RETROFITTING TECHNIQUES OF RC FRAME BUILDINGS

Similar to masonry buildings various methodologies are available for analysis and retrofitting of RC frame building structures. The proposed retrofitting schemes are based on predicted behavior of this class of buildings which is based on observed behavior in the past earthquakes. However, these buildings could be brought to seismic safety level recommended by various Building Codes within economic limits. The economically viable option with less intervention would be more desirable though various other intervention options are available worldwide.

The following are the major types of problems observed during earthquakes in RCC frame buildings:

•                absence of ties in beam column joints

•                inadequate confinement near beam column joint

•                inadequate lap length and anchorage and splice at inappropriate position

•                low concrete strength

•                improperly anchored ties (90^o hooks)

•                inadequate lateral stiffness

•                inadequate lateral strength

•                irregularities in plan and elevation

•                irregular distribution of loads and structural elements

•                other common structural deficiencies such as soft storey effect, short column effect, strong beam-weak column connections etc.

Earthquake resistance in RC frame buildings can be enhanced either by

1)      Increasing seismic capacity of the building

This is a conventional approach to seismic retrofitting which increase the lateral force resistance of the building structure by increasing stiffness, strength and ductility and reducing irregularities. This can be done by two ways

i)        Strengthening of original structural members

These include strengthening of o Columns (reinforced concrete jacketing, steel profile jacketing, steel encasement, fiber wrap overlays)

a)       Beams (reinforced concrete jacketing, steel plate reinforcement, fiber-wrap overlays)

b)      Beam Column joint (reinforced concrete jacketing, steel plate reinforcement, fiber wrap overlays)

c)       shear wall (increase of wall thickness)

d)      Slab (increase of slab thickness, improving slab to wall connection) o Infilled partition wall (reinforce infilled walls and anchor them into the surrounding concrete frame members). 

ii)      Introduction of New structural elements  

The lateral force capacity of an existing structure may be increased by adding new structural elements to resist part or all of the seismic forces of the structure, leaving the old structure to resist only that part of the seismic action for which it is judge reliable. Newly added structural elements may be

a)       shear walls in a frame or skeleton structure

b)      Infilled walls (reinforced concrete or masonry located in the plane of existing columns and beams)

c)       wing walls (adding wall segments or wings on each side of an existing column) additional frames in a frame or skeleton structure

d)      trusses and diagonal bracing (steel or reinforced concrete) in a frame or skeleton structure

Establishing sound bond between the old and new concrete is of great importance. It can be provided by chipping away the concrete cover of the original member and roughening its surface, by preparing the surfaces with glues (for instances, with epoxy prior to concreting), by additional welding of bend reinforcement bars or by formation of reinforced concrete or steel dowels.

Perfect confinement by close, adequate and appropriately shaped stirrups and ties contributes to the improvement of the ductility of the strengthening members. Detailed consideration of                 the possibility of significant redistribution of the internal forces in the structures due to member stiffness changes is very important.

2)      Reducing seismic response of the building

Increasing damping in the building by means of energy dissipation devices, reducing mass, or isolating the building from the ground enhance the seismic structural response. A more recent approach includes the use of base isolation and supplemental damping devices in the building. These emerging technologies can be used to retrofit existing RC frame structures; however, their high cost and the sophisticated expertise required to design and implement such projects represent impediments for broader application at recent time. Seismic strengthening measures identified for one RC frame building may not be relevant for another. Retrofit solutions have to be determined building wise. Most of these retrofit techniques have evolved in viable upgrades. However, issues of costs, invasiveness, and practical implementation still remain the most challenging aspects of these solutions. In the past decade, an increased interest in the use of advanced nonmetallic materials or Fiber Reinforced Polymers, FRP has been observed.  

The following retrofit strategies for RC buildings are widely used after recent earthquakes in several places:

3.2.1 Jacketing

Jacketing of existing structural members may be of reinforced concrete, steel case or carbon fiber reinforced polymer (CFRP).

