1800

RESTORATION OF AN EIGHT STOREYED APARTMENT BUILDING - A CASE STUDY

R. JAGADISH Reader in Civil Engg., Dept. of Civil Engg., U. V.C.E.,

Bangalore University, Bangalore 560 056, INDIA

H. SHARADA BAI Lecturer in Civil Engg., Dept. of Civil Engg., U. V. C. E.,

Bangalore University, Bangalore 560 056, INDIA

ABSTRACT

An eight storeyed apartment building which was completed in alI respects and was partially occupied was investigated for its strength and serviceability. It was found after a thorough check that many footings were over stressed some columns under designed and most of the columns possessed less strength than that was assumed in the designo A feasible rectificatioin scheme was proposed and executed successfully and the building was put into use with satisfactory strength and serviceability requirements.

INTRODUCTlON

The apartment building under study (name of the building is not mentioned

deliberately) consists of Basement Floor plus Ground Floor and Seven Upper

Floorsj measuring about 500 Sq.m . at each floor leveI. The layout plan of

typical floors is given in Figure 1. The building comprises of reinforced

concrete frame work with brick masonry infill walls. The basement floor

" is meant for car parking, ground floor for shopping and the upper seven

floors (each floor containing four apartments) for residential purposes. At

terrace leveI two water tanks, each of capacity 30,000 litres are provided.

The building was suspected for adequate strength and serviceability conditions

and an investigation was carried out. A t the time of the investigation the

construction of the building was over in all respects and was partially

occupied. The promoters of the building made available some of the Architec­

tural Drawings, Structural Drawings and Soil Investigation report for scrutiny.

1801

ífe:ony

, I1All . , . --"-,,ç.1! E1i G1Q Ji[I11 40 ' 11 140X.10.o :

IOXO

i 1i 1Z0X19D j ~Ir:-:íl-.. '. i i j 1&"6· X 30 110,l'! Sorvo, ,

Mosler bad

',:,,"0' II [C, ~':J~ I '00. U,,,.,,,",,. "'li',",' 10'0");40' ll_ L ~ ~ G~ r-'== ~18:;-;:::==:JIF//~To=ila=I=~~

, 'I A7 C7 06 FG ~ O K7 ~ 4~ I1 14 'O'X1o'O S'o'XlO'O" 1/ [ ."---'111 ~'f '/~ Kitch.n S.d room Bo~.

10'o'U : : ' , ! ' - '-l~~VllLJ AS C5 0 5 " , , ;5 ~~~===:IK!=:S ===~Mt=:5 :=JTYPica I

c-' !,

Ir---

U o

8 , W------uO

floor H~ )~ K4 M~. plan ~r=--~I!=rF===~=====F~

ro..") m U Sorvonl ho;.! Kilchtn Bad room Bo I. I H] In M]

K] [I r To il.1

I f T oil. I .--

Enl. Varortdah I )] I I

F'

MZ BoI.

Front elevat/OC1 ~

FIGURE 1. THE BUILDING Ar THE TIME OF INVESffiATON

1802

Using the available data structural designs were checked on a computer

and Non-destructive testing (load tests on typical floors and ultrasonic tests

on columns and beam-column joints) was conducted at alI floor levels. After

a thorough check, design and construction deficiencies were identified.

In this paper proposed rectification scheme and the execution of rectification

works are presented briefly. Detailed ['eport on the investigation and execution

of rectification work was submitted elsewhere [1J.

INVESTIGATIONS CARRIED OUT

The building was investigated in three stages viz., (1) Physical inspection,

(2) 5tructural Design Check and (3) Non-destructive testing and was carried

out ' in the same order.

Physical Inspection

First of alI, a thorough inspection of the building, right from Basement

floor to Terrace floor was made. In the entire building no visible structural

cracks were found in any part of the structure. However, some surface

cracks and plaster shrinkage cracks of minor nature were observed. Overall

dimensions of sla bs, beams and columns at alI floor levels and of typical

footings as per actual construction were measured and found to agree with

those proposed in the structural drawings.

