Assessment Of Seismic Performance And Strengthening Of RC Existing Residual Buildings In The Sudan

Assessment Of Seismic Performance And Strengthening Of RC Existing Residual Buildings In The Sudan

M. A. Ismaeil1, M. E. Sobaip, A. E.Hassaballa3

1King Khalid University, KSA. On Leave from Sudan University of Science and Technology, Khartoum, Sudan,

2Alhosn University, UAE. On Leave from Cairo University, Giza, Egypt,

3Jazan University, KSA. On Leave from Sudan University of Science and Technology, Khartoum, Sudan

Abstract

The evaluation of seismic performance of existing buildings has received a great attention in the last decade. A common engineering practice in the Sudan not to consider earthquake effects in the design of all buildings. Therefore, all types of buildings in the Sudan are not earthquake-resistant. The objective of this paper is to assess the seismic performance of existing residual buildings in the Sudan. One case study has been chosen for this purpose. The evaluation has proved that the columns of four-story residual buildings are not seismically safe. A comparative study has been done to choose a suitable strengthening method. An effective method has been proposed by adding RC shear walls. Three cases of same positions for the shear walls with thicknesses of 20 cm, 15 cm and 10 cm have been examined. It has been proved that RC wall with 15 cm thickness is suitable strategy for this case to reduce the seismic vulnerability of exiting (RC) buildings in the Sudan.

1. Introduction

   Seismic Analysis is a subset of structural analysis and is the calculation of the response of a building structure to earthquakes. It is a part of the process of structural design, earthquake engineering or structural assessment and retrofit in regions where earthquakes are prevalent. Sudan and its vicinity, which spans several countries, have diverse geologic and tectonic structure. So that the Sudan is not free from earthquakes. It has experienced many earthquakes during the recent history, and the previous studies in this field demonstrated this argument. This paper is an attempt to study the effect of seismic loading on residual buildings in the Sudan.

2. Description of Study Case.

   The case performed in this study is a typical four- story model for residual building in the Sudan .The buildings are comprised of a reinforced concrete structural frame. The structure members are made of in- situ reinforced concrete .The overall plan dimension is 20 x 17.5 m. Height of the building is 12 m .The floor is a flat slab system. Figures 1- 4 show detailed information of the structural and architectural layout.

3. Current Design.

   It is a common practice in the Sudan to design buildings without any consideration of seismic loads. Therefore, the residual building has been studied first under the effect of gravity loads only without consideration of seismic loads in order to check the current design. Dead and live loads are following the rules given in the (BS 8110, 1997) [2].

   Figure 1. Plan of residual buildings considered

   Figure 2.South Elevation

   Figure 3. Section x-x

   Figure 4.Foundations and columns plan

   1. Numerical model

      Numerical models for the case has been prepared using SAP2000 version 14 (Computers and Structures) [3]. Beams and columns are modelled as frame elements while walls and slabs are modelled as shell elements. In this paper the seismic performance of the considered residual building will be evaluated using the linear static analysis procedure. This procedure uses a simple estimate of the structures fundamental period and the anticipated maximum ground acceleration together with other relevant factors to determine a maximum base shear. Horizontal loading equivalent to this shear is then distributed in some prescribed manner throughout the height of the building to allow a static analysis of the structure. This method is simple and rapid .Figure5 shows the models for the four-story building. The label of columns is shown in Figure 6.

      Figure 5. Model of 4-story residual building

      Figure 6. Label of columns

   2. The modelling assumptions

      The following assumptions are considered:

      1. The cross section of beam and column members are input according to the original design

      2. Rigid diaphragm is assumed

      3. Residual Building is modelled as 3-D frames with fixed supports at the foundation level.

4. The columns in all selected models are assumed fixed at the base and supported on isolated footing.

1. Check of design for gravity loads

   The internal forces obtained from the computer analysis program SAP2000 [3] are used to design the reinforced concrete sections of the structural elements of the residual building using the (BS 8110, 1997) [2] using the limit state design method (Mosley and Bungey, 1997) [6]. It has been found that the existing design of columns under the effect of gravity loads is adequate for the study case. As for the design of columns a computer program called ISACOL (Shehata, 1999) [7] has been used. The paper studied ten columns from thirty for the evaluation. Table 1shows the Straining action for the ten columns due to gravity load and Table 2 shows the present design compared with the original design of critical columns for the studied case. It is clear that the original design of these columns exceeds the present design which means that it is satisfactory for gravity loads. It is worthy to mention that internal forces in beams of the study case have

   been calculated under gravity loads. Then the (BS 8110, 1997) [2] has been used to check the existing design. It has been found that the existing design is adequate for the case.

