EFFECT OF MULTIPLE PLAN IRREGULARITIES ON

MULTISTORIED REINFORCED CONCRETE BUILDING

A. O. Chowdhury1*, P. C. Ghush2, & W. N. Chowdhury3

1 Lecturer, Department of Civil Engineering, Southern University Bangladesh, Chittagong,

<mapleleaf2@gmail.com> 2 Alumnus, Department of Civil Engineering, Southern University Bangladesh, Chittagong,

<pijushghush1@gmail.com> 3 Alumnus, Department of Civil Engineering, Southern University Bangladesh, Chittagong,

<nocturnalwahid@gmail.com>

*Corresponding Author

ABSTRACT

Bangladesh is an overpopulated small country in the world, so it requires high-rise buildings for limited

available land. In present, a multistoried building can be irregular in shape because of architectural

consideration and unavailable regular plot. A building can have plan and vertical irregularities

depending on its configuration. As plan-irregular building experiences torsion during earthquake, its

components must be designed considering it. Recent earthquakes tipped several buildings in

Bangladesh which made this research essential. In this paper, the effect of multiple plan irregularities

e.g. torsional irregularity, re-entrant corners, diaphragm discontinuity, and non-parallel system of shear

wall on multistoried reinforced concrete building is shown. It is shown that 3D modeling of a

plan-irregular building is a must to consider the actual rotational stiffness which affects moment and

reinforcement calculated thereof. It is also shown that slab reinforcement of a plan-irregular building

differs from that of a plan-irregular building with expansion joints because of moment redistribution.

Moreover, the effect of multiple plan irregularities increases cost of slab reinforcement significantly.

Keywords: Plan irregularity; Expansion joint; Reinforced concrete; Building; Cost

INTRODUCTION

Being more fire and corrosion resistant and less susceptible to vibration, reinforced concrete buildings

are used more all over the world as they can have any shape. A strong earthquake of magnitude 6.8

occurred 26 km west of Chauk in Burma located beside the south-east border of Bangladesh in 2016. In

case of horizontal seismic forces, their directions for design shall be those which will produce the most

critical load effects. In Bangladesh, due to irregular plan of a building, the major concern is torque.

Torsional failure occurs where the symmetry is not planned in the location of the lateral force resisting

elements. The analysis of seismic response of a plan-irregular building is complex because the

structural irregularities cause non-uniform load distribution in various members of a building. Plan

irregularities are related to in-plan asymmetrical mass, stiffness, and strength distribution, causing a

substantial increase of the torsional effect when the structure is subjected to lateral forces (Stefano and

Mariani, 2014). In case of a plan irregular building, the asymmetry is measured by the presence of

torsion in the building. A plan irregular building possesses separate centers of mass and of stiffness.

Eccentricity between them causes torsion in the building which in turn is responsible for successive

damage. The seismic force acts at the center of mass of the floor and is counteracted by the resistance at

the center of rigidity of its stiffening members (Gaur et al., 2014). Plan irregularities often entail the

development of brittle collapse mechanisms due to local increases of seismic demand in specific

elements not always provided with sufficient strength and ductility (Stefano and Mariani, 2014). In this

research, a plan-irregular multistoried reinforced concrete building is analysed, and then its ground and

2nd floor reinforcements are estimated. The same building with expansion joints is also analysed, and

then its ground and 2nd floor reinforcements are estimated. Besides, the total cost of ground and 2nd

floor reinforcements of the plan-irregular multistoried reinforced concrete building is compared with

that of the same building with expansion joints.

1st International Conference on Research and Innovation in Civil Engineering (ICRICE 2018), 12 –13 January, 2018, Southern University Bangladesh (SUB), Chittagong, Bangladesh ISBN: 978-984-34-3576-7

METHODOLOGY

A plan-irregular building, shown in Fig. 1 through 3 and table 1, is modeled on ETABS 2015. Four

regular buildings, shown in Fig. 4 through 9 and table 1, are also modeled. Then, each building is

analyzed and its ground and 2nd floor are designed by BNBC 2006.

