Seismic Retrofitting Techniques for Concrete Structures

Seismic Retrofitting Techniques for Concrete Structures:

Seismic Retrofitting Techniques are required for concrete constructions which are vulnerable to damage and failures by seismic forces. In the past thirty years, moderate to severe earthquakes occurs around the world every year. Such events lead to damage to the concrete structures as well as failures.

Thus the aim is to Focus on a few specific procedures which may improve the practice for the evaluation of seismic vulnerability of existing reinforced concrete buildings of more importance and for their seismic retrofitting by means of various innovative techniques such as base isolation and mass reduction.

So Seismic Retrofitting is a collection of mitigation technique for Earthquake engineering. It is of utmost importance for historic monuments, areas prone to severe earthquakes and tall or expensive structures.

Keywords: Retrofitting, Base Isolation, Retrofitting Techniques, Jacketing, Earthquake Resistance

1. Introduction to Seismic Retrofitting Techniques:

• Earthquake creates great devastation in terms of life, money and failures of structures.
• Upgrading of certain building systems (existing structures) to make them more resistant to seismic activity (earthquake resistance) is really of more importance.
• Structures can be (a) Earthquake damaged, (b) Earthquake vulnerable
• Retrofitting proves to be a better economic consideration and immediate shelter to problems rather than replacement of building.

1.1 Seismic Retrofitting of Concrete Structures:

Definition:

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.

1.2 Need for Seismic Retrofitting:

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

1.3 Problems faced by Structural Engineers are:

Lack of standards for retrofitting methods – Effectiveness of each methods varies a lot depending upon parameters like type of structures, material condition, amount of damage etc.,

1.4 Basic Concept of Retrofitting:

The aim is at:

• Upgradation of lateral strength of the structure
• Increase in the ductility of the structure
• Increase in strength and ductility

2. Classification of Retrofitting Techniques:

Fig 1: Retrofitting Techniques for Reinforced Concrete Structures

2.1 Adding New Shear Walls:

• Frequently used for retrofitting of non ductile reinforced concrete frame buildings.
• The added elements can be either cast?in?place or precast concrete elements.
• New elements preferably be placed at the exterior of the building.
• Not preferred in the interior of the structure to avoid interior mouldings.

Fig 2: Additional Shear Wall

2.2 Adding Steel Bracings

• An effective solution when large openings are required.
• Potential advantages due to higher strength and stiffness, opening for natural light can be provided, amount of work is less since foundation cost may be minimized and adds much less weight to the existing structure.

Adding STEEL Bracings:

Fig 3: RC Building retrofitted by steel bracing

2.3 Jacketing (Local Retrofitting Technique):

This is the most popular method for strengthening of building columns.

Types of Jacketing:

1. 1.Steel jacket,
2. Reinforced Concrete jacket,
3. Fibre Reinforced Polymer Composite (FRPC) jacket

Purpose for jacketing:

• To increase concrete confinement
• To increase shear strength
• To increase flexural strength

Fig 4: Column Jacketing

Fig 5: Beam Jacketing

2.4 Base Isolation (or Seismic Isolation):

Isolation of superstructure from the foundation is known as base isolation. It is the most powerful tool for passive structural vibration control technique.

Fig 6: Base Isolated Structures (a)Model Under Test, (b) Diagrammatical Representation

2.4.1 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

2.4.2 Disadvantages of Base Isolation

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

2.5 Mass Reduction Technique of Retrofitting:

This may be achieved, for instance, by removal of one or more storey’s as shown in Figure. In this case it is evident that the removal of the mass will lead to a decrease in the period, which will lead to an increase in the required strength.

Fig 7: Seismic Retrofitting by Mass reduction (removal of Storey)

2.6 Wall Thickening Technique of Retrofitting:

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.

