Bridge load rating determines the safe live load carrying capacity of a bridge, expressed as a rating factor (RF) or metric tonnage. Condition deterioration found during inspection — section loss, cracking, spalling, corrosion — directly reduces load rating, potentially requiring posting or closure. Covers load rating methods (ASR, LFR, LRFR), rating factor calculation, inventory vs operating rating, bridge posting, re-rating after repairs, and the inspection-load rating feedback loop.
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Bridge load rating is defined by the National Bridge Inspection Standards (NBIS) at 23 CFR 650.305 as “the analysis to determine the safe vehicular live load carrying capacity of a bridge using bridge plans and supplemented by measurements and other information gathered from an inspection.” This definition establishes load rating as an analytical process that is inherently dependent on both as-built design data and field-verified condition data obtained through inspection. A load rating is not a static number assigned at construction — it is a living assessment that must reflect the current physical state of the bridge, including any deterioration, damage, or modifications discovered during inspection.
The legal foundation for bridge load rating in the United States originates from the Federal-Aid Highway Act of 1968, which directed the Secretary of Transportation to establish national standards for bridge safety inspection. Congress codified this mandate at 23 U.S.C. 144, requiring the establishment of minimum standards for bridge inspection and the preparation and maintenance of a national bridge inventory. The NBIS regulations at 23 CFR 650 Subpart C implement this statutory mandate. Section 650.315 specifically requires each State transportation department to “prepare and maintain an inventory of all highway bridges,” which includes load rating data as an essential component of that inventory.
The AASHTO Manual for Bridge Evaluation (MBE) is the governing technical standard for load rating in the United States. It is incorporated by reference into federal regulation at 23 CFR 650.317, giving it the force of regulation. The MBE provides the methodology for three load rating methods (ASR, LFR, LRFR), the rating factor equation, load and resistance factors, condition factors, and legal load configurations. The MBE is updated periodically through AASHTO’s Bridge Committee, with interim revisions published between full editions. The current edition is the MBE 3rd Edition (2018) with subsequent interim revisions.
The NBIS applies to all structures defined as “highway bridges” — structures with an opening measured along the center of the roadway of more than 20 feet (6.1 meters) between abutments or extreme ends of openings, located on public roads. All such bridges must have a current load rating on file. Load ratings must be performed or supervised by a licensed Professional Engineer (PE) with bridge load rating experience. The load rating report must be sealed and signed by the engineer of record. For bridges that have not been load rated by formal analysis, the NBIS permits assigned load ratings based on the original design load if certain conditions are met — the bridge must have been designed by LRFD or LFD to at least HL-93 or HS-20, built per plans, with no deterioration that reduces capacity below the design level.
The AASHTO MBE recognizes three load rating methods, each representing a different generation of structural engineering philosophy. The selection of method depends on the design specifications used for the original bridge, the availability of as-built information, and State DOT policy. The FHWA has progressively moved the industry toward Load and Resistance Factor Rating (LRFR) as the preferred method, but continues to accept legacy methods for existing valid ratings.
Allowable Stress Rating (ASR) is the oldest method, rooted in working stress design philosophy. In ASR, the computed stress in each bridge member under the rating vehicle load is compared to an allowable stress — a fraction of the material yield strength or ultimate strength, divided by a single factor of safety. For steel members, the allowable bending stress is typically 0.55Fy (55% of yield stress) for inventory rating and 0.75Fy (75% of yield) for operating rating. The rating factor under ASR is simply the allowable stress divided by the computed stress from the rating vehicle.
ASR was the standard method from the early 20th century through the 1970s and is still applied to timber and masonry bridges where the more sophisticated LFR and LRFR methods are not well-calibrated. The FHWA 2006 policy memorandum on bridge load ratings specifically notes that ASR remains acceptable for timber and masonry bridges as a policy exception. ASR does not differentiate between different types of loads (dead vs. live) with different factors — it applies the same safety margin to all loads. This lack of load-specific differentiation is the primary theoretical weakness of ASR compared to later methods.
The rating factor equation for ASR takes the form: RF = (Allowable Stress − Dead Load Stress) / (Live Load Stress × (1 + I)), where I is the impact factor. This is a simplified single-equation approach that does not separately account for dead load uncertainties, live load variability, or material strength variability. ASR produces the most conservative ratings among the three methods for most bridge types, though the degree of conservatism varies with span length and member type.
