Seismic Analysis and Design of a Multi-Storey Building Located in Haql City, KSA

Document Type: Regular Article

Authors

1 Assistant Professor, Department of Civil Engineering, King Khalid University, KSA. On leave from Sudan University for Science and Technology, Khartoum, Sudan

2 Assistant Professor, Department of Civil Engineering, King Khalid University, KSA. On leave from Structural Engineering Department, Zagazig University, Zagazige, Egypt

3 Lecturer, Department of Civil Engineering, King Khalid University, KSA

Abstract

Recently the design of RC building to mitigate seismic loads has received a great attention. Since Saudi Arabia has low to moderate seismicity, most of buildings were designed only for gravity load. The objective of this paper is to analysis design RC building located in the most active seismic zone region in Saudi Arabia to mitigate seismic loads. A multi-storey reinforced concrete building, in Haql city, was seismically analyzed and designed using the Equivalent Lateral Force Procedure with the aid of SAP200 software. The chosen buildings which was Ordinary Moment Resisting Frame (OMR), was analyzed and designed by using SBC 301 (2007) Saudi Building Code [1], SAP2000 (structural analysis software) [2] and ISACOL "Information Systems Application on Reinforced Concrete Columns" [3]. The results showed that the current design of RC buildings located in the most active seismic zone region in Saudi Arabia, Haql city was found unsafe, inadequate and unsatisfied to mitigate seismic loads.

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1.  Introduction

Haql is a town in the northwest of Saudi Arabia near the head of the Gulf of Aqaba, adjacent to Aqaba across the Jordanian border. The coasts of Egypt, Israel and Jordan can be seen from Haql. Haql city is located in the most active seismic zone region of the Kingdom of Saudi Arabia where there is a complicated geological structures and tectonics. This paper is an attempt to study the effect of seismic loads on RC residential buildings located in the most active seismic zone region of the Kingdom of Saudi Arabia. Saudi Arabia is not free from earthquakes. It has experienced many earthquakes during the recent history, and the previous studies in this field demonstrated this argument. Most of existing buildings in Saudi Arabia do not meet the current design standards due to design shortage or construction shortcomings.

The last major event was the 1995 Haql earthquake in the Gulf of Aqaba (magnitude 7.3) which caused significant damage on both sides of the Gulf and was felt hundreds of kilometres away. As far as Saudi Arabia is concerned, the most active area is along the Gulf of Aqaba (Dead Sea transform fault). On 19 May, 2009, 19 earthquakes of M4.0 or greater took place in the volcanic area of Harrat Lunayyir to the north of Yanbu, including a M5.4 event that caused minor damage to structures [4]. The 1995 Gulf of Aqaba earthquake (also known as Nuweiba earthquake) occurred in November 22 at 06:15 local time (04:15 UTC) and registered 7.3 on the moment magnitude scale. The epicentre was located in the central segment of the Gulf of Aqaba. The earthquake occurred along the Dead Sea Transform (DST) fault system, an active tectonic plate boundary with seismicity that is characterized by long-running quiescent periods with occasional large and damaging earthquakes, along with intermittent earthquake swarms. It was the strongest tectonic event in the area for many decades and caused injuries, damage, and deaths throughout the Levant and is also thought to have remotely triggered a series of small to moderate earthquakes 500 kilometres (310 miles) to the north of the epicentre. In the aftermath of the quake, several field investigations set out to determine the extent of any surface faulting, and the distribution of aftershocks was analyzed. Areas affected: Egypt, Israel, Jordan and Saudi Arabia as shown in Figure 1[4]. Recent studies, historical evidences, geological and geophysical observations indicate that parts of the Kingdom fall within seismic risk regions. In western Saudi Arabia, a design peak ground acceleration (PGA) ranging from 0.03g to 0.26g for an economic life of 50 years was suggested. Seismic zonation was established with zone numbers 0, 1, 2A, and 2B [5]. Saudi Arabia is not free from earthquakes. It has experienced many earthquakes during the recent history, and the previous studies in this field demonstrated this argument. Most of existing buildings in Saudi Arabia do not meet the current design standards due to design shortage or construction shortcomings. Therefore, buildings should be designed regarding their capacity for resisting expected seismic effects. The seismic hazard analysis for the Kingdom was performed [6, 7]. Seismograph stations of the Saudi National Seismic Network as shown in Figure 2 [8], was developed for the Kingdom based on the peak ground acceleration, PGA, values calculated for 50 years service lifetime with 10% probability of being exceeded.

