Seismic Evaluation

Seismic Evaluation of Buildings & Retrofitting Introduction The seismic evaluation of buildings aims at finding the capacity of existing building to resist lateral forces due to seismic acceleration, anticipated in the remaining life time of the buildings. The deficient buildings are retrofitted thereby increasing its capacity for performance under seismic acceleration. The deficiency in the existing building may be due to damage of the building by seismic activity or the design of the building with lower level of seismic design specification. The seismic retrofitting corresponds to up gradation of deficient structures which are in operation. The present Seismic Code of India does not have detailed provisions for seismic evaluation and retrofitting of existing buildings in operation. FEMA 356 outlives the seismic evaluation and retrofitting procedure in the frame work of performance criteria. According to the criteria, the seismic evaluation and upgradation of buildings or structures is to be carried out to achieve a desired objective. The objective thus specified in the FEMA 356 are classified into five categories S-I to S-5 Where: S-I

immediate occupancy structures performance level which is defined as the post earthquake damage state that remain safe to occupy retaining design strength and stiffness.

S-2

is damage control structures performance range. In this range the repair time and operation interruption desired to be minimized.

S-3

is life safety structural performance level – and defined as the post earthquake damage that include damage to structural components but retains a margin against on set of collapse.

S-4

is limited safety structural performance range which is defined as the continuous range of damage state between life safety structural performance and collapse prevention structural performance.

S-5

is collapse prevention structural performance level, defined as post earthquake damage that includes damage to structural component such that structure continue to support gravity load but retains no margin against collapse in compliance with the acceptance criteria in standard for structural performance level.

The seismic hazard due to ground shaking based on which the evaluation are done shall depend on the location of the building with respect to causative faults, the regional and site specific geologic characteristics and selected earthquake level. The probabilistic seismic hazard assessment shall be required to identify seismic hazard level at the site. In absence of adequate

data for supporting specific accurate seismic hazard assessment it may be herculean task to adopt the provision in FEMA 356. Recently, a guideline for seismic evaluation and strengthening of Existing Buildings has been published by Department of Civil Engineering IIT Kanpur in collaboration . The guidelines have been developed with reference to current edition of Seismic Code IS-1893-2002. The present guidelines are derived from similar documents like AT-40, FEM 310, FEMA 356 New Zealand Draft Code and Euro Code. The presentation in this paper is based on the guidelines published by IIT Kanpur.

Seismic Evaluation Criteria The seismic performance of existing buildings is evaluated in relation to the performance criteria in use for new buildings. The minimum evaluation criteria for the expected performance of life safety of existing buildings with appropriate modification to IS-1893-2002, which is applicable for seismic design of new buildings. The main aim of evaluation of existing buildings as per the guideline is to avoid risk of life was in existing building similar to the provision of IS-18932002 for new buildings. The existing buildings may not have seismic design features for seismic resistance but it is expected to meet requirement of other codes of structural safety for loads such as gravity wind etc.

Evaluation Process The evaluation process described in the guidelines is a two level process comprising of increasing detailing and decreasing conservatism. The two level of the process are; Preliminary evaluation and Detailed evaluation.

The preliminary evaluation involves broad assessment of its physical condition, robustness, structural integrity and strength of structure including simple calculations.

The detailed evaluation includes numerical check on stability and integrity of the whole structure as well as the strength of each member.

The detailed evaluation is required only if the results of preliminary evaluations are unacceptable. Flows chart in Fig. 1 summaries the evaluation process as per the guidelines published by IIT Kanpur.

As an introduction to the topic the presentation is limited to the preliminary evaluation. Readers interested on the topics may refer the publication “Seismic Evaluation and Strengthening of Existing Buildings by Dr. Durgesh C. Rai, Department of Civil Engineering IIT Kanpur.

Preliminary evaluation The preliminary evaluation is a quick procedure to establish actual structural layout and assess its characteristics that can affect its seismic vulnerability. The method is primarily based on observed damage characteristics in previous earthquakes, coupled with some back of the envelope calculations. The stages of preliminary evaluation are as given below:

Site Visit A site visit will be conducted by the design professionals to verify available existing building data or collect additional data and to determine the condition of building and its components. The following information are either need to be collected or confirmed during visit. (a)

General information: Number of storeys and dimensions, year of constructions.