   Table 1.Straining action for the ten columns due to gravity load

   ORIGINAL

   Column No N Mx My

   C05	1819.32	-0.47	6.25
   C06	1823.8	-0.23	5.75
   C13	1966.4	-0.78	5.68
   C15	1816.35	-0.34	5.73
   C16	2002.41	-0.26	5.57
   C20	1998.13	-0.79	6.24
   C21	1777.94	0.97	2.52
   C23	2021.05	-0.24	1.88
   C25	1977.63	-0.32	7.19
   C29	1947.29	-0.36	1.04

   1. Design of some columns due to gravity loads only:

      Figure 7. ISACOL Program result for Design of Column No, C21due to gravity loads

      Table 2. Comparison between Original and Present Design For Gravity Loads

      Column

      4-Story Case Study

      Original Design Present Design

      No.	Section*	Reinf.	Section*	Reinf.
      C05	250×500	10 16	250×500	10 16
      C06	250×500	10 16	250×500	8 16
      C13	250×500	10 16	250×500	10 16
      C15	p>250×500	10 16	250×500	8 16
      C16	250×500	10 16	250×500	8 16
      C20	250×500	10 16	250×500	8 16
      C21	250×500	10 16	250×500	10 16
      C23	250×500	10 16	250×500	8 16
      C25	250×500	10 16	250×500	8 16
      C29	250×500	10 16	250×500	8 16

      No.	Section*	Reinf.	Section*	Reinf.
      C05	250×500	10 16	250×500	10 16
      C06	250×500	10 16	250×500	8 16
      C13	250×500	10 16	250×500	10 16
      C15	250×500	10 16	250×500	8 16
      C16	250×500	10 16	250×500	8 16
      C20	250×500	10 16	250×500	8 16
      C21	250×500	10 16	250×500	10 16
      C23	250×500	10 16	250×500	8 16
      C25	250×500	10 16	250×500	8 16
      C29	250×500	10 16	250×500	8 16

      * Section dimensions are in mm.

2. Check of design considering earthquake and wind loads

   The moments obtained from earthquake and wind loads are shown in .Tables3 and 4. It has been found that the

   Figure 8.Comparison between My due to Wind loads and My due to Seismic loads

   Table 4. The Staining actions (Mx) due to Wind loads and Seismic loads

   Column No. WIND SEISMIC

   Mx Mx

   effect of seismic load is much more than the effect of C05				31.45	264.36
   wind load. Fig 8 and 9 show the comparison between C06				31.6	264.52
   				C13	32.54	285.83
   Table 3. The Staining actions (My) due to Wind loads				C15	31.19	261.38
   and Seismic loads C16				32.78	285.58
   C20				32.23	305.43
   WIND SEISMIC Column No. C21				30.79	276.76
   My My C05 5.68 20.24				C23 C25	32.4 32.55	304.11 306.11
   			C29	32.82	286.42
   C06	5.73	5.73
   C13	6.24	26.27
   C15	5.75	5.75
   C16	7.19	7.19
   C20	1.88	21.16
   C21	6.25	39.8
   C23	1.04	1.27
   C25	2.52	2.52
   C29 5.57 5.57

   effect of seismic load is much more than the effect of C05				31.45	264.36
   wind load. Fig 8 and 9 show the comparison between C06				31.6	264.52
   				C13	32.54	285.83
   Table 3. The Staining actions (My) due to Wind loads				C15	31.19	261.38
   and Seismic loads C16				32.78	285.58
   C20				32.23	305.43
   WIND SEISMIC Column No. C21				30.79	276.76
   My My C05 5.68 20.24				C23 C25	32.4 32.55	304.11 306.11
   			C29	32.82	286.42
   C06	5.73	5.73
   C13	6.24	26.27
   C15	5.75	5.75
   C16	7.19	7.19
   C20	1.88	21.16
   C21	6.25	39.8
   C23	1.04	1.27
   C25	2.52	2.52
   C29 5.57 5.57

   moments in columns due to earthquake and wind loads

   Figure 9.Comparison between Mx due to Wind loads

   (y) and Mx due to Seismic loads

   In all directions the effect of seismic loads is govern so, the paper concentrated in the effect of seismic loads on residual buildings in the Sudan.