 

Fig. 1. 1995 Gulf of Aqaba earthquake [4]

 

Fig. 2. Seismograph stations of the Saudi National Seismic Network [8]

2.  Description and Model of the Building

A six-storey residential buildings with plan and elevations as shown in Figures 3 to 6 is considered for study. The building is composed of moment resisting RC frame with solid slab, 140mm thickness, situated in the most active seismic zone region of the Kingdom of Saudi Arabia. The structure members are made of in-situ reinforced concrete .The overall plan of building is square with dimensions 14.5x15m as shown in Figure 3. Height of the building is 16 m and storey height for each floor is 3.2 m. Columns and beams sizes are shown in Table 1. The building is approximately symmetric in both directions. The plan and some frames of the studied building as shown in Figures 3 to 5. Beams and columns have been modelled as frame elements while in-plane rigidity of the slab is simulated using rigid diaphragm action. The columns are assumed to be fixed at the base. The building is analyzed.

as per seismic provisions provided by SBC 301-2007.

Table 1. The cross section of beams and columns

Building

Beams

Level

Columns

Reinforcement

 

mm

 

mm

 

6 Stories

600*250

1st floor -2nd floor

600*250

12 Φ 16

 

(10 Φ 16)

3nd floor-4rd floor

500*250

10 Φ 16

   

5th floor 5th floor

450*250

10 Φ 16

 

 

Fig. 3. Architectural plan of the studied building

 

 

Fig. 4. YZ elevation @ X=5.5 m of the studied building

 

Fig. 5. XZ elevation @ Y=9.5 m of the studied building

 

Fig. 6. XY Plan of studied building

3.   Current Design

It is a common practice in The Kingdom of Saudi Arabia to design buildings without any consideration of seismic loads. Therefore, the one typical case study has been studied first under the effect of gravity loads and without consideration of seismic loads in order to check the current design. Dead and live loads are following the equations and tables given in the SBC-301-2007(Saudi Arabia) and second under the effect of seismic loads.

4.  Modelling and Analysis of RC Residential Buildings due to Earthquake Loads (Equivalent Static Method as per SBC-303-2007)

Most buildings and structures in the kingdom of Saudi Arabia have not yet been designed and constructed in compliance with earthquake provisions or given any consideration for earthquake effect.

The horizontal seismic loads are defined according to Saudi Buildings Code (SBC-303-2007). The lateral force effect on the structure can be translated to equivalent lateral force at the base of the structure which can be distributed to different stories. According to Saudi Buildings Code (SBC-303-2007), the total seismic base shear force V is determined as follows:

V = Cs*W                                                                                                        )1(

Where: Cs is the seismic coefficient, W is the total weight and V is the base shear. The seismic design coefficient (Cs) shall be determined in accordance with the following equation:

Cs = SDS / (R / I)                                                                                              )2(

Where, SDS = Design spectral response acceleration in the short period range

R = Response modification factor

I = Occupancy importance factor determined

The value of the seismic response coefficient, (Cs), need not be greater than the following equation:

Cs = SD1 / [T. (R / I)]                                                                                       )3(

But shall not be taken less than.

T = 0.1N                                                                                                           )4(

Where N = Number of stories

Cs = 0.044SDS I                                                                                               )5(

Where, SDS = Design spectral response acceleration at a period of 1 sec

T = Fundamental period of the structure (sec)

Design earthquake spectral response acceleration at short periods, SDS, and at 1-sec period, SD1, shall be as follows.

SMS= Fa*SS                                                                                                     )6(

SM1= Fv*S1                                                                                                     )7(

SDS= 2/3*SMS                                                                                                 )8(

SD1= 2/3*SM1                                                                                                 )9(

Where:

SS: the maximum spectral response acceleration at short periods

S1: the maximum spectral response acceleration at a period of 1 sec

Fa: acceleration-based site coefficient

Fv: velocity-based site coefficient

SMS: the maximum spectral response acceleration at short periods adjusted for site class

SM1: the maximum spectral response acceleration at a period of 1 sec adjusted for site class

SDS: the design spectral response acceleration at short periods

SD1: the design spectral response acceleration at a period of 1 sec

5.  Vertical Distribution of Base Force

The buildings are subjected to a lateral load distributed across the height of the buildings

based on the following formula specified by Saudi Buildings Code (SBC-303-2007):

 

Where, Fx is the applied lateral force at level ‘x’, W is the storey weight, h is the storey height and V is the design base shear, and N is the number of stories. The summation in the denominator is carried through all storey levels. This results in an inverted triangular distribution when k is set equal to unity. A uniform lateral load distribution consisting of forces that are proportional to the storey masses at each storey level.

k = an exponent related to the structure period as follows:

For structures having a period of 0.5 sec or less, k = 1

For structures having a period of 2.5 sec or more, k= 2

6.  LOAD COMBINATIONS AS PER SBC-303-2007

As per SBC-301 section 2.3, following load combinations should be considered for design of structures, components, and foundations.

