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From ExpEriEncE
“how did you do that?”
Reinforced Concrete Tank Design
By Mark W. Holmberg, P.E. ®
Reinforced concrete tanks have been used Analysis
for water and wastewater storage and treatment Analysis of reinforced concrete slabs
for decades. Structural engineers are asked and walls is a complex finite element
to design a variety of square, round, and exercise. ACI 350 suggests three de-
oval reinforced concrete structures that may sign aids that can simplify the analysis
be above, below, or partially below ground. of liquid containing tanks:
Design of reinforced concrete tanks requires Rectangular Concrete Tanks,
attention not only to strength requirements, Portland Cement Association, Figure 1: Commerce WWTP Aeration Screen, Walkway and
but also to crack control and durability. The 1994 Hoist Bridge.
challenge for the structural engineer is to de- Copyright
sign concrete liquid containing structures that Circular Concrete Tanks Details
will resist the extremes of seasonal temperature Without Prestressing, Portland
changes and a variety of loading conditions, Cement Association, 1993 Proper detailing of reinforcing and joints
and remain watertight. Concrete Manual, U.S. Department of has as much to do with durability of a liquid
th containing structure as properly applying the
Design and construction of the Bear Creek Reclamation, 8 edition, 1981
Waste Water Treatment Plant (WWTP) Ex- The two Portland Cement Association (PCA) provisions of ACI 350 and correctly analyzing
pansion located in Hampton, Georgia and the publications include coefficients for calculating the structure. End fixity of a section affects
Commerce Georgia WWTP are used to illus- moments, shears, tension, and deflection for location and magnitude of maximum moments,
trate code requirements, tank analysis, design a variety of aspect ratios and end conditions and joint location affects the amount of mini-
details, and construction of liquid containing of plate and circular elements. Engineering mum shrinkage and temperature reinforcing
reinforced concrete tanks. judgment of the designer is required to select required. The PCA design tables include both
the proper design tables, detail sections to fixed and hinged base conditions. The com-
Code Requirements resemble the boundary conditions selected mentary states that either extreme is rarely
and recognize that ideal boundary conditions achieved in the field. The detail in Figure 2 is
R U C T U R E
ACI 350-06 Code Requirements for Environ- are rarely achieved in the field. In addition, the typical for a hinged base condition and was
mental Engineering Concrete Structures extends engineer must recognize conditions, beyond used in several of the Bear Creek WWTP
T
the limitations of the PCA data, that require structures. A compression type waterstop was
the basic ACI 318 Building Code Requirements
magazine
for Structural Concrete with additional service- special analysis. installed near the inside face of the wall. A
S Liquid containing tanks often support heavy PVC bulb type waterstop is not practical at this
ability requirements for crack width control
due to flexure, shrinkage of concrete, and tem- equipment requiring special design of equip- location due to the location of slab reinforcing.
perature movement. The commentary of ACI ment support and surrounding walls. The Prior to completing the Bear Creek WWTP
350 makes it clear that ACI 318 requirements structural engineer must obtain vertical and structures wall details, it was determined that
alone will not produce watertight structures. ACI vibration loads of equipment, and design for the maximum allowable length between con-
350 serviceability requirements include: possible restraint of temperature movement of struction joints would be 50 feet, and that
Reduced working load stresses, and supporting concrete. Grade 60 reinforcing steel would be specified.
requirements for size and spacing of Figure 1 is an example of equipment that Figure 3, from the PCA Circular Concrete Tanks
reinforcement restrains or induces temperature movement in Without Prestressing noted above, is a useful aid
Increased minimum reinforcement for the reinforced concrete structure. for proportioning shrinkage and temperature
temperature and shrinkage movement, reinforcing based on joint spacing. ACI 350
which is dependent on the grade notes that concrete sections over 24 inches
of reinforcing steel and the length thick must have shrinkage reinforcing at
between shrinkage dissipating joints each face based on a 12-inch thick section.
Waterstop requirements at all joints
Concrete mix design requirements Concrete
Increased cover requirements Specifying and supplying a concrete mix
compared to ACI 318 that minimizes shrinkage is critical to the
Commentary suggestions for use of durability of liquid containing structures.
shrinkage compensating concrete Concrete shrinkage is dependent on several
per ACI 223 factors, including cement content and aggre-
Properly designed, specified, and detailed gate. To insure that design assumptions are
structures applying the requirements of the valid, the engineer should specify shrinkage
ACI 350, along with engineering judgment requirements (0.04% to 0.03%), and that
and quality construction, should have a the concrete supplier provide shrinkage test
useful life of 50 to 60 years. results prior to approving the mix design.
Figure 2: Hinged Base Detail. Where large tank walls or slabs are required
STRUCTURE magazine 22 July 2009 STRUCTURE magazine
bility of the structure. Almost all form systems
require through-wall form ties that create po-
tential for leaks, and weak planes where cracks
may occur. Proper concrete mix design and
appropriate add mixtures will assure that
concrete is easily consolidated. Proper cur-
ing of concrete will reduce the potential for
shrinkage cracking. ®
Figure 4 shows small temperature/shrinkage
cracks in the Bear Creek WWTP Aeration
Basin Tank wall located at form tie holes. The
contractor injected the cracks and applied
a specified waterproofing membrane to the
inside of the tank walls.
Figure 3: Minimum Temperature and Shrinkage Conclusion Figure 4: Tank Wall Cracks at Tie Hole.
Reinforcement. Proper application of ACI 350 design require-
Copyright
without expansion joints, shrinkage compen- ments, appropriate structural details, properly
sating concrete may be specified; however, the specified and supplied concrete, and quality Mark W. Holmberg, P.E. is Vice President
structural engineer must account for concrete construction will assure long life for reinforced and Civil Manager for Heath & Lineback
expansion movement. concrete tanks that will be subjected to harsh Engineers, Inc. in Marietta, Georgia. Mark is
ACI requirements for hot and cold weather environmental and use conditions. The structural also a member of the STRUCTURE Editorial
concrete placement must be specified because engineer and the contractor must work together Board, representing CASE. He can be reached
construction of a large project can take place to produce reinforced concrete liquid containing at mholmberg@heath-lineback.com.
over a year or more. One concrete mix design tanks that are as watertight as possible.▪
will not be adequate for all pour conditions. Project Team
Construction Process Design Engineer and Construction Observation – Stevenson & Palmer Engineers, Inc.
Formwork, concrete placement and curing Structural Engineer – Heath & Lineback Engineers, Inc.
are obviously critical to the quality and dura- Contractor – Choate Construction
R U C T U R E
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July 2009 SSW09_S_7_1-2x4_3-4.indd 1 STRUCTURE magazine 23 July 2009 6/12/2009 4:16:21 PM
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