   1. Earthquake loads

It is well known that the Sudan has no regulations for the seismic design of buildings. Therefore, in the present paper earthquake loads are calculated following the rules which are given in the Regulations for earthquake resistant design of buildings in Egypt, (ESEE, 1988) [4]. These regulations have been prepared by the Egyptian Society for Earthquake Engineering (ESEE). In order to apply the ESEE regulations a seismic map for the Sudan is required to determine the site seismicity factor. In 2009, Hassaballa.et. al. developed a new seismic hazard maps and seismic zoning map for the Sudan (Hassaballa et al

, 2009) [5] , as shown in Fig.11.

Figure 10. Seismic Hazard Map of The Sudan (Hassaballa et al , 2009).

3.4.1.1 Calculation of base shear

The total weight is given by:

Wi = Di + PLi (1)

Where, p is the incidence factor and is equal to p =

0.25. After analysis for gravity loads, the total floor weight will be as follows:

= 29580 + 0.25 X 2812 = 30283 KN

The equivalent lateral force procedure of (ESEE 1988) was used to calculate the design base shear. The resulting seismic coefficient, Cs, was determined to be

0.125 and the corresponding base shear was approximately 3785 KN.from:

V = Cs*Wt (2)

3.4.1.2Distribution of horizontal seismic force

The period of the building is the same in both directions. Hence, the load in the E-W direction are the same as those for the N-S direction as shown Fig 18.

Figure 11.Distribution of horizontal seismic force

3.42 Check of Seismic design for study case

Numerical analysis for the study case has been performed using SAP2000 (Computers and Structures)

Table 5 and 6 show the Straining action (moments) for the ten columns due to seismic load, and the seismic design compared with the original design of that columns which are chosen respectively. It is clear that most of columns are unsafe due to seismic loads. Therefore, a strengthening scheme is needed for the residual building in order to resist earthquake forces.

Table 5.Straining action for the same ten columns due to seismic load

Column

No. Case Type N Mx My

Column No.

Original Design Present Design

Section* Reinf. Section* Reinf.

C05 ENVEQX Max 1819.3 264.36 20.24

C05 250×500 10 16 300×1000 20 16

C29 ENVEQX Max 1947.3 286.4 5.57

3.4.2.1 Design of some columns due to gravity and seismic load

Figure 12.ISACOL Program result for Design of Column No, C21 due to seismic loads

Table 6. Comparison between Original and Present Design Including Seismic Loads

C25 250×500 10 16 300×1100 24 16

* Section dimensions are in mm.

1. Proposed Strengthening Method.

   There are different methods for seismic strengthening of existing buildings. However, social and economic conditions should be considered to choose the appropriate method. Adding structural walls is one of the most common structure-level retrofitting methods to strengthen existing structures. This approach is effective for controlling global lateral drifts and for reducing damage in frame members.

   Structural walls may be either reinforced concrete or steel plate.

   4.1 Modelling of reinforced concrete shear walls (RCSW)

   The Reinforced Concrete Shear Walls (RCSW) can be modelled using full shell elements and isotropic material. It is suggested that the wall panel be modelled using at least 16 shell elements (4×4 mesh) per panel (Abolhassan, 2001).The lateral force resisting system consists of moment resisting frames with RC Shear Walls . The residual building of the study case was analyzed and designed for gravity and seismic loads as previously explained, i.e., using SAP2000 structural analysis software package (Computers and Structures) , British standard code (BS 8110 , 1997) , and ESEE – Regulations ((ESEE , 1988).

   4.1.1 Comparative Study

   Three cases of same positions for the shear walls have been examined. Reinforced concrete walls with

   thicknesses of 20 cm, 15 cm and 10 cm as shown in Figures 13-16

   The following results have been obtained:

   1. For the two cases and using the shear walls of concrete, with different showed that all the columns in both directions x and y are safe, as shown in Table 4.

