Guidelines for Evaluation of Load Carrying Capacity of Bridges

IRC SP 37: Scope - Key Specifications & Tables

The code provides design bending moments (B.M.) and shear forces (S.F.) for various span lengths under different traffic conditions and vehicle GVW classes, considering impact, lane reduction, and overload factors (OLF = 1.4 or 2.0).

1. Scope Summary:

• Applicable for carriageway widths:
  • Up to 5.3 m (single lane)
  • Between 13.1 m to 19.6 m (4 lanes, steel superstructure)
• Vehicle GVW classes considered: 16.2T, 25T, 35.2T, 40.2T, 49T
• Includes Impact Factor, Lane Reduction Factor, and Overload Factor (OLF)
• Conditions:
  • Moving Traffic
  • Crowded/Traffic Jam

2. Key Formula:

Absolute Governing Moment/Shear = Maximum of

• Governing IRC Load effect
• Effect due to GVW class with OLF (usually 1.4, sometimes 2.0 for extreme cases)

3. Sample Table Extract: Bending Moment (t-m) for 4-Lane Carriageway (Span = 20m, Moving Traffic)

4. Sample Table Extract: Shear Force (tonne) for 4-Lane Carriageway (Span = 20m, Moving Traffic)

Assessment of Condition of Bridge (IRC SP 37 - Clause 3)

Key Points:

• Scientific assessment requires detailed guidelines and a strong database.
• Refer to these IRC guidelines for comprehensive assessment:
  • IRC:SP:35-1990 – Inspection & Maintenance
  • IRC:SP:40-1993 – Strengthening & Rehabilitation
  • IRC:SP:51-1999 – Load Testing
  • IRC:SP:52-1999 – Bridge Inspector's Manual
  • IRC:SP:60-2002 – Remaining Life of Concrete Bridges
  • IRC:SP:74-2007 – Steel Bridge Repair
  • IRC:SP:80-2008 – Corrosion Protection
  • Special Report 17 – Non-Destructive Testing

Structural Condition Assessment Checklist (Clause 3.2.2):

• Cracking, spalling, honeycombing of concrete
• Corrosion of rebars/prestressing cables/steel members
• In-situ material strength
• Condition of joints, bearings, expansion joints
• Settlement, deformation, rotation effects
• Movements of piers, abutments, foundations
• Hydraulic factors: scour, erosion, high flood level (HFL), afflux

Important Notes:

• If design/construction documents are unavailable, prepare as-built drawings by measuring member dimensions.
• Intrusive investigations may be needed for reinforcement/prestressing details.
• Hydraulic safety and foundation stability must be evaluated separately.

Typical Formula Reference for Load Carrying Capacity (from IRC codes):

[ \text{Load Capacity} = \frac{\text{Ultimate Strength}}{\text{Load Factor}} ]

Where ultimate strength is derived from material properties and structural analysis considering deterioration.

Conceptual Flow for Bridge Condition Assessment:

flowchart TD
    A[Start: Document Review] --> B[Field Investigation]
    B --> C{Complete Documentation?}
    C -- Yes --> D[Identify Deterioration Effects]
    C -- No --> E[Measure Structural Dimensions]
    E --> F[Intrusive Investigations]
    D --> G[Material Testing]
    F --> G
    G --> H[Structural Analysis & Load Capacity]
    H --> I[Hydraulic & Foundation Safety Check]
    I --> J[Assessment Report

Traffic Factors per IRC SP 37

The code provides Safe Load (S.F.) values in tonnes for different span lengths and vehicle Gross Vehicle Weights (GVW), considering:

• Moving Traffic Condition
• Crowded/Traffic Jam Condition
• Impact factor, lane reduction factor, and overload factor (OLF) = 1.4 (or 2 for 49-ton GVW)

Key Tables Summary (Carriageway ≤ 5.3 m)

Values include impact, lane reduction, and overload factors.

Important Notes:

• Overload Factor (OLF): 1.4 for general GVWs; 2 for 49-ton GVW.
• Span length influences the magnitude of the traffic factor.
• For higher GVWs (25T to 49T), S.F. increases accordingly.
• Tables provide safe load values to be used in design for different traffic scenarios.

