maintenance and repair of building

1. MAINTENANCE AND REPAIR OF
BUILDINGS
Shear Strengthening Of Structural Members
Name - Ashutosh Maurya
Roll no - 101501009
3rd year Civil Engineering Student
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2. Contents
1. Introduction
2. Need of Structural Strengthening for Structures
3. Factors affecting selection of strengthening method
4. Methods of Shear Strengthening Technique
5. Conclusion
6. Reference​s
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3. Introduction
Many buildings that originally were constructed for a specific use now are being
renovated or upgraded for a different application that may require higher
load-carrying capacity. As a result of these higher load demands, existing
structures need to be reassessed and may require strengthening to meet heavier
load requirements. As any building or structure aged they start deteriorate and
become obsolete due to inadequate strength, behavior or stability.
In general, structural strengthening may become necessary because of deficiencies
that develop because of environmental effects (such as corrosion), changes in use
that increase service loads, or deficiencies within the structure caused by errors in
design or construction.
A reinforced concrete element is normally designed to develop its full flexural
strength to ensure a ductile flexural failure mode under excessive loading. Shear
failure is a brittle type of failure, which is more dangerous and less predictable.
Shear failure is likely to occur suddenly with little advanced warning, while a
flexural failure occurs gradually with large deflections and cracking thus giving
ample warning. In practice both flexural and shear failure modes may occur, and
structural elements may require strengthening for both modes.
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4. Need of Structural Strengthening for Structures
Concrete structures need to be strengthened for any of the following reasons
1. Load increases due to higher live loads, increased wheel loads,
installations of heavy machinery, or vibrations.
2. Damage to structural parts due to aging of construction materials or
fire damage, corrosion of steel reinforcement, and/or impact of
vehicles.
3. Improvements in suitability for use due to limitation of deflections,
reduction of stress in steel reinforcement and/or reduction of crack
widths.
4. Modification of structural system due to elimination of walls/columns
and/or openings cut through slabs.
5. Errors in planning or construction due to insufficient design dimensions
and/or insufficient reinforcing steel.
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5. Factors Affecting Selection of Strengthening Method
There are many factors which govern during selecting material and methods for
strengthening, some of them are following-
1. Environmental conditions- ​When we are using adhesives for
strengthening, then it might be unsuitable for applications in high temperature
environments.
When we are using external steel for strengthening, it may not be suitable in
corrosive environments.
2. Size of project- ​methods involving special materials and methods may be
less cost-effective on small projects
3. In-place concrete strength and substrate integrity- ​ the effectiveness of
methods relying on bond to the existing concrete can be significantly limited
by low concrete strength.
4. Dimensional/clearance constraints- ​Section enlargement might be limited
by the degree to which the enlargement can encroach on surrounding clear
space.
5. Operational constraints- ​Methods requiring longer construction time might
be less desirable for applications in which building operations must be shut
down during construction. Construction cost, maintenance costs, and
life-cycle costs also affect.
6. Availability of materials, equipment, and qualified contractors.
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6. Methods of Shear Strengthening Technique
Shear strengthening is required for structural members when the shear stress
applied is higher than the corresponding design shear resistance.
V​sd​ ≤ ​V​rd
For strengthening there is a lot of method are available which can be applied based
on requirement. For example-
1. External steel reinforcement
2. Section enlargement
3. Internal steel and FRP reinforcement
4. External and internal post Tensioning
5. Supplemental members
6. FRP plates or strips
7. Near-surface-mounted reinforcement
External steel reinforcement- ​The shear capacity of concrete members, such
as columns, beams, slabs, and shear walls, can be increased by attaching steel
plates to the concrete surface with epoxy bonding, mechanical anchorage, or both.
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7. Section enlargement​- ​This method of strengthening involves placing additional
"bonded" reinforced concrete to an existing structural member in the form of an
overlay or a jacket. With section enlargement, columns, beams, slabs, and walls
can be enlarged to increase their load-carrying capacity or stiffness. For example,
columns can be strengthened by using jackets, beams by increasing the section,
shear walls by increasing the wall thickness.
Internal steel and FRP reinforcement - ​Additional steel or fiber-reinforced
polymer (FRP) reinforcement can be installed by drilling holes, inserting steel or
FRP dowels, and grouting . Several grouting materials have been used successfully
to bond the dowels to the concrete.
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8. External and internal post Tensioning - ​The post-tensioning technique has
been used effectively to increase the flexural and shear capacity of both reinforced
and prestressed concrete members. The post-tensioning forces are delivered by
standard pre-stressing tendons or high-strength steel rods. Usually located outside
the original section.
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9. Supplemental members- ​ Supplemental structural members, such as posts or
beams, can be added to reduce shear stress. When architectural constraints allow,
this can often be a cost-effective and practical solution.
FRP plates or strips - ​FRP materials may be used as either externally bonded
plates or strips. The advantage of using strips is that they do not entrap moisture in
the structure and more closely resemble conventional shear reinforcement. The
advantage of FRP material is light in weight- easy to install, High strength- 5 times
of steel, Corrosion resistant- durable structure and highly versatile- it suit for any
project.
Polymer material which can be used with structural fiber - fiberglass(GFRP),
Carbon fiber(CFRP), Aramid fiber. The FRP is typically bonded to concrete with a
structural grade epoxy. FRP can be wrapped around the member by many
mechanism, for example complete wrapping, U-wrap, side bonding.
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10. Near-surface-mounted reinforcement (NSMR)- ​It involves inserting steel
or FRP rods or reinforcing bars into grooves cut into the surface of the concrete
section. The grooves are typically not deeper than the existing reinforcement cover
dimension. The grooves are then filled with a polymer or grout to provide
compatibility (that is, load transfer) of the reinforcement with the concrete. The
advantages of this method are easy access, limited chance of disturbing existing
reinforcing bar as compared with internal steel reinforcement, and increased
protection and bonded surface area of the reinforcement as compared with the use
of external shear reinforcement.
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11. CONCLUSION
Strengthening of reinforced concrete structures is one of the most important tasks
normally associated with the maintenance of concrete structures. There are a
variety of materials and methods available to increase shear capacity, including the
use of external steel reinforcement, section enlargement, internal steel or FRP
reinforcement, supplemental members, FRP plates and strips, both steel and FRP
NSMR, and external prestressing. Depending on the type of application, crack,
crack stitching and epoxy injection could be considered shear strengthening
measures, although these methods would typically be described as repairs rather
than strengthening.
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12. References
1. https://theconstructor.org/structures/strengthening-concrete-structu
res/1576/
2. https://mycourses.aalto.fi/pluginfile.php/198600/mod_resource/con
tent/2/2016-03-14_Strengthing.pdf
3. Peter H. Emmons/Concrete Repair and Maintenance
4. ACI 364.2T-08/​Increasing shear capacity within existing
reinforced concrete structures
5. https://csengineermag.com/article/keys-to-success-structural-repair
-and-strengthening-techniques-for-concrete-facilities/
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