(b)

Structural system description:Framing vertical lateral force resisting system, floor and roof diaphragm connection to wall, basement and foundation system.

(c)

Building type as in IS-1893 (Part-I) site soil classification as in IS-1893 (Part-I).

(d)

Building use and nature of occupancy.

(e)

Adjacent building and potential for pounding and failing hazards.

(f)

General Condition: Deterioration of materials, damage from past earthquakes, alterations and additions that could effect earthquake performance.

(g)

Architectural features that may effect earthquake performance especially location of masonry infill walls.

(h)

Geological site hazards and foundation condition: Susceptibility for liquefaction and conditions for slope failure and surface fault rupture.

Configuration deck load path One of the fundamental attributes required for the proper seismic response of a building during earthquake motion is that its lateral load resisting members should be tied together to act as a single unit. These provisions are intended to provide conditions lateral load system that ties all parts of the structures together. It also provides for proper connection between the members of

the system to transmit additional seismic force safely. A vertical lateral force resisting system should continuous and run from the foundation to the top of the building. The presence of discontinuity in a load path makes a building inadequate for carrying seismic forces.

Fig. 2 Load path

Configuration check:

Redundancy

The numbers of lines of vertical load resisting element in each principal direction shall be greater than or equal to 2. In case of movement or braced frames, the number of bays in each line shall be greater than or equal to 2. Similarly numbers of lines of shear wall in each direction shall be greater than or equal to 2. This provision is recommended because of the uncertainties involved in the magnitude of both seismic load and member capacity. If any member of a lateral force resisting system fails, the redundancy of the structure will help to ensure that there is another member present in the lateral force resisting system that will contribute lateral resistance to the structure.

Fig.3 Redundancy of moment frame


Geometry

No chance in the horizontal dimensions of the lateral force resisting system of more than 50% in a storey relative to adjacent stories excluding pent-houses and mezzanine floors should be made. Geometric irregularities of overall building shape in plan and elevation effect, the seismic

response of the structure by increasing ductility demand at a few locations. The irregular building in plan as per IS-1893 (Part-I) is given below in Fig…

Fig 4 (a) Irregularity in plan and elevation

Fig 4 (b) Building with offset Plan configuration should always be symmetrical in respect of two orthogonal directions. Complex shape of building like H,I, X etc. should be avoided.


Weak Storey

The strength of the lateral force resisting system in any storey shall not be less than 70% of the strength in an adjacent storey. If this requirement is not satisfied, it is called as weak storey. Weak storey is usually found where the vertical discontinuities exist or where the size of the reinforcement has been reduced.

Fig.5 shows some cases of weak storey.


Soft Storey

The stiffness of lateral load resisting system in any storey shall not be less than 60% of the stiffness in an adjacent storey or less than 70% of the average stiffness of three stories above. If this specification is not fulfilled the storey is called as soft storey. The soft storey buildings are well known for their poor performance during earthquakes.

Fig 6 Soft Story with sever deformation demand during seismic shaking


Vertical discontinuities

All the vertical element in lateral force resisting system shall be continuous from roof to the foundation. If such discontinuities exist specific arrangement for transfer of forces should be made.

Fig 7 Vertical irregularities


Mass

There shall be no change in the effective mass more than 100% from one storey to the next. The mass irregularity effect the dynamic response of the structure by increasing ductility demand at a few locations and lead to unexpected higher mode effects.

Fig.8 Mass irregularities


Torsion

The eccentricity between the centre of mass of building and centre of stiffness create a torsional effect thereby increasing the forces in the outer columns. The estimated distance between a storey centre of stiffness shall be less than 30% of the building dimension at right angle to the direction of loading considered.

Fig 9 Torsional irregularities Configuration check:

Adjacent Building

The clear horizontal distance between the building under consideration and any adjacent building shall be greater than 4% of the height of the shorter building except for building that one of same height with floors located at the same level. If this prescription is not adhered, during seismic shaking two such adjacent building may hit each other due to lateral displacement known as pounding.

Fig.10 Pounding situation, (b) is more serious.


