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Seismic Evaluation of Reinforced Concrete Buildings: A Practical Approach

Introduction

Most of the existing buildings, which do not fulfilled the current seismic requirements, may suffer extensive damage or even collapse if shaken by a severe ground motion. The aim of evaluation is to assess the seismic capacity of earthquake vulnerable buildings or earthquake damaged buildings for the future use. The evaluation may also prove helpful for degree of intervention required in seismically deficient structures. The methodologies available so far for the evaluation of existing buildings can be broadly divided into two categories (i) qualitative methods (ii) analytical methods. The qualitative methods are based on the background information available of the building and its construction site, which require some or few documents like architectural & structural drawings, past performance of similar buildings under severe earthquakes, visual inspection report, some non-destructive test results etc. The methods under this category are Field Evaluation Method, Rapid Visual Screening Method, ATC-14 methodology etc. The analytical methods are based on the consideration of the capacity and ductility of buildings on the basis of available drawings. The methods in this category are capacity/demand method, screening method, pushover analysis, nonlinear inelastic analysis etc.

It is often seen that the drawings of buildings are generally not available due to one or more reasons. Moreover, the evaluation of the capacity and ductility of a building is also a cumbersome task, which is difficult for a field engineer and may not be practical in the present Indian scenario. It is important to underline that the methods of evaluation procedure should be very simple and immediate based on synthetic information that can prove suitable for risk evaluation on large populations. Therefore, qualitative evaluation of the buildings is generally being carried out.

The aim of these methods is to direct the evaluating engineer to identify the weak links in the structure that could precipitate the structural or component failure. Traditionally, a qualitative evaluation of a structure is conducted by a visual examination of the structure alongwith some testing of materials. Over the

last decade or more other valuable and practical non-destructive evaluation methods have been developed for relatively rapid inspection of damage and deterioration of structures. The objective of this chapter is to present a general methodology for evaluation of buildings based on the condition assessment, visual inspection and some non-destructive test. This methodology should be treated as a guide to that decision making process and not as the absolute method of evaluation.

Components of Seismic Evaluation Methodology

The evaluation of any building is a difficult task, which requires a wide knowledge about the structures, cause and nature of damage in structures and its components, material’s strength etc. The proposed methodology is divided into three components:

1. Condition Assessment based on (i) data collection or information gathering of structures from architectural and structural drawings (ii) performance characteristics of similar type of buildings in past earthquakes, (iii) rapid evaluation of strength, drift, materials, structural components and structural details. This component of methodology is primarily based on ATC-14 project and is used basically for undamaged existing structures.

2. Visual Inspection / Field Evaluation based on observed distress and damage in structures. Visual inspection is more useful for damaged structures however it may also be conducted for undamaged structure.

3. Non-destructive Evaluation (NDE) is generally carried out for quick estimation of materials strength, determination the extent of deterioration and to establish causes remain out of reach possible from visual inspection and determination of reinforcement and its location. NDT may also be used for preparation of drawing in case of non-availability.

Condition Assessment for Evaluation

The aim of condition assessment of the structure is the collection of information about the structure and its past performance characteristics to similar type of structure during past earthquakes and the qualitative evaluation of structure for decision-making purpose. More information can be included, if necessary as per requirement.

Data Collection/ Information Gathering

Collection of the data is an important portion for the seismic evaluation of any existing building. The information required for the evaluated building can be divided as follows;

Building Data

Architectural, structural & construction drawings

Vulnerability parameters: number of storeys, year of built, total floor area, etc

Specifications, soil reports, & design calculations

Seismicity of the site

Construction Data

Identifications of gravity load resisting system

Identifications of lateral load resisting system

Maintenance, addition, alteration, or modifications in structures

Field surveys of the structure's existing condition

Structural Data

Materials

Structural concept: vertical & horizontal irregularities, torsional eccentricity, pounding, short column and others

Detailing concept: ductile detailing, special confinement reinforcement

Foundations

Non-structural elements

Past Performance Data

Past performance of similar type of structure during the earthquake provides considerable amount of information for the building, which is under evaluation process. Following are the areas of concerns, which are responsible for poor performance of buildings during earthquake

Materials Concerns

Low grade on concrete

Deterioration in concrete and reinforcement

High cement-sand ratio

Corrosion in reinforcement

Use of recycled steel as reinforcement.