Load Factor Rating (LFR) emerged from Load Factor Design (LFD) philosophy, which was adopted in the AASHTO Standard Specifications for Highway Bridges beginning in the 1970s. LFR applies different load factors to different load types — higher factors to live loads (which are more variable) than to dead loads (which are better known). This differentiated treatment is the key advancement over ASR. The LFR rating factor equation is: RF = (φ × Rn − γ_DC × DC − γ_DW × DW) / (γ_LL × (LL + I)), where φ is the resistance factor (typically 1.0 for flexure in steel, 0.90 for shear), γ_DC is the dead load factor (typically 1.30 for inventory, 1.30 for operating), γ_DW is the wearing surface factor (1.30), and γ_LL is the live load factor (2.17 for inventory, 1.30 for operating).
LFR uses the MS18 (HS-20) design truck as the rating vehicle. The MS18 truck has a gross weight of 72,000 lbs (32.4 metric tons) distributed as an 8,000 lb front axle and two 32,000 lb rear axles spaced 14 to 30 feet apart, plus a lane load of 640 lbs per linear foot. The inventory rating live load factor of 2.17 corresponds to a reliability index of approximately 3.5, while the operating factor of 1.30 corresponds to a reliability index of approximately 2.5.
LFR was the dominant rating method in the United States from the 1970s through the early 2000s. Many thousands of existing bridges still carry valid LFR ratings that remain acceptable to FHWA. The FHWA 2006 policy memorandum confirmed that LFR ratings could continue to be reported to the NBI for bridges designed by LFD or ASD specifications. However, for new load ratings performed after October 1, 2010, FHWA policy required that all new bridges designed by LRFD must use LRFR methods. For existing bridges, LFR remains an acceptable alternative to LRFR.
Load and Resistance Factor Rating (LRFR) is the current state-of-the-art method, aligned with AASHTO Load and Resistance Factor Design (LRFD) philosophy. LRFR is based on reliability theory — the load and resistance factors are calibrated using probability-based methods to achieve consistent target reliability indices (β) across different bridge types, span lengths, and limit states. For inventory rating, the target reliability index is β = 3.5 (approximately 1 in 4,000 probability of exceeding a limit state during the evaluation period). For operating rating, the target is β = 2.5 (approximately 1 in 160 probability).
LRFR uses the HL-93 design live load as the rating vehicle for design-load rating. HL-93 consists of either a design truck (HS-20 with 32,000 lb axles) plus a 640 plf lane load, or a tandem (25,000 lb per axle spaced 4 ft apart) plus lane load, whichever produces the worst effect. HL-93 also includes the design truck alone (without lane load) for negative moment between points of contraflexure. This loading was introduced with the LRFD specifications in 1994 and is more representative of modern heavy truck traffic than the older MS18/HS-20.
The LRFR rating factor equation incorporates three additional adjustment factors not present in LFR:
Condition factor (φc) — applied to the member resistance to account for deterioration observed during inspection. Per MBE Table 6A.4.2.3-1, φc = 0.85 for members with “heavy deterioration” (significant section loss, cracking, or spalling), 0.95 for “moderate deterioration,” and 1.0 for “no deterioration” or “minor deterioration.” This factor creates a direct mathematical link between inspection findings and load rating — poorer condition directly reduces the computed capacity.
System factor (φs) — accounts for the level of structural redundancy. Per MBE Table 6A.4.2.4-1, φs ranges from 0.85 for non-redundant (fracture-critical) members to 1.0 for highly redundant multi-girder systems. A two-girder steel bridge (non-redundant) receives φs = 0.85, while a seven-or-more girder system receives φs = 1.0.
Resistance factor (φ) — per MBE Article 6A.4.2.2, varies by material and limit state: φ = 1.0 for steel flexure, 0.90 for steel shear, 0.90 for concrete flexure, 0.85 for concrete shear, 0.85 for prestressed concrete flexure.
The complete LRFR rating factor equation is: RF = (φc × φs × φ × Rn − γ_DC × DC − γ_DW × DW ± γ_P × P) / (γ_LL × (LL + IM))
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The Rating Factor (RF) is the fundamental numerical output of every load rating analysis. It represents the ratio of the available structural capacity (after accounting for dead loads and other permanent loads) to the live load effect produced by the rating vehicle. An RF of 1.0 or greater indicates that the bridge can safely carry the rating vehicle. An RF below 1.0 indicates that the bridge is overstressed by that vehicle and cannot safely carry it.
