🎨 Design Engineer / Consultant — Design Constraints

Clearance preservation is the single most inflexible constraint a design engineer faces on bridge rehabilitation. Unlike strength — which can be increased by adding material — geometric clearances can only be maintained, not expanded, without major structural intervention. Every design decision that adds material to the structure — deeper reinforcing plates, thicker overlay decks, new drainage structures, electrical conduit housings — consumes clearance that may not exist to give. The design engineer must treat clearances as hard design boundaries and check them at every design iteration.

• Three governing clearance types: (1) Vertical roadway clearance — the minimum height from the bridge deck surface to any overhead obstruction, including girder soffits, diaphragm framing, utility conduit housings, and drainage scuppers. AASHTO minimum is 16 feet for new bridges; existing bridges may have exceptions but cannot be further reduced. (2) Horizontal clearance — roadway lane widths and barrier offsets. Rehabilitation that narrows lanes requires CDOT Traffic Operations approval and may require IDOT concurrence. (3) Navigation clearance — vertical and horizontal clearance over the navigable channel. Governed by USCG and USACE permit conditions; typically set at the time of original construction and cannot be reduced without federal regulatory approval.
• Deck overlay — the most common clearance-consuming design decision: A concrete deck overlay is often the most economical deck rehabilitation approach — pour 2–3 inches of concrete over the existing deck surface and gain a new wearing surface. But a 3-inch overlay consumes exactly 3 inches of vertical clearance from the underside. On a bridge with existing clearance of 16 ft 2 in above a street below, a 3-inch overlay produces a post-overlay clearance of 15 ft 11 in — below the 16-foot minimum. The design engineer must check the existing clearance before specifying an overlay, then determine whether the proposed overlay thickness is achievable within the clearance budget.
• USCG navigation clearance — the federal lock-in: The navigation clearance over the channel of the Chicago River was established for each movable bridge in the original USCG permit, issued when the bridge was constructed. For bascule bridges that open to allow vessel passage, the navigation clearance in the closed (lowered) position must not be reduced from the permitted value — because it is the reference from which the bridge tender determines whether a vessel can pass without opening. Any design element that projects below the bridge deck into the navigation clearance envelope — new drainage structure, conduit housing, inspection walkway — requires a new or amended USCG permit. This is a 30–90 day process that should be identified before design is finalized, not after.
• Clearance verification requires field survey, not drawing dimensions: The design engineer must obtain a current clearance survey — measured with a total station, not scaled from drawings — for every clearance-critical design decision. Drawing dimensions of clearances are frequently inaccurate due to deck resurfacing over the years (which reduces vertical clearance incrementally with each successive overlay) and pavement settlement (which reduces clear height above the bridge deck). A bridge whose drawings show 16 ft 6 in of clearance may actually measure 16 ft 1 in after 30 years of overlay accumulation.

Every Chicago river bridge was designed for a specific dead load — the permanent weight of the bridge structure itself, its deck, its wearing surface, its barriers, and its utilities. That design dead load was the basis for the original foundation sizing, the original bearing design, and the original superstructure capacity calculations. When rehabilitation adds material — a new concrete deck overlay, heavier barriers, additional structural reinforcing, new drainage structures — the dead load increases above the original design value. Every dead load increase must be evaluated against the original design capacity. Exceeding that capacity requires either substructure strengthening, superstructure modification, or acceptance of a reduced live load capacity.

