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1
Difficult or Impossible Falsework — Cannot Obstruct River Navigation
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📘 Explanation
Falsework — the temporary structural framework erected to support a bridge during construction or repair — is routine on land-based bridge projects. Over the Chicago River, it becomes one of the most complex engineering and regulatory challenges on the job. The USCG-mandated navigation channel must remain clear at all times, which means the GC cannot simply plant a forest of shoring towers in the river and work from above. Every falsework scheme must be designed around this constraint.
- USCG navigational channel requirements: The Chicago River main stem and South Branch carry commercial and recreational navigation. USCG Ninth District mandates a minimum horizontal clearance (varies by location, typically 40–60 ft on the main stem) and vertical clearance that must be maintained at all times during construction. Any temporary falsework that encroaches on this envelope requires a USCG permit and a Notice to Mariners — and may be flatly denied if the channel cannot be maintained.
- Soil conditions below the river are unpredictable: Even where channel width allows a falsework bent at the edge of the navigation zone, founding a shoring tower in the river bottom requires subsurface investigation. River bottom soils in Chicago vary dramatically — soft organic silts and legacy industrial fill in some reaches, dense glacial till in others. A falsework foundation design without subsurface data is a structural liability that no engineer should accept.
- Most Chicago river bridge rehab work avoids in-water falsework entirely: The combination of navigational restrictions, permit requirements, marine mobilization cost, and foundation uncertainty drives most GCs and engineers toward in-water falsework avoidance as a primary design goal. Repairs are redesigned to be executed from the structure itself — using cantilever techniques, work platforms hung from existing members, or barge-mounted lifting rather than tower shoring.
- Bascule bridge falsework is uniquely constrained: On movable bridges, any falsework attached to or bearing against the movable leaf must be completely removable before the bridge can open. This means falsework for bascule bridge leaf repairs must either be self-contained on the leaf (no ground support), hung from the leaf structure itself, or designed for rapid removal with the leaf locked in the down position — after which the bridge cannot open until the falsework is struck.
- Temporary bridge closures for falsework-dependent repairs: For repairs that genuinely require in-span support — deck replacement, major girder repair, truss panel replacement — the most practical solution is often a total temporary bridge closure with no falsework in the river channel, replacing the falsework function with structural shoring anchored to the adjacent approach span or abutment. This approach requires detailed structural analysis but avoids the navigation conflict entirely.
⚠️ Key Planning Reality: The time and cost to engineer, permit, install, and remove in-water falsework on the Chicago River frequently exceeds the time and cost of redesigning the repair to eliminate the need for in-water falsework. Engineers who approach Chicago river bridge rehab with a standard "falsework under the work" mindset will consistently overbid and underperform relative to teams that design for no in-water support from the outset.
📍 Real Project Example
State Street Bridge — Trunnion Bearing Replacement: Original repair design called for falsework towers in the river to support the bascule leaf during counterweight removal. USCG review rejected the in-channel falsework placement as it reduced the navigable channel below the minimum 42-ft horizontal clearance required at this location. The design team spent 9 weeks redesigning the support scheme, ultimately using a custom steel spreader frame bearing entirely on the bridge deck and abutment walls — eliminating in-water elements entirely. Redesign cost: $78,000. However, the new scheme was faster to install and removed in half the time of the original falsework plan, ultimately compressing the overall schedule by 3 weeks.
North Branch Pratt Truss Bridge — Chord Repair Falsework: A lower chord repair required temporary vertical support under the repaired panel. The river channel here was only 35 ft wide — no room for any in-water bent. The GC designed a cantilevered timber crib falsework system anchored against the existing abutment wingwall, projecting horizontally over the water to support the chord from the side rather than from below. System was engineered by a PE, reviewed by IDOT, and installed in 4 days without any river obstruction or USCG permit required. Cost: $34,000 versus an estimated $110,000 for an in-water alternative.
✅ Solutions & Alternatives
Design-Phase Falsework Avoidance Study: At the earliest design stage, challenge the need for any in-water support. For every work activity that would normally require falsework, ask: can this be done from the structure itself? Can we sequence the work to use the existing bridge as the support platform? Can we pre-fabricate the repair in a shop and install it as a complete unit requiring only minimal temporary support? A systematic falsework avoidance study at design phase saves months of permitting and tens of thousands in falsework cost.