Structural Design Check

In the Architectural drawings it was noticed that boudary walls were proposed

to be of 230mm thick and alI internal walIs of 115mm thick. But during

the inspection of the building, it was obse rved that all walIs were of brick

masonry 230mm thick. This forced to carry out structural design check.

Typical frames were analysed on a computer for alI the dead loads and

live loads as per 15:875-1964, [ 2J i.e., on Ground floor 4 KN/ m2

, typical

floors 2 KN/m2

and on Roof 1.5 KN/m2

• 5tructural designs were made

as per 15:456-1978 [3J and compared with the detailing in the original st ruc­

tural drawings. From this investigation the folIowing observations were

made.

1. Detailing of slabs shown in the original structural drawings were quite

adequate for M15 grade concrete.

2. Quantity of reinforcement as shown in the drawings was sufficient for

M 15 grade concrete in alI beams.

3. At Basement floor and Ground floor levels columns A2'

B 2' C 7' C 9' D 2' E 8' E 11' G 2' G 8' G 10' 18, 111 , J 2'

A 3' A 7' A9'

J3, J4, K3' K4' K9

-

1803

and L 2 were inadequately reinforced by 15-25% for concrete grade of

M20.

4. For the provided dimensions except footings C4, C 5' D 5' D 6' F 1 to F 6 and

K 3 all the other footings were stressing the soU by about 15-40% more

than the recommended safe bearing capacity of the sou of 210 KN/ m 2.

Non-Destructive Testing

To assess the structural soundness of beams and slabs load tests were conduc­

ted; and the existing strength of hardened concrete in columns and beam­

column junctions was determined by Ult rasonic and Rebound hammer tests[4}.

Lood Test on Beams and Slabs

Even though the detailing of beams and slabs sa tisfied the design check,

the strength of concrete (M 15) assumed in the design was suspected in the

actual structure. Hence load tests were conducted on typical portion of

floors as per the requirements in Clause 16 of IS:456-1978, for verifying

the structural soundness of slabs and beams at every {Zoor leveI of the

building. As construction of the building was over completely all the dead

loads were already acting and hence only live loads were applied on {Zoors

for load test (1.25 X 4 = 5 KN/ m2

on CF and 1.25 X 2 = 2.5 KN/ m2

on

typical floors).

Sand bags of 0.5 KN weight were used for loading purposes (Figure

2). Maximum deflections were measured using dial gauges of least count

O.Olmm at criticaI sections of beams and slabs after 24 hours of loading

and recovery of de{Zections was recorded wherever necessary. Measured

deflections were compared with the maximum allowable deflections.

1t was satisfying to note from the experimental results that almost

all the beams and slabs fulfilled the IS codal requirements and hence declared

as structurally sound.

Ultrasonic Test and RebOtmd Hammer Test

To assess the cor lpressive strength of hardened concrete in columns both

Ultrasonic and Rebound Hammer Tests were conducted (Figure 2). Plaster

on the columns at the se lected points (top, centre, and bottom) was removed

and the experiments were conducted on bare concrete. An abstract of

Lest results of typical columns is presented in Table 1.

It is seen that most of the columns tested possess concrete strength

lower than that was suggested in the original struc tural designs. In addition

beam-column joints were found to be honeycombed with lower compressive

st rength.

a) TYl2ical floor loaded with sand bag s

c) ultrasonic test

1804

""

b)Measurernent of deflections

d) Rebound harrmer test

FIGURE 2. NON OESTRUCTIVE TESTS IN PROGRESS

-

1805

TABLE 1

ABSTRACT OF NON-DESTRUCTIVE TEST RESULTS ON COLUMNS

Number of columns in the Building = 57 2 Oêu = Cube compressive strength of concrete in N/mm .

SI No

1 2 3 4 5 6 7 8 9

Floor leveI

Basement Ground First Second Third Fourth Fifth Sixth Seventh

No. of columns tested

31 43 23 20 22 21 24 23 24

Minimum CTêu used in crêu design

recorded

15.3 16.4 12.6 12.8 12.0 11.6 12.2 11.6 11.6

20.0 20.0 15.0 15.0 15.0 15.0 15.0 15.0 15.0

RESTORATION SCHEME

No. of columns with (Jêu lower than design OCu

26 19 15 17 15 21 23 22 24

Certain structural design inadequacies and construction deficiencies were

identified in the building from the investigations carried out. Most of the

footings were found to be overstressingthe soil beneath and so me columns

were underdesigned in addition to possessing lower concrete compressive

strength. To take care of these defects a feasible rectification scheme

was proposed and later on executed accordingly.

