Contents
FAILURES AND REPAIR OF SILOS AND BUNKERS
STRUCTURAL DISTRESS OF SILOS AND BUNKERS :
-DESIGN ERROR
-OR INADEQUACY
-CONSTRUCTION ERRORS
-MISUSE
-EXPLOSION
-OR OTHER UNEXPECTED LOADING
TYPES OF STRUCTURAL FAILURES :
-FOUNDATION FAILURE
-FAILURES IN WALLS
-ROOFS FAILURE
-FAILURES IN HOPPER SYSTEMS
FOUNDATION FAILURES :
TILTING OF CANADIAN PACIFIC RAILROAD (CPR) ELEVATOR,
TRANSCONA, CANADA.
FAILURES OF CONCRETE SILO WALLS :
DESIGN ERROR OR INADEQUACY
-Failure to consider Overpressure
-Effect of horizontal variation of pressure
-Shock Loading
-Thermal Stresses
-Pressure due to Flow improving devices
-Use of incorrect material properties
DESIGN ERROR OR INADEQUACY
CONSTRUCTION ERRORS
× Missing or misplaced hoop reinforcement
× Difficulties with slip forming
-DESIGN ERROR OR INADEQUACY
-CONSTRUCTION ERRORS
-USAGE ERROR
× Storing materials other than the material
for which the structure was designed.
× Installing additional discharge openings.
-DESIGN ERROR OR INADEQUACY
-CONSTRUCTION ERRORS
-USAGE ERROR
-FOUNDATION MOVEMENT
-SHRINKAGE AND CREEP
CORE REASON :
Nothing will happen, we are designing from so
many years its always like this only
“TAKE IT FOR GRANTED ATTITUDE“
-SINGLE CIRCULAR SILO, horizontal bending
will occur whenever lateral pressure vary
around the circumference at any particular
level
This
-variation will be present under all condition
except that of a single central discharge
opening with a symmetrical centrally located
flow channel
-Since
many circular silos have been
designed assuming the wall to be in
tension only,only,(ignoring bending)
It
is not surprising that walls of
circular silos often have bed vertical
cracks
-When
multiple openings are used, the
horizontal bending may subject to
reverse
ECONOMY :
A SAILENT KILLER
-Resistance to failure under combined bending
and
tension is highly dependent on
reinforcement
position in the wall
-where the bending moment is reversible, two
layers
of reinforcing is preferred
-then also, in the interest of economy, engineers
frequently
specify single layer reinforcement
-The single layer is easier to handle than two
layers
during slip forming
-Two layer reinforcing requires more labor to
place
than the single layer
-However, TWO LAYER DESIGN PROVIDES
MUCH
BETTER PROTECTION AGAINST WALL
CRACKING
FAILURES OF CONCRETE SILO WALLS :
A CASE STUDY 1
FAILURE OF POCKET BIN WALL :
-Group of Cement Storage Silos
CONSTRUCTION
ERROR
-The vertical spacing of the horizontal steel in
the
bin wall varied and most sections exceeded
the
specifiecd spacing
-Location of horizontal bars were close to
outside
face of the wall, some were near the
center
and some near the inside face
DETAILING ERROR
-The incorrect tie detail shown by the drawing
-Inadequate anchorage of the Main horizontal
steel
detail in the drawing
FAILURES OF CONCRETE SILO WALLS : A CASE STUDY 2
DAMAGED GRAIN ELEVATOR
-CONSTRUCTION PROBLEM Missing Bars
-DESIGN DEFICIENCY Ignoring Over pressure
-USAGE PROBLEM Storing Peas rather than
wheat
as assumed in
design
FAILURES OF CONCRETE SILO WALLS :
A CASE STUDY
3
-CEMENT SILO WITH SERIOUS WALL CRACKS
-REASON Stored Material Over pressure
-REPAIR METHOD Post Tensioning using wire winding
method
FAILURES OF CONCRETE SILO WALLS :
A CASE STUDY
4
COLLAPSED BIN AT AUSTRIA
(WEAT STORAGE) :
-Continuous emptying and
refilling kept the grain constantly
in motion creating high flow
induced overpressure
-Other silos subjected serious
cracks
-Repaired by External wire
winding
FAILURES OF CONCRETE SILO WALLS :