Fig. 1 3D View of Irregular Building Fig. 2 Plan View of Ground Floor of Irregular Building

Fig. 3 Plan View of 2nd Floor of Irregular Building Fig. 4 Plan View of Ground Floor of

Regular Building-1

1st International Conference on Research and Innovation in Civil Engineering (ICRICE 2018), 12 –13 January, 2018, Southern University Bangladesh (SUB), Chittagong, Bangladesh ISBN: 978-984-34-3576-7

Fig. 5 Plan View of 2nd Floor of Fig. 6 Plan View of Ground Floor of

Regular Building-1 Regular Building-2

Fig. 7 Plan View of 2nd Floor of Fig. 8 Plan View of Ground Floor of

Regular Building-2 Regular Building-3

Fig. 9 Plan View of 2nd Floor of Regular Building-3

S3 S13

1st International Conference on Research and Innovation in Civil Engineering (ICRICE 2018), 12 –13 January, 2018, Southern University Bangladesh (SUB), Chittagong, Bangladesh ISBN: 978-984-34-3576-7

Table 1 Data of a Typical Building

I. Seismic Force Resisting System Concrete Shear Wall with Concrete

Special Moment Resisting Frame

II. Seismic Zone 2, Chittagong

III. Seismic Zone Coefficient, Z 0.15

IV. Response Modification Coefficient, R 12

V. Structural Importance Factor, I 1

VI. Wind Load

Basic Wind Speed

Exposure Type

Windward Coefficient

Irregular Building

Leeward Coefficient (X Direction)

Leeward Coefficient (Y Direction)

Regular Building-1

Leeward Coefficient (X Direction)

Leeward Coefficient (Y Direction)

Regular Building-2

Leeward Coefficient (X Direction)

Leeward Coefficient (Y Direction)

Regular Building-3

Leeward Coefficient (X Direction)

Leeward Coefficient (Y Direction)

Regular Building-4

Leeward Coefficient (X Direction)

Leeward Coefficient (Y Direction)

161.5 mph

A

0.80

0.46

0.55

0.56

0.30

0.57

0.29

0.58

0.33

0.50

0.50

VII. Soil Factor, S 1.50

VIII. Height of Building 73 ft

IX. Wall Thickness

Internal Masonry Wall

External Masonry Wall

Shear Wall

5 in

10 in

12 and 15 in

X. Unit Weight of Brick 120 pcf

XI. Unit Weight of Concrete 150 pcf

XII. Slab Thicknesses

Waffle Thickness

7, 7.5, 8, 8.5, and 10.5 in

4 in

XIII. Floor to Floor Height

Base to Ground Floor

Ground Floor to 1st Floor

1st Floor to 5th Floor

10 ft

15 ft

12 ft

XIV. Column Sizes

C20D

C20X20

C25X30

C25X38

C27X27

C30X30

D = 20 in

20 in x 20 in

25 in x 30 in

25 in x 38 in

27 in x 27 in

30 in x 30 in

XV. Beam Sizes

B1, B1A, B2, B2A, B2B, B6, B8, B9,

B10, B10A, B10B, B10C, B11, B12,

B12A, B12B, B12C, B12D, B12E,

B12F, B12H, B13, B15, B17, B21A,

B23, B24, B25

B3, B3B, B22, B22A

B3A, B3C, B33

B4, B4A, B7, B7A, B12G, B16

B16A, B18

15 in x 24 in

20 in x 24 in

24 in x 24 in

18 in x 24 in

1st International Conference on Research and Innovation in Civil Engineering (ICRICE 2018), 12 –13 January, 2018, Southern University Bangladesh (SUB), Chittagong, Bangladesh ISBN: 978-984-34-3576-7

Table 1 Data of a Typical Building (Continued)

XV. Beam Sizes (continued)

B5, B10K

B15X17

B15X31.25

B15X34

B15X38.50

B18X17

B18X34

B18X58

B19, B20, B21, LB

10 in x 24 in

15 in x 17 in

15 in x 31.25 in

15 in x 34 in

15 in x 38.50 in

18 in x 17 in

18 in x 34 in

18 in x 58 in

12 in x 24 in

XVI. Reinforcement Yield Strength 60000 psi

XVII. Concrete Compressive Strength

Column

Slab and Beam

4000 psi

3500 psi

XVIII. Superimposed Dead Load

Floor Finish

Roof Finish

20 psf

40 psf

XIX. Live Load

Cabin

Corridor

Storage

42 psf

82 psf

103 psf

RESULTS AND DISCUSSIONS Results of ground and 2nd floors of irregular building are shown in table 2 and 3. Slab panel

reinforcement of 3D model of irregular building is compared with that of the same building computed

by existing methods in BNBC 2006.