Spelling of concrete by the corrosion of embedded reinforcing bars

Corrosion is related to insufficient concrete cover

Poor concrete placement and porous concrete

Structural Concerns

The relatively low stiffness of the frames - excessive inter-storey drifts -damage to nonstructural items

Pounding - column distress - possibly local collapse

Unsymmetrical buildings (U,T,L,V) in plan - torsional effects & concentration of damage at the junctures (i.e., re-entrant corners)

Unsymmetrical buildings in elevation - abrupt change in lateral resistance - performed poorly

Vertical strength discontinuities - concentrate damage in the "soft" stories.

Short column

Detailing Concerns

Large tie spacing in columns - lack of confinement of concrete core - shear failures.

Insufficient column lengths - concrete to spell.

Locations of inadequate splices - brittle shear failure.

Insufficient column strength for full moment hinge capacity- brittle shear failure.

Lack of continuous beam reinforcement - hinge formation during load reversals.

Inadequate reinforcing of beam-column joints or location of beam bar splices at columns - joint failures.

Improper bent-up of longitudinal reinforcing in beams as shear reinforcement - shear failure during load reversal

Foundation dowels that are insufficient to develop the capacity of the column steel above - local column distress.

Seismic Evaluation Data

Seismic evaluation of data will provide a general idea about the building performance during an earthquake. The criteria of evaluation of building will depend on materials, strength and ductility of structural components and detailing of reinforcement.

Materials Evaluation

Buildings height >3 stories – Min. grade concrete M20, desirable M 30 to M40 in particularly in columns of lower stories

Maximum grade of steel should be fe 415 due to adequate ductility

No significant deterioration in reinforcement

No evidence of corrosion or spelling of concrete

Structural Components

Evaluation of columns shear strength and drift – check for permissible limits

Evaluation of plan irregularities – check for torsional forces and concentration of forces

Evaluation of vertical irregularities – check for soft storey, mass or geometric discontinuities

Evaluation of discontinuous load path – check for ground floor columns, projected cantilever beam and ductile detailing at beam-column joints

Evaluation of beam–column joints – Check for strong column – weak beams

Evaluation of pounding – check for drift control or building separation

Evaluation of interaction b/w frame & infill – check for force distribution in frames and overstressing of frames

Structural Detailing

Flexural Members

Limitation of sectional dimensions

Limitation on min. and max. flexural reinforcement – at least two continuous at top and bottom of the members

Restriction of lap splices

Development length requirements: for longitudinal bars

Shear reinforcement requirements: stirrup and tie hooks, tie spacing, bar splices,

Columns

Limitation of sectional dimensions

Longitudinal reinforcement requirement

Transverse reinforcement requirements: stirrup and tie hooks, Column tie spacing, Column bar splices,

Special confining requirements

Foundation

Column steel doweled into the foundation

Non-structural components

Cornices, parapet, and appendages are anchored

Exterior cladding and veneer are well anchored Note: Structural detailing in the structural member should comply with IS: 13920: 1993

Field Evaluation/Visual Inspection Method

Visual inspection is an integral part of evaluation, and in fact, is the most widely used form of non-destructive evaluation (NDE). Visual inspection can provide a wealth of information that may lead to positive identification of the cause of observed distress. Before performing a detailed visual inspection, the investigator must be familiar with type of damage and its cause and be able to distinguish between recent damage and pre-existing damage, ensuring that the observed damage may prove to be dangerous for the structures or not. It is not always true that the observed damage is due to earthquake; it may be due to some other reasons as well.