   2. For economy, Reinforced concrete wall with thicknesses of 15 cm have been chosen for this case study.

   Figure 13. The RC shear walls positions

   Figure 14. Modelling of shear wall in both directions x and y

   Figure 15. Modelling of shear wall in x direction

   Figure 16. Modelling of shear wall in y directions

   Table. 7 show straining action for the ten columns that which are chosen due to seismic load before and after strengthening. It has been found that all columns in the

   Figure 17.Straining action for the ten columns which are chosen due to seismic load before and after strengthening.

   Table 8. Comparison between Original and

   Strengthened Design for Study Case

   study case became safe after strengthening.

   Table 7. Straining action for the ten columns that

   Colu mn

   Seismic Loads in direction (x) and direction (y)

   Original Design After Strengthening

   which are chosen due to seismic load before and after

   strengthening.

   No.

   Section* Reinf. Section* Reinf. C05 250×500 10 16 250×500 10 16

   Column

   Original Column

   Shear wall

   Shear wall

   Shear wall

   No. 0.2 0.15 0.1

   Mx Mx Mx Mx Mx

   C06 250×500 10 16 250×500 10 16

   C05	-0.47	264	19	22.06	28	C13	250×500	10 16	250×500	10 16
   C06	-0.23	265	14	16.36	21	C15	250×500	10 16	250×500	10 16
   C13	-0.78	286	5.3	6.56	9	C16	250×500	10 16	250×500	10 16
   C15	-0.34	261	14	16.49	21 C20 250×500 10 16 250×500 10 16

   C16 -0.26 286 4.9 6.22 8.7

   C20 -0.79 305 5.4 6.78 9.3

   C23	-0.24	304	5	6.42	9
   C25	-0.32	306	4.8	6.27	8.9

   C23	-0.24	304	5	6.42	9
   C25	-0.32	306	4.8	6.27	8.9

   C21 0.97 277 14 13.51 14

   C29 -0.36 286 4.6 5.97 8.5

   C21 250×500 10 16 250×500 10 16 C23 250×500 10 16 250×500 10 16 C25 250×500 10 16 250×500 10 16 C29 250×500 10 16 250×500 10 16

   * Section dimensions are in mm

2. Conclusion.

   The present study represents the first attempt to investigate the seismic resistance of residual buildings in theSudan. Due to the lack of knowledge about the seismic activity in this country buildings are designed and constructed without any seismic load consideration. Seismicity of The Sudan 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 residual buildings in the Sudan does not consider earthquake loads.

   2. It has been found that the current design of residual buildings in the Sudan is not safe for the current seismicity of the Sudan.

   3. A proposed methodology has been presented for evaluation of seismic resistance of existing residual buildings in the Sudan.

   4. A strengthening technique for existing residual buildings in the Sudan has been presented. It has been proved that RC walls with 15 cm thickness are suitable strategy for this case to reduce the seismic vulnerability of exiting (RC) buildings in the Sudan.

3. References.

1. Abolhassan, P.E. (2001). Seismic Behaviour and Design of Steel Shear Walls.ASI, Steel TIPS, First

   Print, California.

2. BS 8110. (1997). The Structural Use of Concrete, British Standard Institution, London.

3. Computers and Structures. (2001). SAP2000: Three Dimensional Static and Dynamic Finite Element Analysis and Design of Structures, Computers and Structures Inc., Berkeley, California, U.S.A.

4. Egyptian Society for Earthquake Engineering (ESEE). (1988) :Regulations for Earthquake-Resistance Design of Buildings in Egypt. Cairo. Egypt.

5. Hassaballa, A. E , Sobaih, M. E & A. R. A. Mohamed "Sensitivity Analysis in Estimating Seismic Hazard for Sudan" Proc., 14th European Conference on Earthquake Engineering, 30 Aug.-3 Sept., 2010, Ohrid, Republic of Macedonia

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

7. Shehata, A .Y. (1999). Information Systems Application on Reinforced Concrete Columns. M.Sc. Thesis, Faculty of Engineering, Department of Structural Engineering, Cairo University, Giza, Egypt.

8. Sobaih, M.E. (2002): Introductory Earthquake Engineering, 2nd Edition, Giza, Egypt.

9. Sobaih, M.E. and Ismaeil, M. A. " A Proposed Methodology for Seismic Evaluation and Strengthening of

Existing School Buildings in The Sudan", 15th WCEE, Portugal, September, 2012. Paper No.0 571.