Table 2 Result of Ground Floor of Irregular Building

Slab

Panel

Direction

Factored

Moment

(k-ft)

Reinforcement

Provided

(in2/ft)

Ultimate

Moment

(k-ft)

Reinforcement

Required

(in2/ft) Remark

X Y +ve -ve Top Bottom +ve -ve Top Bottom

S3

6.3 0 0.27 6.86 0 0.25 Overreinforced

8.5 0.61 0.13 15.31 0.33 0.13 Overreinforced

7.25 0 0.27 6.86 0 0.28 Underreinforced

22.7 0.61 0.13 15.31 0.97 0.13 Underreinforced

S4

8.57 0 0.27 6.86 0 0.34 Underreinforced

11.3 0.61 0.13 15.11 0.45 0.13 Overreinforced

6.94 0 0.27 6.86 0 0.27 Overreinforced

10.75 0.61 0.13 15.11 0.43 0.13 Overreinforced

S10

4.8 0 0.53 13.25 0 0.19 Overreinforced

6.71 0.8 0.27 19.3 0.25 0.27 Overreinforced

5.57 0 0.48 12.09 0 0.22 Overreinforced

10.84 0.37 0.24 9.62 0.42 0.24 Underreinforced

Table 3 Result of 2nd Floor of Irregular Building

Slab

Panel

Direction

Factored

Moment

(k-ft)

Reinforcement

Provided

(in2/ft)

Ultimate

Moment

(k-ft)

Reinforcement

Required

(in2/ft) Remark

X Y +ve -ve Top Bottom +ve -ve Top Bottom

S18

5.28 0.21 0.44 11.81 0.21 0.19 Overreinforced

21.06 0.65 0.21 15.96 0.89 0.21 Underreinforced

5.36 0.21 0.44 11.81 0.21 0.19 Overreinforced

25.25 0.65 0.21 15.96 1.09 0.21 Underreinforced

S19 5.97 0.24 0.48 12.1 0.24 0.22 Overreinforced

21 0.72 0.24 17.53 0.88 0.24 Underreinforced

1st International Conference on Research and Innovation in Civil Engineering (ICRICE 2018), 12 –13 January, 2018, Southern University Bangladesh (SUB), Chittagong, Bangladesh ISBN: 978-984-34-3576-7

Table 2 shows that all slab panels are overreinforced except S3, S4, and S10. As rotational stiffness is

not well-considered in existing methods in BNBC 2006 to find the provided reinforcement, it is less.

Besides, table 3 shows that all slab panels are overreinforced except S18 and S19 for the same reason.

The most slab panel reinforcements of ground and 2nd floors of regular building 1 and 3 differ from

those of irregular buildings due to moment redistribution. Moreover, most slab panel reinforcement of

ground floor of regular building-2 differs from that of irregular building except 2nd floor of regular

building-2 of which all slab panel reinforcements differ due to moment redistribution. The total costs of

ground floor reinforcement of irregular and regular buildings are 36,81,192.76 and 23,10,405.11 BDT

respectively. Thus, the effect of multiple plan irregularities increases the total cost of ground floor

reinforcement by 59.33% with respect to the total cost of ground floor reinforcement of regular

buildings. Furthermore, the total costs of 2nd floor reinforcement of irregular and regular buildings are

41,53,795.23 and 22,45,107.22 BDT respectively. Therefore, the effect of multiple plan irregularities

increases the total cost of 2nd floor reinforcement by 85.02% with respect to the total cost of 2nd floor

reinforcement of regular buildings.

CONCLUSIONS

The effect of multiple plan irregularities e.g. torsional irregularity, re-entrant corners, diaphragm

discontinuity, and non-parallel system of shear wall on multistoried reinforced concrete building is

studied. The following outcomes are found.

▪ 3D modeling of a plan-irregular building is a must to consider the actual rotational stiffness which

affects moment and reinforcement calculated thereof.

▪ Slab reinforcement of a plan-irregular building differs from that of a plan-irregular building with

expansion joints because of moment redistribution.

▪ The effect of multiple plan irregularities increases cost of slab reinforcement significantly.

ACKNOWLEDGMENTS

The authors acknowledge Professor Engr. M. Ali Ashraf, PEng., Head, Department of Civil

Engineering, Southern University Bangladesh, for his cordial support and guidance during the research.

REFERENCES 1. Bangladesh National Building Code (BNBC) 2006. pp. 10557-10655 and 10797-10822.

2. ETABS 2015. Computers and Structures, Inc., California, USA.

3. Gaur, H; Goliya, RK, Murari, K, and Dr. Mullick, AK. 2014. A parametric study of multi-story R/C

buildings with horizontal irregularity. International Journal of Research in Engineering and

Technology, 3(4): 360-364.

4. Stefano, MD; Mariani, V. 2014. Pushover analysis for plan irregular building structure.

Geotechnical, Geological, and Earthquake Engineering, 34: 429-448.

1st International Conference on Research and Innovation in Civil Engineering (ICRICE 2018), 12 –13 January, 2018, Southern University Bangladesh (SUB), Chittagong, Bangladesh ISBN: 978-984-34-3576-7