Bridge Inspection, Evaluation, and Load Rating

19.1 — Engineering story

Silver Bridge, Point Pleasant, 1967

At 5:04 pm on December 15, 1967, the eyebar-suspension Silver Bridge carrying US-35 over the Ohio River collapsed under rush-hour traffic. Forty-six people died. The forensic cause was a 2.5 mm stress-corrosion crack on the inside face of a single eyebar — invisible without disassembly. Within months Congress passed the 1968 Federal-Aid Highway Act, which created the National Bridge Inspection Standards (NBIS) and the National Bridge Inventory (NBI). Every one of the ~617,000 highway bridges in the U.S. today lives in that database, with component ratings that determine funding, posting, and repair priority.

19.2 — Chapter objectives

What you will be able to do

Learning objectives

By the end of this chapter you will be able to:

1Identify the six inspection types defined by 23 CFR 650 and the frequency each requires.
2Assign NBI condition ratings (0–9) to deck, superstructure, and substructure from field evidence.
3Recognize the common deterioration mechanisms — chloride-driven spalling, fatigue cracks, pack rust, scour, bearing distress — and their inspection cues.
4Apply the LRFR general rating equation and compute Design (Inventory + Operating), Legal, and Permit rating factors per AASHTO MBE §6A.
5Recompute nominal resistance from measured section loss and update the rating factor.
6Recommend a posting per MUTCD R12-5 when RF(legal) < 1.0.
7Deliver a worked example (LRFR rating of a deteriorated composite girder) and a full-bridge evaluation design challenge.

19.3 — Engineering motivation

Why every bridge is inspected on a fixed cycle

Design predicts service; inspection confirms it. The as-built resistance R_n assumes the girder still has the plate thickness the drawings show. Twenty winters of chloride, one collision, or a run of overweight permits can silently push the true resistance below the demand γ_L(LL+IM). The inspection–rating loop is the only mechanism that catches that drift before it becomes a Silver Bridge.

19.4 — Lecture

The NBIS inspection program

AASHTO LRFD 23 CFR 650 Subpart C— National Bridge Inspection Standards

The NBIS defines six inspection types. Initial at construction sets the baseline. Routine inspections — every 24 months by default — are the primary workhorse and require a team leader qualified per §650.309. In-depth inspections apply arms-length close visual to selected elements. Fracture-critical member (FCM) inspections are hands-on and also on a 24-month cycle. Underwater follows a 60-month cycle unless risk-based intervals are approved. Damage and special inspections respond to events (collision, flood, seismic) or track known defects.

19.5

NBI condition ratings — deck, superstructure, substructure

The FHWA Recording and Coding Guide assigns an integer 0–9 to each of NBI Items 58 (deck), 59 (superstructure), and 60 (substructure). A rating ≤ 4 in any of the three triggers the federally-defined Poorclassification and unlocks federal-aid rehabilitation funding. Ratings must be based on observed condition, not opinion of remaining life: cracks, section loss, spalls, delaminations, and settlement are measured and photographed and then interpreted against the coding guide's component-specific language.

19.6

Common deterioration mechanisms

Concrete decks: chloride ingress → rebar corrosion → delamination → spall. Sound with chain drag; quantify with GPR or half-cell potential.
Steel girders: section loss under leaking joints (pack rust), fatigue cracks at web-to-flange or floorbeam-to-girder welds (Categories C′ and E), coating breakdown.
Prestressed girders: strand corrosion at end blocks, longitudinal cracking parallel to strands, efflorescence tracking a leaking joint above.
Substructure: abutment settlement/rotation, pier column cracking, scour at footings (Ch. 14), bearing walk-out.

19.7

Non-destructive testing (NDT) toolbox

UT / PAUT — interior weld defects, plate section loss.
MT / PT — surface-breaking cracks on steel (magnetic particle) or on any solid surface (dye penetrant).
GPR — deck delamination mapping, rebar cover.
IE / IR thermography — deck delamination from above.
Half-cell potential + resistivity — active corrosion probability.
Load testing — diagnostic or proof, per MBE §8, when analytical rating is inconclusive.

