250 kPa. Thermal load analysis, in accordance with the standard’s provisions for uniform temperature change of ±25°C, produced secondary stresses of 8-12 MPa in the continuous deck — well within the concrete tensile capacity when properly reinforced with temperature steel. This design example demonstrates how the standard’s simplified procedures can produce a complete, code-compliant bridge design suitable for construction in areas without access to advanced analysis software.
One of the most cost-effective design strategies from ISO 28842 is the use of standard span lengths (15 m, 20 m, 25 m, 30 m) that allow reuse of formwork and standardized reinforcement detailing. For the rural highway example, using a standard 25 m span design with modular abutments saved approximately 20% in construction costs compared to a custom design.
A second design example illustrates the standard’s treatment of seismic loads for bridges. A three-span continuous RC bridge in a moderate seismic zone (PGA = 0.25g) was designed using the standard’s simplified lateral force procedure. The seismic analysis showed that the central pier would experience a ductility demand of 3.2 under the design earthquake, requiring special detailing per the standard’s provisions for seismic zones. Confinement reinforcement in the plastic hinge zone (top and bottom 1.0 m of the 8 m tall pier) was specified as 12 mm spiral at 75 mm pitch, providing a volumetric confinement ratio of 1.2% — exceeding the 0.8% minimum. The cap beam was designed for capacity protection, with design forces amplified by 1.4 times the column plastic moment capacity to ensure a strong-beam weak-column mechanism.
Frequently Asked Questions
Q: What is the maximum span length covered by ISO 28842?
A: The simplified procedures are validated for spans up to 40 m. For longer spans, more detailed analysis accounting for creep, shrinkage, and prestress effects is recommended.
A: The simplified procedures are validated for spans up to 40 m. For longer spans, more detailed analysis accounting for creep, shrinkage, and prestress effects is recommended.
Q: Does this standard apply to prestressed concrete bridges?
A: No, it specifically addresses reinforced concrete (non-prestressed) bridges. Prestressed concrete design follows different principles for stress control and tendon layout.
A: No, it specifically addresses reinforced concrete (non-prestressed) bridges. Prestressed concrete design follows different principles for stress control and tendon layout.
Q: How does thermal loading affect bridge design per this standard?
A: The standard specifies uniform temperature changes of ±25°C for concrete bridges and temperature gradient effects (top surface 5°C warmer than bottom for positive gradient). These produce significant forces in continuous structures.
A: The standard specifies uniform temperature changes of ±25°C for concrete bridges and temperature gradient effects (top surface 5°C warmer than bottom for positive gradient). These produce significant forces in continuous structures.
Q: What seismic detailing is required for bridges in low seismic zones?
A: Even in low seismic zones (PGA < 0.1g), minimum transverse reinforcement in plastic hinge regions is still required — typically #10 stirrups at 300 mm spacing in columns and minimum confinement in cap beams.
A: Even in low seismic zones (PGA < 0.1g), minimum transverse reinforcement in plastic hinge regions is still required — typically #10 stirrups at 300 mm spacing in columns and minimum confinement in cap beams.