The general form of the RF equation, applicable to all three rating methods with appropriate modifications, is:
RF = (C − γ_DC × DC − γ_DW × DW ± γ_P × P) / (γ_LL × (LL + IM))
Where:
C = capacity of the member. For LRFR, C = φc × φs × φ × Rn. For LFR, C = φ × Rn. For ASR, C = Allowable Stress × Section Modulus.
DC = dead load effect from structural components and attachments (girders, deck, diaphragms, cross-frames, stiffeners). This is calculated from the as-built cross-sectional dimensions and material unit weights (steel = 490 pcf, reinforced concrete = 150 pcf, prestressed concrete = 160 pcf, asphalt = 140 pcf per AASHTO LRFD Table 3.5.1-1).
DW = dead load effect from wearing surfaces (asphalt overlay, concrete overlay, thin polymer overlay) and utilities (water pipes, gas lines, communication cables, signage). The actual thickness of the wearing surface must be based on field measurements, not design plans, because overlays are typically thicker than designed. A 25 mm difference in overlay thickness adds approximately 60 kg/m² to dead load — significant enough to affect load ratings on longer spans.
P = permanent loads other than dead loads, including earth pressures, soil surcharge, and prestressing forces. The sign convention (±) depends on whether the permanent load adds to or subtracts from the live load effect being evaluated.
LL = live load effect (moment, shear, or axial force) from the rating vehicle at the critical position on the influence line for the member being rated. For continuous bridges, the live load must be positioned to produce the maximum load effect at the section under consideration. The live load distribution factor (DF) per AASHTO LRFD Article 4.6.2.2 accounts for the portion of the live load carried by each girder. The DF depends on girder spacing, span length, deck thickness, and girder stiffness. For a typical interior girder on a multi-girder bridge with 2.4 m girder spacing, the DF is approximately S/3.3 (per the Lever Rule for one lane loaded) or S/4.3 (per AASHTO approximate formulas for two or more lanes).
IM = dynamic load allowance (impact factor). Per MBE Article 6A.2.5.1, IM = 33% of the static live load for strength limit states (IM = 0.33). For service limit states for fatigue, IM = 15%. The dynamic allowance is applied only to the static live load, not to the lane load component. The impact factor accounts for the dynamic amplification of live load from truck bounce, roadway roughness, and bridge vibration.
γ_DC, γ_DW, γ_P, γ_LL = load factors for the respective load components. For LRFR design-load rating (HL-93): inventory — γ_DC = 1.25, γ_DW = 1.50, γ_LL = 1.75; operating — γ_DC = 1.25, γ_DW = 1.50, γ_LL = 1.35. For LRFR legal-load rating (AASHTO legal trucks), the live load factor varies from 1.15 to 1.80 depending on the Average Daily Truck Traffic (ADTT) and the load factor specified in MBE Table 6A.4.5.4.2-1.
The RF is computed for each limit state that governs the design of the member. The Strength I limit state (basic load combination for vehicular live load) typically governs for most bridge members. However, other limit states may be critical for specific configurations:
Strength II limit state — governs for permit vehicles (special hauling vehicles with weights exceeding legal limits). The RF for permit loads uses a live load factor of 1.35 for routine permits and 1.15 for special permits.
Service I limit state — governs for prestressed concrete members where tensile stress limits must be satisfied. Service I uses a live load factor of 1.0 and limits concrete tensile stress per AASHTO LRFD Table 5.9.2.3.1-1 (typically 0.19√f’c to 0.5√f’c depending on the member classification).
Service II limit state — governs for steel members where permanent deflection must be controlled. Service II uses a live load factor of 1.30.
Fatigue limit state — governs for steel details subjected to repetitive truck loading. The Fatigue I load factor is 1.50 (for infinite life design) or 1.75 (for finite life design) per AASHTO LRFD Table 3.4.1-1.
The RF must be computed for every critical section and every member type. The minimum RF across all members and all limit states determines the overall rating of the bridge. If any single member has an RF below 1.0 for a particular vehicle, the bridge cannot safely carry that vehicle, and posting or closure must be considered.
Every bridge load rating produces two distinct values: the Inventory Rating and the Operating Rating. These represent different levels of stress at which the bridge can safely operate, and they serve different purposes in bridge management.