Formula (Conceptual):

[ \text{Safe Load} = \text{Base Load} \times \text{Impact Factor} \times \text{Lane Reduction Factor} \times \text{Overload Factor} ]

Visual Summary:

flowchart TD
    A[Span Length] --> B[Select GVW Class]
    B --> C{Traffic Condition}
    C -->|Moving| D[Apply Impact + Lane Reduction + OLF=1.4]
    C -->|Crowded| E[Apply Impact + Lane Reduction + OLF=1.4]
    D --> F[Determine Safe Load from Table]
    E --> F

Use these tables and factors for accurate load estimation in bridge design per IRC SP 37.

IRC SP 37: Vehicle Dimensions & Load Classifications

1. Vehicle Dimensions (Clause 4.3)

• Axle Types & Max Safe Loads (Tonnes):

2. Load Classifications (Clause 4.2.1 & 5.3)

• GVW (Gross Vehicle Weight) Classes: 16.2 t, 25 t, 35.2 t, 40.2 t, 49 t.
• Span Length vs Bending Moment (BM) for Moving & Crowded Traffic:

• Load Governing Conditions for Lanes:

Analytical Method of Bridge Rating (IRC SP 37 - Clause 6)

Key Points:

• Applicability: When as-built drawings/specs are available or can be accurately prepared.
• Analysis Methods: Use period-appropriate methods (e.g., Courbon's for girders, Guyon-Massienet for slabs) for preliminary strength distribution.
• Modern methods: Use FEM or grillage analysis for detailed force distribution and compare with old methods.

Important Tables & Formulas

Table 3: Safe Axle Load for RCC Slab Bridges

• Notes:
  • Slab thickness includes 25 mm cover + 75 mm wearing coat.
  • Impact allowance is included in safe axle loads.

Masonry & Plain Concrete Arch Bridges

• Use period-specific methods or nomograms (see Fig. 1 in code).
• Calculate provisional safe axle load from nomogram.
• Apply factors (profile, material, joint, support) from Annex 2 to get allowable loads.

Summary of Rating Methods (Clause 5.1)

• **Anal

Design Method - IRC SP 37 Key Points

1. Design Philosophy (Clause 6.3):

   • Use Working Load Allowable Stress Method for rating existing bridges, except those originally designed by Limit State Design.

2. Analytical Methods (Clause 6.5b):

   • Use period-appropriate analysis methods for strength assessment:
     • Courbon's method for girder bridges
     • Guyon-Massienet method for slab bridges
     • Equivalent plate method for wide bridges
   • Modern methods (Grillage, FEM) can refine load distribution and compare with old methods.

3. Slab Bridges - Safe Axle Load (Clause 6.5c, Table 3):

   • Safe axle loads depend on effective span and slab thickness.
   • Includes 25 mm cover + 75 mm wearing coat; impact already accounted.

4. Masonry/Plain Concrete Arch Bridges (Clause 6.5d):

   • Use historical methods (nomograms, rules of thumb).
   • Adjust provisional axle loads by factors (profile, material, joint, support) from Annex 2.
   • Material factor for plain concrete arches = 1.5.

5. Substructure & Foundation (Clause 6.5 2 & 3):

   • Condition surveys critical for bearings, piers, foundations.
   • Repairs or jacketing preferred over derating if feasible.
   • Scour treated by filling and bed protection to avoid derating.

Summary Flow of Design Methodology

flowchart TD
    A[Start: Existing Bridge Assessment]
    B[Gather Details: Drawings / Investigations]
    C{Bridge Type?}
    D[Girder Bridge - Courbon's Method]
    E

IRC SP 37 - Load Combinations with Other Loads of IRC:6

Key Points from Clause 7.3 and Related Clauses:

• GVW loading replaces only IRC:6 live loads; other loads/combinations per IRC:6 and relevant codes.
• Permissible stresses increase by 15% for substructure/foundation when crowded traffic governs wind/water current combos.
• Seismic checks may be omitted in some cases (e.g., infrequent heavy GVW or old bridges).
• Old bridges: wind loads can be reduced by 50% in wind+seismic combos; seismic checks may be omitted.
• OD/OW vehicles: Refer Section 11 for special provisions.
• Load combinations should follow IRC:6, considering Class AA and 70R loads as per original design.
• Working Stress Design: Use allowable stresses per original design or Annex 1 of IRC:6, limited by test results.

