EVALUATION AND REHABILITATIONINTRODUCTION

1.1 The purpose of the preliminary visual survey was to make an initial assessment of the fire effects, the structural damage, and to devise an outline plan for deliberate forensic investigations. The plan aimed at estimating reasonably accurate and pragmatic residual strength of the structure and evolving a rehabilitation scheme as economic as possible with the desired levels of safety for the future use of the building.

1.2 A brief description of the required parameters and the possible means to obtain them had been given in Part-I of the report. It must be mentioned that the processes involved require an in-depth knowledge of the dynamics of fire and its effects. In addition, the means and methods for evaluation of fire affected structures are empirical in nature based on a limited data. The problem is further accentuated by the unpredictable concrete behavior under varying as well as similar circumstances. An effort has been made to organize the structural investigations in a manner expected and required of a sound professional approach to the problem in hand.

1.3Scope. This report has been divided into two volumes as described below:

1.3.1 Volume I. It contains:

1.3.1.1 Forensic investigation scheme.

1.3.1.2 Structural Survey.

1.3.2 Volume II. It comprises of:

1.3.2.1 Material tests.

1.3.2.2 Load tests.

1.3.2.3 Interpretation of test results.

FORENSIC INVESTIGATION SCHEME

2.1 These investigations are required to assess the residual material properties to ascertain the structural evaluation in its damaged state. The required parameters include the temperatures attained in fire, the fire duration, temperatures to which the structural members and materials were exposed. The plan is described in subsequent paragraphs.

2.2 Structural Survey

2.2.1 Line and Level Survey. The residual deformations were found in slabs of almost all the floors affected by fire. It was desirable to ascertain deflections in all affected slab panels but it was not possible due to time constraints and colossal work involved in removal of floor components and exposing the slab surfaces.

2.2.2 Spalling of Concrete. Spalling was investigated to ascertain the thickness of spalled concrete and the extent of damaged concrete due to heating.

2.2.3 Concrete Colour. The property of concrete to change the colour when exposed to high temperatures was used to arrive at residual strengths and collated with other field data.

2.2.4 Recording and Measurements of Cracks

2.2.4.1 Cracks of each individual member was recorded on drawings.

2.2.4.2 Widths of cracks was measured and recorded with the help of crack detection microscope (precision level 0.02 mm).

2.2.4.3 The variation of crack depths along their axes was noted for severely damaged members.

2.3 Material Testing

2.3.1 Concrete

2.3.1.1 Destructive Testing. The concrete cores were extracted and tested from the structural members as under:

a. Non Affected Members

i.Slabs - 5

ii.Beams-3

iii.Core wall-3

b. Affected Members

i.Slabs - 11

ii.Beams-7

iii.Core wall-3

2.3.1.2 Non-Destructive Testing (NDT)

a. Ultrasonic Pulse Velocity Measurements were made with PUNDIT equipment as under:

i.Slabs - 151

ii.Beams-170

iii.Column-147

b. Schmidt Rebound Hammer tests were carried out on concrete members. The number of readings taken on different members are given below :

i.Slabs - 3020

ii.Beams-2040

iii.Core wall- 136

iv.Columns-1168

v.Columns concrete core- 48

2.3.2 Reinforcing Steel. Coupons from the following RC members were tested for their tensile strength:

2.3.2.1Slab -6 Samples

2.3.2.2Core wall-3 Samples

2.4Load Tests. These were conducted on three floors. Two slab panels were loaded to record and analyze behavior of two floor panels and the intermediate beam. The floor selected for the load tests were:

2.4.1 Ninth Floor was selected because of excessive calcination on slab soffits and the beam.

2.4.2 Tenth Floor was selected because of extensive deflection of slab and cracking of the beam.

2.4.3 Roof Top was selected because slab had its cover spalled off.

STRUCTURAL SURVEY

3.1 General. This part presents the detailed examination of fire affected structural elements in the building. A structural element identification system was devised to designate each member with a unique number based upon location of the member.

3.2Structural Element Identification System. Grid lines running in E-W direction are numbered from 1 to 11 and those running parallel to N-S direction were labelled with alphabets from A to L as shown on typical plan attached as Drawing Sheet No 1/35. Floors were also numbered from Basement to Roof. The identification of each element is explained in subsequent paragraphs

3.2.1 Slab. Each slab panel was identified by two digits and two alphabets arranged alternately starting with a digit. Slab designated as 6S4H, for example, indicates:

a.6 - Floor number (sixth floor).

b.S - Slab element.

c.4H - Two point grid reference of the slab panel right bottom corner.

3.2.2 Beam identified as 6B8(D-E)S indicates:

a.6 - Floor number (sixth floor).

b.B - Beam element.

c.8 - Beam is spanning on grid line 8.

d.D-E - Beam runs between grid lines D and E.

e.S - Observation direction is southwards.