A CASE STUDY
5
SERIOUS DEMAGE OF SILOS
AT UNITED STATES :
×REASON Reinforcement
were inadequate and faulty
construction
REPAIR TECHNOLOGIES :
REPAIR TECHNOLOGIES :
REPAIR SCHEMES :
-Determination of full extent of damage its
effects
on Safety
-Cause of Damage
-Various possible Methods
-Economic Analysis
ALWAYS REMEMBER
“
An effective repair should not merely return the
structure to its original unfailed condition, but
should also
•
Eliminate the cause of the Structure
•
So strengthen the structure that those
causes
no longer pose a threat to it
GANERALIZED METHODS :
×EPOY GROUTING
×SHOTCRETING
×CAST IN SITU LINER CONSTRUCTION
×EXTERNAL PRESTRESSING WIRE WINDING)
EPOXY GROUTING :
×Filling the Cracks with epoxy where there is no
other damage
×Spalled area of walls were reinforcing steel will
exposed, should be patched with grout
SHOTCRITING :
×For patching lifts, fallouts and other holes and
indentations in the silo walls
×It is applied under high pressure
×It sticks very well to a existing surface,
×More dense than regular concrete
STRENGTHN BY LINER :
×It is the construction of concrete wall over existing
wall
at internal or external face of the silos
×It may be
-cast in place concrete
-slip formed
-shotcrete
STRENGTHN BY LINER :
A CASE STUDY :
EXTERNAL REPAIR OF
CRACKED GRAIN SILOS.
ABOUT CASE STUDY :
-The main grain storage :
-60 concrete silos
-Arranged in 3 clusters
-One cluster consists of 20 silos
Configurations :
-Capacity : 3100 tons
-Diameter : 12 m
-Height : 37 m
Draw
off openings are located concentrically at
the base of the each silo.
ABOUT CASE STUDY :
DISTRESS PROBLEM :
3 Years after Construction :
LARGE VERTICAL CRACKS
WERE DISCOVERED.
×Silos are located in a
region
of high rainfall
×Subjected to salt water
spray
from nearby ocean
CRACK PATTERN :
×Cracks were typically 350 mm
apart
×Most cracks were wider than 0 2 mm
×Some cracks were wider than 1 mm
×A few cracks were so wide that
water had penetrated into the silos
REPAIR SCHEME :
-POST TENSIONE THE SILOS WITH EXTERNAL
TENDONS
-CONSTRUCTING NEW RCC WALLS WITHIN THE
EXISTING SILOS
×The repair scheme that was
chosen
involver adding
reinforced
concrete to the
exterior
walls of the silo
clusters
×No repair was made to any
internal
walls as they are
surrounded
by external silo
walls
POINT OF INTEREST :
THE
THE EXTERIOR REPAIR OF
SINGLE SILO IS WELL
KNOWN
THE EXTERNAL JACKETING
OF AN ENTIRE SILO CLUSTER
IS INNOVATIVE
POINT TO BE TAKEN CARE
× With the exterior walls repaired, the WEAK LINK in
the
structure would seem to be the buttress walls
between
exterior silos
×However, the buttress walls have two layers of
reinforcement,
and thus have batter crack control
characteristics
than the original exterior walls which
had
only one layer of reinforcement
×Critical condition when two silos
are unloaded simultaneously.
×During withdrawal, the horizontal
pressure exerted by the grain is
believed to increase by about
60 %
from the static case.
×To prevent overloading, an
operational limitation was
implemented
STRENGTHNING TECHNIQUE
CONSTRUCTION METHODOLOGY
Method of Construction :
SHOTCRITING
×The silos were to be full of grain when the new
concrete
was applied to the outside in order to
reduce
the effect of restrained shrinkage
×The silos had to remain full until the shotcrete
had
cured sufficiently
×An appropriate shotcrete for this repair project is
one that provides
•Required Strength
•Good bond to the original Concrete
•Corrosion protection for new
reinforcement