Typical Load Combinations (per IRC:6 working stress design):

Note: For rare combinations (max live load + design flood + max wind or earthquake), lower safety factors may be used within elastic limits.

Allowable Stress Selection (Working Stress Design):

[ \text{Allowable Stress} = \max \left( \text{(i) Original design allowable}, \text{(ii) Annex 1 values} \right) \leq \text{(iii) Strength test values} ]

Summary Diagram:

flowchart TD
    A[GVW Load replaces IRC:6 Live Load] --> B[Use IRC:6 Load Combinations]
    B --> C{Special Cases?}
    C -->|Crowded Traffic| D[Increase Permissible Stress by 15%]
    C -->|Infrequent Heavy GVW| D
    C -->|

Load Testing for Rating & Posting (IRC SP 37 - Clause 8)

Purpose:

• Load Test for Rating: When analytical rating isn't possible due to missing data (esp. masonry arches).
• Load Test for Posting: When strength verification by analysis is unreliable.

Key Specifications (Clause 1.25):

Rating Load Determination:

• Load for rating = ½ the axle load causing:
  • New visible cracks, or
  • Noticeable widening of existing cracks.

Summary Table for Load Test Criteria

Notes:

• Load tests must be carefully instrumented to measure deflections and crack developments.
• For masonry arches, load testing is recommended due to complex behavior.

flowchart TD
    A[Start Load Test] --> B{Type of Test?}
    B -->|Rating| C[Apply Load Incrementally]
    B -->|Posting| D[Apply Load Incrementally]
    C --> E{Crack observed?}
    E -->|Yes| F[Record Load]
    E -->|No| G[Increase Load]
    F --> H[Load for Rating = ½ Recorded Load]
    D --> I[Measure Deflections & Spreads]
    I --> J{Deflection/Spread limits exceeded?}
    J -->|Yes| K[Record Load & Behavior]
    J -->|No| L[Increase Load]

Reference: IRC SP 37, Clause 8 and Clause 1.25 for deflection and crack criteria.

Bridge Posting as per IRC SP 37 - Clause 9.4 & Related Specifications

Posting Signs Requirements (Fig. 4):

• Load Regulatory Sign:

  • Placed ≥ 100 m from bridge abutments on both ends.
  • Indicates maximum allowable vehicle/axle load.
  • Helps truckers plan detours or reduce loads.

• Advance Warning Sign:

  • Placed ≥ 200 m from abutments on both ends and at all road junctions leading to the bridge.
  • Displays "LOAD LIMIT BRIDGE AHEAD" warning.

Sign Specifications (per IRC:67-1977 & SP:31):

• Color: Black writing on yellow-black background.
• Information includes:
  • Cross vehicle weight limit (e.g., 162 tonnes).
  • Axle load limit (e.g., 102 tonnes).
  • Bridge rating class (e.g., CL 30 R).
  • Date of posting.
  • Railing post on entry and exit.

Key Notes on Bridge Posting:

• Signs must be visible on both sides of the bridge.
• Install at all approach roads and junctions leading to the bridge.
• Posting ensures safety and load regulation compliance.

Related Load Rating Criteria (Clause 8.6.2 for Girder Bridges):

• Rating load ≤ minimum of:
  1. Load causing deflection ≤ span/1500 (simply supported) or cantilever span/800.
  2. Load causing tension cracks > 0.3 mm (normal) or 0.2 mm (severe conditions).
  3. Load causing visible new diagonal cracks > above widths or crack widening near supports.
  4. Load with deflection recovery ≥ 75% (RCC) or 85% (prestressed concrete) after unloading.

flowchart LR
    A[Bridge Abutment] -->|≥ 200 m| B[Advance Warning Sign]
    B -->|≥ 100 m| C[Load Regulatory Sign]
    C --> D[Bridge Entry]
    D --> E[Bridge]
    E --> F[Load Limit Enforcement]

This diagram shows the relative positioning of posting signs on bridge approaches.