Barge-Mounted Shoring as the In-Water Alternative: When vertical support over the water is unavoidable, a work barge with installed shoring towers provides the support function without placing any fixed structure in the river bottom. The barge can be positioned, held with spud anchors, and its deck height adjusted to provide precise bearing elevation. USCG requires the barge to maintain channel clearance requirements, but a barge is movable — it can be repositioned for bridge openings without the removal cost of fixed falsework.
Cantilevered Strongback and Spreader Frame Systems: Engineer a horizontal cantilevered steel spreader frame that bears on the bridge's own superstructure — diaphragms, floor beams, cross-frames — and projects the support point outward to where the repair is located. This transfers all repair loads back into the existing structure with no ground contact. Requires careful load analysis but eliminates all navigation and foundation issues.
Post-Tensioned Tie-Back Support from Abutments: For major span-level repairs, a post-tensioned tie-back system anchored at the abutment or pier cap can provide upward or horizontal support to the repair zone from outside the channel — no in-water elements. This approach is engineering-intensive but can support significant loads and is fully outside the navigation envelope. Used successfully on several CDOT projects where standard falsework was not feasible.
Early USCG Pre-Application Coordination on Falsework Scheme: If in-water falsework is unavoidable, submit the falsework scheme to USCG Ninth District for informal review before finalizing the design. USCG can indicate whether the proposed channel restriction will be approved or not — before the GC has invested in detailed engineering. Informal coordination costs nothing and can prevent a 3-month redesign cycle after a formal permit denial.
📝 Your Notes
2
Crane Picks Over Water Require Marine Equipment, Barges, or Aerial Lifts
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📘 Explanation
Lifting structural components, equipment, and materials to a bridge deck over the Chicago River is fundamentally different from the same operation over land. A land crane sets up on solid ground, lifts over the structure, and sets its load. A river bridge crane must either set up on land and reach over a channel of water — with the associated reduction in rated capacity from the extended radius — or be mounted on a marine platform in the river, with all the complexity that entails.
- Over-water lifts require ASME B30.5 and OSHA 1926.1400 compliance for crane operations over water: Any crane lift where the load will travel over water triggers additional ASME and OSHA requirements — including a pre-lift plan documenting the load weight, center of gravity, rigging arrangement, crane configuration, and contingency plan for load drop. For marine crane operations, USCG regulations additionally apply.
- Crane radius vs. capacity reduction: A land-based crane reaching across a 60-ft river channel to make a pick at the center of the bridge span will be operating at maximum radius — typically 50–70% capacity reduction from its rated capacity at minimum radius. A 100-ton crane operating at full extension may have only 30–45 tons of capacity at the required radius. This either limits the weight of components that can be lifted in a single pick or requires a larger, more expensive crane than the bare weight of the components would suggest.
- Barge-mounted crane advantages: A crane mounted on a work barge in the river operates at short radius to picks on the bridge structure — maximizing lifting capacity and precision. The barge is positioned by spud anchors or mooring lines for stability during the lift. However, the crane barge must be mobilized through the river system (limited beam and draft at certain locations), river current and boat wake can affect barge stability during picks, and USCG navigation requirements must be maintained throughout the lift operation.
- Aerial work platforms (AWPs) have weight and reach limitations: Large telescoping boom lifts can access bridge members from the deck or from street level at the bridge approach, avoiding the need for a crane in many maintenance scenarios. However, AWPs have strict load ratings (typically 500–1,000 lbs platform capacity), limited reach in the over-water direction, and require level, stable ground or deck support — conditions that are not always available at Chicago river bridges.
- Load drop planning over water is mandatory: OSHA 1926.1424 requires a written plan for crane operations over water, including the procedure if the load is dropped. Over the Chicago River, a dropped load creates environmental (lead paint contamination), navigation hazard (vessel collision risk), and recovery cost implications that must be anticipated and addressed in the pre-lift plan.
Crane Operations Over Water — Key Requirements
OSHA
1926.1400 Cranes in Construction: Requires operator certification, pre-shift inspection, assembly/disassembly director, signal person, and written pre-lift plan for all critical lifts. Lifts over water trigger additional requirements for load drop contingency planning.
