150
X2
Category 2: Civil Structures
The tank is a monolithic structure in reinforced concrete,
supported directly on the ground, for the storage of
treated water. Its capacity is 2,000 m³. As the water is
treated with chlorine, the internal environment of the tank
is extremely aggressive due to the presence of chlorine
gas in the air cushion between the water surface and
the lower face of the tank lid. Due to the presence of the
chlorine gas, a flat slab with capitals was chosen as the
tank lid to avoid the formation of a gas retention chamber
under the lid with the enhancement of the aggressive
effect on the structure. Another design challenge was
to adapt the tank to ground conditions to obtain a more
economical solution in the direct foundation (raft), within
the limits of the admissible soil tension.
Structure Description
The tank has an inner diameter of 22.00 m and a height
of 5.37 m. The bottom slab, which acts as a raft, has a
thickness of 20 cm, with the capitals thickness of 30 cm
and dimensions of 2.40 m x 2.40 m under the columns.
Along the outer circumferential wall, the bottom slab
thickness is increased to 30 cm, with a width of 1.00 m.
This forms an outer flange cantilevered projection 50 cm
beyond the outer face of the wall. This flange has the
function of reducing the contact pressure between the
structure and the foundation soil beneath the exterior
walls. The nine columns have internal dimensions of
30 cm x 30 cm, with eight of them radially distributed and
one central column. The tank lid is 20 cm thick, supported
by columns via capitals with a thickness of 30 cm and
dimensions of 2.40 m x 2.40 m. The entire structure has
been designed in reinforced concrete.
Standard Codes Applied
Eurocode (Scia), NBR-6118:2003 - Design of concrete
structures, NBR-6120 - Loads for calculation of edification
structures and other Brazilian norms.
Exposure Class, Materials and Concrete Cover
According to Table 6.1 of NBR-6118, the structure
is classified as aggressive with a very strong risk
of structural deterioration, Class IV Environmental
Aggression. According to Table 7.1 of the same norm,
Class C40 concrete with a water/cement ratio mass
≤ 0.45 was adopted. According to Table 7.2 and
specifications of the contractor, all of the adopted
concrete covers are 45 mm. The used steel was CA-50.
Loads considered
The loads applied to the bottom slab were 5.37 tf/m²,
equivalent to a height of 5.37 m of water and a constant
load of 0.25 tf/m² for adjustment and trims. A permanent
load of 0.25 tf/m² for the settlement and trims and an
accidental load of 0.15 tf/m² were applied to the tank lid.
Modeling and Analysis
The structure was modelled in 3D Scia. Various stress
diagrams, areas of rebar and contact stresses of the
model were obtained. After some adjustments to the
dimensions of the structure and soil parameters, the final
model was settled and it provided parameters for the
design and detailing of the structure. According to the
diagrams of the contact stresses, the maximum tension in
the soil reached a value of around 1.00 kgf/cm².
Design and Detailing
From the information obtained from Scia, the design
of the reinforced concrete members conducted by the
Eurocode program were checked using the criteria and
requirements of the Brazilian standard for critical sections,
making sure of the compatibility between standard norms.
After the definition of the design of the structural parts,
the detailing of the most important pieces was made and
the general criteria of drawings from other parts were
established. This information was relayed to the engineer
responsible for the use of Allplan, who developed all the
detailing of all the reinforcement in the program. Scia
proved a powerful tool that caters for issues such as
the reliability of results and productivity gains. There is
great ease in the creation and display of input data as
well as results obtained by the settable graphical user
choice. The interpretation of results is quite intuitive and
interactive.
Drawing and final presentation
With all the reinforcement detailed in Allplan 3D, specific
drawings were developed for each structural piece as
forms and reinforcements.
Supported Tank 2,000 m³ Cascavel - Paraná, Brazil
Software: Allplan Engineering, Scia Engineer