For detailed sign dimensions and design, refer to **IRC:67

IRC SP 37: Guidelines for Permitting Over-Dimensioned/Over-Weight Vehicles

Key Definitions (Clause 2.2)

• Rating of a bridge: Safe permissible load capacity as per IRC standard loads.
• Posting of a bridge: Limits on vehicle dimensions/weights allowed without special permission.
• Over-Dimensioned/Over-Weight Vehicles (ODC/OWC):
  • ODC: Vehicles exceeding legal height, width, or length limits (see Section 4.3).
  • OWC: Vehicles carrying loads >100 tons or special multi-axle vehicles.

Vehicle Classification & Overload Factors (Clause 4.2.2 & Table 2 summary)

• Overload Factor = Actual GVW / Theoretical Max GVW (based on axle and tyre config)
• Overload factors are mean values from surveys; adjust for local conditions.

Recommendations for Permitting ODC/OWC (Clause 11 summary)

• Conduct special local traffic surveys for heavy industries.
• Assess actual loading per IRC:5 General Design Features.
• Use overload factors and vehicle classification for bridge rating and posting.
• Provide information to traffic authorities and users on safe limits.
• Permit ODC/OWC with special permissions and escorts if required.

Visual: Vehicle Load Distribution Concept

graph LR
A[Axle 1] -->|Load| B[Bridge]
C[Axle 2] -->|Load| B
D[Axle 3] -->|Load| B
E[Axle n] -->|

IRC SP 37 - Impact and Lane Reduction Factors Summary

Key Points from Clause 1.4 & 5.3 (IRC SP 37):

• Impact Factor & Lane Reduction Factor are incorporated as per IRC:6.
• Overload Factor (OLF): Typically 1.4; also considered for 2.0 for higher GVWs.
• Traffic Conditions:
  • Moving Traffic: Includes impact & lane reduction factors.
  • Crowded/Traffic Jam: Includes lane reduction but excludes impact factor.

Impact & Lane Reduction Factors (per IRC:6):

• Impact Factor (I) depends on span length ( L ) (m):

  [ I = \frac{50}{L + 125} \quad \text{(for spans up to 30m)} ]

• Lane Reduction Factor (K) for multiple lanes:

  [ K = 1.0 \quad \text{for single lane loaded} ] [ K = 0.85 \quad \text{for two lanes loaded} ] [ K = 0.75 \quad \text{for three lanes loaded} ]

Overload Factor (OLF):

• Applied on GVW to account for possible overload:

  • Normal: ( \text{OLF} = 1.4 )
  • Severe: ( \text{OLF} = 2.0 )

Sample Shear Force (S.F.) Values (tonne) for GVW 16.2 T (Excerpt):

Values include overload factor 1.4.

Usage:

• Use IRC:6 formulas for impact and lane reduction.
• Apply overload factor on GVW.
• Choose impact factor inclusion based on traffic condition.
• Refer to tables in IRC SP 37 for exact

IRC SP 37: Key Formulas, Tables & Specifications for Bending Moments & Shear Forces

1. Basic Parameters:

• GVW (Gross Vehicle Weight): 49.0 tonnes (T)
• Overload Factor (OLF): 2.0 (for absolute governing moments/shear)
• Span Location for BM: 0.5 L (mid-span)
• Span Location for SF: 0.0 L (support)

2. Key Tables Summary:

(Refer to Tables 14, 23, 24 for detailed values by lane width and conditions.)