The Inventory Rating is defined by the AASHTO MBE as the live load that “can safely utilize the bridge for an indefinite period of time.” It is based on a lower allowable stress level and a higher reliability index, representing the capacity level at which the bridge can carry normal, repetitive daily traffic without accumulating fatigue damage or experiencing excessive permanent deflection. For steel members in ASR, the inventory allowable stress is 0.55Fy. For LRFR, the inventory rating uses a reliability index of β = 3.5 with corresponding load factors (γ_LL = 1.75 for HL-93). The inventory rating is the conservative value and is used for routine load evaluation and bridge management decisions.
The Operating Rating is defined as the “maximum permissible live load to which the structure may be subjected.” It is based on a higher allowable stress and a lower reliability index, representing the capacity level at which the bridge can carry occasional heavy loads. For steel members in ASR, the operating allowable stress is 0.75Fy. For LRFR, the operating rating uses a reliability index of β = 2.5 with corresponding load factors (γ_LL = 1.35 for HL-93). The operating rating is the higher value and is used for legal load evaluation and posting decisions.
The ratio of operating rating to inventory rating varies by method and material but is typically in the range of 1.3 to 1.67. For LRFR design-load rating, the operating live load factor (1.35) divided by the inventory live load factor (1.75) gives a ratio of 1.30. For ASR, the ratio of operating stress (0.75Fy) to inventory stress (0.55Fy) gives 1.36. For concrete in ASR, the ratio is approximately 1.6 to 1.67 because concrete working stresses have a wider separation between inventory and operating levels.
Practical significance: A bridge with an inventory RF of 0.80 and an operating RF of 1.15 for the HS-20 design vehicle cannot safely carry HS-20 loads on a routine basis (inventory RF < 1.0) but can carry them occasionally (operating RF > 1.0). This distinction allows bridge owners to restrict traffic rather than close the bridge. The operating rating governs posting decisions — if the operating RF for any legal load is below 1.0, the bridge must be posted.
Both inventory and operating ratings are reported to the National Bridge Inventory (NBI) . Under the legacy Coding Guide, Items 63 and 64 recorded the inventory rating (method and value), and Items 65 and 66 recorded the operating rating (method and value). Under the new Specifications for the National Bridge Inventory (SNBI) , effective from 2025, these fields are designated as B.LR.01 through B.LR.06 with reporting in Rating Factor (RF) format preferred over metric tonnage.
The direct relationship between bridge condition and load rating is one of the most critical concepts in bridge engineering. Every inspection finding that documents deterioration — section loss, cracking, spalling, corrosion — potentially reduces the load rating of every affected member. The AASHTO MBE provides explicit methods for incorporating inspection findings into load rating calculations, creating a mathematically rigorous feedback loop between the inspection report and the bridge safe capacity.
Section loss — the reduction in cross-sectional area of a steel member due to corrosion — is the most common inspection finding that directly reduces load rating. When a steel girder web or flange loses thickness to corrosion, its section modulus (S) decreases, reducing the moment capacity (Mn = Fy × S) and shear capacity (Vn = 0.6 × Fy × Aw × Cv). The reduction is not linear — a 15% section loss in a flange can reduce section modulus by 15-20% depending on the flange-to-web area ratio, because the flange area is at the extreme fiber where it contributes most to bending resistance.
The FHWA Bridge Inspector’s Reference Manual (BIRM) requires inspectors to measure section loss by ultrasonic thickness gauging or mechanical calipers. Measurements are taken at the worst section of each member — typically at bearing locations (where trapped moisture and debris accelerate corrosion), at midspan (where bending stresses are highest), and at any location with visible corrosion. The measured remaining thickness is compared to the as-built thickness to calculate the percentage of section loss.
The MBE addresses section loss through the Condition Factor (φc) . For LRFR, Article 6A.4.2.3 specifies φc = 0.85 for members with “heavy deterioration,” 0.95 for “moderate deterioration,” and 1.0 for “minor or no deterioration.” However, for section loss beyond approximately 10-15%, the reduced measured section properties (rather than the condition factor alone) govern the analysis. The load rating engineer must calculate the actual section modulus and moment capacity using the measured remaining cross-section.