Foundation Rectification

Rectification of footings was necessary to see that the resultant soil pressure

any where did not exceed the recommended safe bearing capacity of soil

210 KN/m2 .

All the footings which were overstressed were first widened by 300mm

alround in order to reduce the soil pressure. The soil around the footing

to be rectified was excavated for a width of 300mm alround and upto

the bottom of the base concrete. 200mm width of the existing footing

was chipped off such that the reinforcement was clearly exposed. The

existing reinforcement was extended by 300mm by welding additional reinfor­

cement of the same diameter (Figure 3). A layer of epoxy paint was applied

on the chipped vertical surface after thoroughly cleaning it. lt was then

concreted with M 15 concrete.

lt was also decided to convert all infill walls into load bearing walls,

in tum transfering the wall load dif'ectly to the ground to a considerable

~

§

o ~

~

1806

..-, ..-, Ali ClI ElI 111 Kll M Pl PI PI PI

GIO pt PI PI

A9 CIl PI PI Pl E8 G8 18 PI K9 M 9

PI P2 W2 PI

A7 C7 06 F6 é!F{ K7 M

li ! I !! l! ~ L1FT

A5 C5 05 F5 r~ K5 M5

7

Pl = SINGLE PIPE SUPPORT ~ 8. 8. MASONRY WALLS P2 = OOU8LE PIPE SL.JPFmT AT 8. F. 31.5MM THICK

A 10M COWMNS AT G·F. 230MM THICK PI Hi. Jl. Kl. MI.

AI. Ci. 04 PI El. PI P2 P2 PI Pl

PI PI PI PI P2 P2 PI Pl

A"l C) 03 F) H3 P2 'j] K3 M3 I

PI PI PI P2 PI A2 82 02 62 J2 L2

PI PI P2 PI M2

AI 1'\1 nl FI HI JI LI Ml ,

M15 CONCRETE 1//1 '-; OVER EXISTING FOOTING

EXISTING FOOTING

c;k 750 h b) neral \VOU f~ tion. b) Wall fOlJll<bt íon rmr a collJJlO FIGURE 1. CEtAlLS DF ADOOI(}.JAL lOAD BE.ARlf\JG WAL LS

-

a) Footirg opened for wldening

C)100mmJap left between mason top & beam 50fh t

1807

b)Additional brickmasonry wall constructed at B F

FIGURE 4. CONSTRUCTION OF AOOITIONAL lDAO BEARING BRICK MASONRY WALLS

1808

extent. To effect this, additional table moulded burnt brick masonry walls

of 345mm thick in 1:6 C.M. with appropriate size stone masonry foundation

in 1:8 C.M. (Figure 3) was constructed in Baserr.ent floor and 230mm thick

masonry wall in 1:6 C.M. in Grow1d floor.

To see that complete load transfer from beams to walls was affected,

initially a 100mm gap was left deliberately between the top of masonry

wall and the beam soffit . This was then filled and well compacted with

M15 concrete mixed with 2% expansive cement (Figure 4).

Rectification of Columns

The columns were rectified in two different ways depending upon the degree

of deficiency.

1. 50me of the columns which carried load more than their capacity were

reli eved of the extra load by providing additional mild stee l pipe supports

around them and

2. Columns and beam-column joints possessing low concrete c ompressive

strength due to honeycombing were pressure grouted to fil! up the pores

thereby increasing the strength.

Providing Additional Pipe Supports Around Deficient Columns

Mild steel pipes of 150mm diameter conforming to 15:1161-1968 were provi­

ded around 011 deficient columns as additional supports at basement as

well as ground floors (Figure 5 and 6). This was provided to reduce the

column loads by load equivalent to two floors and bring back the columns

within the safe ty limits. The rectification schem e corresponding to this

was executed in the following steps.