3. General Formulas:

• Bending Moment for Simply Supported Span at mid-span (0.5L):

[ M = \frac{wL^2}{8} ]

Where:

• (w) = total load per unit length (including IRC load, GVW with OLF, impact, and lane reduction factors)

• (L) = span length (m)

• Shear Force at support (0.0L):

[ V = \frac{wL}{2} ]

4. Load Factors:

• Impact factor and lane reduction factors are applied as per IRC guidelines.
• Overload Factor (OLF) of 2.0 is used for absolute governing moments and shear forces due to GVW 49T.

5. Graphical Representation:

graph LR
A[Span Length (L)]

Special Considerations for Old Bridges (IRC SP 37, Clause 6.5)

Key Points:

a) Detailing of Steel in Old Bridges

• Use existing detailed drawings if available.
• If not, use NDT (e.g., magnetic covermeter) and semi-destructive methods (chipping cover).
• Extrapolate bar spacing and diameters based on construction practices of the period.
• Compare with similar period structures for curtailment and corner detailing.

b) Analytical Methods

• Use period-appropriate methods for initial strength estimation:
  • Courbon's method for girder bridges.
  • Guyon-Massienet method for slab bridges.
  • Equivalent plate method for wide bridges.
• Modern FEM or grillage analysis can refine load distribution and compare with old methods.

c) Slab Bridges: Safe Axle Load (Table 3)

• Slab thickness includes 25 mm cover + 75 mm wearing coat.
• Impact factor is included in safe axle loads.

d) Masonry & Plain Concrete Arch Bridges

IRC SP 37: Testing Procedures & Deflection Monitoring - Key Points

1. Theoretical Deflection Calculation (Clause 8.5.7 & 8.5.1)

• Calculate theoretical deflections at critical points before testing using section properties and concrete modulus.
• Plot these deflections vs. load stages as a baseline graph.

2. Load-Deflection Monitoring

• During testing, plot actual deflections on the same graph.
• Check for linearity:
  • If two successive deflections exceed 10% deviation from linear trend ⇒ possible plastic behavior → stop loading temporarily and consult design office.
• Continue marking deflections for next 24 hours to monitor creep or delayed effects.

3. Stopping Criteria for Load Increment

• For arch bridges and other types, loading increments must stop if non-linear behavior or excessive deflections occur (detailed in Clause 8.5.7(a)).

4. Testing & Inspection References

• Follow IRC:SP:51 for detailed load testing procedures.
• Use inspection and testing methods from IRC:SP:35, IRC:SP:74, IRC:SP:80 for non-destructive testing and assessment techniques.

Typical Theoretical Deflection Formula for Concrete Girders:

[ \delta = \frac{PL^3}{48EI} ]

Where:

• (P) = applied load
• (L) = span length
• (E) = modulus of elasticity of concrete
• (I) = moment of inertia of the section

graph LR
A[Calculate Theoretical Deflections] --> B[Plot Deflection vs Load]
B --> C[Apply Load & Measure Actual Deflections]
C --> D{Is Deflection Linear?}
D -- Yes --> E[Increase Load]
D -- No --> F[Stop Load & Consult Design Office]
F --> G[Continue Monitoring for 24 hrs]

Summary:

• Pre-calculate and plot theoretical deflections.
• Monitor actual deflections for linearity during loading.
• Stop loading if deflections deviate >10%.
• Use referenced IRC codes for detailed procedures and advanced testing methods.

IRC SP 37 Key References & Annexures Summary

1. Bending Moment & Shear Force Tables

For 4-lane steel superstructure bridges with carriageway width 13.1m to 19.6m, inclusive of impact, lane reduction, and overload factor (OLF = 1.4):

• Overload Factor 2.0 considered for 49T GVW.
• Governing moments and shears are tabulated for design and rating.

2. Safe Axle Loads for RCC Slab Bridges (Table 3, Clause 6.5c)

• Includes 25 mm cover + 75 mm wearing coat.
• Impact allowance included in safe axle load.

3. Analytical Methods for Existing Bridges (Clause 6.5a,b,d)

• Use period-appropriate methods (Courbon, Guyon-Massienet) for load distribution.
• Modern FEM or grillage analysis recommended for detailed assessment.
• For masonry/plain concrete arches, use nomograms and apply factors from Annex 2.

4. Code Structure & Annexures