Standard Specifications and Code of Practice for Road Bridges, Steel Road Bridges (Limit State Method) (Third Revision)
2010 Edition

IRC 24 defines its extent in Clause 501.1, encompassing general guidelines for the design and construction of steel road bridges. It standardizes symbols for structural parameters (Clause 501.5), such as cross-sectional area, depth, and factored loads, while specifying units for calculation (Clause 501.7) like kN for forces and kNm for moments. The design framework is grounded in Limit State principles (Clause 503), and material requirements are elaborated in Clause 502. Multiplying factors for stress ranges in hollow sections are detailed in Clause 511.2.2.1 (Tables 16 and 17), ensuring consistent and reliable bridge design practices.

Clause 501.3 outlines the references utilized throughout IRC 24, including the code's structure and associated sections. Clause 501.5 provides a comprehensive list of symbols defining areas, forces, and geometric parameters vital for bridge design. Clause 501.7 recommends standardized units for all design calculations to maintain uniformity, including forces in kiloNewtons, mass in kilograms per cubic meter, and moments in kiloNewton-meters. The code also references IS 786 for unit conversions and ensures these conventions support coherent calculations across the code.

Core design considerations are detailed primarily in Clauses 503 and 504. The design philosophy is presented in Clause 503.1 and 512.2, emphasizing the Limit State Design methodology (Clause 503.2) and defining design loads (Clause 503.3). Classification of cross-sections (Clause 503.7) and the calculation of geometric properties (Clause 503.6) are key aspects. Effective span and depth parameters critical to structural analysis are specified in Clauses 504.1 and 504.2. Collectively, these clauses establish the basis for safe and serviceable bridge structures.

As specified in Clause 503.7.2, cross-sections are categorized into four classes based on plate width-to-thickness ratios, which influence local buckling behavior:

• Class 1 (Plastic): Sections capable of forming plastic hinges with adequate rotation capacity, with width/thickness ratios below Class 1 limits.
• Class 2 (Compact): Sections that can develop plastic moments but have limited rotation due to local buckling, lying between Class 1 and Class 2 limits.
• Class 3 (Semi-compact): Sections where the extreme compression fiber yields but plastic moment capacity is not achieved, falling between Class 2 and Class 3 limits.
• Class 4 (Slender): Elements prone to local buckling before yield; effective section properties must be employed.

The section adopts the classification of the most critical (lowest) class among its elements. Table 2 provides limiting width-to-thickness ratios, incorporating the factor ε = (250/f_y)^0.5 where f_y is yield stress in MPa. Additional guidance on web buckling and stress ratios is included to ensure accurate classification.

Specific detailed content for the design of compression members is not available in the provided context.

Tension members, subjected to axial tensile forces causing elongation and potential rupture, are addressed in Clause 506.1. The factored design tension must not exceed the member's design strength. Failure modes include rupture at critical sections and block shear failures in bolted or riveted connections. General provisions for design and detailing are covered in Clauses 507.6 to 507.9. First-order elastic analysis assumes no geometric deformation, while second-order analysis may be employed to consider stiffness changes due to axial forces. Although specific formulas are not provided here, design strength is typically calculated as the product of design yield strength and gross cross-sectional area.

Trusses and open-web girders are treated as triangulated skeletal girders (Clause 508.1). Design aspects include member and connection design per Clauses 506, 507, and 512. Camber is recommended for spans exceeding 50 meters (Clause 8.3.1), with deformation stresses disregarded for cambered girders. Camber calculations use member length changes and modulus of elasticity, with camber ordinates obtainable through the Williot-Mohr diagram (Annex-B). The width of cap stiffeners is computed using load, permissible bearing stress, and stiffener thickness (Clause 3.1.3). Elastic lateral torsional buckling moments for doubly symmetric beams are calculated using formulas involving section properties and effective lengths (Annex-C).

For beams and plate girders, key points include:

• Effective shear area calculations differ for rolled sections and plate girders (Clause 8.2.3).
• Maximum stiffener outstand varies with steel grade (Clause 11.2.6).
• Flange splices must have an area at least 105% of the flange area spliced, with load transfer ensured by rivets or welds (Clause G4.12).
• Web splices require design for shear and moment, with riveted and welded options specified (Clause G4.13).
• End connections must allow flexibility for simply supported beams (Clause G4.14).
• Lateral bracing is mandatory to counteract lateral forces (Clause G4.15).
• Thermal effects from temperature and live load changes must be incorporated in design (Clause G4.16). General design parameters such as span, girder spacing, and deflection limits are outlined in Clauses 504.1 to 504.7.