• The compounding overlay problem: Chicago river bridges are typically 80–120 years old. Over their service lives, they have received multiple deck overlays — each adding permanent dead load without a corresponding structural evaluation. A bridge that was designed with a 7-inch concrete deck in 1922 may currently have a 7-inch original slab, a 2-inch 1960s overlay, and a 1.5-inch 1990s latex overlay — for a total of 10.5 inches of concrete deck. The additional 3.5 inches of accumulated overlay represents approximately 42 psf of unaccounted-for dead load — enough to measurably reduce the bridge's live load capacity. The design engineer must account for this accumulated dead load in the baseline analysis before specifying additional overlay.
• Foundation sensitivity — the substructure constraint: The original foundation design for a masonry pier bridge was likely sized to carry the original dead load with a factor of safety of 2.0–3.0. Multiple cycles of accumulated dead load may have reduced this factor of safety. When rehabilitation adds further dead load, the geotechnical and structural capacity of the foundation must be re-evaluated. Adding dead load to an old masonry pier without checking bearing capacity is one of the most common structural errors on historic bridge rehabilitation projects.
• Bascule bridge counterweight sensitivity — a uniquely Chicago constraint: On bascule bridges, the dead load balance between the main span leaf and the counterweight is precisely calibrated — the bridge is designed to open with minimum machinery force by balancing the leaf weight against the counterweight. Adding dead load to the main span without adjusting the counterweight changes this balance, increasing the machinery load required to open the bridge. On hydraulically or electrically operated bascule bridges, this can overload the drive machinery or exceed the rated capacity of the hydraulic cylinders. Any dead load addition to a bascule bridge main span must be evaluated for counterweight balance impact, not just structural capacity.
• Alternative materials — the weight reduction strategy: When dead load capacity is constrained, the design engineer has a range of lighter-weight alternative materials available: Ultra-High Performance Concrete (UHPC) deck overlays at 1.5 inches versus 3-inch conventional; FRP (fiber-reinforced polymer) deck panels replacing concrete slabs; lightweight CFRP (carbon fiber) structural reinforcement rather than steel doubler plates; aluminum barrier systems versus concrete barrier. These alternatives cost more per unit than conventional materials but may be the only structurally acceptable approach when the dead load budget has been exhausted by accumulated previous overlays.

Chicago's historic steel bridges were designed and fabricated with member geometries that do not correspond to any current AISC standard shape catalog. The main girders are custom built-up sections — specific combinations of flange plates, web plates, and cover plates that were designed to the bridge's exact loading and span requirements. When rehabilitation requires replacing or supplementing any of these members, the replacement must match the original geometry — because the connections, the load distribution, and the structural compatibility all depend on precise dimensional continuity. There is no catalog section you can order.

• Built-up sections — fabricated from plates, not rolled shapes: A typical Chicago river bridge from 1920 has main girders fabricated from: a web plate (specific width and thickness, e.g., 60 in × 1/2 in), top flange plates (specific width and thickness), bottom flange plates (specific width and thickness, sometimes with cover plates that taper in thickness), and stiffener plates at regular intervals. None of these are standard AISC sections. If a flange splice requires replacement, the replacement plate must be cut from standard plate stock to match the exact profile — which requires custom shop fabrication, certified welding procedures for the specific thickness, and close-tolerance fit-up in the field.
• Rivet hole patterns — the invisible geometry constraint: Every connection in a riveted bridge is defined by its hole pattern — the spacing, gauge, and end distance of rivets in each connected element. When a connection plate requires replacement, the new plate must match the existing hole pattern exactly. The original hole pattern was laid out by the fabricator's draftsman, not from any standard specification — and the actual holes may have been field-reamed to different diameters than the original drawings show. The design engineer must field-measure every rivet hole in a connection before designing its replacement, and specify the replacement using actual measured hole geometry rather than drawing dimensions.
• The built-up section advantage — customizable to the exact section required: The fact that historic bridge members are built-up from plates is, paradoxically, an advantage for rehabilitation. Unlike rolled shapes (which have fixed cross-section properties), built-up sections can be supplemented with additional plates at exactly the dimensions required to restore the original section properties. A bottom flange with 22% section loss can be restored by welding a precisely dimensioned doubler plate that exactly compensates for the lost section — producing an as-repaired section that matches the original design section. This precision rehabilitation is what makes restoration of 100-year-old bridges both technically feasible and structurally sound.
• Field measurement precision requirement: Because all replacement details must match existing geometry, the field measurement campaign (see Existing Conditions module) must produce dimensional data at a precision appropriate for fabrication: plate thickness to ±0.010 in, hole diameters to ±1/32 in, hole spacing to ±1/16 in. A caliper, not a tape measure, is the appropriate tool for fabrication-relevant dimensional data. The design engineer who specifies replacement details using tape-measure-precision field data will generate fitting problems during installation.

