FAILURES AND  REPAIR OF   SILOS AND BUNKERS

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FAILURES AND  REPAIR OF   SILOS AND BUNKERS

 

 

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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

 

 

 

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