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

The scope of IRC 24 is defined in Clause 501.1, covering general provisions for bridge design and construction. Key specifications include the use of standard symbols for structural parameters as per Clause 501.5, which defines areas, forces, and dimensions relevant to bridge members. Units recommended for design calculations are given in Clause 501.7, including forces in kN, moments in kNm, and unit weights in kN/m³. Design considerations follow limit state principles (Clause 503), and materials specifications are detailed in Clause 502. For example, symbols like A (area of cross-section), d (depth of web), and Fd (factored design load) are standardized for clarity. Multiplying factors for stress ranges in hollow sections are provided in Tables 16 and 17 (Clause 511.2.2.1). This framework ensures consistent, reliable bridge design under IRC 24.

Sources: Clause 501.1, Clause 501.5, Clause 501.7, Clause 503, Clause 502, Clause 511.2.2.1, Tables 16 and 17

The key references section in IRC 24 is covered under Clause 501.3, which lists the structure of the code and related sections. Important specifications include:

• Clause 501.5 provides a comprehensive table of symbols used throughout the code, defining areas, forces, dimensions, and factors relevant to bridge design.
• Clause 501.7 recommends units for design calculations: forces in kN, unit mass in kg/m³, unit weight in kN/m³, and moments in kNm.

These references ensure uniformity in notation and units across the code. For detailed design, refer to the symbols table in Clause 501.5 and unit conventions in Clause 501.7. The code also refers to IS 786 (Supplement) for unit conversions.

No explicit formulas or additional tables are provided in the references clause itself, but the symbols and units form the basis for all calculations in the code.

Sources: Clause 501.3, Clause 501.5, Clause 501.7

General design considerations in IRC 24 are primarily covered under Clauses 503 and 504. The basis of design is outlined in Clause 503.1 and 512.2, emphasizing limit state design principles (Clause 503.2) and design loads (Clause 503.3). Key aspects include classification of cross-sections (Clause 503.7) and geometrical properties (Clause 503.6). Effective span and effective depth, critical for structural analysis, are specified in Clauses 504.1 and 504.2 respectively. These clauses collectively guide the structural design process ensuring safety and serviceability. Unfortunately, the exact formulas and tables are not provided in the retrieved context. For detailed formulas and tables, refer to Clauses 503 and 504 in IRC 24, which cover design strength, factors governing ultimate strength, and geometrical properties essential for bridge design.

Sources: Clause 503.1, Clause 503.2, Clause 503.3, Clause 503.6, Clause 503.7, Clause 504.1, Clause 504.2, Clause 512.2

As per IRC 24 Clause 503.7.2, cross-sections are classified into four classes based on width-to-thickness ratios of plate elements, which govern their local buckling behavior:

• Class 1 (Plastic): Sections can form plastic hinges with sufficient rotation capacity. Width/thickness ratio must be less than Class 1 limits in Table 2.
• Class 2 (Compact): Sections can develop plastic moment but have limited rotation due to local buckling. Width/thickness ratio lies between Class 1 and Class 2 limits.
• Class 3 (Semi-compact): Extreme compression fiber can yield but plastic moment cannot develop due to local buckling. Width/thickness ratio lies between Class 2 and Class 3 limits.
• Class 4 (Slender): Elements buckle locally before yield. Width/thickness ratio exceeds Class 3 limits; effective section properties must be used.

When different elements fall under different classes, the section is classified by the most critical (lowest) class.

Table 2 Limiting Width to Thickness Ratios (excerpt):

Where ε = (250/f_y)^0.5, f_y = yield stress in MPa.

Additional notes:

• Elements exceeding semi-compact limits are slender.
• Webs must be checked for shear buckling if d/t > 67ε.
• Stress ratios r1 and r2 are used for some web classifications.

This classification guides design for local buckling and plastic hinge formation.

For buckling classes (a, b, c, d) and related design factors, see Clause 507.1.2.2 Table 4.

Sources: Clause 503.7.2, Table 2, Clause 507.1.2.2, Table 4

Detailed content not available.

For the design of tension members as per IRC 24, the key points are:

• Tension members are linear members subjected to axial tensile forces causing elongation and potential rupture at critical sections (Clause 506.1).
• The factored design tension T in the member must satisfy the design strength Ta of the member (Clause 506.1).
• Failure modes include rupture at critical sections and block shear failure in bolted or riveted end connections (Clause 506.1).
• For design and detailing, refer to Clauses 507.6 to 507.9 for general provisions (Clause 507.8).
• First-order elastic analysis assumes undeformed geometry and neglects stiffness changes due to axial forces; second-order analysis can be used to account for these effects (Clause 505.2.4).

