Lake Street Bridge. A Road, Rail, and Pedestrian Bridge in Chicago’s Infrastructure History. A Photographic Collection. Part One: Superstructure, Bridge Tender’s Houses, and Substructures.

Article and photographs by Divi Logan. Assisted by Copilot Smart AI.

I am pleased to share my photographic collection with bridge restoration companies, bridge crews, Chicago transportation departments, steel fabrication companies, and fans of bascule bridges.

I. About Lake Street Bridge

Lake Street Bridge is a double-leaf, double-deck bascule bridge, that carries commuter trains on its upper deck and vehicles and pedestrians on its lower deck. Completed in 1916, the bridge is now undergoing major maintenance.

Features of the bridge are shown in photographs taken between 2019 and 2025. A kid-friendly glossary is included to help our engineers of the future learn and appreciate the importance of bridges in our national infrastructure.

Chicago in 1830 — This is the 1830 survey plat done by James Thompson. The Chicago River winds through the city, and at historic Wolf Point (near the center), separates into three branches. The survey’s east boundary ended at Fort Dearborn, a United States government installation dating to 1803. Because of the location of the fort, the grid did not extend to Lake Michigan.

II. The Bridge’s Superstructure

bifurcation of the Chicago River at Wolf Point — Lake Street Bridge is basically the portal to the South Branch of the Chicago River. Here is the bifurcation of the river at Wolf Point. Here it separates from the Main Branch (foreground) into the South Branch (left) and North Branch (right). 27 April 2024. Viewed from the Franklin Street Bridge.

Lake Street Bridge. clear span of a bridge — The entire span of Lake Street Bridge is shown here in this view near Wolf Point. Both bridge tender’s houses are present. Features include train deck approach spans, wing walls, abutments, parts of the substructure, and the divided Warren Truss design. 2 August 2024. Hazy conditions added to the unusual lighting conditions. The day was pleasant and there was busy urban traffic.

Lake Street Bridge clear span — The Lake Street Bridge is a double-deck, double-leaf bridge that spans the beginning of the Chicago River’s South Branch. It is in the divided Warren Truss style. The beams form a W shape along the clear span. Structural features include wing walls, abutments, and train deck approach spans. Bridge tender’s houses serve as control centers during bridge lifts and construction projects. They are an essential aspect of public safety. Bridge date: 1916. This view was taken during an architecture cruise. Photo date: 4 May 2019. Lighting challenge: reflections from glass-faced buildings and the calm water surface.

train deck approach span of a bascule bridge — An eastbound CTA Green Line train crosses the Lake Street Bridge. Features of this view from the Franklin Street Bridge include the Warren Truss design, a bridge tender’s house, retaining walls for Wacker Drive and the bridge’s wing wall, the Wacker Drive Improvements monument, and part of the Riverwalk. 4 May 2019.

This is the east train deck approach span over Wacker Drive and Lake Street. Warren Truss divided. Photo date: 11 July 2020.

CDOT bridge crews — Bridge crews gather under the east train deck approach span before a bridge lift. 19 October 2019.

III. The Train Deck and Superstructure

X-bracing on a through-truss bridge — The superstructure of Lake Street Bridge is held together by intricate bracing systems that include X-bracing and lateral bracing. X‑bracing keeps the truss acting as a single, stiff unit throughout that motion. The diagonals are not decorative—they’re the reason the leaf behaves as a rigid body rather than a floppy plate. The curved steel bar near the portal is the bridge orientation indicator. The view looks west along the clear span. Photo date: 27 March 2019.

Lake Street Bridge superstructure. East view from near 150 N. Riverside. 4 May 2019.

🔧 What X‑Bracing Does on a Bascule Bridge

1. Controls Lateral Sway and Racking

X‑bracing is the bridge’s way of saying: “I will not twist, I will not rack, even when the leaf is moving or when a train is pounding across the approach span.”

Bascule leaves experience asymmetric loading during opening and closing.
Wind loads hit the superstructure from the side, especially on Chicago’s river corridor.
X‑bracing triangulates the frame so that lateral forces are redirected into tension in one diagonal and compression in the other, depending on load direction.

X-bracing on members of the Warren through truss superstructure, along with floor beams and lateral braces of the train deck. 18 June 2019.

2. Stiffens the Truss During Leaf Rotation

Superstructure elements include intricate X-bracing for the Warren Truss, floor beams, and lateral bracing for the train deck. 18 June 2019. Lighting elements converge in the distance in this eastbound view towards the intersection of W. Lake and W. Wacker.