3. Indian Standard Codes for Earthquake Design of Structures:

• IS: 1893-2002 (part-1) Criteria for Earthquake Resistant Design of Structures (Part 1 : General Provision and Buildings) – Code of Practice
• IS: 4326-1993 Earthquake Resistant Design and Construction of Buildings – Code of Practice
• IS: 13920-1993 Ductile Detailing of Reinforced Concrete Structures subjected to Seismic Forces – Code of Practice
• IS: 13935-1993 Repair and Seismic Strengthening of Buildings – Guidelines
• IS: 13828-1993 Improving Earthquake Resistance of Low Strength Masonry Buildings – Guidelines
• IS: 13827-1993 Improving Earthquake Resistance of Earthen Buildings – Guidelines

4. Conclusion – Seismic Retrofitting Techniques for concrete structures:

• Seismic Retrofitting is a suitable technology for protection of a variety of structures.
• It has matured in the recent years to a highly reliable technology.
• But, the expertise needed is not available in the basic level.
• The main challenge is to achieve a desired performance level at a minimum cost, which can be achieved through a detailed nonlinear analysis.
• Optimization techniques are needed to know the most efficient retrofit for a particular structure.
• Proper Design Codes are needed to be published as code of practice for professionals related to this field.

5. References:

• Agarwal, P. and Shrikhande, M., 2006, Earthquake Resistant Design of Structures, 2nd Edition, Prentice-Hall of India Private Limited, New Delhi.
• Cardone, D. and Dolce, M., 2003, Seismic Protection of Light Secondary Systems through Different Base Isolation Systems, Journal of EarthquakeEngineering, 7 (2), 223-250.
• Constantinou, M.C., Symans, M.D., Tsopelas, P., and Taylor, D.P., 1993, Fluid Viscous Dampers in Applications of Seismic Energy Dissipation and Seismic Isolation, ATC-17-1, Applied Technology Council, San Francisco.
• EERI, 1999, Lessons Learnt Over Time – Learning from Earthquakes Series: Volume II Innovative Recovery in India, Earthquake Engineering
• Research Institute, Oakland (CA), USA.Murty, C.V.R., 2004, IITK-BMTPC Earthquake Tip, New Delhi.

Article By: SHAIK NASREEN, M.Tech Structural Engineering

Performance and Behavior of Masonry Structures during Earthquakes

Masonry structures are most vulnerable during earthquake. Performance and behavior of masonry structures during earthquakes is discussed in this article.

Many human fatalities have depended on masonry constructions from the past. The condition is same at the present. As the main problem concerned is earthquakes, it is important to improve the seismic behavior of masonry buildings.

The most common materials used for the construction of masonry buildings are brick and hollow concrete block. The types of materials used for construction of masonry buildings are:

• Brick: It is a clay that is fired to a hard consistency.
• Hollow concrete block: Known as “cinder block.”
• Hollow clay tile: Concrete block shaped with hollow cells, but brick-color.
• Stone: Used in its natural shape, “dressed” or cut into rectangular blocks
• Adobe: Formed by pouring mud into the form of walls or made of sun-dried bricks.

Influence of Material Properties on Behavior of Masonry Structures during Earthquakes

The behavior of masonry structures during earthquake depend on the properties of its materials like mortar and masonry units. The properties of these materials vary due to variation in raw materials and construction methods, which in turn depends on the source of the resources.

Burnt clay bricks are most commonly used for construction of masonry building. These are naturally porous and they absorb water. Excessive porosity is harmful to good masonry behavior because bricks absorb water from the adjoining mortar. This results in a poor bonding between brick and mortar causing difficulty in positioning masonry units.

To avoid this problem, bricks with low porosity are to be used, and they must be soaked in water before. This would minimize the amount of water drawn away from the mortar.

Various mortars are used in building construction, e.g., mud, cement-sand, or cement-sand-lime. Among these mud, mortar is the weakest. Mud mortar crushes easily when dry, flows outward and have very low earthquake resistance.

Cement-sand mortar with lime is the most suitable. This mortar mix provides excellent workability for laying bricks, stretches without crumbling at low earthquake shaking, and bonds well with bricks.

The earthquake resistance of masonry walls depends on the relative strengths of brick and mortar. Bricks must be stronger than mortar. Excessive thickness of mortar is not desirable.

Behavior of Masonry Structures during Earthquakes

The ground motion or ground vibrations due to earthquakes results in higher amount of inertia forces at the floor or at the location of the mass of the whole building. A building will remain safe, if the forces emerged finds a path to transfer into the ground, without any obstruction which in turn minimizes the damage or collapse.

Among the elements that involve in transferring these forces i.e. roof, wall and foundation, it is seen that walls are the one found most vulnerable to the damage (by the horizontal forces emerged due to the earthquake forces).

We will assume two possibilities in direction of horizontal forces acting on a masonry wall. Let the initial condition be the force which is acting horizontally at the top, which is in a direction perpendicular to its plane, as shown in figure.1. below

Fig.1: The wall is Pushed perpendicular to the plane of the wall

This direction is considered as ‘weak’, as the wall undergo toppling or a form of overturning.