Procedure for Visual Inspection Method

The procedure for visual inspection method and its limitations are describe in the following sections (FEMA 306, 1999)

Description

Perform a walk through visual inspection to become familiar with the structure;

Gather background documents and information on the design, construction, maintenance, and operation of the structure;

Plan the complete investigation;

Perform a detailed visual inspection and observed type of damage - Cracks, spells and delaminations, permanent lateral displacement, and buckling or fracture of reinforcement, estimating of drift

Observed damage documented on sketches - interpreted to assess the behaviour during earthquake

Perform any necessary sampling - basis for further testing

Equipments

Optical magnification - allows a more detailed view of local areas of distress

Stereomicroscope - that allow a three dimensional view of the surface. Investigator can estimate the elevation difference in surface features by calibrating the focus-adjustment screw

Fiberscopes and bore scopes - allow inspection of regions that are inaccessible to the naked eye.

Tape - to measure the dimension of structure, length of cracks

Flashlight - to aid in lighting the area to be inspected, particularly in post earthquake evaluation, power failure

Crack comparator - to measure the width of cracks at representative locations - two types - plastic cards and magnifying lens comparators

Pencil - to draw the sketch of cracks

Sketchpad - to prepare a representation of wall elevation, indicating the location of cracks, spelling, or other damage, records of significant features such as non-structural elements.

Camera - for photographs or video tape of the observed cracking

Execution

To identify the location of vertical structural elements - columns or walls

To sketch the elevation with sufficient details - dimensions, openings, observed damage such as cracks, spelling, and exposed reinforcing bars, width of cracks

To take photographs of cracks - use marker, paint or chalk to highlight the fine cracks or location of cracks in photographs

Observation of the non-structural elements - inter-story displacement

Limitations

Applicable for surface damage that can be visualised

No identification of inner damage - health monitoring of building - change of frequency and mode shapes etc

Identification of Damages in Building Components

Possible damages in building component, which are frequently observed after the earthquakes (Penelis and Kappos, 1997) are as follows

Columns: Damaged mainly due to lack of confinement, large tie spacing, insufficient splices length, inadequate splicing at the same section, hook configurations, poor concrete quality, less than full height masonry infill partitions, and a combinations of many of the above compounded with vertical and geometrical irregularities. Failure of column has catastrophic consequences for a structure.

Failure sketch

Type 1: Damage at the top and bottom section of column (oftenly occurs in long columns)

Type 2: Damage in weakest part of column in the form of X-shaped cracks (oftenly occurs in short columns)

Characterization of failure type 1 Characterization of failure type 2

Crushing of compression zone manifested by spelling of concrete cover to the reinforcement

Hoop fracture - smaller number of ties Buckling of longitudinal bars - shortening

of columns - redistribution of stress Loses its ability to carry vertical load

Diagonal Cracking in central part of columns

Shear type failure Spectacular collapse of the building Generally occurs in columns of ground

floor Loss of equilibrium

Beams: There is little evidence that the buildings have collapsed due to beam failure. Only a few examples exist in which buildings have exhibited plastic hinging in the beam. The probable regions of hinging are at and near their intersections with supporting columns. An exception may be where a heavy concentrated load is carried at some intermediate point on the span. The causes of hinging are lack of confinement of concrete core and support for the longitudinal compressive reinforcement against inelastic buckling.

Failure sketch

Type 1: Orthogonal to beam axis along the tension zone of the span

Type 2: Shear failure near the supports of beam due to superposition of shear forces due to vertical loading and seismic loading


Most common type of damage

Existing micro cracks, due to bending of the tension zone, widened due to vertical component of earthquake.

Does not jeopardize the safety of structure

Second most frequent type of damage

Move serious than the previous one

Brittle in character

Sometimes jeopardize the overall stability of the structure

Type 3: Flexural cracks on the upper and lower face of beam at the supports

Type 4: Shear failure at the location of indirect support/secondary beams


Flexural cracks on the upper and lower face of the beam

Cracking at lower face due to bad anchorage of the bottom reinforcement in to the support, in that case one or two cracks from close to support

Due to the vertical components of earthquake which amplified the concentrated load

Type 5: X-shaped shear cracks in short beams or coupling beams which connect shear wall

Characterization of failure type 5

Shear failure is similar to that which occurs in short column: inadequate stirrups reinforcement and may have heavy horizontal (flexure) reinforcement, concentrated at one or more cracks

Flexure failure: lack of diagonal reinforcement, but has sufficient stirrups to prevent diagonal tension failure

Not so dangerous for the stability of building

Beams- Column Joint: Beam-column joints are critical element in frame structures and are subjected to high shear and bond-slip deformations under earthquake loading. Account for cross-sectional properties of the joint region, amount and distribution of column vertical steel, inadequate or absence of reinforcement in beam-column joints, absence of confinement of hoop reinforcement, inappropriate location of bar splices in column are the common causes of failure of beam-column joints.