Unfortunately, specific formulas or tables for tension member design are not provided in the retrieved context. Typically, design strength Ta is calculated as:

Ta = f_yd × A_g

where f_yd is the design yield strength and A_g is the gross cross-sectional area.

For block shear and rupture checks, refer to detailed clauses in IRC 24 beyond the retrieved text.

Sources: Clause 506.1, Clause 507.8, Clause 505.2.4

Design of trusses and open-web girders per IRC 24 involves treating them as triangulated skeletal girders (Clause 508.1). Key design aspects include:

• Member and connection design per Clauses 506, 507, 512.
• Cambering for spans > 50 m is recommended (Clause 8.3.1), with deformation stresses ignored for cambered girders.
• Camber calculation uses member length changes under load, with modulus of elasticity per Clause 502.2.4.1; camber ordinates can be obtained via Williot Mohr Diagram (Annex-B).
• Cap stiffener width is calculated as b = (N) / (σ_bg * t_c), where N = load, σ_bg = permissible bearing stress, t_c = stiffener thickness (Clause 3.1.3).
• Elastic lateral torsional buckling moment for doubly symmetric beams is given by the formula in Annex-C (Clause 509.2.2.1), involving section properties (I_y, I_w, G, I_t), effective length L_LT, and modulus of elasticity E.

Summary Table for Cap Stiffener Width:

Formula: b = N / (σ_bg * t_c) (Clause 3.1.3)

For detailed cambering and erection procedures, refer to Annex-B. For lateral torsional buckling, use the elastic critical moment formulas in Annex-C.

This concise framework covers key formulas, tables, and specifications for truss and open-web girder design under IRC 24.

Sources: Clause 508.1, Clause 8.3.1, Clause 3.1.3, Annex-B [Clause 504.6.2], Annex-C (Clause 509.2.2.1)

For the design of beams and plate girders as per IRC 24, key points include:

• Effective Sectional Area for Shear (Clause 8.2.3):

  • Rolled beams/channels: web thickness × overall depth.
  • Plate girders: web thickness × full web depth.
  • Special cases with variable web thickness or large openings require special consideration.

• Stiffener Outstand (Clause 11.2.6):

  Steel Grade	Max Outstand
  IS 2062 up to E250	16t
  IS 2062 Grade E300+	14t
  Flats (all steels)	12t
  (t = thickness of section or flat)

• Flange Splices (Clause G4.12):

  • Splice area ≥ 105% of flange area spliced.
  • Centre of gravity of splice to coincide with element spliced.
  • Rivets/welds to develop load + 5%, minimum 50% effective strength.

• Web Splices (Clause G4.13):

  • Designed for shear and moment at splice.
  • Riveted: splice plates both sides.
  • Welded: full penetration butt welds preferred.

• End Connections (Clause G4.14):

  • Must provide flexibility for simply supported beams.

• Lateral Bracing (Clause G4.15):

  • Required for all spans to resist lateral forces (wind, seismic).

• Thermal Effects (Clause G4.16):

  • Design must accommodate thermal stresses and length changes due to temperature and live loads.

• General Design Considerations:

  • Effective spans, depths, girder spacing, deflection, and minimum sections refer to Clauses 504.1 to 504.7.

This summary is based on IRC 24 Clauses 509.6, 8.2.3, 11.2.6, and G4 series.

Sources: Clause 509.6, Clause 8.2.3, Clause 11.2.6, Clause G4.12, Clause G4.13, Clause G4.14, Clause G4.15, Clause G4.16

According to IRC 24, Clause 512.5.2, connections and splices in flexural members must ensure full transmission of longitudinal shear and vertical loads between flange and web. Flange splices should use the same steel grade, possibly with different cross-sections. For bolted or riveted flange splices, the total splice plate area must be at least equal to the flange area spliced. The splice must transmit the greater of: (i) 1.10 times the flange force at the splice from factored loads, or (ii) 0.80 times the maximum capacity of the weaker flange (Clause 1.10). Web splices must resist shear, design moment, and moment from shear eccentricity, with symmetrical splice plates extending nearly full web depth and at least two rows of rivets/bolts per side. Per Clause 4.2.1, tension member splices require 5% extra sectional area over the load demand, with cover material arranged to suit stress distribution. Connections include all joints and splices; fasteners include bolts, rivets, and pins (Clause 512.1.1).

