Microsoft Word - CIVL990531.doc Science and Technology, 5 (2000) 25-34 © 2000 Sultan Qaboos University 25 Earthquake Response of a Historical Castle Mohammed Qamaruddin and Ali Salim Al-Harthy Department of Civil Engineering, College of Engineering, Sultan Qaboos University, P.O. Box 33, Al Khod 123, Muscat, Sultanate of Oman هزات أرضيةبتاريخية ال نزوىقلعةتأثر علي بن سالم الحارثيومحمد قمرالدين م و 977 إذ أن هناك عدة تقارير لهزات أرضية وقعت بين سنة ، إن خطر وقوع زلزال في سلطنة عمان يعتبر حقيقة :خالصة مان وجمع بيانات أخرى من علهناك حاجة إلى عمل خارطة زلزالية . ولكن دون حدوث أضرار على اإلنشاءات م 1998سـنة قلعة نزوى مدى تأثر ففي هذه الورقة العلمية تم تقييم . أجـل وضـع مواصفات عامة لتصميم اإلنشاءات ضد القوى الزلزالية وبغياب المعلومات عن هذا). Uniform Building Code(ستخدم المواصفات العامة للمباني بإقـوى الزلزال بالتاريخـية تشير لهذه المؤثرات و زالية ومعامل الموقع فإن دراسة رد الفعل الناتج عن الهزة تمت بإفتراض قيم مناسبة عامل المنطقة الزل . المحتملة مقاومة الزالزل من لتتمكن للقلعة إلى الحاجة إلى إجراء بعض أعمال تقوية التى توصلنا إليهاالنتائج ABSTRACT: The earthquake hazard potential in the Sultanate of Oman is considered real as there are several reports of felt earthquakes in the Sultanate of Oman during the period from years 977 to 1998, although no damage to the existing structures has been reported due to earthquakes. In Oman, seismic zoning map and other basic data are required for the development of a standard code for seismic design of structures. In this paper, seismic response of a historical Nizwa castle in Oman is assessed employing the methodology of Uniform Building Code (UBC, 1994). Also, in the absence of data pertaining to the seismic zone factor and the site coefficient, the seismic response study is carried out by assuming appropriate values of these parameters. The results of such investigation are presented in this paper. The results show that some retrofitting measures have to be undertaken in order that the existing historical building may survive in a possible damaging earthquake. KEYWORDS: Earthquake Records, Seismic Response, Uniform Building Code, Masonry Wall and Equivalent Static Approach. he assessment of earthquake hazard involves collecting and evaluating a wide range of data pertaining to the history and occurrence of earthquakes in a region and to their origin. There are several reports of felt earthquakes in the Sultanate of Oman (Figure 1) during the period from years 977 to 1998 (Table 1). Although Oman does experience earthquake ground motions from time to time, no structural damage has been reported due to earthquakes. The historical records, however, suggest a possibility of such damage during the Qalhat earthquake of 15th century (Dickson, 1986). Some major events, however, that occurred at a great distance away in southern Iran, Pakistan or Yemen have been reported in Oman. For example, several individuals reported (Dickson, 1986) the effects of 27 November 1945 earthquake (M = 8.2) whose epicenter was just off the southeastern Iranian Makran Coast about 470 km from Muscat (in Oman). One witness reported damage, casualties, and a Tsunami surge. Also, 18th April 1983 earthquake event (M = 6.5) that occurred in Baluchistan was felt by some persons residing in multistory buildings in Muscat (590 km from place of earthquake occurrence) and was also felt in Buraimi (in Oman) at an even greater distance (Dickson, 1986). The Yemen earthquake (M = 5.7) of 13th December 1982 was also felt by residents in Salalah region (Southern Oman) but not in Northern Oman (Dickson, 1986). Some faults were reported in the Batinah Coastal Plain in Northen Oman north to the Daymaniyat islands (Figure 2). Khaboura fault zone (Dickson, 1986) is located in south of Sohar about 80 km to the vicinity of Ras A’ssawadi along the coast (Figure 2). No systematic investigations concerning the seismic QAMARUDDIN and AL-HARTHY 26 activity in Oman was undertaken before 1985. Earthquakes magnitudes recorded along the northeastern margin of the Arabian Plate have reached the most severe record magnitude of 8.2 (27 November 1945) (Dickson, 1986). Figure 1. Felt Earthquake Locations