Failure sketch

Type 1: Corner Joint Type 1: Exterior Joint Type 3: Cross shaped Joint

Characterization of failure type 1 Characterization of failure type 2 Characterization of failure type 3

Cyclic moments subjected the inner face to compression and tension

Cyclic moments inducing compression at the lower face and upper face of beam

Failure due to infrequent transverse hoops & beam longitudinal bar located outside the column cage

Slab: Generally slab on beams performed well during earthquakes and are not dangerous but cracks in slab creates serious aesthetic and functional problems. It reduces the available strength, stiffness and energy dissipation capacity of building for future earthquake. In flat slab construction, punching shear is the primary cause of failure.

Failure Sketch

Type 1: Cracks parallel or transverse to the reinforce at random locations

Type 2: Cracks at critical sections of large spans or large cantilevers, transverse to the main reinforcement


Most frequent

Most of time it is due to the widening of already existing micro-cracks due to bending action/temperature changes/ shrinkage. It became visible after dynamic excitation

Sometimes due to differential settlement of columns

Mostly due to the vertical component of the earthquake action

Type 3: Crack at locations of floor discontinuities, such as the corner of large openings accommodating interval stairways, light shaft and so on

Type 4: Cracks in areas of concentration of large seismic load effects, particularly in the connection zones of slab to shear walls or to column in flat plate system


Mostly due to the vertical component of the earthquake action

Punching shear failure

Aggregated by the cyclic bending moment caused by an earthquake

Slab on column seismically vulnerable structures. It should be avoided

Shear wall: Generally performed well, sometimes showed diagonal flexural-shear cracking and significant damage to coupling beams and short piers between openings

Failure Sketch

Type 1: X-shaped shear cracks - limited spelling

Type 2: Sliding at the construction joint

Type 3: Damage due to flexure and compression


Shear type of brittle failure

Under the action of vertical loads, the isosceles triangles formed on the two sides tend to separate from the structure and therefore cause of collapse

Most frequent type of failure

Old concrete is not properly bounded with fresh concrete

Not too serious because the structure still carries vertical load

Very rarely occurs because of bending moment developed at the base of the wall much smaller when those calculated for the design.

Infill wall: Basically due to stiffening effect of infill panels which is cause of i) unequal distribution of lateral forces in the different frames of a building - overstressing of some frames ii) vertical irregularities in strength and stiffness - soft storey iii) horizontal irregularities - torsion and iv) failure of infill itself.

Failure Sketch

Type 1: full interaction between the frame and infill wall

Type 2: Infill wall fail in shear along the bed joint near the mid height level

Type 3: Detachment of the masonry from the frame except the vicinity of the two corner


At low magnitude of lateral forces

Cracks appear along the line of contact of the masonry with the frame

At the intermediate lateral load

The formation of the shear crack separates the panel into two parts

At the high magnitude of lateral load

Infill wall fails in shear (diagonal tension) and columns in the frame yield in flexure

Load Bearing Walls: The most common mode of failures observed in the masonry buildings are out-of-plane failure and in plane failure. Out-of-plane failure is due to inadequate anchorage of the masonry wall into the roof diaphragm and limited tensile strength of the masonry and its mortar. Excessive bending or shear may produce in-plane failures of the walls. The other types of failure are: diaphragm failure, pounding, connection and non-structural components.