The steel used in Chicago's historic bridges is not the same as today's structural steel. Bridge steel from 1900–1950 was produced by manufacturing processes — primarily the Bessemer and open-hearth methods — that yielded different mechanical and chemical properties than modern electric arc furnace (EAF) steel. The differences affect weldability, fracture toughness, and yield strength in ways that are critical for rehabilitation design. The design engineer must characterize the original steel before specifying any welded repair — because welding to historic steel without understanding its weldability can produce hydrogen-induced cracking (HIC) that is more dangerous than the original corrosion damage.

• Pre-1950 carbon steel — the weldability challenge: Bessemer-process steel from the early 20th century can have carbon equivalents (CE) of 0.45–0.65% — significantly higher than modern structural steel (CE typically 0.35–0.45% for ASTM A709). High carbon equivalent steel requires higher preheat temperatures during welding to prevent hydrogen cracking. AWS D1.5 provides a formula for calculating required preheat based on carbon equivalent — but the design engineer must first determine the actual carbon equivalent through chemical testing of a sample from the bridge, since the original steel specification may be unknown or unreliable.
• Historic ASTM grades — what was actually used: Chicago bridges from different eras used different steel specifications. Bridges from 1900–1930 used ASTM A7 or A9 structural carbon steel, with nominal yield strengths of 24,000–33,000 psi. Bridges from 1930–1960 used ASTM A7 with higher-strength variants. Bridges from 1960–1980 used ASTM A36 (36 ksi yield). Post-1980 bridges use ASTM A709 in various grades. The design engineer must determine which specification applies to the specific bridge — not assume modern A36 properties for a 1920s structure, which would overestimate strength and potentially produce an unconservative repair design.
• The wrought iron distinction — the oldest bridges: The very oldest Chicago river bridges (pre-1900) may contain wrought iron members — a material that predates structural steel entirely. Wrought iron has very low carbon content (0.02–0.08%), excellent corrosion resistance, and high ductility — but significantly lower tensile strength than steel (typically 28,000–36,000 psi). More critically, wrought iron is practically unweldable by standard arc welding processes — it requires specialized forge welding or brazing techniques, or must be connected mechanically (bolted/riveted) rather than welded. A design engineer who specifies standard arc welding repairs on a wrought iron member will produce defective, potentially dangerous connections.
• CVN toughness — the temperature constraint for Chicago: Chicago experiences temperatures as low as -27°F (the 2019 polar vortex). AASHTO specifies Charpy V-Notch (CVN) fracture toughness requirements for bridge steel based on the minimum service temperature. Modern ASTM A709 bridge steel meets these requirements. Historic bridge steel from before 1960 was not produced to CVN requirements — it may have very low fracture toughness at low temperatures, meaning that a crack that would be stable at 50°F can propagate catastrophically at -20°F. This temperature-fracture behavior is particularly critical for FCM bridges and must be considered in repair design.

The protective coating system is the primary defense against corrosion — the dominant deterioration mechanism for Chicago's steel river bridges. Selecting the right coating system is a design decision with 15–25 year lifecycle consequences: an appropriate system applied correctly provides decades of corrosion protection; an inappropriate system fails in 3–5 years. The design engineer's coating specification must address: surface preparation profile (anchor pattern for coating adhesion), primer selection (the critical first coat that bonds directly to steel), intermediate coat, and topcoat — all with climate-appropriate characteristics for Chicago's freeze-thaw, de-icing salt, and river humidity environment.

