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 After the contract for the new metal bearing had been initiated, another
bearing was investigated. This bearing is a rubber pad in a "pot" used under
bridge girders. The specific bearing investigated was manufactured by the
Andre Rubber Company Limited, and the bearing was called "Andre Rota Bridge
Bearing". An estimated cost at that time for a bearing equivalent to the
steel bearing used under the tower would have been approximately $4000. As a
result of these possible big savings, some research work was done at NCEL to
check the claims of the bearing manufacturer. Results were promising.
Two recent towers, one a new 600 foot radiator design and the other an
existing 1200 foot radiator have had rubber "pot" bearings placed under them.
was made at the Navy-Air Force test facility at Forestport, NY. The bearing
performed satisfactorily. The particular bearings used were manufactured by
Spencer Dynamics Corporation of Providence, Rhode Island. The antennas have
been in operation, one for about a year and a half and the other for over six
months. Both appear to be operating okay.
Solution: The "pot bearing" is the choice over a spherical metal bearing for
the base of a "hinged" base, guyed tower.
CASE S17 - Drydock Analysis, A. Wu
Problem: In the U. S., there are over 50 graving drydocks which need to be
certified at a certain point in time. Naval shipyards are usually located in
areas with unfavorable soil conditions. There have been many hydraulic fills
which utilize uniform, fine sands which are subject to liquefaction. These
drydocks were mostly built during the past 20 to 50 years.
Loose, uniform, fine grained sands are subject to liquefaction when exposed to
earthquake motions. Because of the upgraded seismic criteria, there is
concern about drydock failures resulting from soil liquefaction. In the event
of liquefaction, the drydock may float and/or tilt. Liquefaction also
combined with high seismic inertia force may overstress or overturn the
drydock wall. All of these possibilities of failure are not acceptable to the
Navy.
The drydocks need an adequate structural analysis. One obvious reason is the
safety requirements. A graving drydock is a stationary drydock which is built
below the ground surface. The drydock is used to maintain or repair the Navy
ships which include nuclear powered aircraft carriers or submarines. The
seismic criteria also has been upgraded recently and a higher seismic load is
considered in the analysis. Absolute safety of the ship while in dock is
necessary.
Symptoms: Analysis indicating structural distress may occur under large
earthquake loads.
Collection of Facts:
The drydock can be classified according to the hydrostatic pressure conditions
when the dock is dry. These are: (1) full hydrostatic, meaning that the
hydrostatic pressure is acting both to the floor and to the wall; (2) fully
relieved type, meaning that no hydrostatic pressure to the floor and wall; and
(3) partially relieved type which has only hydrostatic uplift pressure acting
to the floor.
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