ASME
B30.5 Mobile Cranes: Load chart compliance, ground bearing pressure calculation for land-based cranes. For barge cranes, ASME B30.8 Floating Cranes and Derricks applies — requires marine surveyor certification of barge stability before each lift campaign.
USCG
33 CFR Part 162 — Inland Waterway Navigation: Barge-mounted crane operations must maintain navigational clearances. USCG may require a vessel safety plan and 24-hour radio watch during extended crane barge operations that restrict the channel.
USACE
Section 10 Permit: Crane barge operations in the Chicago River require USACE Section 10 authorization if the barge footprint restricts the navigable channel below minimum clearance thresholds.
📍 Real Project Example
Kinzie Street Bridge — Main Girder Segment Replacement: A deteriorated 28-ft main girder segment (estimated 14,000 lbs) needed replacement. Land-based crane option: a 200-ton crawler crane on Kinzie Street with a 95-ft radius to reach the center of the channel — analysis showed only 12.8-ton capacity at that radius, insufficient for the lift. The GC mobilized a barge-mounted 50-ton hydraulic crane, positioned 15 ft from the girder with a 20-ft radius — well within capacity. Marine mobilization premium over land crane: $68,000. However, the barge crane completed the removal and installation in one 14-hour shift versus an estimated 3 days for the land crane approach with partial picks.
South Branch Swing Bridge — Machinery Room Equipment Replacement: A 4,200-lb motor-generator set needed replacement in the bridge's below-deck machinery room. Access was only possible from a hatch in the bridge deck over the open channel. A telescoping boom aerial work platform could not reach the drop point at the required capacity. Solution: a pick-and-carry rough terrain crane on the bridge deck itself (deck load rating verified by structural engineer — the deck could support the crane's wheel loads). The crane used a 40-ft boom from a deck-mounted position to lower the equipment through the hatch without going over the water edge at all — eliminating the over-water lift classification entirely. Saved the $68,000 marine mobilization cost.
✅ Solutions & Alternatives
Lift Radius Analysis Before Equipment Selection: For every major lift on a river bridge project, calculate the required crane radius before selecting equipment. Do not assume the crane that would work on land will work over the river. Determine required radius → look up capacity at that radius → if insufficient, evaluate barge crane, larger land crane, or component pre-assembly to reduce lift weight. This analysis takes 2 hours and prevents mobilizing the wrong crane to a live project.
Pre-Position and Pre-Qualify a Marine Crane Subcontractor: Identify and pre-qualify a marine crane subcontractor (barge-mounted crane operator) before project bidding. Obtain a firm price for mobilization, standby rates, and per-lift cost. Include this as a defined allowance in the bid. Having a pre-qualified marine lift sub on standby eliminates the 4–6 week mobilization delay that occurs when the GC discovers mid-project that a land crane cannot make the required pick.
Pre-Assembly to Minimize Over-Water Lift Weight: Design repair components to be assembled in maximum possible sub-assemblies on land before the pick. A single pre-assembled girder segment lifted in one pick over water is safer, faster, and lower-risk than multiple smaller picks each requiring over-water crane positioning. Work with the fabricator to ensure the maximum sub-assembly weight is within the barge crane's capacity at the required working radius.
Written Critical Lift Plan for Every Over-Water Pick: For any lift over water exceeding 75% of the crane's rated capacity at the required radius, prepare a full critical lift plan per ASME B30.5 — documenting load weight (with 10% contingency for rigging hardware), center of gravity, crane configuration, boom angle and radius, actual capacity at that configuration, sling angles and rigging hardware capacities, signal person assignment, and load drop contingency. The plan is submitted to the CM/engineer for review 72 hours before the lift. This is not optional on CDOT projects with federal oversight.
Deck-Mounted Pick Points with Engineered Beam Reinforcement: For repetitive lifts of moderate weight (5,000–20,000 lbs), install temporary engineered steel pick beams spanning between existing bridge girders on the deck surface. Rigging runs through the deck grating to picks below the deck. This converts an over-water lift into a vertical hoist from a fixed structural point — eliminating crane radius, stability, and over-water classification concerns for routine material handling throughout the project.
📝 Your Notes