17 Seismic Retrofitting in Disaster management

Dr. Rajnish Ranjan

 Natural Disasters are serious disruptions caused by environmental impacts of hazards like earthquakes, floods, hurricanes and volcano eruptions . They creates great devastation in terms of life, money and failures of structures.These have a major and long lasting impact on the people and infrastrures of the community long after the immediate effect is over . Earthquake is one of the major natural disasters affecting mankind all over the world which had a huge loss of impact in terms of structural failures. See the below Fig a.1.

In order to overcome this kind of failures in the structural settings of buildings and other infrastructures ,‘Seismic Retrofitting’ technique are used for existing reinforced concrete buildings. Retrofitting proves to be a better economic consideration and immediate shelter to problems rather than replacement of building. In recent years the seismic requirements in building standards have been getting distinctly more rigorous. Because of widespread disregard of these regulations, the question of seismic safety is not only applicable for older buildings but also for newer ones.

SEISMIC RETROFITTING

   It is the modification of existing structures to make them more resistant to seismic activity, ground motion, or soil failure due to earthquakes.

The retrofit techniques are also applicable for other natural hazards such as tropical cyclones, tornadoes, and severe winds from thunderstorms.

1.1 Why it is needed?

• To ensure the public safety and security of a building
• Essential to reduce the hazard and losses from non-structural elements
• Predominantly concerned with structural improvement to reduce seismic hazard
• Important building must be strengthened whose services are assumed to be essential just after an earthquake like hospitals.

    1.2 Assessment of existing buildings

Existing buildings have to be assessed thoroughly for the basic properties and causes of deterioration of masonry, concrete, and steel buildings. To know more about the material properties, various tests can be conducted to check the quality of the materials like destructive and non-destructive load tests. Based on the results of test we can find the quality of the materials used in the construction.

Retrofitting technique is used for the upgradation of lateral and ductility of the structure. It was classified into two types. One is global and the other one is local.

In global method adding of shear wall , infill wall, wing wall, bracing , wall thickening, mass reduction, base isolation, mass dampers are commonly used. But in case of local method jacketing of beams, jacketing of columns, jacketing of beams-columns joints, strengthening of individual footings are done.

2.0 Adding of shear wall

Shear walls can be classified as load-bearing or non-load bearing, depending on whether they carry gravity loads also in addition to lateral load. They can be classified based on the type of masonry and construction used, for example brick or concrete, reinforced and unreinforced. Most commonly used shear walls have rectangular or flanged configuration. This are frequently used for non-ductile concrete frame buildings. The cast-in-place or precast concrete elements are placed only on the exterior of the buildings to overcome shear forces.

Solid shear walls are most efficient so it is highly desirable. Often openings are required in shear walls for functional necessity in doors and windows. The portion of a shear wall between two adjacent openings is called a pier, whereas the segment of shear wall above the adjacent openings is called a spandrel or a beam.

Shear walls meeting each other at right angles result in flanged configurations and are referred to as flanged walls. Such walls are normally required to resist earthquake forces in both principal directions of the building.

    Fig a.2 shows the shear wall aligned from bottom to top of the building

2.1 Adding steel braces

A braced frame is a structural system commonly used in structures subject to lateral loads. The addition of a bracing frame increases the structural stability against lateral loads such as wind loading and seismic pressure. The members in a braced frame are generally made of structural steel which can work effectively both in tension and compression.

The beams and columns that form the frame, carry vertical loads and the bracing system carries the lateral loads. Braced frames reduce lateral displacement, as well as the bending moments in columns, they are economical, easily erected and have the design flexibility to create the strength and stiffness required

The resistance to horizontal forces is provided by two bracing systems.

2.2.1 Vertical bracing

Bracing between column lines (in vertical planes) provides load paths for the transference of horizontal forces to ground level. Framed buildings required at least three planes of vertical bracing to brace both directions in plan and to resist torsion about a vertical axis.