| Section Loss (%) | Condition Factor (φc) | Reduction in Moment Capacity | Typical Action Required |
|---|---|---|---|
| < 5% | 1.0 | < 5% | Monitor, clean and coat |
| 5–10% | 0.95 | 5–15% | Re-rating required, schedule repair |
| 10–20% | 0.85 | 10–30% | Immediate re-rating, posting evaluation |
| > 20% | Not applicable | > 30% | Critical finding, posting or closure |
State DOT practice varies on how section loss is modeled. The Rhode Island DOT Load Rating Guidelines (Section 6.4.1.1) provide a specific method for deteriorated steel beam ends: the average remaining web thickness measured over the deteriorated zone is used to compute reduced shear capacity. If uniform corrosion has reduced the web thickness to 6 mm from an original 10 mm, the shear capacity is computed using 6 mm (60% of original). The FHWA 2024 Peer Exchange Report (FHWA-HIF-24-113) documented that most states apply the condition factor AND also model the reduced section geometry directly — a “double counting” concern that is resolved by using the reduced geometry in the capacity calculation and applying φc = 0.85 only when the deterioration is severe enough to affect the performance beyond simple section loss.
Cracking in concrete bridge members affects load rating in multiple ways. Flexural cracking in reinforced concrete T-beams or box girders reduces the effective moment of inertia, increasing deflection and potentially reducing the section’s ability to distribute loads. The cracked section analysis per AASHTO LRFD uses the effective moment of inertia (Ie) computed by Branson’s equation. For load rating, the engineer must determine if the observed cracking is consistent with the design assumptions or indicates that the member is overstressed.
Diagonal (shear) cracking in concrete beams is particularly significant because shear failures are brittle and occur without warning. The AASHTO LRFD shear capacity of concrete members depends on the concrete tensile strength (√f’c) and the horizontal and vertical reinforcement. If shear cracks wider than 0.40 mm are observed near supports, the load rating engineer must evaluate whether the existing shear reinforcement is yielding — a condition that would reduce the nominal shear capacity (Vn) and the member RF.
Spalling and delamination remove concrete cover and reduce the effective cross-section. The MBE requires that spalled areas be physically measured (area and depth) and that the remaining concrete section be used in rating calculations. Delamination detected by hammer sounding or Impact-Echo testing reduces the effective section even when the concrete has not yet fallen away. For prestressed concrete members, spalling over tendon paths is a critical finding that can indicate tendon corrosion, which must be investigated by NDT before a valid load rating can be determined.
Corrosion of reinforcement reduces the effective steel area in the tension zone of reinforced concrete members. The steel area (As) in the RF equation is reduced by the percentage of section loss measured from exposed bars. If stirrups in a concrete beam have lost 25% of their cross-sectional area from corrosion, the shear RF is reduced proportionally.
Bearing deterioration — seized rocker bearings, corroded roller nests, failed pot bearing seals — can reduce the load rating by introducing unintended restraint forces. A seized expansion bearing at an abutment prevents thermal movement, inducing horizontal forces that must be resisted by the substructure. The load rating engineer must assess whether the substructure (abutment, pier, foundation) has adequate capacity to resist these forces. If not, the load rating must be reduced.
Connection deterioration — corroded or loose bolted connections, cracked welds at stiffener-to-flange junctions, failed shear connectors in composite construction — reduces the ability of the structure to transfer forces between elements. A bridge with failed shear connectors (studs) cannot develop full composite action, and the effective section modulus is based on the non-composite steel section only, which is typically 30-50% less stiff than the composite section.
Fatigue cracks in steel members — typically at welded details such as diaphragm-to-girder connections, cover plate ends, and stiffener-to-flange welds — reduce the fatigue life and, in advanced cases, the load rating. For load rating, the fatigue limit state is evaluated separately from the strength limit state. The Fatigue Serviceability Index (FSI) per MBE Article 6A.5.2 provides a measure of fatigue performance. If active fatigue cracking is documented, the load rating engineer must determine whether the crack reduces the section enough to affect the strength limit state RF. Fatigue Category E details (cover plate ends, weld attachments) have a fatigue threshold of 31-56 MPa (4.5-8 ksi) depending on the detail category.
Bridge posting is the installation of regulatory traffic signs that communicate the maximum safe vehicular weight for a bridge. Per the NBIS definition at 23 CFR 650.305, “Load posting” means “regulatory signs installed in accordance with 23 CFR 655.601 and State or local law which represent the maximum vehicular live load which the bridge may safely carry.” Posting is required whenever a bridge’s operating rating for any legal load is less than the legal load for that vehicle type in the State.