3.2.1.a Reinforced Concrete Jacketing

This method involves addition of a layer of concrete, longitudinal bars and closely spaced ties on existing structural elements. The jacket increases both the flexural strength and shear strength of the column and beam. It helps to basket the member, hence improve its shear strength and ductility. This method also improves integrity and deformability. Main improvements in different structural elements of the building by this method are as follows:

Columns: The jacketing not only increases the flexural strength and shear strength of the column but also increases its ductility. The thickness of the jacket also gives additional stiffness to the concrete column. Since the thickness of the jacket is small, casting self-compacting concrete or the use of short Crete are preferred to conventional concrete. During retrofitting, it is preferred to relieve the columns of the existing gravity loads as much as possible, by propping the supported beams.

Beams: Beams are retrofitted to increase their positive flexural strength, shear strength and the deformation capacity near the beam-column joints. The lack of adequate bottom bars and their anchorage at the joints needs to be addressed. Usually the negative flexural capacity is not enhanced since the retrofitting should not make the beams stronger than the supporting columns. The strengthening involves the placement of longitudinal bars and closely spaced stirrups.

3.2.1.b Steel Profile Jacketing

Steel profile jacketing refers to encasing frame elements with steel plates and filling the gap with non-shrink grout. This is generally used for improving ductility and shear strength and it provides confinement to structural element.

Columns: Steel profile jacketing of column consists of four longitudinal angles profiles placed one at each corner of the existing reinforced concrete column and connected together in a skeleton with transverse steel straps. They are welded to the angle profiles. The angle profile size should be no less than 50x50X5 mm. Caps and voids between the angle profiles and the surface of the existing column must be filled with non-shrinking cement grout or resin grout. A covering with concrete or shotcrete reinforced with welded fabrics is efficient for corrosion or fire protection. In general, an improvement of the ductile behavior and an increase of the axial load capacity of the strengthened column is achieved. However, the stiffness remains relatively unchanged. If the plates are carried continuous to the floor slab, steel jacketing also improves flexural strength of the strengthened member, though not extensively.

Beams: Steel plate reinforcement is a new technique which can be used for beams subject primarily to static loading to improve their shear strength or mid-span flexural strength. The steel external plates are attached to concrete surfaces of the reinforced concrete members by gluing with epoxy resin. During the epoxy hardening, the steel plates must be clamped to the concrete member. It is recommended that the steel plates also be anchored by either nails shot into the concrete or anchor bolts. Special attention must be paid to corrosion or fire, especially considering the total loss of epoxy resin strength at temperature higher than 250^o C. This procedure is not recommended for beams subject to cyclic loading due to earthquake forces.

3.2.1.c CFRP Jacketing (Fiber Reinforced Polymer)

Seismic resistance of frame buildings can be improved significantly by using Fiber Reinforced Polymer overlays on RC elements of the building. Strengthening with FRP is a new approach. FRP is light weight, high tensile strength material and has a major advantage of fast implementation. This method could be effectively used to increase strength and stiffness of RC frames. The effectiveness is strongly dependent on the extent of anchorage between the FRP strips and the frame.

3.2.2 Addition of Reinforced Concrete Shear Walls

Adding shear walls is one of the most popular and economical methods to achieve seismic protection. Their purpose is to give additional strength and stiffness to the building and could be added to existing and new buildings. They are positioned after careful planning and judgment by the structural engineer as to how they would affect the seismic forces in a particular building. However, it is desired to ensure an effective connection between the new and existing structure.

3.2.3 Bracing

In this method diagonal braces are provided in the bays of the building. Diagonals stretch across the bay to form triangulated vertical frame and as triangles are able to handle stresses better than a rectangular frame the structure is also supposed to perform better. Braces can be configured as diagonals, X or even V shaped. Braces are of two types, concentric and eccentric. Concentric braces connect at the intersection of beams and columns whereas eccentric braces connect to the beam at some distance away from the beam-column intersection. Eccentric braces have the advantage that in case of buckling the buckled brace does not damage beam- column joint. The steel bracings secure the view, natural light and ventilation allowing retrofitting without removing the openings at the periphery of the building that made the structure more vulnerable to earthquakes.  The steel bracings are installed to limit the displacement as well as improve the strength and rigidity.