2.2.2 Horizontal bracing

The bracing at each floor level (in horizontal planes) provides load paths for the transference of horizontal forces to the planes of vertical bracing. Horizontal bracing is needed at each floor level, however, the floor system itself may provide sufficient resistance. Roofs may require bracing.

Types of bracing

•  Single diagonals
• Cross-bracing
• K- bracing
• V- bracing
• Eccentric bracing

   2.a. Single diagonals

Trussing, or triangulation is formed by inserting diagonal structural members into rectangular areas of a structural frame, helping to stabilise the frame. If a single brace is used, it must be sufficiently resistant to tension and compression

2.b. Cross bracing

Cross-bracing (or X-bracing) uses two diagonal members crossing each other. These only need to be resistant to tension, one brace acting to resist sideways forces at a time depending on the direction of loading. As a result, steel cables can also be used for cross-bracing.

    However, this provides the least available space within the facade for openings and results in the greatest bending in floor beams

2.c. K- Bracing

Braces connect to the columns at mid-height. This frame has more flexibility for the provision of openings and results in the least bending in floor beams. K-bracing is generally discouraged in seismic regions because of the potential for column failure if the compression brace buckles.

2.d. V-bracing

This involves two diagonal members extending from the top two corners of a horizontal member and meeting at a centre point at the lower horizontal member, in the shape of a V. Inverted V- bracing (also known as chevron bracing) involves the two members meeting at a centre point on the upper horizontal member. Both mean that the buckling capacity of the compression brace is likely to be significantly less than the tension yield capacity of the tension brace. This can mean that when the braces reach their resistance capacity, the load must instead be resisted in the bending of the horizontal member.

     2.e. Eccentric bracing

This is commonly used in seismic regions and allows for doorways and corridors in the braced bays. It is similar to V-bracing but instead of the bracing members meeting at a centre point there is space between them at the top connection. Bracing members connect to separate points on the beam or girder. This is so that the ‘link’ between the bracing members absorbs energy from seismic activity through plastic deformation. Eccentric single diagonals can also be used to brace a frame.

    3.0 Jacketing

Jacketing is the most popularly used method for strengthening of building columns. The most common types of jackets are steel jacket, reinforced concrete jacket, fibre reinforced polymer composite jacket, jacket with high tension materials like carbon fibre, glass fibre etc.

The main purposes of jacketing are –

1.  To increase concrete confinement by transverse fibre reinforcement, especially for circular cross-sectional columns.

2.  To increase shear strength by transverse fibre reinforcement.

3.  To increase flexural strength by longitudinal fibre reinforcement provided.

The main objective of jacketing is to increase the seismic capacity of the moment resisting framed structures.Jacketing serves to improve the lateral strength and ductility by confinement of compression concrete. It should be noted that retrofitting of a few members with jacketing or some other enclosing techniques might not be effective enough to improve the overall behaviour of the structure, if the remaining members are not ductile.

3.0.1 Column jacketing

Strengthening of reinforced concrete columns is needed when the load carried by the column is increased due to either increasing the number of floors or due to mistakes in the design.The compressive strength of the concrete or the percent and type of reinforcement are not according to the codes’ requirements.The inclination of the column is more than the allowable.The settlement in the foundation is more than the allowable.

There are two major techniques for strengthening reinforced concrete columns. One is column jacketing and the other one is steel jacketing.

The size of the jacket and the number and diameter of the steel bars used in the jacketing process depend on the structural analysis that was made to the column. In some cases, before this technique is carried out, we need to reduce or even eliminate temporarily the loads applied to the column; this is done by the following steps-

•  Putting mechanical jacks between floors
• Putting additional props between floors

    Moreover in some cases where corrosion in the reinforcement steel bars was found, the following steps should be carried out-

• Remove the concrete cover
• Clean the steel bars using a wire brush or sand compressor
• Coat the steel bars with an epoxy material that would prevent corrosion
• If there was no need for the previous steps, the jacketing process could start by the following steps:

1. Adding steel connectors into the existing column in order to fasten the new stirrups of the jacket in both the vertical and horizontal directions at spaces not more than 50 cm. Those connectors are added into the column by making holes 3-4 mm larger than the diameter of the used steel connectors and 10-15 cm depth.