• Surface preparation — the most critical factor in coating performance: No coating system performs better than the surface it is applied to. SSPC surface preparation standards define the cleanliness (removal of rust, mill scale, and contamination) required before coating application. For new or fully stripped bridge steel, SSPC-SP 6 (Commercial Blast) is the minimum and SSPC-SP 10 (Near-White Blast) is the standard for high-performance systems. The surface profile (anchor pattern depth, measured in mils with Testex tape) must match the primer manufacturer's specification — typically 1.5–3 mils for zinc-rich primers. Too little profile and the primer lacks adhesion; too much profile and coating peaks are exposed.
• System selection — the three-coat standard: IDOT's standard three-coat bridge coating system for Chicago-area bridges consists of: (1) Inorganic zinc (IOZ) primer — provides galvanic sacrificial protection; maximum corrosion protection of any primer type. Requires SSPC-SP 10 surface preparation. Sensitive to moisture and temperature during application. (2) High-build epoxy intermediate — provides barrier protection and builds film thickness over the zinc primer. Critical for chemical resistance in industrial/urban environments. (3) Polyurethane topcoat — UV resistance, color retention, and final barrier. The combination provides a theoretical service life of 15–25 years when correctly applied.
• Moisture-cure urethane (MCU) — the cold-weather alternative: Standard IOZ primers require surface temperatures above 50°F for application. This severely limits the painting season in Chicago (see Project Controls module). Moisture-cure urethane zinc-rich primers can be applied at temperatures as low as 0°F and at relative humidity up to 98% — allowing year-round or extended-season painting operations. MCU systems cost approximately 15–25% more than standard IOZ systems but can dramatically expand the painting window on projects where schedule is a constraint.
• Spot painting vs. full repaint — specification complexity: When only localized areas of the coating system have failed, a spot painting specification may be more cost-effective than a full repaint. However, spot painting on a bridge with multiple coating layers — including lead-containing historic layers (see XRF module) — requires careful specification: the spot-painted area must be cleaned to a compatible surface preparation, and the new coating must be compatible with the adjacent existing coating system chemically and aesthetically. Incompatible overcoating can cause adhesion failure in the spotted areas within 1–2 years.

The Americans with Disabilities Act (ADA) and its implementing regulations — particularly the PROWAG (Public Rights-of-Way Accessibility Guidelines) and FHWA's Design Standards for Accessible Transportation Facilities — require that any federal-aid transportation project that alters a facility must bring the altered portions of the facility into ADA compliance. For CDOT bridge rehabilitation projects funded by FHWA, this means that rehabilitating a bridge deck or approach triggers an obligation to address ADA compliance throughout the pedestrian access route on the bridge — not just the portion being directly rehabilitated.

• The alteration trigger — scope expansion is automatic: When a CDOT bridge rehabilitation project includes work on the bridge deck, sidewalk, approach, or any element of the pedestrian route, FHWA's ADA requirements are triggered automatically. The alteration must bring "the altered portion of the facility and the path of travel to the altered area" into compliance — which typically means the entire pedestrian route on the bridge, including approaches at both ends, must meet current PROWAG standards. A bridge rehabilitation that focuses exclusively on structural steel and ignores the sidewalk system is in ADA compliance jeopardy if the sidewalk is in the project limits.
• Grade transition at bridge approaches — the persistent challenge: Chicago's historic bridges frequently have non-ADA-compliant grades at their approach transitions — where the bridge deck meets the approach pavement. Historic movable bridge approaches may have grade changes of 5–8%, compared to PROWAG's maximum 5% cross slope and 8.33% running slope for accessible routes. The design engineer must survey the approach grades before design and determine whether grade correction is achievable within the existing structure geometry or requires approach ramp redesign — a potentially significant scope addition on historic bridges with constrained approach geometry.
• Curb ramps and detectable warning surfaces — common non-compliances: Most pre-2000 Chicago bridge approaches have curb ramps that do not meet current PROWAG standards: insufficient landing areas at the top of the ramp (minimum 5 ft × 5 ft clear), insufficient detectable warning surface area (2 ft depth, full ramp width), improper flare grades, or inadequate cross slope control. Retrofitting compliant curb ramps at bridge approaches in the dense urban environment of Chicago frequently requires utility relocation, signal pole repositioning, and pavement reconstruction — each with its own coordination and cost implications.
• Sidewalk width on bridge — a structural constraint with ADA implications: PROWAG requires a minimum 5-foot clear width for accessible pedestrian routes. Some historic Chicago bridge sidewalks, designed for foot traffic in an era when wheelchairs were not considered, are narrower than 5 feet — particularly on ornamental bridges where the railing structure reduces the clear walkway width. Widening these sidewalks requires structural modifications to the deck and railing systems — and on landmark bridges, those modifications may conflict with preservation requirements. The design engineer is caught between ADA compliance and historic preservation — a conflict that must be documented and resolved with FHWA, CCL, and IHPA.