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Godfrey, Kneeland, Jr. (ed.) / The Wisconsin engineer
Volume 59, Number 4 (January 1955)

White, Richard N.
The Mackinac Straits bridge,   pp. 20-22


Page 21


per year were failing in the U.S. alone, and this weak-
ness ushered in a new era in bridgebuilding and mate-
rial-steel.
  The increased strength of steel made it possible to
build the first examples of today's long, majestic spans.
The suspension bridge, which is now the leader of all
types, was built in ever increasing span lengths until
the completion on the Golden Gate Bridge in 1937 over-
took all bridges with a span of 4200 feet. This will be
eclipsed by the Messina Straits Bridge which is pres-
ently in the planning stages and will have a 5000-foot
main span. New techniques in the use of reinforced
concrete, and even more recently, prestressed concrete,
have resulted in much longer spans for concrete arch
construction. This is a type used widely in building
the many shorter bridges needed on all highways and
railways.
  The U.S. has around 400,000 bridges in its truly
great, though inadequate, transportation system. Both
civil defense plans and the ever-increasing number of
vehicles on America's roads cry out the need for bigger
and better highway systems. Under President Eisen-
hower's 50 billion dollar highway improvement plan
(which has recently been recommended to be cut to
26 billion dollars) many more bridges will be designed
and built by the engineers of America.
  As for the bridges of the future, it is safe to say that
span lengths will be increased still further with the
perfection of super high-strength alloy steels and
lightweight structural materials such as aluminum and
magnesium alloys. Dr. David B. Steinman says, "In
many ways the story of bridgebuilding is the story of
civilization."
           The Mackinac Straits Bridge
  Ever since 1884 farsighted Michigan businessmen
and newspaper editors have realized the necessity and
eventual construction of either a bridge or tunnel
across the Straits of Mackinac which would link the
tipper and lower peninsulas of the state. Now the
dream is coming true with the construction of a five-
mile bridge of steel and concerete featuring the second
longest suspension span in the world-3800 feet. This
center span, linked with two 1800-foot side suspension
spans, two 472-foot unloaded backstay spans, and two
135-foot anchorages, makes a suspension bridge of
8,614 feet-the world's longest of this type. Replacing
the old ferry system now in use, the bridge will allow
the largest Great Lakes boats to pass under the center
span (minimum clearance height is 148 feet).
  The Mackinac Bridge Authority, which is behind
the building of this great transportation link, was
started 20 years ago and made numerous studies
through the 1930's and 1940's with the help of the
state highway department and Army engineers. How-
ever, limited funds and then World War II made it
impossible to carry out the actual building of the
bridge, and the Authority was abolished in 1947 by the
Michigan S'ate Legislature.
  The people of Michigan were not to be denied.
Through their efforts the Authority was recreated in
1950, but this time only with the power to determiiine
feasibility. They reported that the cost of the bridge
would be $86,000,000, and in 1952 the Authority was
granted the powers to finance and build the structlue.
While the R.F.C. was studying the Authority's request
to purchase $85,000,000 worth of bonis, a group of in-
vestment dealers offered to underwrite the sale of the
bonds. The Authority accepted, and by the end of
1953, $99,800,000 worth of bonds had been sold on a
nationwide market. Merritt-Chapman and Scott Cor-
poration was awarded the contract to build all the
foundations for a total of $25,700,000, andI the Amer-
ican Bridge Division of United States Steel Corporation
is building the superstructure for $44,500,000. On \la\t
7, 1954, the bridge building job was begun, and will be
completed in the latter part of 1957.
         Design and Construction Details
  The bridge will be supported by 33 piers, some of
which extend to 195 feet below the lake level. The
anchorage piers (the suspension cables are anchored
by embedding the ends in huge masses of concrete,
each containing 85,000 cubic yards of concrete and ca-
pable of resisting a pull of 60,000,000 pounds) are
among the most massive ever constructed, having foun-
dations 135 feet long by 115 feet wide.
  Thirty of the 33 piers are being built by the coffer-
dam method. This means that a watertight enclosure
made of interlocking sheet piling will be driven into
the lake bottom around the space occupied by the
pier. After the enclosure is braced adequately, the
bottom is excavated to the depth desired and concrete
is poured into the cofferdam building the base of the
pier. The bracing is concreted into the pier with the
sheet piling acting as an exterior form. The cofferdam
is then pumped out and the pier is finished by pouring
the remainder of the concrete. This method is usually
used for relatively shallow depths and stable bottom
conditions.
  The two tower piers and the southern cable rest pier
are using the caisson method because of their extreme
depth. A caisson is similar to the cofferdam in that it
serves as an enclosure and matches the shape of the
pier. It is fabricated on the shore, towed to the pier
site, and stink to the bottom. The lower section of the
caisson is a cutting edge built up of steel plates and
structural shapes. This edge cuts into the bottom when
the caisson is lowered. The bottom is then removed,
usually by dredging, and concrete is poured into the
spaces around the dredging wells causing the caisson
to cut deeper into the bottom. This process is continued
until the desired depth is reached.
  The caissons used for the tower piers will be 116
feet in diameter and will require 2,530 tons of steel for
their construction. They are designed in the shape of
a giant doughtnut with an 86-foot diameter center for
dredging out the bottom and a 15-foot space into which
concrete is poured.
JANUARY, 1955
21


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