Sources: Clause 512.5.2, Clause 1.10, Clause 512.1.1, Clause 4.2.1

The key formulas and specifications for fatigue assessment as per IRC 24 are:

• Fatigue Assessment for Variable Stress Ranges (Clause 511.5.2.4):

  • For normal stresses (f): [ \sum_{j=1}^r \frac{n_i f_j^5}{5 \times 10^6 (u_f / Y_{mft})^5} \leq 1 ]
  • For shear stresses (τ): [ \sum_{j=1}^r \frac{n_i \tau_j^5}{5 \times 10^6 (u_\tau / Y_{mft})^5} \leq 1 ]
  • Here, summation limits exclude stress ranges below 0.55 of the fatigue strength (M_fm or M_τ).

• Necessity for Fatigue Assessment (Clause 511.6): Fatigue assessment is NOT required if any of the following hold:

  • Maximum normal stress range ( f_{max} \leq 0.27 M_{fm} / Y_{mft} )
  • Maximum shear stress range ( \tau_{max} \leq 0.67 M_{fm} / Y_{mft} )
  • Total number of stress cycles ( N_{sc} ) satisfies a certain limit involving equivalent constant amplitude stress range ( f_{eq} ).

• Equivalent constant amplitude stress range (f_eq): [ f_{eq} = \left[ \sum_{i=1}^r n_i f_i^2 + \sum_{j=1}^s n_j f_j^2 \right]^{1/2} ]

These formulas ensure that the cumulative damage from variable stress cycles does not exceed fatigue capacity. The factors ( u_f, u_\tau, Y_{mft} ) relate to fatigue strength and safety factors.

This summary is based on Clauses 511.4, 511.5, 511.5.2.4, and 511.6 of IRC 24.

Sources: Clause 511.4, Clause 511.5, Clause 511.5.2.4, Clause 511.6

Key welding procedures and details in IRC 24 refer extensively to IS 9595 for metal arc welding of carbon and carbon manganese steels. As per Clause 512.4.4.4, welding procedures must comply with IS 9595 unless otherwise specified. Clause 513.5.8.2 mandates that general welding procedures, including fusion face preparation, follow IS 9595. Further, Clause 513.5.8.3 requires written submission of welding procedures for shop and site welds, including edge preparation, per Clause 22 of IS 9595 for Engineer's approval before fabrication.

Additional relevant IS codes for welding quality and inspection include:

• IS 822: Inspection of welds
• IS 1024: Welding in bridges and dynamic structures
• IS 1182 & IS 4853: Radiographic examination of welds
• IS 5334: Magnetic particle flaw detection
• IS 7307, IS 7310, IS 7318: Approval tests for welding procedures and welders

These codes collectively govern welding quality, inspection, and approval procedures.

Sources: Clause 512.4.4.4, Clause 513.5.8.2, Clause 513.5.8.3

Key specifications and formulas for fasteners and bolting per IRC 24 include:

• Fasteners must comply with relevant IS standards such as IS 1364 (hexagon head bolts, screws & nuts), IS 3757 (high strength structural bolts), IS 6623 (high strength structural nuts), and IS 4000 (code of practice for high strength bolts in steel structures) as per Clause 1.6.

• All bolted joint surfaces must be clean and free from defects; slope of contact surfaces shall not exceed 1:20, else tapered washers are required (Clause 513.5.6.1).

• Washers are mandatory under nuts or bolt heads.

• Tightening of bolts is done by hand or calibrated wrenches to 70% of minimum tensile strength. The "turn of nut" method specifies additional nut rotation after snug tightness based on bolt length and face disposition:

• For High Strength Friction Grip (HSFG) bolts, the formula for flange design includes:

  ₿ = 2 for non pre-tensioned bolt, 1 for pre-tensioned bolt; n = 1.5;

  b = effective flange width per pair of bolts;

  fo = proof stress;

  t = thickness of end plate (Clause 512.6 and 1.1).

These form the core requirements for fasteners and bolting in steel structures per IRC 24.