The legal load is defined per 23 CFR 650.305 as “the maximum load for each vehicle configuration, including the weight of the vehicle and its payload, permitted by law for the State in which the bridge is located.” Each State has its own legal load limits based on Federal bridge formula B and State-specific exceptions. When the operating RF < 1.0 for a legal load combination, the bridge cannot safely carry that legal load and must be posted.
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The posting sign (R12-1 per the Manual on Uniform Traffic Control Devices, MUTCD) displays the maximum legal load for up to three vehicle types:
Single Unit (SU) trucks — typical 2-axle trucks, dump trucks, garbage trucks, delivery trucks. A typical posting might read “SU 15 TONS.”
Combination (C) trucks — tractor-trailer combinations, typically 3- or 4-axle trucks. The posting reads “C 23 TONS.”
Semi-Trailer (ST-5) trucks — 5-axle tractor-semi-trailer combinations. The posting reads “ST-5 25 TONS.”
If the operating rating for any vehicle type falls below 3 tons, the bridge must be closed to all traffic — not just heavy vehicles. The FHWA position is that a bridge with a rating below 3 tons has insufficient capacity for even emergency vehicles (fire trucks, ambulances) and must be closed. The closure must be physically enforced with barriers or permanent closure structures.
Posting procedures vary by State but typically follow this sequence:
The NBI Item 70 (SNBI B.PS.01) records bridge posting status. Codes range from 0 (bridge closed to all traffic) through 5 (posted for load restriction) to 9 (no restriction — bridge load capacity exceeds legal loads). If a bridge is not posted but the operating RF is below 1.0, the bridge owner is not in compliance with NBIS requirements.
Load rating is a core input to bridge management systems (BMS) . The Pontis/BrM system, used by most State DOTs, incorporates load rating data to model the consequences of deterioration on bridge capacity and to prioritize rehabilitation projects. A bridge with a low load rating but high traffic volume and detour length receives higher priority for strengthening or replacement than a similar bridge on a low-volume road.
The relationship between load rating and bridge management is governed by the concept of Level of Service (LOS) . Bridge owners define a target LOS for each bridge class — for Interstate highways, the target is typically that all bridges carry legal loads without restriction (operating RF ≥ 1.0 for all legal loads). For local roads, a lower LOS may be acceptable if the bridge is posted and alternate routes exist.
When inspection reveals condition deterioration that reduces the load rating below the target LOS, the bridge management system flags the structure for action. The options are:
Do nothing — acceptable only if the reduced load rating remains above legal loads (no posting required). Even if the rating has dropped, if RF ≥ 1.0 for all legal loads, the bridge remains functional. However, the trend must be monitored.
Post the bridge — if the operating RF for any legal load falls below 1.0, posting is mandatory. The posting may be for specific vehicle types only. Posting preserves the bridge for lighter traffic while maintaining public safety.
Strengthen the bridge — structural strengthening (steel cover plates, external post-tensioning, FRP wrapping, supplemental beams) can restore or increase the load rating. Strengthening is typically 30-60% of the cost of replacement and can extend bridge life by 15-25 years.
Replace the bridge — when the load rating is critically low and strengthening is not cost-effective or technically feasible. Replacement is typically triggered when the cost of repeated repairs approaches 50% of replacement cost.
Close the bridge — when the operating rating falls below 3 tons for any vehicle type, or when the condition is Critical (rating 2) with imminent failure risk. Closure must be enforced with physical barriers.
The Bridge Load Rating and Posting Determination Flow per the NBIS requires that load rating data and posting status be reviewed for every bridge during each inspection cycle. If inspection reveals new deterioration, the bridge owner must determine within 30 days whether the existing load rating remains valid. If it does not, a re-rating must be initiated.
When a bridge is repaired, strengthened, or rehabilitated, the load rating must be updated to reflect the new condition. The NBIS at 23 CFR 650.315 requires that initial inspection data be recorded for new, replaced, or rehabilitated bridges. The corresponding load rating must be completed “within 3 months” of opening to traffic per FHWA guidance.
Rehabilitation — structural repairs that restore original capacity — requires a re-rating to verify that the target capacity has been achieved. Common rehabilitation actions that trigger re-rating:
Steel girder repair — welding of cover plates over corroded sections, bolted splice repairs, replacement of deteriorated beam ends. The re-rating must verify that the repair has restored at least the original section modulus. A cover plate repair typically restores 90-110% of the original flexural capacity.