Short Column

The reduced height of column due to surrounding parapet infill will etc shall not be less than five times the dimensions of the column in the direction of parapet, infill wall etc or 50% of the nominal height of typical column in that storey. Short column are relatively stiffer than other columns in a storey and tend to attract higher seismic forces because of their high stiffness relating to other columns. Adequate ductile detailing by proving confining reinforcement to full height of column should be made in short column.

Fig.11 Short column with variable stiffness

Strength Related Check Approximate and quick check shall be used to compute the strength and stiffness of the building. The seismic base shear and storey shear for the building shall be computed in accordance with IS 1893 (Part-I) for comparing with the capacity. Shear stress in R.C. frame columns The average shear stress in concrete columns τ col computed in accordance with the following equation shall be lesser of(a)

0.4 MPa and

(b)

0.10 fck where fck is the characteristic cube strength of concrete.

⎛

n

c τ col = ⎜⎜ − n ⎝ c nf

⎞⎛ V j ⎟⎜ ⎟⎜ A ⎠⎝ c

⎞ ⎟⎟ ⎠

Where, nc

=

total numbers of columns

nf

=

total number of frames in the direction of loading.

Vj

=

storey shear at level j and

Ac

=

total cross section area of columns.

Axial stress in moment frames The maximum compressive axial stress in columns of moment frames at base due to overturning forces along ( Fo ) as calculated using following equation shall be less than 0.25 f ck

Fo =

2 ⎛⎜ Vs 3 ⎜⎝ n f

⎞⎛ H ⎞ ⎟⎜ ⎟ ⎟⎝ L ⎠ ⎠

Where

nf

=

total numbers of frames in the direction of loading.

VB

=

base shear

H

=

total height

L

=

length of building.

The detailed evaluation shall be carried out when the requirement in preliminary checks are not satisfied. A worked out example of preliminary check are given below for illustration of the method.

Retrofitting of building

Based on the results of the preliminary and detailed evaluation the retrofitting strategies are decided. The removable plan irregularities should be removed. This may reduce the force and deformation demand of the building to acceptable level. The eccentric mass due to location of water tank can also be relocated. Further unwanted non structural mass can also be removed which will reduce the seismic weight of the building thereby the reducing the base shear capacity demand. If the building is highly deficient in lateral load resisting capacity the retrofitting of the component of building should be retrofitted. Local retrofit strategies refer to retrofitting of beams, columns, slabs, joints, walls and foundations, without significantly affecting the overall response of the building. The local retrofit strategies are grouped according to the elements.

Column Retrofitting:

Concrete jacketing

Concrete jacketing is a popular method of column retrofit. This involves addition of a layer of RC in the form of a jacket, using longitudinal reinforcement and closely spaced ties with seismic detailing (Figure 12). The method is comparatively straightforward and increases both strength and ductility. But, the composite deformation of the existing and the new concrete requires adequate dowels in the existing column. Also, the additional longitudinal bars need to be anchored to the foundation and should be continuous through the slab. The disadvantages are that the size of the column increases and the placement of the ties at the beam-to-column joints is difficult. Although there are disadvantages, use of concrete jacket is relatively cheap. =350mm Additional longitudinal reinforcement.

=350mm

Dowel bars to be inserted into existing concrete upto a depth full anchoring length. Existing column

Jacket Pocket of dowel bar to be filled with grout.

CONCRETE JACKETING

Fig 12 Retrofitting of Column Column Retrofitting:

Steel Jacketing

Steel jacketing refers to encasing the column with plates and filling the gap with non-shrink grout (Figure13). Steel jacketing is an effective method to remedy deficiencies such as inadequate shear strength and faulty splicing of longitudinal bars at the potential hinge regions.

Welding

New concrete/non-shrink grout

Welding

Shear lugs

Existing column

Steel plate

Steel angle STEEL JACKETING

Shear lugs

Fig 13 Steel jacketing of deficient column

Column Retrofitting:

Fibre Reinforced Polymer Sheet Wrapping

Fibre Reinforced Polymer (FRP) has desirable physical properties like corrosion and fatigue resistance and high tensile strength to weight ratio. FRP sheets are thin, light and flexible enough to be inserted behind pipes, electrical cables and other service ducts, thus facilitating installation. In retrofitting of a column there is no significant increase in the size. The main drawbacks of FRP are the high cost, brittle behaviour and fire resistance.

