The Ely miner. [volume] (Ely, Minn.) 1895-1986, 1935-04-26

The building of this bridge involved some of
the most difficult engineering problems ever at
tempted by man. Never before has bridge
been reared above piers which were sunk by the
casson method from the surface of the water
down through both mud and water to rock bot
tom, sight unseen. Never before have suspen-
sion towers reared themselves Gifted by their
own bootstraps," one might say until their
hollow steel frames rose sos feet above the
ter and their concrete bases sank in some cases
235 feet below water level.
No other bridge yet built has called for
many piers, 81 in number. In fact, 1t was the
belief for more than half century that lack of
adequate foundations in the bay for piers, as
well the impossibility of stretching span
from Yerba Buena to San Francisco, would pre-
vent the bay from being bridged. But in 1929 a
survey of the bottom of the bay, directed by
state engineers, revealed a high ridge of bed
rock extending between San Francisco and the
island which would provide foundation at
higher levels than surrounding bedrock and
make a practicable route. The water along this
route is from so to 105 feet deep and bedrock
lies from 100 to 200 feet below the mud on the
bottom of the bay.
So the most serious problem was the work
below the water level and the building of the
five major piers between the island and San
Francisco. Because of the depth of the water
and the mud 'sand-hogs" as underwater labor
ers are called who work in pressure chambers
to clear away the mud to the rock bottom, could
not be used. The solution of the problem was in
a compressed air-fotation caisson method which
enabled the bridge builders for the first time to
construct their piers from the surface of the
water down, using the jaws of huge clamshell
buckets instead of men to do the excavating.
Each casson consists of cluster of huge steel
tubes Is feet in diameter, held together by an
outer casing. Compressed air made the caisson
buoyant 1t was towed into place and an
chored. In the space between the tubes, con-
crete was poured, forming enormous box
something like an egg carton, except that the
openings were round. Each tube was sealed with
an alright cap The concrete poured around the
tubes caused the caisson to sink slowly and as
It sank additional lengths were added to the
tubes and more concrete poured around them.
When the bottom of the caisson sank to the
mud in the bottom of the bay, a steel cutting
edge pushed down through the mud. The caps
were cut off the tubes and dredge buckets were
dropped down through them to scoop out the
mud below the caisson. Gradually each caisson
was worked down through the mud to rock bot
tom, in one case 235 feet below the surface Of
the water. Thus the piers were built.
Another problem was the two-mile Suspension
between San Francisco and Yerba Buena which
had been deemed impossible. So the engineers
decided upon two suspension spans in tandem,
anchored in the middle of the bay to gigantic
pier. 1t is 1877 feet long (nearly 66 yards) by
90 feet wide and rises sos feet from the rock
bottom of the bay, nearly twice as large the
biggest skyscraper in San Francisco. Since the
two spans anchored to this they actually pull
against each other. The San Francisco anchor
age is huge mass of concrete containing 6S,-
600 cubic yards of cement
In spinning the cables the entire 10000 miles
of cable are pulled in place by shuttle wheels
which run over the towers all the way from
San Francisco to the concrete center anchorage
on the west suspension bridge and from the
center anchorage to Yerba Buena on the east
suspension bridge Spinning the cables is by
far the most protracted job on the bridge and
will require a year. Each piece of wire in the
cable is approximately two miles long A total
of 34,96S strands must be drawn over the sus-
pension towers. Each cable will exert pull of
100000 pounds against its anchorage in San
Francisco and Yerba Buena island.
Just as the building of the Bay bridge pre-
sents knotty problems TO be solved, so does the
building of the Golden Gate bridge bring up diff
fculties never before encountered In such work.
Outstanding among these were the great length