The second possibility is that the wall being pushed in the same plane, and the result is shown in figure.2. This is considered as the strong direction because it offers greater resistance when pushed along its length.

Fig.2: The wall is Pulled in the plane of the wall

It is not always the case that only a single possibility can occur. The ground can shake simultaneously in horizontal as well as vertical directions. Hence both the possibilities have a chance to occur.

Horizontal inertia forces evolved because of ground motion are the most damaging response of normal masonry buildings. The transfer of the forces can take place from the roofs then to the walls. This transfer of horizontal forces can take place either in weak or stronger direction.

Fig.3: Walls A is considered to be loaded in the strong direction and the Walls B (loaded in weak direction). The wall B undergoes toppling.

Remedy to Prevent Damage of Masonry Structures during Earthquakes

When a measure to tie up the walls together like a box is not undertaken, there arise chances of the toppling of walls that are loaded in the weaker direction. So, the remedy for this problem is to join the walls together which will ensure good seismic performance.

This procedure would help the walls loaded in a weaker direction to seek the lateral resistance that is offered by the walls that are loaded in the stronger direction. As each form separate built elements, a rigidity in totality must be bought to ensure resistance as a single unit. So, to enable this, walls to have to be connected to the roof and the foundation.

Fig.4: Wall B properly connected to Wall A

Wall units made up of masonry behave like slender units because of their small thickness compared to their height and length. The simplest way of making these masonry wall units to behave appreciably during earthquake motion is by letting them act together like a box as mentioned before, along with the roof at the top and with the foundation at the bottom.

Box Action of Masonry Building to Prevent Earthquake Damage

The formation of box action as put forward requires several construction aspects. This can be ensured by undergoing following features of ensuring good connections between the walls. This can be achieved by:

• Ensuring good interlocking of the masonry courses at the junctions
• Employing horizontal bands at various levels, particularly at the lintel level.
• Smaller the openings, larger is the resistance offered by the wall.

The tendency of a wall to topple when pushed in the weak direction can be reduced by limiting its length-to-thickness and height-thickness ratios as shown in fig.5. Hence it is recommended to keep the sizes of door and window openings small.

Fig.5: Vulnerability of slender Masonry Walls

Design codes specify limits for these ratios. A wall that is too tall or too long in comparison to its thickness, is particularly vulnerable to shaking in its weak direction.

EARTHQUAKES EFFECTS ON REINFORCED CONCRETE BUILDINGS

Reinforced Concrete Buildings

In recent times, reinforced concrete buildings have become common in India, particularly in towns and cities. Reinforced concrete (or simply RC) consists of two primary materials, namely concrete with reinforcing steel bars. Concrete is made of sand, crushed stone (called aggregates) and cement, all mixed with pre-determined amount of water. Concrete can be molded into any desired shape, and steel bars can be bent into many shapes. Thus, structures of complex shapes are possible with RC.

A typical RC building is made of horizontal members (beams and slabs) and vertical members (columns and walls), and supported by foundations that rest on ground. The system comprising of RC columns and connecting beams is called a RC Frame. The RC frame participates in resisting the earthquake forces. Earthquake shaking generates inertia forces in the building, which are proportional to the building mass. Since most of the building mass is present at floor levels, earthquake-induced inertia forces primarily develop at the floor levels. These forces travel downwards – through slab and beams to columns and walls, and then to the foundations from where they are dispersed to the ground. As inertia forces accumulate downwards from the top of the building, the columns and walls at lower storeys experience higher earthquake-induced forces (Figure 1) and are therefore designed to be stronger than those in storeys above.

Figure 1: Total horizontal earthquake force in a building increases downwards along its height.

Roles of Floor Slabs and Masonry Walls

Floor slabs are horizontal plate-like elements, which facilitate functional use of buildings. Usually, beams and slabs at one storey level are cast together. In residential multi-storey buildings, thickness of slabs is only about 110-150mm. When beams bend in the vertical direction during earthquakes, these thin slabs bend along with them (Figure 2a). And, when beams move with columns in the horizontal direction, the slab usually forces the beams to move together with it. In most buildings, the geometric distortion of the slab is negligible in the horizontal plane; this behaviour is known as the rigid diaphragm action (Figure 2b). Structural engineers must consider this during design.