in Oman. The National Geophysical Data Center in Denver, Colorado (U.S.A) has some seismic data from about almost all regions of the world. It turns out from such seismic data that all the earthquakes that occurred in an area between 45.0º E to 65.0º E and 10.0º N to 30.0º N (Figure 3) show that a significant seismic activity is concentrated along the Zagrous fault zone in Iran. Also, it can be seen from Figure 3 that there are numerous epicenters along the Batinah Coastal Plain and offshore of Oman. Prior to 1980 very few earthquakes had been recorded in Oman mainly because of the lack of the local seismographic stations or sensitivity of the world seismic network or perhaps it was difficult to receive signals from events in the Oman area because of placement of seismic stations (Dickson, 1986). It is important to note that nearly all of the earthquakes after 1980 were reported by new stations in Norway, Sweden and England. The magnitude of these earthquakes ranges from 3.9 to 4.9 on modified Ritcher scale. Considerable efforts have already been made for assessing seismic hazard in the Sultanate of Oman and various other aspects of seismic monitoring in the Sultanate of Oman. Through these efforts, it turns out that several recent earth movements have occurred in Jabal Salak, Jabal Khubayb and Batinah coastal plains area (Dickson, 1986). An investigation has been undertaken to assess the response to seismic forces of an existing historical masonry structure (Nizwa castle) in EARTHQUAKE RESPONSE OF A HISTORICAL CASTLE 27 the Sultanate of Oman. Seismic analysis of the existing building has been made employing the methodology of the Uniform Building Code (UBC), 1994. In the absence of data concerning the seismic zone factor and the site coefficient, the seismic response study is carried out selecting the appropriate basic data for the earthquake response computation. The results of such investigation are presented in this paper. Methodology for Earthquake Response The method for earthquake response determination presented in this section is based on the requirements of the UBC, 1994. The UBC is most extensively used in the U.S.A., particularly in the western part of that country. This building code is intended to provide guidelines, and formulas, which constitute minimum legal requirements for design and construction within a particular region. These requirements are intended to achieve satisfactory performance of the structure when subjected to earthquake ground motion. The safety of the structure is not assured in the event of a major earthquake. The objective of the code is that a minor or moderate earthquake will not damage the structure and that a major earthquake will not produce collapse of the structure.Whenever a building is subjected to an earthquake, its foundation will start to move randomly because of soil movement of the site under the building. Then, the whole building moves due to the movement of its foundation, which causes deformations in the building. The earthquake forces can be divided into three components: vertical and two horizontals. In the present investigation, the vertical component of the ground motion is ignored. Only horizontal earthquake motion is considered in two orthogonal directions, but considering one component at a time. Table 1: Felt Earthquakes in Oman (Dickson, 1986) Site Place/City Year(s) of Felt Earthquake(s) 1 Shinas 1935,1945,1955,1965 2 Liwa 1969 3 Saham 1944,1945 4 Buraimi 1965, 1983 5 Al Qabil 1934, 1968, 1971 6 Nakhal 1965 (2)* 7 Seeb 1983 8 Muscat 1958, 1965 (4) *, 1967, 1969, 1983 (2) * 9 Quriyat 1965 (3) * 10 Musara 1965 11 Sumayyah 1965 12 Ghugrat Al-Tam 1965 (2) * 13 Sinaw 1945 14 Qalhat Late 15th Century 15 Al Kamil 1949 16 Al Wafi 1955 17 Bilad Bani Bu Hasan 1969 18 Masirah 1965 19 Salalah 1982, 1994 20 Musandam 977, 1184, 1483 21 Al Hallaniyat 1995 * Numbers in parenthesis indicate number of felt earthquakes in a particular year. Building Idealization Details of the Nizwa castle’s structural system are reported elsewhere (Al-Shariqi, 1998). Load bearing masonry walls have been employed as a structural system in the