Fig 14 Retrofitting using FRP Beam Retrofitting:

Concrete Jacketing

Concrete is added to increase the strength or stiffness of a beam. Several options are available for adding concrete. There are some disadvantages in this traditional retrofit strategy. First, addition of concrete increases the size and weight of the beam. Second, the new concrete requires proper bonding with the existing concrete. In beam soffits, the bleed water from the new concrete creates a weak cement paste at the interface. Restrained shrinkage at the interface induces tensile stress in the new concrete. Third, the drilling of holes in the existing concrete can weaken the section, if the width is small and the concrete is not of good quality. Instead of regular concrete, fibre reinforced concrete can be used for retrofit. In addition to strength, this leads to the increase of energy absorption capacity.

Fig15 Enlarging the size of beam

Beam Retrofitting: Bonding Steel Plates.

The technique of bonding mild steel plates to beams is used to improve their flexural and shear strengths. The addition of steel plate is simple and rapid to apply, does not reduce the storey clear height significantly and can be applied while the structure is in use. Glued plates of course are prone to premature debonding which can limit the application of this technique.

Steel Plate Fig. 16 (a) Bonding of steel plate

Fig. 16 (a) Bonding of steel plate Beam Retrofitting:

FRP Wrapping

Like steel plates, FRP laminates are attached to beams to increase their flexural and shear strengths.

Beam-to-Column Joint Retrofitting

Under seismic excitation, the beam-to-column joints are subjected to high shear forces. Hence, the joint should be sufficiently stiff to reduce shear deformation. Also, the formation of hinges in the adjacent beams may deteriorate the joint. The different methods of retrofitting are as follows.

Beam-to-Column Joint Retrofitting:

Concrete Jacketing

A joint can be strengthened by placing ties through drilled holes in the adjacent beams (Stoppenhagen et al., 1995). A simpler option is a concrete fillet at the joint to shift the potential hinge region of the beam away from the column face (Bracci et al., 1995).

Beam-to-Column Joint Retrofitting:

Steel Jacketing

If space is available, steel jacketing can be used to enhance the performance of joints (Ghobarah et al.,1997). A simpler option is to attach plates in the form of brackets at the soffits of the beams and sides of the column. The retrofitted joints were found to perform better because of the following conditions.

a) Pullout of the discontinuous bottom rebar in the beams was prevented. b) Damage of the joints was shifted from the zone where the bottom rebar was embedded. c) The deterioration of the joint under cyclic loading was reduced. Beam-to-Column Joint Retrofitting:

FRP Jacketing

FRP sheets can be used to strengthen beam-to-column joints.

Wall Retrofitting

A concrete shear wall can be retrofitted by adding new concrete with adequate boundary elements (bolster columns). For the composite action, dowels need to be provided between the existing and new concrete (Figure 17) New bolster colum n M inim um Ø 10

To be filled with potym er concrete/expoxy grout

New wall

Existing wall

S TR E N TH E N IN G O F A W A LL U S IN G C O N C R E TE

Fig. 18 Wall retrofitting Steel braces or strips can be bolted to strengthen a wall. FRP or steel sheets can be used to strengthen walls for out-of-plane bending. External prestressing or reinforced grouted core can be introduced for strengthening unreinforced masonry wall.

Foundation Strengthening Foundation strengthening is done by strengthening the footing as well as the soil (FEMA 356, 2000). The following measures may be effective in the rehabilitation of footings. 1.

New isolated or spread footings may be added to existing structures to support new structural elements such as shear walls or frames.

2.

Existing spread footings may be enlarged to increase the capacity.

3.

Existing spread footings may be underpinned to increase the bearing capacity.

4.

Uplift capacity may be improved by increasing the soil mass above the footing.

5.

Differential lateral displacement of the footings can be mitigated by interconnecting them with plinth beams or the beams.

Typical details of strengthening of foundation is shown below in Fig 19.

Existing Column A

A Added Reinforcement

Added Reinforcement B

Reinforced Jacket

B

Existing Column Existing Foundation

Added Concrete

A-A

Added Reinforcement

B-B

TYPICAL DETAIL OF STRENGTHENING OF FOUNDATION

Fig.19 Retrofitting of foundation

Seismic Evaluation

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