Figure 2: Floor bends with the beam but moves all columns at that level together.

After columns and floors in a RC building are cast and the concrete hardens, vertical spaces between columns and floors are usually filled-in with masonry walls to demarcate a floor area into functional spaces (rooms). Normally, these masonry walls, also called infill walls, are not connected to surrounding RC columns and beams. When columns receive horizontal forces at floor levels, they try to move in the horizontal direction, but masonry walls tend to resist this movement. Due to their heavy weight and thickness, these walls attract rather large horizontal forces (Figure 3). However, since masonry is a brittle material, these walls develop cracks once their ability to carry horizontal load is exceeded. Thus, infill walls act like sacrificial fuses in buildings; they develop cracks under severe ground shaking but help share the load of the beams and columns until cracking. Earthquake performance of infill walls is enhanced by mortars of good strength, making proper masonry courses, and proper packing of gaps between RC frame and masonry infill walls. However, an infill wall that is unduly tall or long in comparison to its thickness can fall out-of-plane (i.e., along its thin direction), which can be life threatening. Also, placing infills irregularly in the building causes ill effects like short-column effect and torsion.

Figure 3: Infill walls move together with the columns under earthquake shaking.

Horizontal Earthquake Effects are Different

Gravity loading (due to self weight and contents) on buildings causes RC frames to bend resulting in stretching and shortening at various locations. Tension is generated at surfaces that stretch and compression at those that shorten (Figure 4b). Under gravity loads, tension in the beams is at the bottom surface of the beam in the central location and is at the top surface at the ends. On the other hand, earthquake loading causes tension on beam and column faces at locations different from those under gravity loading (Figure 4c); the relative levels of this tension (in technical terms, bending moment) generated in members are shown in Figure 4d. The level of bending moment due to earthquake loading depends on severity of shaking and can exceed that due to gravity loading. Thus, under strong earthquake shaking, the beam ends can develop tension on either of the top and bottom faces. Since concrete cannot carry this tension, steel bars are required on both faces of beams to resist reversals of bending moment. Similarly, steel bars are required on all faces of columns too.

Strength Hierarchy

For a building to remain safe during earthquake shaking, columns (which receive forces from beams) should be stronger than beams, and foundations

Figure 4: Earthquake shaking reverses tension and compression in members – reinforcement is required on both faces of members.

(which receive forces from columns) should be stronger than columns. Further, connections between beams & columns and columns & foundations should not fail so that beams can safely transfer forces to columns and columns to foundations.

When this strategy is adopted in design, damage is likely to occur first in beams (Figure 5a). When beams are detailed properly to have large ductility, the building as a whole can deform by large amounts despite progressive damage caused due to consequent yielding of beams. In contrast, if columns are made weaker, they suffer severe local damage, at the top and bottom of a particular storey (Figure 5b). This localized damage can lead to collapse of a building, although columns at storeys above remain almost undamaged.

Figure 5: Two distinct designs of buildings that result in different earthquake performances –columns should be stronger than beams.

Relevant Indian Standards

The Bureau of Indian Standards, New Delhi, published the following Indian standards pertaining to design of RC frame buildings: (a) Indian Seismic Code (IS 1893 (Part 1), 2002) – for calculating earthquake forces, (b) Indian Concrete Code (IS 456, 2000) – for design of RC members, and (c) Ductile Detailing Code for RC Structures (IS 13920, 1993) – for detailing requirements in seismic regions.

.S CODES ON EARTHQUAKE RESISTANT BUILDING DESIGN

Earthquake resistant building design guidelines are provided by set of Indian Standard codes (IS Codes). After observing Indian earthquakes for several years Bureau of Indian Standard has divided the country into five zones depending upon the severity of earthquake. IS 1893-1984 shows the various zones.

The following IS codes will be of great importance for the structural design engineers:

• IS 1893–2002: Criteria for Earthquake Resistant Design of Structures (5th revision).
• IS 4928–1993: Code of practice for Earthquake Resistant Design and Construction of Buildings. (2nd revision).
• IS 13827–1992: Guidelines for Improving Earthquake Resistance of Low Strength Masonry Building.
• IS: 13920–1997: Code of practice for Ductile Detailing of Reinforced Concrete Structures Subjected to Seismic Forces.
• IS: 13935–1993: Guidelines for Repair and Seismic Strengthening of Buildings.