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Choosing Storage OptionsPART I: CONCRETE VS. STEEL BINS
Michael Blough
Project Engineer, Clear Creek & Associates
Presentation OverviewStorage History
• Brief Summary
Design and Planning
• Site Layout
• Design & Applied Loading
• Cost
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Presentation Overview
Construction
• Concrete Construction Process
• Steel Construction Process
• Time Lapse Videos
Presentation Overview
Grain Management
• Filling and Unloading
• Flow of Materials
• Aeration
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Presentation Overview
Structural Maintenance
• Corrosion
• Damage
• Repair Methods
• Life Expectancy
Storage History
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History of Grain Storage• Facilities that handled over 2 million bushels
used to be concrete clusters
• Grain bin storage was maxing out at 750k
bushels per bin
• Conveying at 40k bph was high speed
• Filling large dia. bins took days to weeks
• Soil loads at 4000psf allowable bearing
pressure were great to build on.
Grain Facilities Today• Facilities can have over 2 million bushels in 2
corrugated bins
• A single grain bin is capable of storing 1.3
million bushels
• Conveying has moved toward 100k bph
• Now able to fill large diameter bins in hours,
not days.
• Geopiers, pilings, soil additives, & soil with
5000psf allowable bearing pressure is being
required.
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Grain Facilities Today• Grain facilities have evolved to a highly
complex operation compared to the past.
• More automation for high volume incoming &
outbound grain.
• Higher level of consideration to operations,
safety, cleanliness, & maintenance.
Design and Planning
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Design and Planning:Site Layout
Site Layout – Concrete
• Located in areas with limited space
• Built taller than steel bin structures
• Towers often not required for conveying equipment
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Site Layout – Concrete• Circular bin layout
• Capacity increased by using the interstice
bins (star bins)
Site Layout – Concrete• Hexagonal bin layout
Reference: www.rmdkwikform.com
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Site Layout – Steel• Located throughout the site to allow space
for vehicular traffic, flow of materials, and
rail lines
• Don’t require soil to be as firm (lower bearing
capacity) as concrete in order to support the
weight of the structure
Design and Planning:Design & Applied
Loading
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Typical Grain Bin Loads• Filling
• Unloading
• Temperature Cables
• Wind
• Seismic (Earthquake)
• Snow
• Aeration
Design & Applied Loading
Concrete
• Able to withstand more intense filling and
unloading cycles
• Better for resisting high impact loading
• Typically able to better resist wind and snow
loading
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Design & Applied Loading
Steel
• Flexible structural behavior
• Empty steel bin generally provides better
behavior in seismic event than concrete silo
• Provides poor resistance for lateral bracing for
towers or equipment
Design and Planning:Costs
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Costs
• The current difference in cost per
bushel for construction is low as
compared to historical differentials.
• Steel silos generally provide a
greater storage capacity per dollar
and tend to be more cost-effective
as compared to concrete.
Factors to Consider
• Increasing diameter – larger
foundation, larger and/or greater
capacity roof, increased catwalk
length
• Increasing height – longer bucket
elevator, larger tower foundations
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Cost per Bushel vs. Bin Capacity
Reference: Kenkel, P. Oklahoma State University
Construction
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Construction:Concrete Construction
Process
Construction Process• Concrete silos are typically
constructed by jump-forming or
slip-forming.
• Slip-forming
• Complicated process requiring extensive
supervision because it is continuous.
• Rebar placement and concrete affect the
quality of the final product.
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Production of Materials• Concrete is batched in a controlled
environment, but variability of strength is
introduced with variations in ingredients size
and quantity.
• Individual strength tests of concrete can fall
below up to 10% of f’c (compressive
strength) as long as the average of tests
equal or exceed f’c.
Reference: ACI 318-11, Sec. 5.6.3.3
Site Storage Before Installation
• Concrete is greatly affected by delivery time
and weather.
• Concrete produced must be placed very
soon after arriving on site. In general,
concrete starts to lose its optimal
characteristics after 2 hours (From time of
production at batch plant).
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Concrete Silos Time Lapse
Concrete Silos Time Lapse
Reference: YouTube.com, “Trupointe Cooperative, Inc.” YouTube
channel
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Construction:Steel Construction
Process
Construction Process• Erection of bin can be done with minimal
supervision.
• Installation of openings in bins (e.g. doors)
and accessories (e.g. ladders) are easy to
complete.
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Production of Materials• Steel bin components are manufactured in
a controlled environment . Slight variance of
quality.
• ASTM specifications require minimum yield
and ultimate steel strength used in design
• To meet the ASTM specification (e.g. A36,
A992) the steel is produced to be greater
than or equal to minimum yield and ultimate
steel strength.
Site Storage Before Installation• Steel bin
components that are not installed immediately maintain structural strength over a long period of time on site.