2. Filling the holes with an appropriate epoxy material then inserting the connectors into the holes.

3.  Adding vertical steel connectors to fasten the vertical steel bars of the jacket following the same procedure in step 1 and 2

4. Installing the new vertical steel bars and stirrups of the jacket according to the designed dimensions and diameters

5. Coating the existing column with an appropriate epoxy material that would guarantee the bond between the old and new concrete.

6. Pouring the concrete of the jacket before the epoxy material dries. The concrete used should be of low shrinkage and consists of small aggregates, sand, cement and additional materials to prevent shrinkage.

    Column jacketing

3.0.2 Steel jacketing

The technique is chosen when the loads applied to the column will be increased, and at the same time, increasing the cross sectional area of the column is not permitted

This technique is implemented by the following steps

1. Removing the concrete cover

2. Cleaning the reinforcement steel bars using a wire brush or a sand compressor.

3. Coating the steel bars with an epoxy material that would prevent corrosion.

4. Installing the steel jacket with the required size and thickness

5. According to the design and making openings to pour through them the epoxy material that would guarantee the needed bond between the concrete column and the steel jacket.

6. Filling the space between the concrete column and the steel jacket with an appropriate epoxy material.

4.0 Base Isolation

Base Isolation is a collection of structural elements of a building that should substantially decouple the building’s structure from the shaking ground thus protecting the building’s integrity and enhancing its seismic performance.

This earthquake engineering technology, which is a kind of seismic vibration control, can be applied both to a newly designed building and to seismic upgrading of existing structures. Normally, excavations are made around the building and the building is separated from the foundations. Steel or reinforced concrete beams replace the connections to the foundations, while under these, the isolating pads, or base isolators, replace the material removed. While the base isolation tends to restrict transmission of the ground motion to the building, it also keeps the building positioned properly over the foundation. Careful attention to detail is required where the building interfaces with the ground, especially at entrances, stairways and ramps, to ensure sufficient relative motion of those structural elements

This technology can be used for both new structural design and seismic retrofit. In process of seismic retrofit some of the most prominent U.S. monuments, e.g. Pasadena city Hall San Francisco city hall LA City Hall were mounted on base isolation systems.

Base isolation is also used on a smaller scale—sometimes down to a single room in a building. Isolated raised-floor systems are used to safeguard essential equipment against earthquakes. The technique has been incorporated to protect statues and other works of art

Base isolation system

   Advantages of Base Isolation

• Isolates Building from ground motion – Lesser seismic loads, hence lesser damage to the structure, -Minimal repair of superstructure.
• Building can remain serviceable throughout construction.
• Does not involve major intrusion upon existing superstructure

   Disadvantages of Base Isolation

• Expensive
• Cannot be applied partially to structures unlike other retrofitting
• Challenging to implement in an efficient manner

   5.0 Mass isolation technique

Ideally it is impossible to isolate the whole mass of the building from the lateral stiffness of the structure. However, there are some practical means to be able to isolate a large amount of the mass of the system. A simple approach in mass isolation is to vertically isolate the mass of the system from the lateral stiffness of the structure

Application of mass isolation techniques brings some phenomenal changes in the structural characteristics of the building. The dominant distinction between mass isolation and other techniques is in its indirect method of isolation. Mass isolation transfers a large amount of the mass of the structure to the zone of lower earthquake forces without using a sophisticated isolation layer carrying the whole weight of the building. Therefore some of the difficulties associated with design and implementation of isolation layers (compromising on the lateral and vertical stiffness as well as damping characteristics and cost of the system) can be easily resolved.

