Pushover Case Study

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Case Study The case study to validate the Pushover analysis is taken from part of the round robin exercise (experiment) conducted by Reactor Safety Division (RSD), of the Bhabha Atomic Research Centre (BARC), Mumbai, at Central Power Research Institute (CPRI), Bangalore. Experimental Building description The building is a portion of a four storey office building assumed to be in seismic zone IV. Complete details of the structure including modelling concepts and their effect on the analysis results have been supplied by RSD. A brief summary of the building is presented in Table below

Type of structure Ordinary moment resisting RC frame

Grade of concrete M 20

Grade of reinforcing steel Fe 415

Plan size 5 m × 5 m

Number of stories G + 3 storey

Building height 12 m above ground storey

Type of foundation Raft foundation which is supported on rock bed using rock grouting.

Structural system and members 1. Structural system The building is an RC framed structure. The floor plan is same for all floors. The beam arrangement is different for the roof. It is symmetric in both the direction. The concrete slab is 120 mm thick at each floor level. Overall geometry of the structure including the beam layout of all the floors is as shown in Fig below.

2

5000

BR20

CL15 CL16

CL19 CL20

2500

BR6

BR6

BR7

BR21

5000

5000

CL15 CL16

CL19 CL20

3000

BF205

BF205

BF204 BF223 BF225

Roof Plan Floor Plan

A A′

5000

4000

4000

4000

4000

700

Roof

Third

Second

First

RAFT 7m x 7m

BR6

BF205

BF205

BF205

CL15 CL16

Section A-A′ Figure 2.1 Overall Geometry of the Structure

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Table below shows the size and reinforcement details for beam sections at the column face.

Longitudinal Reinforcement

Beam Size (mm) Length (m)

Top Bottom

Transverse Reinforcement

BF 204 230 × 1000 5 2#16 3#16 2#8@200 c/c

BF 205 230 × 1000 5 2#25 2#25 2#10@125 c/c

BF 223 230 × 1000 5 2#25 2#25, 1#16

2#10@125 c/c

BF 225 230 × 1000 5 2#20 2#25 2#10@150 c/c

BR 6 230 × 1000 5 2#20 3#20 2#8@200 c/c

BR 7 230 × 600 5 2#16 3#16 2#8@120 c/c

BR 20 230 × 1000 5 2#20 2#25 2#8@175 c/c

BR 21 230 × 1000 5 2#20 2#20 2#8@175 c/c

Note: # represents Torsteel Fig below shows the size and reinforcement details for column at the beam face (as given in round robin exercise). 2. Foundation The structure is resting on a 700 mm thick raft resting on rock below, with rock anchors provided 3. Loading The test was conducted under gradually increasing static lateral load in an inverted triangular pattern till failure. The ratio of force at “first level : second level : third level : fourth level” was kept as “1: 2 : 3 : 4”.

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Computational Building model 1. Material properties The material properties considered for the analysis are given in Table below

Material Characteristic strength (MPa)

Modulus of Elasticity (MPa)

Concrete (M20) fck = 20 Ec = 22360

Reinforcing steel fy = 415 Es = 2 E+5

900

400

700

400

700

300

900

350

900

350

Long. Reinft: 12-28Φ Trans Reinft: 10Φ@100 c/c

Long. Reinft: 4-25Φ + 6-20ΦTrans Reinft: 10Φ@100 c/c




(d) CL 16 and 20 Ground to 2nd Floor

(e) CL 16 and 20 3rd and 4th Floor

(a) CL 15 and 19 Ground to 2nd Floor

(b) CL 15 and 19 3rd Floor

(c) CL 15 and 19 4th Floor

Fig 2.4 Details of Columns at various levels

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2. Structural modelling The analytical model was created in such a way that the different structural components represent as accurately as possible the characteristics like mass, strength, stiffness and deformability of the structure. Non structural components were not modelled. The various primary structural components that were modelled are as follows:

(a) Beams and columns: Beams and columns were modelled as 3D frame elements. The characteristics like strength, stiffness and deformability of the members were represented through the assignment of properties like cross sectional area, reinforcement details and the type of material used. The modelled effective moment of inertia for the beams is as in Table below.

Sections Effective Mom. of Inertia (Ieff)

Rectangular beam 0.5 Igross

Columns 0.7 Igross

where Igross represents the gross moment of inertia

(b) Beam-column joints: The beam-column joints were assumed to be rigid and were modelled by giving end-offsets to the frame elements. This was intended to get the bending moments at the face of the beams and columns. A rigid zone factor of 1 was considered to ensure rigid connections of the beams and columns.

(c) Slab: The slabs were not modelled physically, since modelling as plate elements would have induced complexity in the model. However the structural effects of the slabs i.e., the high in-plane stiffness giving a diaphragm action and the weight due to dead load were modelled separately.

(d) Foundation modelling: The foundation was modelled based on the degree of fixity which is provided. The effect of soil structure interaction was ignored in the analysis. In the model, fixed support was assumed at the column ends at the end of the footing.

3. Pushover analysis The result of the experimental pushover as provided by BARC is as below

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0

100

200

300

400

500

600

700

800

900

1000

0 100 200 300 400 500 600Displacement (mm)

Base

She

ar (k

N)

Pushover analysis was performed in orthogonal x- and y- direction for the building. The summary of pushover results of various institutes along with the analytical result as done, and experimental results provided by BARC is shown in Fig below.

Regards, Rahul Leslie

Pushover Case Study

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