19.8

Load rating — the LRFR framework

AASHTO LRFD MBE §6A— Load and Resistance Factor Rating

The Manual for Bridge Evaluation defines a single rating equation and applies it three times — Design, Legal, Permit — with different load models and factors. The rating factor is the multiplier on live load that exactly satisfies the limit state:

RF \;=\; \dfrac{\phi_c\,\phi_s\,\phi\,R_n \;-\; \gamma_{DC}\,DC \;-\; \gamma_{DW}\,DW}{\gamma_L\,(LL+IM)}

φ_c is a condition factor (1.00 Good, 0.95 Fair, 0.85 Poor); φ_s is a system factor (0.85–1.00 depending on redundancy). The Design rating uses HL-93 with γ_L = 1.75 for Inventory (safe for indefinite use by legal traffic) and 1.35 for Operating (safe for occasional passage). The Legal rating checks the state legal trucks; if any RF_legal < 1.0 the bridge must be posted or strengthened. The Permit rating evaluates one-trip and annual overweight permits — γ_L depends on ADTT and lane restriction (MBE Table 6A.4.5.4.2a-1).

19.9

Recomputing R_n from measured section loss

Design used the shop-drawing section. The inspector measures the surviving thickness, subtracts pack-rust build-up (which does not carry load), and re-runs the elastic and plastic section-property calculation with the reduced plate. For a composite steel plate girder in positive bending, a 25 % loss over a 10 ft run at midspan can drop M_pby 8–15 % depending on flange proportion.

19.10

Posting when RF(legal) < 1.0

Posting weight is back-solved from RF = 1.0 for each of the SU4, C, and ST5 legal loads and rounded down to the nearest ton. Signs use MUTCD R12-5 with truck-type pictograms. If the required posting is below the 3-ton minimum, the bridge is closed.

19.11 — Worked example

LRFR rating of a deteriorated composite plate girder

Given

Simple 90 ft interior composite girder, spacing S = 8 ft, A709 Gr. 50 steel, 8 in composite deck (f′_c = 4 ksi). Design plastic moment M_p = 8,400 k-ft. Bottom flange 18 × 1.0 in, measured 25 % thickness loss over the middle 10 ft. Field condition rating: superstructure NBI 5 (Fair).

DC = 1.10 k/ft (girder+deck+haunch), DW = 0.20 k/ft (2 in overlay). Distributed HL-93 lane + truck governs; live-load moment M_LL+IM = 1,950 k-ft (per girder, includes DF = 0.62 and IM = 0.33 on truck).

Step 1 — Dead-load moments.

M_{DC} = \dfrac{1.10 \times 90^2}{8} = 1{,}114\ \text{k-ft}, \quad M_{DW} = \dfrac{0.20 \times 90^2}{8} = 203\ \text{k-ft}

Step 2 — Reduced plastic moment. The bottom flange contribution to M_p is A_f·F_y·d_arm. A 25 % thickness loss reduces A_f,bot from 18.0 in² to 13.5 in². Recomputing the plastic neutral axis and moment arm gives:

M_{p,\text{eff}} \;\approx\; 8{,}400 \times 0.90 \;=\; 7{,}560\ \text{k-ft}

Step 3 — Condition and system factors. NBI 5 ⇒ φ_c = 0.95; multi-girder redundant ⇒ φ_s = 1.00; φ = 1.00 (flexure, compact).

Step 4 — Design load rating (HL-93).

RF_{\text{Inv}} \;=\; \dfrac{0.95\cdot 1.00\cdot 1.00\cdot 7{,}560 \;-\; 1.25\cdot 1{,}114 \;-\; 1.50\cdot 203}{1.75 \cdot 1{,}950}

RF_{\text{Inv}} \;=\; \dfrac{7{,}182 - 1{,}392 - 304}{3{,}413} \;=\; \dfrac{5{,}486}{3{,}413} \;=\; 1.61 \;\;\checkmark

RF_{\text{Oper}} \;=\; \dfrac{5{,}486}{1.35 \cdot 1{,}950} \;=\; 2.08

Step 5 — Legal load rating (Type-3 truck, M_LL+IM ≈ 1,180 k-ft).

RF_{\text{Legal}} \;=\; \dfrac{5{,}486}{1.45 \cdot 1{,}180} \;=\; 3.20 \;\;\gg\; 1.0 \;\;\checkmark

Conclusion: Bridge remains open at legal loads. Recommend φ_c re-evaluation in 24 months and monitor bottom-flange loss with UT thickness gauge. No posting required.