Sources: Clause 1.6, Clause 513.5.6.1, Clause 512.6, Clause 1.1

Key specifications for Fabrication, Erection, and Inspection per IRC 24 include the following:

Fabrication Tolerances (Clause 3.5, Table 20):

• Width of built-up girders: ±3 mm
• Deviation in width of members inserted in others: 0 to -3 mm
• Depth deviation of solid/open web: +3 mm to -2 mm
• Straightness deviation of columns: L/300 (max 15 mm)
• Deviation of centre line of web from flanges in built-up members: 3 mm
• Deviation from flatness of plate webs: 0.005d (max 2 mm)
• Tilt of flange of plate girders at splices/stiffeners/supports: 0.005b (max 2 mm), elsewhere 0.015b (max 4 mm)
• Squareness deviations for columns, base plates, beams: ranges from D/500 to D/1000 depending on element
• Ends of members at joints: 1/600 of depth (max 1.5 mm)

Inspection and Repairs (Clause 513.6.5, Table 21):

• Edge discontinuities ≤3 mm depth: no repair needed
• 3-6 mm depth (up to 100 mm thick) or 6-12 mm depth (100-200 mm thick) over certain lengths: remove, no welding needed
• 6-25 mm depth: remove and weld; no single repair >20% of edge

• 25 mm depth: remove to 25 mm depth and weld with Engineer approval

• Edges cut in fabrication with ≤12 mm depth discontinuities: no repair needed

These ensure accurate fit-up and structural integrity during fabrication and erection stages.

Sources: Clause 3.5, Table 20, Clause 513.6.5, Table 21

Key specifications and design rules for Load Carrying Stiffeners and Diaphragms per IRC 24 are as follows:

• Load bearing stiffeners must be provided in pairs, symmetrically on both sides of the web; eccentricity effects must be considered if not.
• Ends of stiffeners should fit closely or be adequately connected to both flanges, allowing clearance up to 5 times the web thickness for weld root fillets (Clause 509.7.14a,b).
• Stiffeners shall be solidly packed, not joggled (Clause 509.7.14c).
• Bearing stress on the stiffener legs in contact with the flange must not exceed design bearing strength (Clause 509.7.14d).
• Two-legged stiffeners are designed as struts including the stiffeners plus web length equal to 20 times web thickness, limited by edge distance and adjacent stiffener spacing (Clause 509.7.14e).
• For four or more legs, the effective section includes stiffeners plus web plate between outer legs and adjacent web portions as above.
• Adequate rivets, bolts, or welds must transfer the full load to the web (Clause 509.7.14f).
• Web panel unsupported clear dimensions must not exceed 270t (greater dimension) and 180t (lesser dimension), where t is web thickness (Clause 509.7.14g).

These rules ensure stiffener effectiveness in load transfer and web stability.

Sources: Clause 509.7.14

Quality control and testing in IRC 24 focus on inspection, maintenance, and structural testing to ensure safety and durability. Key points include:

• Inspection of connections and corrosion: Pins, rivets, bolts, and corrosion-prone areas must be checked regularly (Clause E7.3.2, E8).

• Maintenance of steel decks and expansion joints: Check for corrosion, weld soundness, and free movement of expansion joints (Clauses E9, E10).

• Documentation: Maintain detailed records of inspections, repairs, and load capacity (Clauses E14).

• Standard tools and assessment methods: Use tools like ultrasonic testing, strain gauges, paint film gauges, and accelerometers for various assessments (Table E-2).

• Testing procedures (Annex F):

  • Acceptance test load: F = (1.0 × self weight) + (1.15 × test permanent load) + (1.25 × variable load) (Clause 1.0).
  • Strength test confirms calculated resistance; coupon tests determine actual yield strength (F2.2).
  • Test to failure determines ultimate resistance with at least three specimens (F2.3).
  • Check tests on production batches ensure consistency (F2.4).
  • Test conditions require calibrated devices, representative loading, and deflection measurements (F3).
  • Test loads must be maintained for at least 1 hour with readings every 15 minutes (F4.2).
  • Acceptance criteria: no failure under strength test load for 15 minutes; serviceability limits on deformation (F5).

• Table F.1 provides factors for test load multipliers based on number of units tested:

This comprehensive approach ensures structural integrity through systematic inspection, testing, and documentation as per IRC 24 clauses E7 to E17 and Annex F.

Sources: Clause E7.3.2, Clause E8, Clause E9, Clause E10, Clause E14, Table E-2, Annex F (Clauses F1 to F5), Table F.1