🔩 General Contractor — Structural & Field Conditions

Many of Chicago's steel bridges were designed and built between the 1890s and 1940s — long before the era of digital records, standardized filing systems, or robust as-built documentation. Original construction drawings, if they exist at all, may be microfilmed, deteriorated, dimensionally inaccurate, or simply missing from CDOT's bridge records archive. For the GC, this creates a fundamental problem: you cannot plan, price, or safely execute a rehabilitation project when you don't know what's actually in the field.

• Missing structural drawings: The GC arrives on site and finds that plate thicknesses, connection geometry, member sizes, and section properties cannot be verified from drawings — they must be field-measured. This requires physical access to every element, which takes time and money, and sometimes reveals conditions that change the entire repair strategy.
• Incomplete details on connections: Older Chicago bridges use unique, non-standard connection configurations — pin-and-hanger assemblies, built-up riveted plate girders, eyebar chains — that have no modern equivalent drawing standards. Without original details, the repair engineer must reverse-engineer every connection from field measurements.
• Load rating gaps: Without accurate section dimensions, the bridge load rating cannot be reliably verified. IDOT requires an updated load rating after major repairs — if the as-built section cannot be confirmed, the load rating analysis must rely on conservative assumptions that may result in a lower allowable capacity than the bridge actually has.
• Scope creep from hidden conditions: Differing site conditions (DSC) claims — the most common and costly type of construction claim on bridge rehab projects — originate almost entirely from unknown existing conditions. When the field doesn't match the contract documents (or there are no contract documents for existing conditions), the GC has grounds for a claim but also faces a project that is suddenly larger than bid.
• Utility drawings often absent: Conduits, gas lines, fiber optic bundles, and water mains attached to or embedded in older bridge decks are frequently not on any drawing. Cutting into a deck or column without knowing what's inside is a safety and utility damage risk.

Corrosion is the dominant deterioration mechanism in Chicago's steel river bridges. The combination of freeze-thaw cycles, road salt runoff migrating through deck joints, river humidity, and decades of deferred maintenance creates corrosion pockets hidden beneath intact-looking paint surfaces. The GC cannot know the true extent of section loss until the paint is removed — by which point the project is already underway and the scope is locked.

• Paint surface masks subsurface corrosion: A bridge can look structurally sound from the deck level while having severe pack rust, section loss, and pitting on the lower flanges and webs — areas not visible without scaffold access and paint removal.
• Section loss thresholds: AASHTO and IDOT define allowable corrosion limits. Once section loss exceeds defined thresholds (typically 10–15% net section loss), structural repair — not just paint — is required. This triggers a complete design change, fabrication lead times, and additional contract modification procedures.
• Pack rust in built-up members: Built-up riveted sections (cover plates stacked on top of each other) trap moisture between plies. Corrosion between layers forces the plies apart — a phenomenon called pack rust — which can cause visible distortion and requires either plate replacement or engineered repair. Pack rust is almost never fully visible until the section is disassembled.
• Bearing seat and sole plate section loss: Bearing areas are corrosion hot-spots. Runoff concentrates at bearings, and steel-to-steel contact creates crevice corrosion. Sole plates, masonry plates, and anchor bolts are frequently found corroded well beyond the condition assumed at bid — sometimes requiring bearing replacement rather than cleaning and repainting.
• Geometric distortion: Severe section loss in compression members can produce buckling. Tension members with section loss below the AASHTO limit may still have reduced fatigue life. Both conditions require engineering evaluation that was not in the original scope.

Most of Chicago's historic steel bridges — built between 1890 and 1940 — are riveted structures. Riveting was the standard steel connection method before high-strength bolts became available in the 1950s. Every primary member connection, gusset plate, splice, and web stiffener is held together by rows of driven rivets. This creates a host of complications during rehabilitation that a contractor unfamiliar with historic bridge work will not anticipate.

• Rivet removal is labor-intensive and destructive: Removing a driven rivet requires either flame cutting (thermal), pneumatic chiseling, or drilling — each method risks damaging the parent metal and the rivet holes. Damaged holes must be reamed to the next larger diameter or sleeved, adding scope to every connection repair.
• Direct rivet-for-rivet replacement is a vanishing trade: Hot-driven riveting requires a 4-person crew (heater, catcher, bucker-up, riveter) and specialized equipment that virtually no American contractor maintains. Only a handful of shops in the country still perform hot-riveting, and they command premium pricing and scheduling.
• AISC and IDOT approval required for rivet-to-bolt substitution: The standard industry solution is to replace driven rivets with high-strength A325 or A490 bolts (ASTM F3125). However, this substitution must be explicitly reviewed and approved by the engineer of record because the connection geometry, slip resistance, and load path may change — especially at bearing-type vs. friction-type connection classifications.
• FHWA historic bridge considerations: On FHWA-funded projects involving bridges on or eligible for the National Register of Historic Places — which includes many Chicago river bridges — rivet replacement with bolts may require a Section 106 consultation with the Illinois Historic Preservation Agency (IHPA). This adds a regulatory review cycle that has no defined timeline.
• Loose or cracked rivets hidden under paint: Rivets that have lost clamping force (loose) or developed fatigue cracks are virtually impossible to identify without removing the paint and performing visual or magnetic particle inspection. A bridge that appears to have intact rivets may have 10–30% loose or defective rivets beneath the coating.

A Fracture-Critical Member (FCM) is defined by AASHTO as a steel tension member or steel tension component whose failure would be expected to result in collapse of the bridge or loss of load-carrying ability. The designation is binary — a member either is or is not fracture-critical — and it triggers a completely different set of inspection, repair, and construction protocols that the GC must follow at all times.

• FCM inspection is federally mandated: FHWA requires FCM bridges to receive hands-on inspection every 24 months (versus 48 months for routine bridges). Any GC activity that could affect an FCM — welding, drilling, impact loading, thermal cutting — must be performed under a special inspection regime with an AASHTO/AWS-certified fracture-critical inspector present at all times during the operation.
• Welding on FCMs requires AASHTO fracture control plan: Repairs to FCMs must be performed per the AASHTO Guide Specifications for Fracture Critical Non-Redundant Steel Bridge Members. This requires a project-specific Fracture Control Plan (FCP) submitted and approved before any welding begins. The FCP specifies preheat temperatures, interpass temperatures, filler metal certification, NDT requirements, and inspector credentials.
• No redundant load path — zero tolerance for defects: By definition, an FCM has no alternate load path. A fatigue crack or weld defect that would be tolerable in a redundant system must be repaired immediately in an FCM. This means the GC must have immediate response capability — not "we'll add it to the next change order."
• Access restrictions: Traffic cannot be permitted on the structure while an FCM repair is in progress — even if the repair is on one side and traffic could physically use the other. This mandatory closure requirement is sometimes unknown to GCs who assume they can maintain partial traffic during FCM work.
• Inspector qualifications: The fracture-critical inspector must hold current AWS Certified Welding Inspector (CWI) credentials with documented FCM-specific experience. This is a specialist who commands $125–175/hr. On a complex FCM repair, inspector fees alone can reach $40,000–80,000.