• Moisture between galvanized sheets
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Steel Bin Time Lapse
Steel Bin Time Lapse
Reference: YouTube.com, “Grass Roots Media” YouTube channel
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Filling and Unloading – Concrete• Concrete has the rigidity to withstand the
effects of eccentric loading
• Concrete wall thickness
• Bracing against other bins
Filling and Unloading – Steel• The sheets and
stiffeners of steel bins are susceptible to damage and/or collapse if they experience off-center unloading or loading
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Maintaining Unloading Conditions
Free Flowing
• Clean
• Dry
• In Condition
Issues with Poor Conditions
• Grain develops clumps
• Foreign items end up in the
conveyors
• Grain goes out of condition and
becomes non-free flowing
• Gates plug
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1. Core bin from
center gate
2. Unload from
center gate
until grain
stops flowing
3. Unload from
intermediate
gates, moving
out from bin
center
Correct Unloading - Floor Gates
Off-Center Unloading
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Concrete Hopper Bottoms• The construction process lends itself to
hopper bottoms underneath making complete
cleanout possible without sweep augers
Filling and Unloading – Steel• Large bins require quality, commercial-
grade sweep auger for complete cleanout
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Steel Hopper Bottom• Hopper bottoms typically 36 ft. diameter or less
• Require structural columns to elevate tank
• Generally 10 rings tall or less
Grain Management:Flow of Materials
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Flow of Materials
Reference: ASABE EP 433
Dec. 1988 (R2006)
Funnel Flow
Reference: Jenike & Johanson (www.jenike.com)
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Mass Flow
Reference: Jenike & Johanson (www.jenike.com)
Flow of Materials
Function of:
• Finish of the interior surface (smoothness)
• Angle of the hopper
• Height/Diameter ratio
• Grain material properties
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Flow of Materials – Concrete
• Higher coefficient of friction
of grain
• Mass flow difficult to achieve,
likely funnel flow
• Funnel Flow = First-In,
Last-Out (FILO)
Flow of Materials – Steel
• Lower coefficient of friction of grain
• Mass flow or funnel flow
• Mass Flow = First-In, First-Out
(FIFO)
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Grain Management:Aeration
Aeration
• The horsepower required to
operate an aeration system is a
non-linear function of grain depth
• Often impossible or impractical
to provide the same airflow in
concrete silos that could be
achieved in shorter, wider
steel bins.
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Corrosion
• Corrosion is a concern for both
concrete and steel
• Concrete protects steel in
concrete silos
• Sheets on steel bin are exposed
Corrosion –Concrete Silos
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Stiffener Buckling Caused by Corrosion
Steel Material Loss• Bins are not designed with a
corrosion allowance.
• Many older bins were not designed using
current, typical grain loads (ASABE 433).
• Recommendations for material losses:
• 0% to 5% - Acceptable for regular use
• 5% to 10% - Close monitoring and
regular inspections
• > 10% - Replace component/sheet
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Delamination Test
Solid sound • Good bond between
rebar and concrete
Slapping/Popping sound • Delamination of the
rebar and concrete
Delamination Test
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Delamination Test
Reference: YouTube.com, “Tyler Smith” YouTube channel
Crack Monitoring
Recommended to better understand
damage and to determine if there is
cracking growth
• Marked with marker or paint for growth
progress
• Crack monitors installed across cracks
• Easy to install
• Very inexpensive
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Crack and Patch Repair
Remove Concrete Damage
• Remove loose, delaminated concrete or
debris in the affected region.
• Undercutting of corroded reinforcing steel
may be required if concrete is delaminated
behind the reinforcement.
• Cut the spalled area perpendicular to
the surface.
• Reference: ICRI 310.1R-2008
Delamination Repair
Reference: International Concrete Repair Institute
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Crack and Patch RepairPrepare Steel Reinforcement
• Review and check steel reinforcement to determine if
the reinforcement will still meet the original diameter
specification after the cleaning the bar of rust.
• If it is suspected that the reinforcement will not meet
the original diameter specifications, contact a
structural engineer to replace or reinforce the steel.
• Remove rust from steel reinforcement using a wire
wheel or sandblasting.
Reference: ICRI 310.1R-2008
Crack and Patch Repair
Prepare Concrete Surface
• Prepare the surface with a wire wheel
grinder to remove dust, debris, damaged
concrete that prevent proper bonding.
• For cracking, grind out and open cracks.
Reference: ICRI 310.1R-2008
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Crack and Patch Repair
Repair Concrete and Steel
• Treat the steel reinforcement with a
rust inhibitor.
• Apply bonding agent to concrete surface.
• Patch small concrete areas with high-yield
non-shrink cement grout
• Patch large areas with Shotcrete/Gunite
• Inject high-yield, non-shrink caulk into cracks.
Reference: ICRI 310.1R-2008
Sealing Cracks
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Surface Repair vs. Structural Repair
• Surface Repair
• Patch repair
• Crack injection
• Structural Repair
• Shotcrete/Gunite liner in silos
• Carbon fiber
Shotcrete Installation
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Steel Repair
Steel Bin Repair
• Refer to manufacturer’s
recommendations
• Remove damaged bin components
and replace with new components
• If bolts are removed always
replace with new bolts
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Structural Maintenance :
Life Expectancy
Life Expectancy - Concrete
• Concrete silos have a longer
useful life expectancy than steel
bins. With proper maintenance
concrete silos can last 70+ years
• Repair methods (e.g. silo liners)
can extend this life
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Life Expectancy - Steel
• A steel bin’s useful life is around 40
to 50 years even with proper
maintenance
• Repair methods like replacing
components usually cost as much
or more than replacing the
structure at this age
Thank youQuestions can be emailed to [email protected]