Failure sketch

Type 1: Out-of-plane failure Type 2: In-plane failure


1. Vertical cracks in the corner and/or T walls

2. Horizontal cracks along the facade

3. Partial collapse of an exterior wall

4. Wythe separation

5. Cracks at lintel and tops of slender piers

6. Cracks at the level of the roof

7. Masonry ejection

1. Vertical cracks on openings

2. Diagonal shear cracks at doors and window lintels

3. Diagonal shear cracks in the masonry piers of walls between openings (weak floor mechanism)

4. Crushing of corners of walls due to excess of compression

5. Horizontal flexure cracks on top and/ or base

6. Vertical cracks at wall intersections

7. Spelling of material at the location of floor beam

8. Separation of two corner s

Concrete Distress and Deterioration other than Earthquake

Table 1 (Poston, 1997) summarizes the list of other causes or reasons of distress and deterioration observed in buildings other than earthquake.

Table 1: Forms of concrete distress and deterioration other than earthquake

Description Typical Causes

Cracking Plastic shrinkage, Drying shrinkage, Restraint, Sub-grade support deficiencies, Vapour barrier, Expansion, Corrosion of reinforcing steel, Thermal loading, Overloading, Aggregate reaction,

Scaling Inadequate air content, Finishing problems, Freeze-thaw cycling, Chemical de-icers

Spalling Aggregate reaction, Corrosion, Freeze-thaw cycling, Construction problems: poor preparation of construction joints, early age loading

Disintegration Frozen concrete, Freeze-thaw cycling, Low strength, Chemical attack, Sulfate attack

Discoloration and straining

Different cement production, Different water-cement ratios, Corrosion, Aggregates, Use of calcium chloride, Curing, Finishing, Non-uniform absorption of forms

Honeycombing and surface voids

Poor placement, Poor consolidation, Congested reinforcement

Non-Destructive Testing (NDT)

Visual inspection has the obvious limitation that only visible surface can be inspected. Internal defects go unnoticed and no quantitative information is obtained about the properties of the concrete. For these reasons, a visual inspection is usually supplemented by NDT methods. Other detailed testing is then conducted to determine the extent of deterioration and to establish causes.

NDT Tests for Condition Assessment of Structures

Some methods of field and laboratory testing (FEMA, 1999 and Nawy, 1997) that may assess the minimum concrete strength and condition and location of the reinforcement in order to characterize the strength, safety, and integrity are described here.

Rebound hammer/ Swiss hammer

The rebound hammer is the most widely used non-destructive device for quick surveys to assess the quality of concrete. In 1948, Ernest Schmidt, a Swiss engineer, developed a device for testing concrete based upon the rebound principal strength of in-place concrete; comparison of concrete strength in different locations and provides relative difference in strength only

Limitations

Not give a precise value of compressive strength, provide estimate strength for comparison

Sensitive to the quality of concrete on the outer several; carbonation increases the rebound number

More reproducible results from formed surface rather than finished surface; smooth hard-towelled surface giving higher values than a rough-textured surface.

Surface moisture and roughness also affect the reading; a dry surface results in a higher rebound number

Not take more than one reading at the same spot

Penetration Resistance Method – Windsor probe test

Penetration resistance methods are used to determine the quality and compressive strength of in-situ concrete. It is based on the determination of the depth of penetration of probes (steel rods or pins) into concrete by means of

powder- actuated driver. This provides a measure of the hardness or penetration resistance of the material that can be related to its strength.

Limitations

Both probe penetration and rebound hammer test provide means of estimating the relative quality of concrete not absolute value of strength of concrete

Probe penetration results are more meaningful than the results of rebound hammer

Because of greater penetration in concrete, the probe test results are influenced to a lesser degree by surface moisture, texture, and carbonation effect

Probe test may cause of minor cracking in concrete

Rebar locator/ convert meter

It is used to determine quantity, location, size and condition of reinforcing steel in concrete. It is also used for verifying the drawing, and preparing as-built data if no previous information is available. These devices are based on interaction between the reinforcing bars and low frequency electromagnetic fields. Commercial covermeter can be divided into two classes: those based on the principal of magnetic reluctance and those based on eddy currents.