1.4 (D + F)

1.2 (D + F + T) + 1.6 (L + H) + 0.5 (Lr or R)

1.2 D + 1.6 (Lr) + (f1L)

1.2D + f1L + 0.5 (Lr)

1.2D + 1.0 E + f1L

0.9D ± 1.0E

Where:

E      =   ρQE + 0.2SDSD

1.0 ≤ ρ ≤ 1.5

f1      =  1.0 for areas occupied as places of public assembly, for live loads in excess of 5.0 kN/m2, and for parking garage live load.

f1     =   0.5 for other live loads.

SDS = the design spectral response acceleration in the short period range as determined from Section.

QE = the effect of horizontal seismic (earthquake-induced) forces.

Table 6.1 shows the design parameters taken from both codes for analysis of buildings.

7.  Seismic Map for the Kingdom of Saudi Arabia

The Saudi Buildings Code (SBC-303-2007) provides seismic maps for the Kingdom of Saudi Buildings, as shown in Figures 7 and 8.

 

Fig. 7. Maximum Considered Earthquake Ground Motion for the Kingdom of 1 SEC Spectral Response Acceleration (S1 in %g) (5 Percent of Critical Damping), Site Class B. (Region 1) [1].

 

 

Fig. 8. Maximum Considered Earthquake Ground Motion for the Kingdom of 0.2 SEC Spectral Response Acceleration (Ss in %g) (5 Percent of Critical Damping), Site Class B. (Region 1) [1].

8.  Mapped acceleration parameters

The design parameters that are used in the equivalent static method are illustrated as following: The parameters Ss and S1 shall be determined from the 0.2 and 1 second spectral response accelerations shown on country maps

Where S1 is less than or equal to 0.04 and Ss is less than or equal 0.15, the structure is permitted to be assigned to seismic design category A So,

S1= the mapped spectral accelerations for a 1- second period

Ss= the mapped spectral accelerations for short period.

v  On lack of a map of spectral accelerations of S1 and SS, the following can be assumed: S1= 1.25 Z, Ss= 2.5 Z (amendment no. 3 to SI 413 (2009)) or from maps as shown in Figures 7 and 8.

9.  The Results and Discussions

Figures 9 and 10 shows the label of columns and beams of the selected frames

 

Fig. 9. Label of beams and columns in direction XZ@Y=9.5 m

 

Fig 10. Label of beams and columns in direction YZ@X= 5.5 m

 

9.1      Results of analysis of considered buildings due to gravity loads

This part presents the results of analysis and design of considered RC buildings due to gravity loads. We selected one frame in each direction X and Y as shown in figures 9 and 10 for columns and beams.

1. Beams

Table 2 shows the Straining action of some beams in the selected frames at direction YZ @ X = 5.5

 

Table 2. The Straining action of some beams in the selected frames at direction YZ @ X = 5.5

Direction Y-Z @ X=5.5

Load Case Ultimate (1.4DL+1.6LL)

Beam No.

SHEAR

MOMENT 3-3 (KN.m)

 

KN

END

START

B-03

-9.81

0.24

-6,81

B-09

21.7

-29.54

-1.62

B-11

-11.91

4.5

-10.15

B-17

21.42

-29.11

-1.61

B-19

-13.16

7.37

-12.3

2. Columns

Tables 3 shows the Straining action of some columns in the selected frames at direction YZ @ X = 5.5

 

Table 3. The Straining action of some columns in the selected frames at direction YZ @ X = 5.5

Direction Y-Z @ X=5.5

Load Case Ultimate (1.4DL+1.6LL)

Column No.

