Following
the errors occurred during the erection and continued
exploitation, the membrane of single deck floating roof has been
deformed. One
part of the water does not reach roof drain, leaves on the roof and
accelerates
corrosion process. The installation of additional stiffening girders
approximates the shape (the slope) of the membrane toward the projected
one. These
girders stabilize the roof in the case of wind blows and suction.
Radial girders on membrane of single deck floating roof
When
the tank has the diameter D > 40 m the classical
single deck
floating
roofs without any stiffening on the membrane is not recommended
solution,
because during the exploitation big deformations will appear in the
membrane. Serious deviations from the projected form of the membrane are
possible
especially if they are situated in the area with increased wind speed
or membrane
is made by rolls method.
Purpose
of using of the stiffening girders:
- decrease the vibrations and deformations in the central part of the
roof;
- the installation of girders which are high enough and which are
tightly
connected to stiff ring in the center of roof, separates membrane on
sections
and it prevent the pouring of the product all over the membrane if
there is any
gap in the membrane. When this solution is preferred it is necessary
every new
formed section to have its own water drain which through the pipes
under the
roof is connected to the central roof water drain;
- when the girders have big stiffness and their joints
are rigid the work of the central part
of the roof is changed. It is not the round hanging membrane any more
and
becomes round orthotropic plate. The complementary stiffening probably could be
necessary if the girders and pontoons work together.
Radial stiffening girders
The external floating roof which membrane is stabilized by radial
girders exist in Bulgaria, Netherlands,
Germany,
Austria and USA.
The kind
of the girders and their joints depend on the diameter of the tank. The
kinds
of the stiffening girders are:
- girders from steel sheets - it can be complementary cold rolled in
order to become stiffer. It is used for the tanks which have smaller
volumes;
- girders from hot-rolled sections - it is used angular, U-sections
and I-sections. Their
mounting is very convenient and easy;
- the steel sheets or the gilders with gaps in the web - they have
increased constructive height and the anti corrosion defense become
more
difficult;
- girders with box section - they usually are used when the tanks have
bigger
dimensions. When the box section has space it is possible to decrease
volume of
the pontoons, because these girders enable roof to float when there is
the
liquid upon the roof . In other words they are complementary pontoons;
- the radial girders on the roof can have different positions. Some
possible solutions are shown on the figure bellow.
Location of the radial stiffening girders on the
membrane-
girders which are not connected between them - this option is used when
the girders are made from steel sheets. They can be connected to the
pontoons
but can be situated at several distance from them;
- girders connected through the central ring - when this solution is
used
almost always the girders are connected with the pontoons and work with
them.
The central ring can be formed as a cylinder toward which the girders
are connected
frontal or to be cut as a
flat disk which lay above the
ribs. When this solution is used, the girders upon the membrane are high enough and the central ring has the shape of cylinder, independent
sections can be formed;
- girders which are not connected and are dislocated between them - the
girders
stabilizing the membrane have the length = 0,25.Dr. Due to
this
construction roof becomes more stiff, but works as a hanging membrane.
The
forming of the independent sections above the membrane is impossible.
Stiffening girders as rings
The stiffening girders as rings
must be put under the membrane according to the following scheme:
Location of the circular stiffening girders
Many
tanks which volumes are V = 50 000 m3, which are
stiffened with girders made as a rings, and the girders dimension are
L100x100x110 mm, are constructed in Poland. When the
measurements for
determining the moving in the center of the membrane have been done
this mode
for stiffening showed very good effectiveness.
Circular stiffening girders are used in 7 steel tanks with V = 20 000 m3 capacity in PB "Polykraiste", Bulgaria, see picture bellow:
Stiffening of membrane with circular girders
The
disadvantage of this kind of stiffening is that the girders must be
put under the membrane. It causes the following consequences:
- all joints made on site must be done in upper condition – the welders
are below ceiling when the welding joint must be done;
- it is impossible to form smaller sectors on the membrane.
Radial
and circular stiffening girders
When the radial and circular stiffening ribs
are used the consumption of the metal and labor is bigger comparing
with the
other methods for membrane stiffening.
Research of the influence of the girders
upon
the deformation of roof membrane.
The author made 3D computer
models of roof membrane
and stiffening girders and after that he made science research and
analysis.
Conclusions from the obtained results:
- when the membrane is not stiffened with diameter D
≥ 40 m the
deviation from loading and deformation are considerable and the
complementary
stiffening of the construction is necessary. When the stiffening
girders are
put correctly the deformations can be decreased two times;
- decreasing of angle between the radial put girders (increasing of
their density) leads to the stiffening of the membrane and decrease of
the deformation
in it;
- when the stiffening girders are not connected to pontoons and when
joints between pontoons and girders are flexible, the deviation from
loading of
the membrane is the same. More favorable for the mounting is to put the
girders
which are not connected to the pontoons, with which the stiffness which
is
achieved is approximately the same as with the flexible joint ribs;
- rigid joint between the stiffening girders and the pontoons decrease
the deviation from loading of the membrane. This is better expressed
when the
tank diameters are bigger. In other part the rigid joint between the
girders and
the pontoons changes their static scheme and tensioned status. So if
the
pontoons are not prepared for complementary bending from these
additional girders
it is better to avoid the rigid joints to the pontoons;
- the putting of central stiffening ring, joints all radial girders is
more favorable for deviation from loading of the membrane then the
rigid joint
of this sections to the pontoons;
- the circular stiffening girders under the membrane decrease its
deviation
from loading but more material is used and the weld joints which must
be done
are on the ceiling.
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