Limitations

Difficult to interpret - at heavy congestion of reinforcement or depth of reinforcement is too great

Embedded metals sometimes affect the reading

Used to detect the reinforcing bars closest to the face

Ultrasonic Pulse velocity

It is used for determining the elastic constants (modulus of elasticity and Poisson’s ratio) and the density. By conducting tests at various points on a structure, lower quality concrete can be identified by its lower pulse velocity. Pulse-velocity measurements can detect the presence of voids or discontinuities within a wall; however, these measurements cannot determine the depth of the voids.

Limitations

Moisture content: an increase in moisture content increase the pulse velocity;

Presence of reinforcement oriented parallel to the pulse propagation direction- the pulse may propagate through the bars and result is an apparent pulse velocity that is higher than that propagating through concrete

Presence of cracks and voids- these can increase the length of the travel path and result in a longer travel time

Impact Echo

Impact echo is a method for detecting discontinuities within the thickness of a wall. An impact- echo test system is composed of three components: an impact source, a receiving transducer, and a waveform analyzer or a portable computer with a data acquisition.

Limitations

Accuracy of results highly dependent on the skill of the engineer and interpreting the results

The size, type, sensitivity, and natural frequency of the transducer, ability of FFT analyzer also affect the results

Mainly used for concrete structures

Spectral Analysis of Surface Waves (SASW)

To assess the thickness and elastic stiffness of material, size and location of discontinuities within the wall such as voids, large cracks, and delimitations of surface waves have used.

Limitations

Interpretation of results is very complex

Mainly used on slab and other horizontal surface, to determine the stiffness profiles of soil sites and of flexible and rigid pavement systems, measuring the changes in elastic properties of concrete slab

Penetrating Radar

It is used to detect the location of reinforcing bars, cracks, voids or other material discontinuities, Verify thickness of concrete;

Limitations

Mainly used for detecting subsurface condition of slab-on-grade

Not useful for detecting the small differences in materials

Not useful for detecting the size of bars, closely spaced bars make difficult to detect features below the layer of reinforcing steel

References

1. ATC (1987). “Evaluating the Seismic Resistance of Existing Buildings” Applied Technology Council (ATC-14 Project), 555 Twin Dolphin Drive, Suite 550, Redwood City, California 94065

2. Nawy. G. Edward (1997). “Concrete Construction Engineering Handbook”, CRC Press, New York.

3. Poston, W. R. (1997) “Structural Concrete Repair: General Principles and a Case Study” Concrete Construction Engineering Handbook (editor-in-chief: Edward G. Nawy), Chapter 19, CRC Press, New York.

4. FEMA-306 (1999). “Evaluation of Earthquake Damaged Concrete and Masonry wall Buildings” Applied Technology Council (ATC-43 Project), 555 Twin Dolphin Drive, Suite 550, Redwood City, California 94065.

5. Carino, N. J. (1997) “Non-destructive Test Method” Concrete Construction Engineering Handbook (editor-in-chief: Edward G. Nawy), CRC Press, New York.

6. Penelis, G G. and Kappos, A. J., (1997) “Earthquake-Resistant Concrete Structures” E & FN SPON an imprint of Chapman & Hall.

7. Malhotra, V.M. and Carino, N.J. (1991) “ Handbook on Non-destructive Testing of Concrete” CRC Press.

8. Pasquale. G. D. and Orsini, G. (1999), “ Damage Scales in Macro seismic and in GNDT forms for Damage” Seismic Damage to Masonry Buildings, Bernardini (ed.), Balkema, Rotterdam.

9. Zuccaro, G. and Papa, F. (1999), “ Inspection Minutes Process’ for Measurement of the Seismic Damage” Seismic Damage to Masonry Buildings, Bernardini (ed.), Balkema, Rotterdam.

10. IS: 13935:1993, (1993). “Ductile Detailing of Reinforced Concrete Structures Subjected to Seismic Forces”, Bureau of Indian Standards, Manak Bhawan, 9 Bahadur Shah Zafar Marg, New Delhi.

11. IS: 1893:1984, (2002). “Criteria for Earthquake Resistant Design of Structures”, Bureau of Indian Standards, Manak Bhawan, 9 Bahadur Shah Zafar Marg, New Delhi.

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