Influence of diagonal braces in RCC multi-storied … approach to the analysis of dynamic response of lightly damped structures is by ... The sixteen storied building is modeled using

INTERNATIONAL JOURNAL OF CIVIL AND STRUCTURAL ENGINEERING

Volume 3, No 1, 2012

© Copyright by the authors - Licensee IPA- Under Creative Commons license 3.0

Research article ISSN 0976 – 4399

Received on June, 2012 Published on August 2012 214

Influence of diagonal braces in RCC multi-storied frames under wind

loads: A case study

Suresh P1, Panduranga Rao B

2, Kalyana Rama J.S

3

1- Post Graduate Student, Civil Engineering Department, V. R. Siddhartha

Engineering College, Vijayawada, Andhra Pradesh, India

2- Professor and Head, Civil Engineering Department, V. R. Siddhartha Engineering

College, Vijayawada, Andhra Pradesh, India

3- Assistant Professor, Civil Engineering Department, V. R. Siddhartha Engineering

College, Vijayawada, Andhra Pradesh, India

[email protected]

doi:10.6088/ijcser.201203013021

ABSTRACT

Structures are classified as rigid and flexible. Tall structures are more flexible and susceptible

to vibrations by wind induced forces. In the analysis and design of high-rise structures

estimation of wind loads and the inter storey drifts are the two main criteria to be positively

ascertained for the safe and comfortable living of the inhabitants. Estimation of wind loads is

more precise with gust factor method. Inter storey drift can be controlled through suitable

structural system. The present investigation deals with the calculation of wind loads using

static and gust factor method for a sixteen storey high rise building and results are compared

with respect to drift. Structure is analyzed in STAAD Pro, with wind loads calculated by gust

factor method as per IS 875-Part III with and without X- bracings at all the four corners from

bottom to top.

Keywords: Wind analysis, gust factor, inter-storey drift, drift.

1. Introduction

In the case of static structures the flow interacts only with the external shape of the structure.

When the structure is very stiff deflections under the wind loads will not be significant, and

the structure is said to be "Static''. The only design loading parameter of importance is the

maximum load likely to be experienced in its lifetime. The parameters most relevant to the

assessment of design loading are Influence functions, size parameters of the structure, load

duration and assessment of load for design. In the case of dynamic structures, there is an

additional interaction with the motion of the structure. When the structure is sufficiently

flexible, the response to wind loads is significant to the design of the structure. The

conventional approach to the analysis of dynamic response of lightly damped structures is by

resolving the response into the natural modes of vibration, characterizing each normal mode

as a set of model parameters: 1) Model shape, 2) Model mass, 3) Model stiffness and 4)

Model damping. Using these parameters a frequency response function can be defined that

describes the dynamic characteristics of the structures. Gust Effectiveness Factor method is a

more realistic and rational approach to deal with wind loads on tall building towers. Various

studies have also been made regarding the provision of steel braces and infill walls. (T. El-

Amoury et.al, 2005) developed two rehabilitation techniques were proposed to improve the

dynamic response of framed structures. Fiber reinforced polymer (FRP) jackets were used as

a local rehabilitation technique to enhance the joint shear strength and ductility. As another

Influence of diagonal braces in RCC multi-storied frames under wind loads: A case study

Suresh P, Panduranga Rao B, Kalyana Rama J.S

International Journal of Civil and Structural Engineering 215

Volume 3 Issue 1 2012

option, X-steel braces were installed in the middle bay of the frame along its height as an

alternate lateral load resisting system. For each frame, failure sequence and inter storey drift

were examined. It was found that FRP wrapping eliminated the brittle failure modes without

significant change in the structural response. However, steel bracing significantly contributed

to the structural stiffness and reduced the maximum inter storey drift of the frames.

(Mahmoud R. Maheri, 2005) adopted various local and global retrofitting techniques of RC

frames and used various external and internal steel bracing systems in a structure. (Mahtab

et.al, 2008)] made an economical comparison for steel plate shear wall and cross bracing

system for a ten storey bending frame using push over analysis as per FEMA 356 and it was

observed that using steel plate shear walls, the consumed steel volume is 30% lower than that

of bracings. (Pravin B. Waghmare, 2011) compared the retrofitting of RC building using steel

bracing and infill walls. Of both the methods use of structural wall in the ground storey panel

gave the maximum strength and ductility. (Cengizhan Durucan et.al, 2010) focused on a

proposed seismic retrofitting system (PRS) configured to upgrade the performance of

seismically vulnerable reinforced concrete (RC) buildings. The PRS is composed of a

rectangular steel housing frame with chevron braces and a yielding shear link connected

between the braces and the frame. The retrofitting system is installed within the bays of an

RC building frame to enhance the stiffness, strength and ductility of the structure.