Unlike a fixed truss bridge, a bascule leaf is a moving truss.

During rotation:

The truss wants to deform out of plane.
The counterweight pit and heel trunnion impose concentrated forces.
The live load path changes as the leaf transitions from horizontal to partially lifted.

X-bracing on a bascule bridge — Part of the Warren Truss design of Lake Street Bridge’s superstructure is shown here. X-bracing is crucial to the design of this bridge. Photo date: 18 June 2019.

Sidebar. Kid‑Friendly Bridge Glossary: X‑Brace • Portal Frame • Lateral Bracing

X‑Brace

What it is:
Two long metal bars that cross each other to make the shape of an “X.”

Why it matters:
An X‑brace helps a bridge stay strong when the wind pushes on it or when trains and cars make it shake.
The “X” shape keeps the bridge from wobbling sideways, the same way crossing your arms makes your body steadier.

How to picture it:
Imagine two friends holding hands and leaning in opposite directions. Because they’re connected, they don’t fall over. That’s what the X‑brace does for the bridge.

A close-up of X-bracing on the superstructure. Lateral bracing and the train deck are visible. 18 June 2019. The view faces southwest.

Portal Frame

train deck approach span and portal knee braces — The train deck approach span for the westbound portal of Lake Street Bridge is designed in the same divided Warren Truss that is the structure of the clear span. Gigantic portal knee braces built up with gusset plates support the train deck and distribute loads from the upper to the lower portions of the superstructure. Photo date: 18 June 2019.

What it is:
A big, strong doorway made of steel beams at the entrance to a bridge.

Why it matters:
The portal frame helps the bridge stand tall and keeps the opening from bending or tilting. It’s especially important on bascule bridges, where the moving leaf needs a sturdy “doorway” to connect to.

How to picture it:
Think of the frame around your front door. It keeps the doorway square so the door can open and close smoothly. A portal frame does the same thing for a bridge.

portal knee girders for a bascule bridge — The gigantic portal knee braces of the westbound portal of Lake Street Bridge are shown here. These structural elements are built up with gusset plates. They distribute loads between the train deck and the superstructure. 8 April 2019.

Lateral Bracing

What it is:
A set of beams that run side‑to‑side under or inside a bridge.

Why it matters:
Lateral bracing keeps the bridge from sliding or swaying sideways.
It helps the whole structure act like one strong piece instead of lots of separate parts.

How to picture it:
If you’ve ever built a block tower and put your hands on both sides to keep it from tipping, you’ve acted like lateral bracing.

train deck with lateral bracing — The massive lateral braces of the east connecting train deck approach span are shown in this view looking west through the clear span. Lake Street Bridge is built with a through-truss, divided Warren Truss design.18 June 2019.

lateral bracing on a train deck approach span — Gigantic lateral bracing members of the train deck approach span for the westbound portal are an essential part of the bridge’s engineering structures.

Purpose of the Lateral Braces Under the Train Deck Approach Span

(Lake Street Bridge — Lake–Wells Loop Junction)

1. Resisting Side‑to‑Side Forces from Train Movement

Trains entering and exiting the Lake–Wells Loop Junction generate powerful sideways forces when they brake, accelerate, and negotiate curves. The lateral braces tie the longitudinal girders together so the entire deck acts as one stiff, unified structure rather than a collection of independent beams.

2. Preventing Racking, Twisting, and Sway

Without these braces, the approach span would be vulnerable to racking (parallelogram distortion), torsion, and lateral sway. The triangulated bracing system locks the geometry in place, keeping the deck square and stable even under heavy vibration.

3. Transferring Loads into the Portal Frame and Main Truss

The lateral braces channel side forces into the portal knee braces and the Warren through‑truss of the movable leaf. This clean load transfer prevents stress from accumulating in the deck and ensures the fixed span and movable span work together as a coordinated system.

4. Stabilizing the Transition Between Fixed Span and Movable Leaf

Because the bascule leaf rotates and changes stiffness during operation, the fixed approach span must remain absolutely steady. The lateral bracing provides the rigidity needed for the moving machinery to operate safely and predictably.