AXIAL

SHEAR

MOMENT 3-3 (KN.m)

   

KN

END

START

C-01

-907.8

-0.61

-0.62

-2.57

C-03

-1307.75

-10.94

21.51

-13.51

C-09

-586.33

0.56

-1.46

-0.33

C-11

-855.93

-15.98

26.44

-24.68

C-17

-285.18

0.63

-1.17

0.84

C-19

-421.77

-11.1

18.29

-17.22

 

9.2      Results of analysis of considered buildings due to seismic loads

This part presents the results of analysis and design of considered RC buildings due to seismic loads. Using the Saudi Buildings Code (SBC-301-2007) provisions, the following parameters have been calculated to be used as input data for seismic analysis of the selected model with notice that the Haql City falls in region 6. The calculated results of these parameters are as follows:

 

Table 4. Seismic parameter for Haql City according to SBC301

SDS

0.14

CS =

0.0714

SD1

0.04

CS (max.) =

0.0678

I

1.00

CS(min.) =

0.0057

R

2.00

   

W=

812.0 KN

V=

55.1 TON

Take CS= 0.678

 

 

Table 5. Calculation of Base Shear and lateral load distribution with height

Storey

W (ton)

h (m)

w*h

cv

Fx (TON)

Sixth Floor

16.66

21

350

0.015

0.9

Fifth Floor

142.67

18

2568

0.114

6.3

Fourth Floor

268.69

15

4030

0.179

9.8

Third Floor

394.70

12

4736

0.210

11.6

Second Floor

520.72

9

4686

0.208

11.4

First Floor

646.73

6

3880

0.172

9.5

Ground Floor

772.75

3

2318

0.103

5.7

   

SUM(W*H)

22570

SUM FX

55.1

 

1. Beams

Tables 6 shows the Straining action of some beams in the selected frames at direction YZ @ X = 5.5 m due to load case Group-Y

Table 6. The Straining action of some beams in the selected frames at direction YZ @ X = 5.5 m due to load case Group-Y

Direction Y-Z @ X=5.5 m

Load case: GroupY

Beam No.

SHEAR

MOMENT 3-3 (KN.m)

 

KN

END

START

B-03

53.39

-93.73

90.13

B-09

22.07

-30.12

-1.65

B-11

49.02

-86.03

82.88

B-17

21.78

-29.68

-1.64

B-19

32.05

-52.51

49.84

 

Where:

Load Case Group-Y is load combination included seismic loads at Y direction.

Load Case Ultimate is load combination included dead and live loads only

2. Columns

Tables 7 shows the Straining action of some columns in the selected frames at direction YZ @ X = 5.5 m

Table 7. The Straining action of some Columns in the selected frames at direction YZ @ X = 5.5 m due to load case Group-Y

Direction Y-Z @ X=5.5

Load Case: GroupY

Column No.

AXIAL

SHEAR

MOMENT 3-3 (KN.m)

   

KN

END

START

C-01

-922.95

1.23

-2.68

-2.62

C-03

-1331.32

-11.18

21.93

-13.8

C-09

-596.12

1.74

-3.28

2.3

C-11

-871.43

-18.32

27.01

-25.21

C-17

-289.96

0.74

-1.33

1.04

C-19

-429.44

-11.33

18.68

-17.59

10.          Design of structural elements against gravity loads

The reinforced concrete sections were designed according to the BSI 8110 [9] using the limit state design method (Mosley and Bungey, 1997) [10].

10.1. Design of columns

(a) Calculation of internal forces in columns

The columns were designed to resist axial compression forces and bending moment due to gravity load. The design forces in columns obtained from the computer analysis program SAP2000 are shown in Table 8.

*Direction YZ@X=5.5

Table 8. Internal forces in columns due to gravity loads

Column No.

Output Case

Shear Force (KN)

Bending Moment (KN.m)

Axial Force (KN)

C04

1.4DL+1.6LL

11.26

13.99

1372.02

C03

1.4DL+1.6LL

10.94

13.51

1307.75

C02

1.4DL+1.6LL

1.64

3.78

997.52

C01

1.4DL+1.6LL

-0.61

2.57

907.80

 

(b) Design of columns before adding seismic loads

* Direction YX@X=5.5

The design of columns has been performed using a computer program called ISACOL [5]. Figures 11 and 13 show the main window of ISACOL program and sample of a column design.

 

Fig. 11. ISACOL program results for C40 [3].

 

Table 9. Design of columns before adding seismic Loads

Column No.