The objective of this paper is to study the response of sixteen storied building with and

without the provision of corner bracings. Variations of Axial loads, moments, total drift and

inter-storey drift at different heights are presented and discussed.

2. Problem modeling

The sixteen storied building is modeled using STAAD Pro software. The structure consists of

2 bays along Z-direction and 8 bays along X-direction with a total height of 45 m.

.

Figure 1: Plan (Top view Size of the beams - 230 X 300;

Size of the Columns - 600 X 600)

2.1 Loading Conditions

Dead Load: Self weight + floor load 4KN/m2 + UDL of 12 KN/m for fifteen floors + UDL of

6 KN/ m on the roof. Live Load: Floor load 2 KN/m2. Wind pressures on the structure are

calculated manually using static and gust factor method.





Figure 2(a): Structure without Bracings Figure 2(b): Structure with Bracings

Figure 3: Bracings in X-direction Figure 4: Bracings in Z-direction

3. Calculation of wind load

Wind load is calculated based on two different methods a) Static method and b) Gust factor

method.

3.1 Static method

The basic wind speed Vb of a region correspond to certain reference conditions as highlighted

in the earlier section, and shall be modified to include the effects of risk level, terrain





roughness, height, structure size and local topography to obtain design wind speed, Vz in m/s

at height z for the chosen structure as given below

Vz = Vbk1 k2 k3; Where Vz = Design wind speed (in m/s) at height z

Pz = 0.6Vz2

Vb = Basic wind speed for the site (50m/s)

k1 = Probability factor or risk coefficient with return periods

k2 = Factor for the combined effects of terrain (ground roughness) height and size of the

component on structure. Table-2, Terrain category3, Class B- from IS 875 Part III- 1987)

k3 = Factor for local topography (hills, valleys, cliffs, etc.)

The following table shows Static pressures calculated using IS: 875 (Part III)

Table 1: Wind Pressure at different elevations using static method

Level Elevation

(m) 2K ZV (m/s) )/( 21 mKNPZ

1 48 1.084 54.20 1.762

2 45 1.075 53.75 1.733

3 42 1.066 53.30 1.704

4 39 1.057 52.85 1.675

5 36 1.048 52.40 1.647

6 33 1.039 51.95 1.619

7 30 1.030 51.50 1.591

8 27 1.015 50.75 1.545

9 24 1.000 50.00 1.50

10 21 0.985 49.25 1.455

11 18 0.964 48.20 1.394

12 15 0.940 47.00 1.325

13 12 0.904 45.20 1.225

14 9 0.904 45.20 1.225

15 6 0.904 45.20 1.225

16 3 0.904 45.20 1.225

17 0 0.904 45.20 1.225

3.2 Gust Factor method

According to IS: 875 (Part III) the following table is calculated using different relations.





Table 2: Wind Pressure at different elevations using gust factor method

Level Elevation

(m)

1K 1

ZV (m/s) )/( 21 mKNPZ

1 48 0.694 37.70 2.348

2 45 0.685 34.25 2.287

3 42 0.676 33.80 2.227

4 39 0.667 33.35 2.168

5 36 0.658 32.90 2.110

6 33 0.649 32.45 2.053

5 30 0.640 32.00 1.996

6 27 0.625 31.25 1.904

7 24 0.610 30.50 1.814

8 21 0.595 29.75 1.725

9 18 0.574 28.70 1.606

10 15 0.550 27.50 1.474

11 12 0.520 26.00 1.318

12 9 0.500 25.00 1.219

13 6 0.500 25.00 1.219

14 3 0.500 25.00 1.219

15 0 0.000 0.00 0.00

4. Results and discussion

A sixteen storied structure is modeled and analyzed for wind pressures using STAAD PRO

with and without braces. The following inferences have been made.

1. Figure 5 shows the values of wind pressures computed by static method and code

based gust factor method, it can be seen that there is marginal difference between

the two methods up to three floors from the ground level, but from the fourth floor

the gust pressures are more than static pressures.

2. Figure’s 6 and 7 shows the variation of inter-storey drift with respect to the height

of the structure considered in X and Z directions. Fig. 6 shows that the inter-storey

drifts in the X- direction is controlled well due to corner bracing, but has no

influence of braces on the top floors. In the unbraced structure, inter-storey drift is

much less than that of braced structure in the top floors. The reason behind this is,

while performing serviceability check for total drift corner bracings will have

much influence on the structure. Figure.7 clearly implies that with the provision of

corner bracings in Z-direction the inter-storey drift gradually decreases compared

to that in X-direction.