3.2.3Column identified as 6C7H means:

a.6 - Floor associated with bottom of the column element.

b.C - Column element.

c.7H - Two point grid reference of column location.

3.2.4Core Wall identified as 6WE corresponds to:

a.6 - Floor number (sixth floor).

b.W - Core wall element.

c.E - Observation direction is eastwards.

3.3Line and Level Survey

3.3.1Line Survey

3.3.1.1Vertical Plane

a. Expansion joint between main building and emergency staircase had widened at the top by 32 mm (Fig. 3.1).

b. No other member in vertical plan had significant changes in their geometry.

3.3.1.2Horizontal Plane. None of the structural element in horizontal plane had significant changes in their geometry.

3.3.2Level Survey. Slab and beam elements were studied for their vertical deflection in level survey. Typical observations are as follow:

3.3.2.1 Slab Elements

a. Panel 9S4E had a maximum deflection of 40 mm.

b. Panel 10S4F had a maximum deflection of 43 mm.

3.3.2.2 Beam Elements

a. Beam 10BH(5-6) had 14 mm vertical deflection at center.

b. Beam 7B8(D-E) had 12 mm vertical deflection at center.

Fig. 3.1: Widened vertical expansion joint at roof level.

3.4Spalling of Concrete

3.4.1 Slabs. Spalling of concrete cover on eleven slab panels in the main building and at one location in emergency lift well was observed (Fig. 3.2 to 3.7). Location of the affected portion is given in drawing sheets 2/35, 4/35 and 31/35. Depth of observed spalling is as under:

a. Slab panel 5S3G25 mm.

b. Slab panel 6S7E 20 mm.

c. Slab panel RS4E37 mm.

d. Slab panel RS5D37 mm.

e. Slab panel RS5E62 mm.

f. Slab panel RS6E25 mm.

g. Slab panel RS7D37 mm.

h. Slab panel RS7E75 mm.

i. Slab panel RS6H50 mm.

j. Slab panel RS7H56 mm.

k. Slab panel RS7J37 mm.

l. Slab panel RS9E50 mm.

3.4.2Beams. Spalling of concrete cover on beam 14B7(G-H)N was observed as 50 mm deep (Fig. 3.8).

3.4.3Columns. Concrete cover of column 8C8F had spalled with max depth of 40 mm (Fig. 3.9).

3.4.4Core Walls. Spalling of concrete cover at three locations was observed as under (Fig. 3.10):

a. 6WS 50 mm.

b. 8WN30 mm.

c. 12WN80 mm.

Fig. 3.2: Spalling of concrete cover from slab panels 5S3G and 6S7E.

Fig. 3.3: Spalling of concrete cover from slab panels RS4E and RS5D.

Fig. 3.4: Spalling of concrete cover from slab panels RS5E and RS6E.

Fig. 3.5: Spalling of concrete cover from slab panels RS7D and RS7E.

Fig. 3.6: Spalling of concrete cover from slab panels RS6H and RS7H.

Fig. 3.7: Spalling of concrete cover from slab panels RS7J and RS9E.

Fig. 3.8: Spalling of concrete cover from beam 14B7(G-H).

Fig. 3.9: Spalling of concrete cover from column 8C8F.

Fig. 3.10: Spalling of concrete cover from core wall 8WN and 12WN.

3.5Concrete Colour

3.5.1Concrete colour on various floors was observed as below:

a. Floors 4 to 6Carbon deposition

b. Floors 7, 13 and 16Light grey

c. Floors 8, 9 and 12Yellowish grey

d. Floors 10, 11, 14 and 15Pink grey

3.5.2Concrete elements on ninth, tenth and eleventh floor were found calcinated.

3.6Recording and Measurement of Cracks. The cracks were marked on the affected elements with highlighter as per their true locations. Subsequently, these were measured with the help of crack detection microscope (precision level 0.02 mm) and recorded on the drawings appended as drawing sheets 2/35 to 32/35. Variation of crack widths along their longitudinal axes had also been noted and recorded on drawing sheets 33/35 to 35/35. Summary of distressed members is listed in Table 3.1. Slab top surface cracks were also observed (Fig. 3.11).

3.7Related Observations. Debris were examined to collect additional information that could be helpful for subsequent investigations. Following was observed:

3.7.1Glass. Glass panes were observed melted and disfigured (Fig. 3.12).

3.7.2Steel. Steel almirahs, lockers, computer casings, furniture and tube rod shades were found disfigured (Fig. 3.13).

3.7.3Wood. Wooden members used for false ceiling and partitions were burnt. The charring depth was observed from 25 mm to 37 mm on different floors (Fig. 3.14).

3.7.4Aluminium Alloys. Most of locks were made of aluminium alloy and were found melted (Fig. 3.15).

