MACKINAC BRIDGE

ANUJ AGGARWAL MOHIT BOBAL NIKHIL AGGARWAL RAMANDEEP SINGH

Total Length of Bridge (5 Miles) 26,372 Ft.Length of Suspension Bridge (including Anchorages)

8,614 Ft.

Length of Main Span (between Main Towers) 3,800 Ft

Height of Main Towers above Water 552 Ft.Maximum Depth of Water at Mid span 295 Ft.Under clearance at Mid span for Ships 155 Ft.

Total Length of Wire in Main Cables 42,000 MilesMaximum Tension in Each Cable 16,000 TonsWeight of Cables 11,840 TonsDiameter of Main Cables 24 1/2 Inches

BRIEF

DESIGN

THE MACKINAC BRIDGE WAS THE WORLD’S FIRST TO INCORPORATE STEINMAN’S PRINCIPLES OF AERODYNAMIC STABILITY.

DESIGNING A BRIDGE TO CROSS THE MACKINAC STRAITS POSED THREE SIGNIFICANT CHALLENGES:

1. HIGH VELOCITY WINDS2. DEEP WATER 3. EXTREME PRESSURES OF ICE ACCUMULATION

TO DEAL WITH THE WIND, STEINMAN UTILIZED OPEN STIFFENING TRUSSES IN PLACE OF SOLID SHEETS BENEATH THE ROADWAY TO INCREASE STABILITY, THUS IMPROVING THE CRITICAL WIND VELOCITY FROM 40 MPH TO 632 MPH. STEINMAN FURTHER IMPROVED BRIDGE STABILITY BY USING AN OPEN GRID ROAD SURFACE TO FURTHER INCREASE THE CRITICAL WIND VELOCITY .

THE TOWERS/ANCHORAGES

THE TOWERS ARE THE STRUTS FOR THE SUSPENSION BRIDGE. THEY RECEIVE ALL OF THE COMPRESSIVE FORCES.

THESE MEMBERS HAVE TO BE THICK ENOUGH RESIST BUCKLING, FLEXURE, AND OSCILLATION.

THEY HAVE TO WITHSTAND MINOR CHANGES AS A RESULT OF LIVE LOADS AND TEMPERATURE CHANGES.

THE MAIN JOB OF THE TOWERS IS TO WITHSTAND THE FORCES THAT ARE EXERTED ON IT BY THE CABLES.

THE ANCHORAGES PULL THE SIDE SPANS TO THE GROUND AND FIXES THEM IN PLACE.

THE ANCHORS MUST BE ABLE TO HOLD DOWN THE CABLE WITH ITS WEIGHT OR BY TRANSFERRING THE TENSION IN THE ROPES TO THE GROUND.

THE CABLES ALL OF THE TENSION FORCES IN THE

BRIDGE IS TRANSFERRED TO THE MAIN CABLE THROUGH THE SUSPENDERS.

THE CABLES HAVE TO SUPPORT THE DECK AND LIVE LOADS WITHOUT HAVING TO OVERSTRESS ANY MEMBER.

THE CABLES NEED TO ALLOW VIBRATION AND BE RESISTANT TO CORROSION.

THE DECK• THE DECK NEEDS TO BE

RIDGED TO PREVENT ANY DIPPING AS LIVE LOADS PASS, YET IT NEEDS TO BE AS LIGHT AS POSSIBLE.

• THE DECK IS A MAJOR FACTOR IN CALCULATING THE DEAD LOAD AND TENSION IN THE MAIN CABLE.

STRUCTURAL PRINCIPLES A MAIN CABLE OF A SUSPENSION BRIDGE RESEMBLES A PARABOLA. THE

HANGERS, OR SUSPENDERS, ARE PLACED AT EQUAL INTERVALS FROM EACH OTHER.

• THE FIRST STEP WAS TO SINK THE LARGE, DOUBLE-WALLED CYLINDERS THAT FORM THE BASES OF THE TWO MAIN TOWER PIERS. THESE CYLINDERS ARE CALLED CAISSONS.

• THE CABLES WERE CLOSE TO 25 INCHES IN DIAMETER. EACH CABLE CONSISTS OF 340 WIRES BANDED INTO A SINGLE STRAND; 37 STRANDS ARE THEN ASSEMBLED INTO A SINGLE CABLE. MORE THAN 42,000 MILES OF WIRE WERE USED IN THE TWO MAIN BRIDGE CABLES!

CONSTRUCTION

• THE LAST STEP WAS TO CONSTRUCT THE ROAD SURFACE. THE INNER LANE ON THE MIDDLE SPAN OF THE BRIDGE INCLUDES AN OPEN GRID RIDING SURFACE. THIS WAS INSTALLED TO ALLOW WIND FORCES TO MOVE THROUGH THE BRIDGE.

• THE ROADWAY OF THE BRIDGE IS SUSPENDED FROM STEEL CABLES STRUNG BETWEEN TWO CONCRETE-AND-STEEL TOWERS, AND ANCHORED AT BOTH ENDS IN 170,000-TON CONCRETE BLOCKS THAT DESCEND TO BEDROCK 100 FEET BELOW THE WATER AND RISE 119 FEET ABOVE THE SURFACE.

SADDLE

HANGER ATTACHMENT

TO PROTECT AGAINST ICE PRESSURE

STEINMAN DESIGNED THE TOWERS’ CONCRETE PIERS USING A MOST-SECURE SAFETY FACTOR OF 20 (20 TIMES THE MAXIMUM ICE PRESSURE ACHIEVED UNDER LABORATORY-CONTROLLED CONDITIONS). EACH OF THE CONCRETE PIERS WAS SHEATHED IN ARMOR PLATE ABOVE AND BELOW AREAS OF POTENTIAL ICE CONTACT.

THE DESIGN OF THE MACKINAC BRIDGE WAS DIRECTLY INFLUENCED BY THE LESSONS OF THE FIRST TACOMA NARROWS BRIDGE, WHICH FAILED IN 1940 BECAUSE OF ITS INSTABILITY IN HIGH WINDS. THREE YEARS AFTER THAT DISASTER, STEINMAN HAD PUBLISHED A THEORETICAL ANALYSIS OF SUSPENSION-BRIDGE STABILITY PROBLEMS, WHICH RECOMMENDED THAT FUTURE BRIDGE DESIGNS INCLUDE DEEP STIFFENING TRUSSES TO SUPPORT THE BRIDGE DECK AND AN OPEN-GRID ROADWAY TO REDUCE ITS WIND RESISTANCE. BOTH OF THESE FEATURES WERE INCORPORATED INTO THE MACKINAC BRIDGE. THE STIFFENING TRUSS IS OPEN TO REDUCE WIND RESISTANCE. THE ROAD DECK IS SHAPED AS AN AIRFOIL TO PROVIDE LIFT IN A CROSS WIND, AND THE CENTER TWO LANES ARE OPEN GRID TO ALLOW VERTICAL (UPWARD) AIR FLOW, WHICH FAIRLY PRECISELY CANCELS THE LIFT.

CONCLUSION