3. Figures. 8 and 9 shows the variation of total drift with respect to the height of the

structure in X and Z directions. Figure.8 shows that total drift in X-direction is

controlled with the provision of braces due to corner bracing and high stiffness

values. Whereas in Z- direction Figure.9 the total drift is fairly high because of

insufficient stiffness in the X-direction although corner bracings are provided.





Figure 5: Variation of Static and Gust pressure with height

Figure 6: Interstorey drift in X-direction Figure 7: Interstorey drift in Z-direction

Figure 8: Total drift in X-direction Figure 9: Total drift in Z-direction





For the structure analyzed, three frames i.e. Frame-1(figures 10 to 17), Frame-3(figures 18 to

25), and Frame-5(figures 26 to 33) from Figure 3 were considered parallel to the X-Z plane

and checked for the variation of axial loads and column moments in X and Z directions with

respect to the height of the structure and following inferences were made.

Frame-1

Figure 10 and 11: Variation of axial loads and height for internal and external columns in X-

direction

Figure 12 and 13: Variation of axial loads and height for internal and external columns in Z-

direction





0

5

10

15

20

25

30

35

40

45

50

0 50 100 150 200 250

HE

IGH

T

(m)

MOMENTS -Z kN-m

BRACED

UNBRACED

0

5

10

15

20

25

30

35

40

45

50

0 50 100 150 200 250

HE

IGH

T

(m)

MOMENTS-Z kN-m

BRACED

UNBRACED

Figure 14 and 15: Variation of moments and height for internal and external columns in X-

direction

0

5

10

15

20

25

30

35

40

45

50

0 20 40 60 80 100 120

HE

IGH

T

(m)

MOMENTS-Z kN-m

BRACED

UNBRACED

0

5

10

15

20

25

30

35

40

45

50

0 5 10 15 20 25 30

HE

IGH

T

(m)

MOMENTS-Z kN-m

BRACED

UNBRACED

Figure 16 and 17: Variation of moments and height for internal and external columns in Z-

direction

Frame 3


direction






direction

0

5

10

15

20

25

30

35

40

45

50

0 50 100 150 200 250

HEIG

HT

(m)

MOMENTS-Z kN-m

BRACED

UNBRACED

0

5

10

15

20

25

30

35

40

45

50

0 50 100 150 200 250 300

HE

IGH

T

(m)

MOMENTS-Z kN-m

BRACED

UNBRACED


direction

0

5

10

15

20

25

30

35

40

45

50

0 5 10 15 20

HEIG

HT

(m)

MOMENTS-Z kN-m

BRACED

UNBRACED

0

5

10

15

20

25

30

35

40

45

50

0 1 2 3 4 5 6

HE

IGH

T

(m)

MOMENTS-Z kN-m

BRACED

UNBRACED


direction





Frame 5


direction


direction

0

5

10

15

20

25

30

35

40

45

50

0 50 100 150 200 250

HEIG

HT

(m)

MOMENTS-Z kN-m

BRACED

UNBRACED

0

5

10

15

20

25

30

35

40

45

50

0 50 100 150 200 250 300

HE

IGH

T

(m)

MOMENTS-Z kN-m

BRACED

UNBRACED


direction





0

5

10

15

20

25

30

35

40

45

50

0 0.02 0.04 0.06 0.08 0.1 0.12

HE

IGH

T

(m)

MOMENTS-Z kN-m

BRACED

UNBRACED

0

5

10

15

20

25

30

35

40

45

50

0 0.05 0.1 0.15

HEIG

HT

(m)

MOMENTS-Z kN-m

BRACED

UNBRACED


direction

4. Conclusions

The following conclusions may be drawn based on the analysis carried out

1) Frame 1

a) Figures 10 to 13 shows that axial loads in the external frame exterior columns in

braced structure are high when compared with unbraced structure, reduced

gradually and at top floor both are almost same. No significant variation is found

in the external frame interior column.

b) Figures 14 to 17 show that Column moments in the braced structure in the

external frame exterior columns are very high upto to 13m and reduced drastically

upto roof when compared with unbraced structure. In the internal column,

moments are high in the upper floor in the braced structure when compared with

unbraced structure.

2) Frame 3

a) Figures 18 to 21 show that axial loads are almost same in both braced and

unbraced structures.

b) Figures 22 to 25 show that moments in X-direction have reduced significantly in

the braced structure compared to the unbraced structure. In Z-direction moments

are high in the lower floors and then reduced when compared to unbraced

structure. Member forces were controlled in the interior frame columns in braced

structure and reduced drastically when compared with unbraced structure right

from bottom to top.