Connections and splices in flexural members must guarantee full transmission of longitudinal shear and vertical loads between flange and web components (Clause 512.5.2). Flange splices should utilize steel of identical grade, even if cross-sections differ. For bolted or riveted flange splices, the splice plate area must at least equal the flange area being spliced, transmitting the larger of 1.10 times the factored flange force or 0.80 times the capacity of the weaker flange. Web splices must be symmetrical with adequate fasteners to resist shear, design moments, and eccentric moments. Tension member splices require a sectional area exceeding design load requirements by 5%, with cover materials arranged to distribute stresses suitably (Clause 4.2.1). Fasteners include bolts, rivets, and pins.

Fatigue assessment formulas in IRC 24 involve summations of stress ranges raised to the fifth power, normalized against allowable fatigue strengths and safety factors (Clause 511.5.2.4). For normal stress ranges f and shear stress ranges τ, the cumulative damage must not exceed unity. Fatigue evaluation can be omitted if maximum stress ranges are below specified fractions of fatigue strength adjusted by safety factors or if the total number of cycles remains within defined limits. An equivalent constant amplitude stress range is computed as the root sum square of stress ranges weighted by their occurrences. These provisions ensure cumulative fatigue damage remains within safe bounds.

Welding practices for steel bridge construction must comply primarily with IS 9595, particularly for metal arc welding of carbon and carbon manganese steels (Clause 512.4.4.4). General procedures, including preparation of fusion faces, must adhere to IS 9595 (Clause 513.5.8.2). Written welding procedure specifications covering edge preparation must be submitted for Engineer approval prior to fabrication (Clause 513.5.8.3). Additional relevant Indian Standards address inspection and quality control of welds, including radiographic testing, magnetic particle inspection, and welder qualification tests, ensuring welding quality and reliability.

Fasteners must conform to applicable Indian Standards such as IS 1364 for hex bolts and nuts, IS 3757 for high strength structural bolts, IS 6623 for nuts, and IS 4000 for high strength bolt practice (Clause 1.6). Bolted joints require clean contact surfaces with slopes less than 1:20; otherwise, tapered washers are necessary (Clause 513.5.6.1). Washers are mandatory beneath nuts or bolt heads. Bolt tightening is performed manually or by calibrated tools to 70% of minimum tensile strength, with specified 'turn of nut' increments after snug tightening depending on bolt length and face orientation. For High Strength Friction Grip bolts, flange design incorporates factors accounting for bolt pretensioning, effective flange width, proof stress, and end plate thickness (Clauses 512.6 and 1.1).

Fabrication tolerances prescribed in Clause 3.5 and Table 20 include limits on girder width deviations, member insertion tolerances, web depth variations, straightness of columns, and flatness of webs. Squareness tolerances for columns, base plates, and beams, as well as end joint tolerances, are specified to ensure precise assembly. Inspection and repair guidelines (Clause 513.6.5, Table 21) define acceptable limits for edge discontinuities, stipulating when removal, welding, or no repair is required. These standards ensure structural integrity and proper fit during fabrication and erection phases.

Load-carrying stiffeners must be installed in symmetrical pairs on either side of the web to prevent eccentric loading effects (Clause 509.7.14). Stiffener ends should fit closely or be adequately fastened to both flanges, allowing up to five times web thickness clearance for weld root fillets. Stiffeners are required to be solidly packed without joggling, with bearing stresses on flange contact areas limited to design bearing strengths. Two-legged stiffeners are modeled as struts including the stiffeners and an adjacent web length of 20 times web thickness, constrained by edge distances and spacing. Multiple-legged stiffeners incorporate the web plate between outer legs and adjacent regions. Adequate rivets, bolts, or welds must facilitate complete load transfer to the web. Unsupported web panels are limited in size based on web thickness to maintain stability.

Quality control emphasizes inspection, maintenance, and testing to preserve structural safety and longevity. Regular inspection of pins, rivets, bolts, and corrosion-prone areas is mandated (Clauses E7.3.2, E8). Steel decks and expansion joints require monitoring for corrosion, weld integrity, and free movement (Clauses E9, E10). Detailed records of inspections and repairs must be maintained (Clause E14). Various assessment tools including ultrasonic and magnetic particle testing, strain gauges, and accelerometers support thorough evaluation (Table E-2). Acceptance testing involves load combinations with specified factors for self-weight, permanent, and variable loads (Annex F). Testing includes strength verification, coupon testing for yield strength, failure tests on samples, and batch consistency checks. Test loads are sustained for at least one hour with periodic readings, and acceptance criteria ensure no failure under test loads and compliance with serviceability limits. Factors for test load multipliers vary with the number of units tested (Table F.1).