Concrete beam repair — epoxy injection of cracks, concrete patching of spalls, external post-tensioning. Re-rating must verify that the repaired section meets the target RF. External post-tensioning can increase flexural capacity by 15-30%.
Bearing replacement — new bearings restore the intended movement capability, removing the unintended restraint forces that reduced the previous load rating.
Deck replacement — a new deck may be heavier than the original (thicker overlay, additional reinforcement) or lighter (removal of deteriorated overlay, use of lightweight concrete). The change in dead load (DW) directly affects the RF calculation. A 50 mm increase in deck thickness adds approximately 1.2 kPa of dead load, reducing the RF by 2-5% on typical spans.
Strengthening — structural modifications that increase capacity beyond the original design — requires a full re-rating per the MBE. Strengthening methods include:
Steel cover plates — plates welded or bolted to girder flanges to increase section modulus. A 300 mm × 12 mm cover plate on a 900 mm deep girder increases section modulus by approximately 20-30%.
Fiber Reinforced Polymer (FRP) wrapping — carbon or glass FRP sheets bonded to concrete beams to increase flexural and shear capacity. AASHTO Guide Specifications for FRP repair provide design equations. FRP wrapping can increase flexural capacity by 10-25% and shear capacity by 15-30%.
External post-tensioning — tendons installed external to the concrete section, anchored at the beam ends, and tensioned to induce compressive stresses. This is the most effective strengthening method for prestressed concrete bridges, capable of increasing capacity by 20-40%.
Supplemental beams — additional girders installed between existing girders to reduce the load on the original members. Adding one beam between existing beams at 2.4 m spacing reduces the distribution factor from S/4.3 to (S/2)/4.3, approximately halving the load per girder.
After any strengthening, the re-rating must be sealed by a Professional Engineer and the updated RF values submitted to the NBI. The new load rating becomes the basis for posting decisions and bridge management actions.
TarmacView’s bridge inspection data platform is designed to close the loop between field inspection findings and load rating engineering. The platform captures element-level condition data that directly feeds into the load rating calculation process, addressing the explicit NBIS requirement that load ratings be “supplemented by measurements and other information gathered from an inspection” (23 CFR 650.305).
Quantitative deterioration data captured during TarmacView inspections includes:
Section loss measurements — ultrasonic thickness readings at grid points on steel members, recorded with GPS coordinates for precise location tracking. The data can be exported in a format compatible with AASHTOWare BrR (the standard load rating software used by State DOTs). The engineer can create “deteriorated member alternatives” in BrR using measured thickness values rather than as-built dimensions.
Crack mapping — crack widths (measured to 0.05 mm precision with crack comparator gauges), lengths, orientations, and locations plotted on structural drawings. Crack widths exceeding 0.30 mm are flagged as potentially significant for load rating input.
Spalling and delamination extents — areas and depths of concrete loss, mapped for use in reduced section calculations. The effective remaining concrete section is computed using the measured spall dimensions.
Corrosion areas — photographed and measured, with corrosion product thickness noted. Areas with active corrosion (red rust, exfoliating scale) are distinguished from areas with stable corrosion (patina).
Condition rating data — the inspector’s assessment of each bridge element directly informs the Condition Factor (φc) selection for LRFR. A condition rating of 4 (Poor) or 3 (Serious) on a primary member would typically justify φc = 0.85. TarmacView’s platform links condition ratings to φc recommendations.
Fatigue-sensitive detail documentation — identification and condition assessment of fatigue-prone details (Category C, D, E, E’ per AASHTO LRFD Table 6.6.1.2.3-1). The platform tracks which details require fatigue evaluation per MBE Section 7.
Posting verification — TarmacView inspection reports document the condition of posting signs (legibility, damage, missing signs) and verify that the posted limits match the current load rating. Discrepancies between posted limits and the current rating are flagged as critical findings.
The integration of inspection data with load rating enables proactive bridge management:
Trend analysis — comparing section loss measurements from successive inspections identifies corrosion rates. A steel girder losing 0.5 mm/year of thickness at a bearing location will reach 20% section loss in a predictable timeframe, allowing the bridge owner to schedule repairs before posting becomes necessary.
Condition-based re-rating triggers — when inspection finds section loss exceeding 10% or crack widths exceeding 0.40 mm in primary members, the TarmacView system automatically flags the bridge for re-rating, ensuring no structural change goes unaddressed.