3) Frame 5

a) Figures 26 to 29 show that axial loads are almost same in both braced and

unbraced structures.





b) Figures 30 to 33 show that column moments in X-direction are almost zero in both

braced and unbraced structures. In Z-direction column moments have reduced

significantly in braced structure compared to that of unbraced structure.

4) In high rise buildings the stability can be achieved by suitably adding the dimensions

of the corner columns with corner diagonal X-bracings. Provision of X- bracings

reduces the amount of drift and bending moments in the structure.

5) Provision of corner bracings can also be used as a retrofitting technique to strengthen

the existing structure as X- bracings will act more like shear walls.

5. References

1. El-Amoury T. and A. Ghobarah, (2005), Retrofit of RC Frames Using FRP Jacketing

or Steel Bracing, JSEE: Summer, 7(2), pp 83-94.

2. Mahtab M. and M. Zahedi, (2008), Seismic Retrofit of steel frames using steel plate

shear walls. Asian Journal of Applied Sciences. 1(4), pp 316-326.

3. Mahmoud R. Maheri, (2005), Recent advances in seismic retrofit of RC frames, Asian

Journal of Civil Engineering (Building and Housing), 6(5), pp 373-391.

4. Prof. Pravin B. Waghmare, (2011), A Comparative study of retrofitting of RC

buildings using steel bracings and infill walls, International Journal of Advanced

Engineering Research and Studies, 1(1), pp 20-23.

5. Cengizhan Durucan, Murat Dicleli, (2010), Analytical study on seismic retrofitting of

reinforced concrete buildings using steel braces with shear link, Engineering

Structures, 32(10), pp 2995-3010.

6. Gopen Paul and Pankaj Agarwal , (2011), Experimental verification of seismic

evaluation of RC frame building designed as per previous IS codes before and after

retrofitting by using steel bracing, Asian Journal of Civil Engineering (Building and

Housing), 13(2), pp 165-179.

7. Manish S. Takey Prof. S.S.Vidhale, (2012), Seismic response of steel building with

linear bracing system (A software approach), International Journal of Electronics,

Communication and Soft Computing Science and Engineering, 2(1), pp 17-25.

8. Bungale S. Taranath , Wind and Earthquake resistant buildings: Structural analysis

and design, CRC press, 2nd

edition May 2012.

9. P. Mendis, T.Nago, N.Haritos, A.Hira, (2007), Wind Loading on tall building,

Electronic Journal of Structural Engineering (EJSE), Special issue- Loading on

structures, pp 41-54.

10. Ming Ju, (2009), Study on Wind loads and responses of tall buildings and structures,

November 8-12, 2009, The Seventh Asia-Pacific Conference on Wind Engineering,

Taipei, Taiwan.





11. Y. Tamura, Wind and Tall buildings, 19-23 July, 2009, European and African

conference on wind engineering, Florence, Italy.

12. Fabio Minciarellia, Massimiliano Gioffrec, Mircea Grigoriud, Emil Simiub, (2001),

Estimates of extreme wind effects and wind load factors: influence of knowledge

uncertainties, Probabilistic Engineering Mechanics, 16(4), pp 331-340.

13. Yin Zhou, Tracy Kijewski, S. and Ahsan Kareem, (2002), Along-Wind Load Effects

on Tall Buildings: Comparative Study of Major International Codes and Standards,

Journal of Structural Engineering, ASCE, 128(6), pp788–796.

14. Davenport, A. G., (1993), Gust loading factors, Journal of Structural Engineering,

ASCE, 93, pp 11–34.

15. Simiu, E., and Scanlan, R., (1996), Wind effects on structures: Fundamentals and

applications to design, 3rd edition, Wiley, New York.

16. B. Dean Kumar, B. L. P. Swami, Y. Balakoti Reddy, (2011), Appropriate Design

Wind Speeds for Structures, International Journal of Earth Sciences and Engineering,

4(6), pp 504-507 .

17. Chen, X. and Kareem, A, (2004), Equivalent static wind loads on buildings: New

model, Journal of Structural Engineering, ASCE, 130(10), pp 1425-1435.

18. T. Balendra, (1995), Vibration of Buildings to Wind and Earthquake loads, Springer;

1st edition, July 18, 1995.

19. SP: 64- Explanatory handbook on Indian standard Code of Practice for design loads

(other than earthquake) for buildings and structures. Part-3, Wind Loads.

20. IS 875: Part III – Code of Practice for design loads (other than earthquake) for

buildings and structures.

Documents

Influence of diagonal braces in RCC multi-storied … approach to the analysis of dynamic response of lightly damped structures is by ... The sixteen storied building is modeled using