5. Supporting the Structure in a Dense Urban Environment

The Lake Street corridor experiences wind tunnel effects, heavy pedestrian and vehicle traffic, and constant urban vibration. The lateral bracing absorbs and redistributes these forces, protecting the structure from fatigue and long‑term deformation.

portal knee brace and girders — This massive portal knee brace is built of gigantic steel girders connected by gusset plates. Two of them transfer loads from the train deck to the lower portion of the superstructure. They are at the westbound portal. Photo date: 18 June 2019.

IV. The Bridge Tender’s Houses

The bridge tender’s houses are the control centers of bridge operations. Crew members enter the houses and communicate with others around the bridge area to ensure public safety during all stages of bridge lifts. The two houses are designed with Beaux-Arts and Neoclassical elements used in Chicago for bridge houses between 1900 and the 1930s.

wing wall and bridge house — The wing wall that conceals a staircase connecting the Riverwalk to the bridge superstructure is shown here. The northeast bridge house is part of the crucial public safety infrastructure for the bridge. Crews monitor bridge lifts and construction from these houses. Medium: PRO 400 H 35mm film. Photo date: 25 June 2019.

bridge tender's house with bell — The east bridge tender’s house of Lake Street Bridge is shown here with its bell. An entrance to the Riverwalk is adjacent to it. Photo date: 3 December 2020.

clear span of a through-truss bridge — The clear span, both bridge houses, and retaining walls are shown here in this view from the Riverwalk near River Point. 24 September 2019.

Beaux-Arts bridge tender's house with stone steps — The stone staircase connecting the Riverwalk to the sidewalks past the clear span is shown here. The entire house from foundation to bell to roof line is shown. 24 September 2019.

bridge tender's house with crew members inside — Bridge crew members are in the west house of the Lake Street Bridge preparing for a bridge lift. Behind the house are parts of the train deck approach span and to the left is the beginning of the clear span with sidewalk cantilevers visible. 19 October 2019.

V. The Substructure. The Cast of Supporting Members

substructure of a bascule bridge — The substructure of Lake Street Bridge as viewed from the Riverwalk near River Point. In these gigantic pits are the counterweights. East abutment and part of the Warren Truss superstructure are also shown. 24 September 2019.

A view of the Lake Street Bridge substructure is captured during architecture cruise. 2 June 2019.

Bridge Lifts Show Substructure Elements

superstructure of a through-truss bridge — The elevated west leaf of Lake Street Bridge displays crucial information about the design of the substructure. Elements include stringers, floor beams, truss point panels, gusset plates, and sidewalk cantilevers. The edge of the leaf at bridge center is also shown, as are parts of the train deck. Bridge lift of 11 July 2020.

Sidebar: What Road‑Deck Stringers Do on a Bascule Bridge

Road‑deck stringers are the longitudinal beams that run in the direction of traffic, directly beneath the roadway. On a bascule bridge—especially a heavy Chicago trunnion bascule like Lake Street—they play a quiet but essential role in how the deck carries load and how the leaf behaves during a lift.

West leaf structural elements and edge of the leaf at bridge center. 102 mm telephoto. 10 19 2019. View from the sidewalk at the southeast corner of W. Lake and W. Wacker, along with a crowd stopped on the sidewalk during the lift.

🚗 1. They Carry Wheel Loads to the Floor beams

Stringers are the first structural members to receive the concentrated loads from cars, trucks, buses, and maintenance vehicles.

They spread those loads along their length.
Then they deliver the loads into the transverse floor beams, which in turn transfer them into the truss or girder system of the leaf.

Without stringers, the deck would behave like a thin plate—far too flexible for traffic.

🧱 2. They Stiffen the Deck and Prevent Local Bending

Because they run longitudinally and are closely spaced, stringers:

Prevent the deck from sagging between floor beams
Reduce vibration and bounce
Keep the riding surface smooth and predictable

On a movable leaf, this stiffness is even more important because the deck is not continuously supported like a fixed span.

A CDOT bridge crew member and a crowd watch the descent of the west leaf during the 19 October 2019 boat run.

The time is 11:22 as the west leaf descends, watched closely by CDOT bridge crew members on both sides of the leaf. 19 October 2019.

⚙️ 3. They Help the Leaf Act as a Single Structural Unit During a Lift

When the bascule leaf rotates upward, the entire superstructure is in a different load regime.
Stringers help:

Maintain torsional rigidity of the deck
Keep the floor system acting as a unified panel
Prevent twisting or racking as the leaf rises

West leaf structural elements during the boat run on 07 11 2020.