The second important feature of the mass isolation is the mode shapes of the isolated structures. Depending on the frequency content of the earthquakes, a number of different structural mode shapes contribute in defining the response of the structure. If structural system consists of a well separated mass and stiffness subsystems, modal shapes of these subsystems will be in different range of the frequencies. This phenomenon causes the subsystems to have large relative movements with respect to each other. A damping mechanism located between mass and stiffness subsystems triggers the interaction between these modal shapes. The interaction causes the mass subsystem to receive large reaction forces from the stiffness subsystem, consequently the level of force and deformation on this part of the structure will be substantially reduced. This reaction force possesses a frequency content different from the frequency range of the stiffness subsystem, therefore it cannot provoke a strong dynamic excitation on this subsystem (they can be considered simply as variable static forces).

Mass isolation also has an effective means to increase damping characteristics of the system. The problem of damping mechanism in ordinary structures is the fact that inter-storey relative displacement and velocity in the structural systems is quite small. Therefore, sophisticated damping devices are required to be able to dissipate earthquake-input energy effectively. However, in mass isolation damping devices are subjected to relative movement of the mass and stiffness subsystems, which can be large in certain locations across the height of the structure.

Another feature of this technique is its potential for deformation control of the structure. Since the mass subsystem relies on the stiffness subsystem for resisting its lateral forces, the stiffness sub-system can also control its lateral deformations such as inter-story drift. Deformation control would be possible if the gap between mass and stiffness subsystem in each connection point is furnishedwith a clamping mechanism based on displacement, velocity or acceleration triggers to restrict the structural system from undesirable movements.

In mass isolation the required strength in the stiffness subsystem is not necessarily proportional to the stiffness requirement in this system. The demand for higher stiffness in this part of the structure is necessary to clearly separate the behaviour of the mass and stiffness subsystems but higher stiffness does not indispensably absorb higher loads into the system. This is one of the notable features in this technique and leads to lower structural cost for stiffness subsystem (provides the possibility for using inferior constructional materials, smaller cross sections, smaller footings, etc.).

The constructional cost of the mass subsystem can also be reduced because stiffness of this part of the structure should be in its lowest possible level (defined by the stability requirements in this subsystem) to elongate its natural periods and increase the effect of mass isolation. Deduction of the constructional cost of the mass subsystem would be the results of using smaller cross sections and deployment of simpler types of structural connections (due to reduction in the required rigidity in the joints).

Retrofit of the structures can be considered as one of the potential applications of the mass isolation technique. By separating the lateral load mechanism of the existing structure (e.g. bracing) from the remaining parts of the system (through damping devices) or by adding a new lateral resisting system to the building, it is possible to improve earthquake resistance of the existing structures. Compare to the other retrofitting options and depending on the building, this technique might be justifiable in the tall structures.

  Mass reduction

    6.0 Wall thickening technique

The existing walls of a building are added certain thickness by adding bricks, concrete and steel aligned at certain places as reinforcement, such that the weight of wall increases and it can bear more vertical and horizontal loads, and also its designed under special conditions that the transverse loads does not cause sudden failure of the wall.

  Conclusion

Briefly described about the traditional, conventional and innovative methods of seismic retrofitting techniques. It is a suitable technology for protection of a variety of structures.

Discussed about various techniques to be used for repair and seismic strengthening of damaged buildings during earthquakes.

It was concluded that using mass isolation technique, the main part of the mass of the system can be shifted to the low force and energy zone of the earthquake spectrum.

Reference

• Is 13935: 1993 “Indian standard code for Repair and Seismic Strengthening of Buildings-Guidelines”, Bureau of Indian Standards.
• Hand book on Seismic Retrofit of Buildings- CPWD & Indian building congress
• IS 13935:2009 Seismic Evaluation, Repair and Strengthening of Masonry Buildings- Guidelines
• IS 1893: 2003, Criteria for Earthquake Resistance Design of Structures,”-part 1 General provisions and buildings, Bureau of Indian Standards mwikipedia.org/wiki/Seismic_retrofit
• Alocer S.M. and Jisra J.O. (1993) “Strength of Reinforced Concrete frame Connections Rehabilitated by Jacketing”
• 1893 (Part 1): 2002 Criteria for earthquake design of 2002 structures: Part 1 General Provisions And buildings