Prioritized repair planning — bridges with the lowest load ratings on high-volume routes are prioritized for load rating review and potential strengthening. The combination of TarmacView condition data and load rating results creates a comprehensive risk assessment for each bridge in the inventory.
When a bridge is rated Critical (2) or Serious (3) on the FHWA General Condition Rating Scale (0-9), special load rating procedures apply. Per NBIS Section 650.313(c)(2), critical findings — including “structural or safety related deficiency that requires immediate action to ensure public safety” — must be reported to the bridge owner within 24 hours and documented in the inspection report. For critical condition bridges, the existing load rating is presumed invalid until proven otherwise by re-rating.
Emergency load rating is a rapid assessment performed after an extreme event (earthquake, flood, scour, vehicle or vessel impact, fire, explosion) or when routine inspection identifies a critical deficiency. The purpose is to determine within hours or days whether the bridge can remain open, needs posting, or must be closed, pending a detailed evaluation.
The emergency rating process follows simplified procedures per MBE Section 6A.6 and 6A.7:
Post-earthquake rating — State DOTs typically follow a tiered approach: Level 1 (visual inspection from deck, no closures needed for bridges with minor or no damage, immediate reopening), Level 2 (detailed inspection for bridges with moderate damage, 75% of pre-event capacity assumed pending analysis), Level 3 (emergency load rating analysis for bridges with significant damage, 50% or less of assumed capacity).
Post-impact rating — after a vehicle or vessel strikes a bridge, the damaged members are inspected for section loss, alignment change, and connection damage. The emergency rating assumes that the damaged member carries no load (its entire share is redistributed to adjacent members) unless the inspection confirms otherwise. Redistribution is evaluated using a simplified live load distribution: if one girder of a 5-girder system is impacted, the remaining 4 girders carry the full load, increasing the distribution factor from S/4.3 to S/3.4 (approximately 25% higher).
Post-fire rating — fire damage to concrete or steel is assessed by visual inspection and NDT. For steel, fire damage is classified by color (black/blue scale indicates temperatures above 600°C, requiring replacement). For concrete, fire damage is assessed by hammer sounding (hollow sound indicates spalling risk) and depth of color change. The emergency rating assumes a 50% reduction in capacity for fire-damaged zones unless testing confirms higher residual strength.
Post-flood/scour rating — flood damage may include scour of foundations, debris impact, and saturation of approach fills. The emergency rating addresses whether the substructure has adequate foundation capacity. Scoured foundations are assumed to have reduced vertical and lateral capacity — the Emergency Load Rating for a scoured bridge typically reduces the allowable live load by 30-50% until the scour is remediated and foundations are verified.
The numerical threshold for emergency closure is consistent across all emergency scenarios: if the emergency operating rating for any legal load is less than 3 tons, the bridge must be closed to all traffic, including emergency vehicles. The closure must be maintained until detailed load rating analysis confirms a higher capacity or repairs are completed.
Many State DOTs maintain pre-approved emergency load rating protocols that allow field engineers to make posting and closure decisions without waiting for a full office analysis. The California DOT (Caltrans) uses a color-coded system: Green (open, 100% capacity), Yellow (restricted, 75% capacity), Red (reduced loads, 50% capacity or less), Black (closed). These codes are based on observed damage patterns and pre-computed rating factors for the bridge type.
The 2024 FHWA Bridge Load Rating Peer Exchange documented that several State DOTs now use mobile load rating applications on tablets that allow field engineers to perform simplified load rating calculations at the bridge site during emergency inspections. These tools use pre-loaded bridge geometry, member sizes, and material properties, allowing the field engineer to input measured section loss, crack widths, or damage dimensions and get an immediate RF. While these field ratings are not a substitute for a formal PE-sealed load rating, they provide the rapid capacity assessment needed to make posting and closure decisions during emergency situations.
Bridge load ratings must be updated when inspection reveals deterioration or after rehabilitation. TarmacView integrates inspection data with load rating workflows, helping bridge owners identify posting requirements, prioritize repairs, and maintain safe operations. Contact us for expert load rating and bridge inspection solutions.
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A bridge pier is an intermediate vertical support structure between abutments that transfers superstructure loads to the foundation. Multi-column bents, wall pi...
The bridge deck is the uppermost structural element of a bridge, directly supporting traffic loads and providing the riding surface. Deck condition — cracking, ...