Original design

Present design

Dimensions

Reinforcement

Dimensions

Reinforcement

C04

250 X 500

12 Φ 16

250 X 500

10 Φ 16

C03

250 X 500

12 Φ 16

250 X 500

10 Φ 16

C02

250 X 500

12 Φ 16

250 X 500

10 Φ 16

C01

250 X 500

12 Φ 16

250 X 500

10 Φ 16

 

                  

250*500                                         250*500

     12 Φ 16                                   10 Φ 16 

Fig. 12. Design of some columns before adding seismic Loads

10.2. Design of beams

As for the beams the internal forces due to gravity loads have been calculated first .Then the BSI [9], has been used to check the existing design .It has been found that the existing design is adequate.

11.          Design of structural elements against gravity loads and earthquake loads

The reinforced concrete sections were designed according to the BSI 8110 [9] using the limit state design method (Mosley and Bungey, 1997) [10].

11.1 Design of columns

 (a) Calculation of internal forces in columns

The columns were designed to resist seismic and gravity load. The design forces in columns obtained from the computer analysis program SAP2000 are shown in Table 10.

* Direction YZ@X=5.5 m

Table 10. Internal forces in columns due to seismic loads.

Column No.

Output Case

Shear Force (KN)

Bending Moment (KN.m)

Axial Force (KN)

C04

GROUPX

159.27

298.95

1397.14

C03

GROUPX

137.69

256.36

1331.32

C02

GROUPX

125.60

223.55

1041.80

C01

GROUPX

96.96

173.70

922.95

(b) Design of columns after adding seismic loads

The design of columns has been performed using a computer program called ISACOL [3]. Figures 12 and 14 show the design of some columns before and after adding seismic loads.

 

Fig. 13. ISACOL program results for C04 [3].

 

Table 11. Shows the design of columnsafter adding seismic loads.

Table 11. Design of columns after adding seismic loads-direction (y)

Column No.

Original design

Including seismic loads

Dimensions

Reinforcement

Dimensions

Reinforcement

C04

250 X 500

10 Φ 16

250*1550

18 Φ 20

C03

250 X 500

10 Φ 16

250*1250

14 Φ 20

C02

250 X 500

10 Φ 16

250*1200

14 Φ 20

C01

250 X 500

10 Φ 16

250*850

12 Φ 20

 

 

                 

250*1200                                         250*850

     14 Φ 20                                         12 Φ 20 

Fig.14. Design of some columns after adding seismic loads

 

12.          Conclusion

This paper provides set of seismic analysis and design of RC buildings located in the most active seismic zone region in Saudi Arabia. The building was analyzed and designed before and after considering earthquake loads applied in two directions; XX and YY. From the results obtained it can be clearly seen that:

1. There are slight changes in the values of the bending moments and shear forces on the beams before and after considering earthquake loads as shown in Tables 2 and 6. There is increase in some internal beams, such as B-3, B-11 and B-19.

2. The values of the bending moments and shear forces on the columns due to seismic loads are nearly five times that due to gravity loads as shown in Tables 8 and 10.

3. The values of the axial forces on the columns due to seismic loads are approximately similar to gravity loads as shown in Tables 8 and 10.

4.As an overall trend the results showed that the current design of RC buildings located in the most active seismic zone region of the Kingdom of Saudi Arabia, Haql city were found unsafe, inadequate and unsatisfied to mitigate seismic loads.

The present study represents the first attempt to investigate the seismic resistance of residual buildings in Haql city in Saudi Arabia. Due to the lack of knowledge about the seismic activity in this country some buildings are designed and constructed without any seismic load consideration. Seismicity of The Saudi Arabia may be considered as moderate. Hence, all buildings should be checked against earthquake resistance. The present paper proposes a simple procedure to check the seismic resistance of such buildings.

The obtained results emphasize the following conclusions:

1-         Current design of some residual buildings in the Saudi Arabia does not consider earthquake loads.

2-         It has been found that the current design of residual buildings in the Haql city is unsafe for the current seismicity of the Haql city.

Acknowledgements

The author would like to express his gratitude to King Khalid University, Saudi Arabia for providing administrative and technical support.

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[7] Al-Haddad, M., Siddiqi, G.S., Al-Zaid, R., Arafah, A., Necioglu, A., and Turkelli, N., “A Basis for Evaluation of Seismic Hazard and Design Criteria for Saudi Arabia”, Journal of Earthquake Engineering Research Institute, EERI, Spectra, Vol. 10, No. 2, May 1994, Okland, California.

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[9] BS 8110. (1997). the Structural Use of Concrete, British Standard Institution, London..

[10] Mosley, W. H. and Bungey, J. H. (1997): Reinforced Concrete Design; BS 8110:Part 1, 2nd Ed. Macmillan , London.