🔩 4. They Tie Into the Floor System and Truss Nodes

On Lake Street Bridge’s west leaf:

Each stringer frames into a floor beam
Each floor beam frames into a truss panel point
The truss carries the loads to the trunnion and heel joint

So the stringers are the first link in the load path that ultimately reaches the bascule machinery.

stringers and floor beams of a bridge leaf — The substructure of Lake Street Bridge’s west leaf is shown during a bridge lift of 19 October 2019. Note the stringers, sidewalk cantilevers, and floor beams built in with the truss panel points.

🛠️ 5. They Support the Wearing Surface and Utilities

Stringers also carry:

The concrete deck or steel‑grid deck
Utility conduits
Drainage scuppers
Maintenance walkways (depending on the leaf)

They’re the “backbone” that everything else attaches to.

The Load Path

Stringers → Floor beams → Truss Panel Points → Bascule Machinery → Substructure

This hierarchy is what makes a movable bridge behave like a single, predictable structural unit—even while rotating 70+ feet into the air.

Bridge house (Tender’s House)

A small operational building located at the corner of a movable bridge, housing the controls and personnel responsible for raising and lowering the bridge. Chicago’s early‑20th‑century bridge houses often combine Beaux‑Arts architectural massing with industrial fenestration.

reflectivity in a single pane window of a bridge tender's house — Chicago’s bridges are surrounded by glass-faced buildings. The windows of the bridge tender’s houses reflect and bend the images of these skyscrapers. Photo date: 24 September 2019.

VI. Lake Street Bridge: Historic Infrastructure Set for Major Renovations

These photos were taken as crews prepared the bridge for years of significant renovations. All photos were taken from safe vantage points such as sidewalks and walkways near River Point and 150 N. Riverside.

Lake Street Bridge prepared for renovation — The Lake Street Bridge is prepared for major renovations in this photo from October 2, 2025. The view is taken from a walkway near River Point and looks east-southeast.

Train deck approach span west of the bridge. 2 October 2025. Near River Point.

CTA Green Line train crossing a bridge — A westbound Green Line train crosses Lake Street Bridge as preparations for renovations are underway. 2 October 2025.

Bridge construction crew vehicles and equipment, wiring/cables and lighting on the superstructure. View looks east-northeast from a sidewalk across from 150 N. Riverside.

The superstructure, train decks, and west bridge tender’s house viewed looking east from River Point Park. 2 October 2025.

Conclusion

Lake Street Bridge has served Chicago’s travelers since 1916. The double-deck, double-leaf fixed trunnion bascule span carries CTA trains on its upper deck and vehicles and pedestrians on the lower level.

The bridge is undergoing major maintenance until 2028. With careful monitoring and thoughtful renovations, the bridge will continue to serve Chicagoans for the next century.

1916 dedication plaque on the northeast bridge tender’s house.

Resources

“Thompson’s Plat of 1830.” Encyclopedia of Chicago, Accessed 23 Mar. 2026.
Wikipedia contributors. “Muntin.” Wikipedia, The Free Encyclopedia. Wikipedia, The Free Encyclopedia, 11 Dec. 2025. Web. 23 Mar. 2026.
Copilot. “Schematic Diagram of Load Paths on Lake Street Bridge.” Microsoft Copilot, 24 Mar. 2026.
Copilot. “Kid‑Friendly Glossary: X‑Brace, Portal Frame, and Lateral Bracing.
Wikipedia contributors. “Terne.” Wikipedia, The Free Encyclopedia. Wikipedia, The Free Encyclopedia, 7 Dec. 2025. Web. 25 Mar. 2026.
Copilot. “Explanation of the Lateral Braces Under the Train Deck Approach Span of the Lake Street Bridge.” Microsoft Copilot, 25 Mar. 2026.
Microsoft Copilot. “What Road‑Deck Stringers Do on a Bascule Bridge.” Lake Street Bridge Structural Documentation, 26 Mar. 2026. AI‑generated explanatory text.
Microsoft Copilot. “Floor System Hierarchy: How Loads Move Through a Bascule Leaf.” Lake Street Bridge Structural Documentation, 26 Mar. 2026. AI‑generated explanatory text.
Wikipedia contributors. “191 North Wacker.” Wikipedia, The Free Encyclopedia. Wikipedia, The Free Encyclopedia, 28 Feb. 2026. Web. 31 Mar. 2026.

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