1. Top surface of the footing was hacked such that a rough surface was

obtained and thoroughly cleaned.

2. A thin layer of epoxy resin was applied on the surface as per the manufac­

turer's spec ifications.

3. Plain concrete of M 15 grade was laid upto the top of the existing sloped

footing with proper compaction such that a flat surface was obtained.

4. Foundation bolts as per specification were inserted into the wet concrete

for required depth to r eceive the mild steel pipes.

5. After the concrete hardened, 1507T'm dia mild steel pipe with base plates

at bottom and top was inse rted into the projecting foundation bolts.

While the base plote at bottom was tightened against the foundation

bolts using nuts, the base pIa te at top was welded to the bottom of

the reinforcement of the beam to be supported.

~~.-

. 1 f.-a 1$" " "i 10MM TH. BASE PLATE 230X300 SIZE WELDED TO THE EXISTING 8EAM BARS· TO BE ALLED.UP 1:4 CM. li IjI WITHA LAYER OF CHICKEN MESH

ISO DIA.M . S. PIPE'

8 MM TH· FlliET WEU ( TYP.)

8ASEMENT FLOOR LEVEL

10MM M · S. BASE PlATE

12/viM ANCIiJR' 8OLT6, . : .

.. , .. .-/ \ , : ..

BEAM

_4. • # _

~

PC.C,I:2:4

~

'~ ... , EXISTING FOO11NG SURFACE OF EXISTING FOOllNG TO BE ROUGHENED ANO EPOXY PAINTED .

ADDIn:JNAL REINFORCEMENT WEl.DED

=;:.== =..=-~-=-.:=.. =--=--~--=--=----

a) At basement floor ,

FIGURE 5. TYPICAL AOOITfONA L

FtRST FLOOR ~

8MM FILLET WELD (TYP)

600

BEAM MA.IN E)ARS

L-+-----t-TO SE FILLED -UP 1:4CM.

...

WITH A LAYER OF CHICKEN MESH.

XISTIN<i COlo

·12MM t>NCHOR 80LTS

b) At ground f loor

PIPE SUPPORTS PROVIDED

...... 00 o \O

a) Typ'lcal single pipe support at B F

c) Typica I pípe support at ground floor

1810

b) Typícal double pípe sUDPort

d) Groutu"'g of columns

FIGURE 6. AOOITIOt'--LAL PIFE SUPRJRTS & GROUTlNG

1811

6. The pipe was then jacked from bottom such that it provided firm support

to the beam.

7. The bottom support and the top support were grouted with cement mortar

mixed with expansive cement.

This procedure was repeated for all deficient columns at basement

and steps 4 to 7 for ground {Zoor columns.

Grouting of Columns and Beam-Column Joints

Almost ali honeycombed columns and beam-column joints were grouted

as follows (Figure 6).

1. Holes of 10mm dia, spaced at 500mm c/c were drilled over the entire

'height of column and at beam-column joints.

2. 10mm dia PVC tubes were inserted into the drilled holes.

3. A neat cement paste with 2% expansive cement as admixture was prepared

as per manufacturer's specifications.

4. The cement paste thus prepared was grouted under apressure of 0.4-0.5

N/mm2

into the cavities in concrete, until the paste refused to go in

any more.

This procedure was repeated for ali columns and beam-column joints

at alI floor leveIs.

CONCLUSIONS

An eight storeyed apartment building which was found to be defective

both in structural designs and construction was thoroughly investigated,

causes for deficiencies were identified and later on the building was redesig­

ned as per actuals. A feasible rectification scheme was proposed and execu­

ted successfully to the required safety and serviceability conditions.

REFERENCES

1. Jagadish,R. and Rajanna, B.C., A report on the investigations and restora­tion of the apartment building - Report submitted to the Corporation of the City of Bangalore, Jan.1985.

2. 15:875-1964 Code of Practice for 5tructural 5afety of Buildings. Loading 5tandards (Revised).

3. 15:456-1978 Code of Practice for Plain and Reinforced Concrete.

4. Bungey, H. John. The testing of concrete in structures, 5urrey University Press Publication, New York. 1982.