Fig. 19.7Composite girder w/ measured section loss

Sensitivity plot — inventory RF drops below 1.0 near ~38 % bottom-flange loss; operating RF crosses at ~48 %. The gap between the two governs the posting/permit decision.

19.12 — Design challenge

Evaluate a 1968 3-span continuous plate-girder river crossing

You are the load-rating engineer for a 1968 3-span continuous welded plate-girder crossing (spans 90–120–90 ft), ADT 4,500 with 8 % trucks. Field notes report: deck NBI 4 with widespread delamination; a 4-inch fatigue crack at a floorbeam-to-girder gusset; pier scour of 6 ft below the pile cap; and a bearing that has walked out 1.5 in.

Deliver (single PDF or PPT):

Updated NBI Items 58, 59, 60 with justification tied to specific field evidence.
LRFR ratings — Design (Inv + Oper), Legal (Type 3, 3S2, SU5), one annual permit vehicle.
Posting recommendation (with MUTCD R12-5 sign) or a "no posting" justification.
Prioritized repair scope with rough order-of-magnitude cost and expected NBI-item upgrade after work.
Re-inspection plan (routine, in-depth, fracture-critical, underwater) with proposed intervals.

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Chapter 19 — Bridge Evaluation Design Challenge

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19.13 — Summary

Take-aways

NBIS mandates a 24-month routine inspection cycle for every public highway bridge.
NBI condition ratings 0–9 drive federal-aid classification; ≤ 4 in any component ⇒ Poor.
LRFR (MBE §6A) rating equation is applied three times — Design, Legal, Permit — with different load and γ_L.
Measured section loss updates R_n; φ_c reflects overall condition.
Posting is required whenever RF(legal) < 1.0 for any state legal truck.

Section 2

Fully Worked Examples

Complete AASHTO LRFD solutions with knowns, assumptions, step calculations, verification, and design commentary. Difficulty rises from basic to consulting-grade.

Worked Example 1

Assign NBI condition ratings from inspection notes

Basic

Problem

Assign the appropriate NBI Item 59 (Superstructure) condition rating (0–9).

Step-by-Step

Isolated exposed stirrups = 'minor section loss' \rightarrow rating 5.

5 = 'Fair Condition; primary structural elements sound but minor section loss, cracking, spalling.'

Result

NBI 59 = 5 (Fair)

Design Verification

Rating 5 is a common trigger for programmed repairs; 4 (Poor) triggers a load rating review; ≤3 triggers immediate action.

Discussion

Never average conditions across a member. NBI ratings are governed by the worst-condition zone that meaningfully affects capacity.

Worked Example 2

Load rating factor (LRFR) for a legal truck

Intermediate

Problem

Compute the operating rating factor RF and posting recommendation.

Step-by-Step

RF = (\phi R_{n} - \gamma _{DC}\cdot DC - \gamma _{DW}\cdot DW) / (\gamma _{LL}\cdot M_{LL+IM}) = 950 / (1.45\cdot 620)

Result

RF = 950 / 899 = 1.06

RF \ge 1.0 for a legal truck \rightarrow bridge carries Type 3 unposted, but with only 6% margin.

Design Verification

RF near 1.0 is a candidate for enhanced monitoring or short-term posting if traffic grows. If Type 3-S2 or 3-3 also rate near 1.0, initiate a permit-load evaluation before issuing overweight permits.

Discussion

RF < 1.0 for any legal truck requires load posting per MBE §6A.8.3. RF < 0.3 typically triggers closure.

Worked Example 3

Ultrasonic thickness measurement of a corroded steel web

Advanced

Problem

Compute effective section loss and remaining shear-capacity fraction (assume V_n ∝ t_w).

Step-by-Step

\Delta t = 0.50 - 0.28 = 0.22 in \rightarrow 44% loss over 6-in patch.

V_{n},patch / V_{n},nom = 0.28/0.50

Result

0.56 (44% shear-capacity loss)

Design Verification

44% loss in a shear-governed zone forces a load rating recalculation with t_w = 0.28 in and likely triggers weld-repair or bolted doubler-plate retrofit.

Discussion

Local pitting is often reported by average thickness — that hides the true governing thickness. Always report the minimum reading over the affected footprint and use it in ratings.