3.7.5 Lead. Lead had completely melted (Fig. 3.16).

3.7.6 Masonry Wall. Spalling of plaster and cracking in the walls was observed on all the floors.

Floor

Element Type

No. of Elements Cracked

Size of Cracks

(mm)

Remarks4

Column

06

0.1 to 0.8

5

Beam

05

0.2 to 0.3

Slab

21

0.1 to 0.5

Concrete cover of 1 x slab spalled

Column

08

0.1 to 0.5

6Beam

07

0.1 to 0.2

Slab

13

0.2 to 0.5

Concrete cover of 1 x slab spalled

Column

10

0.14 to 0.4

7Beam

12

0.2 to 1.0

Slab

11

0.2 to 0.4

Column

08

0.14 to 0.4

8Beam

10

0.3 to 0.6

Slab

05

0.2 to 0.4

Column

10

0.12 to 0.4

Concrete cover of 1x column spalled

9

Beam

13

0.2 to 0.4

Slab

11

0.2 to 0.4

Column

10

0.1 to 0.8

10Beam

16

0.12 to 0.2

Slab

04

0.6

Column

09

0.2 to 0.8

11

Beams

19

0.2 to 0.8

Slab

10

0.2 to 0.6

Column

08

0.1 to 0.8

12Beam

11

0.2 to 0.6

Slab

15

0.2 to 0.6

Column

10

0.1 to0.6

13

Beams

20

0.1 to 0.2

Slabs

05

0.2 to 0.6

Column

10

0.4 to 0.6

14Beam

12

0.12 to 0.2

Concrete cover of 1 x beam spalled.

Slab

06

0.2 to 0.6

Column

04

0.1 to 0.4

15

Beam

23

0.2 to 0.8

Slab

28

0.2 to 0.4

Column

4

0.1 to 0.4

16

Beam

22

0.12 to 0.2

Slab

28

0.1 to 0.2

Column

08

0.1 to 0.4

Roof Slab

Beam

04

0.1 to 0.4

Slab

28

0.2 to 0.8

Concrete cover of 10 x slabs spalled

Table 3.1: Summary of distressed members.

Fig. 3.11: Slab top surface cracks in panels 9S5H and 10S4F.

Fig. 3.12: Molten glass.

Fig. 3.13: Disfigured steel ware.

Fig. 3.14: Charring of wood.

Fig. 3.15: Molten Aluminum lock.

Fig. 3.16: Molten lead.

3.8 Propagation/Initiation of Cracks. During measurement of the cracks, it was observed that new cracks had appeared in addition to recorded cracks on the already marked beams or even on the new members. The variation of cracking extent between the initial structural survey phase and final phase is given in Table 3.2.

Floor

No of cracked beams on 30 April

Additional cracking after 30 April

Total

2/5

3/5

4/5

10/5

14/5

5

02

02

6

04

01

01

06

7

09

02

11

8

06

03

01

10

9

11

01

12

10

07

08

01

16

11

10

06

02

18

12

11

11

13

15

04

01

20

14

10

01

01

12

15

20

01

02

23

16

19

01

02

22

Roof Slab

04

04

Table 3.2: Summary of beam cracks showing continuity in crack initiation and propagation.

List of Drawings

S/NoTitle of DrawingsSheet No.

1.Typical Building Plan Showing Orientation,1/35

Grid Lines and Floor Levels

2.Fifth Floor Slab Cracks2/35

3.Fifth Floor Beam Cracks3/35

4.Sixth Floor Slab Cracks4/35

5.Sixth Floor Beam Cracks5/35

6.Seventh Floor Slab Cracks6/35

7.Seventh Floor Beam Cracks7/35

8.Eighth Floor Slab Cracks8/35

9.Eighth Floor Beam Cracks9/35

10.Ninth Floor Slab Cracks10/35

11.Ninth Floor Beam Cracks11/35

12.Tenth Floor Slab Cracks12/35

13.Tenth Floor Beam Cracks13/35

14.Tenth Floor Beam Cracks14/35

15.Eleventh Floor Slab Cracks15/35

16.Eleventh Floor Beam Cracks16/35

17.Eleventh Floor Beam Cracks17/35

18.Twelfth Floor Slab Cracks18/35

19.Twelfth Floor Beam Cracks19/35

20.Thirteenth Floor Slab Cracks20/35

21.Thirteenth Floor Beam Cracks21/35

22.Thirteenth Floor Beam Cracks22/35

23.Fourteenth Floor Slab Cracks23/35

24.Fourteenth Floor Beam Cracks24/35

25.Fifteenth Floor Slab Cracks25/35

26.Fifteenth Floor Beam Cracks26/35

27.Fifteenth Floor Beam Cracks27/35

28.Sixteenth Floor Slab Cracks28/35

29.Sixteenth Floor Beam Cracks29/35

30.Sixteenth Floor Beam Cracks30/35

31.Roof Slab Cracks31/35

32.Roof Beam Cracks32/35

33.Variation of Crack Width in Beams33/35

34.Variation of Crack Width in Beams34/35

35.Variation of Crack Width in Beams35/35

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