castle. In view of this, the castle is idealized as a three-storied lumped-mass-stiffness shear building system. The earthquake response of the building is completely defined by the physical properties of its structural elements, such as its mass, stiffness, damping and load-displacement characteristics on one hand, and time QAMARUDDIN and AL-HARTHY 28 varying accelerations introduced at its foundation support on the other hand. Thus, the evaluation of structural properties and the selection of earthquake input are the most critical factors in the earthquake response analysis. In the present investigation equivalent static approach of earthquake analysis of multistory structures is employed (UBC, 1994). Equivalent Static Approach This method is adopted in most of the building codes for moderately high buildings due to its simplicity and due to the fact that many structures designed on the basis of code coefficients have satisfactorily withstood past earthquakes. Static horizontal forces are applied based on the values of seismic coefficients to simulate the effects of the designed earthquake. The distribution of the shear forces along the height of the building is adopted to be similar to that obtained by dynamic analysis. Design forces specified by most codes are smaller than those indicated by dynamic elastic analysis. However, to design any building in accordance with UBC-94, many factors should be taken into account by which the building shall be designed and constructed to resist a minimum lateral seismic force applied statically and independently in the direction of each of the two main axes of the structure. The equivalent static method used in the present study is briefly described in the following paragraphs. TOTAL BASE SHEAR FORCE: The total base shear force (V) of the structure is determined by using the relation (UBC, 1994): where, W is the seismic weight of the structure, Z, the seismic zone factor, I, the occupancy importance coefficient, Rw, the structural factor and C (dynamic factor) = 1.25S/T 0.66 ≤ 2.75 in which S is the site coefficient, T is the fundamental period of the building and is equal to Ct (hN0.75) while hN is the total height of the building in feet. Ct is the time coefficient factor which depends on the structural system of a structure to be analyzed. DISTRIBUTION OF LATERAL FORCES: The lateral force (Fx) at a height level x above the base is determined as: where Ft = 0 for T < 0.7 sec or Ft = 0.07TV < 0.25 for T > 0.7 sec, N is the total number of stories above the base of the building, Fx, Fi, FN are the lateral forces applied at level x, i and N, Ft is the portion of the base force (V) at the top of the structure in addition to FN, hx, hi are the heights of level x and i above the base of the building, Wx, Wi are the seismic weights of ith level. STOREY SHEAR FORCE: The shear at ith storey (Vx) is calculated using the formula: DRIFT AND STOREY LATERAL DISPLACEMENT: Drift (∆x) and storey lateral displacement (δx) are computed employing ∆x = Vx/ Kx and δx = Σ∆x ,where, Kx is the flexural stiffness for ith storey. The code stipulates that the storey drift should not exceed (0.04/Rw) times the storey height or 0.005 times the storey height. NATURAL PERIOD OF THE BUILDING: The natural period of the building is determined using Rayleigh’s method. OVERTURNING MOMENT: The overturning moment at ith level (Mx) of the building is computed by the following equation: HORIZONTAL TORSIONAL MOMENT: Diaphragms are considered flexible when the maximum wR ZIWC V =       = ∑ = N 1 i iixx t x hW/hW) F- V(F ∑ = += N x i xtx FFV ∑ = −+−= N 1 i xiixNt )hh(F)hh( FM EARTHQUAKE RESPONSE OF A HISTORICAL CASTLE 29 lateral deformation of the diaphragm is more than twice the average storey drift of the associated stories. The provisions (UBC-1994) have been made for the increased shear force resulting from horizontal torsion where diaphragms are flexible. The accidental torsional moment (Tx) is 0.05DVx, where D is the Figure 2. Epicenters and Active Faults along the Batinah Plain and Offshore in Oman. dimension of the building perpendicular to the direction under consideration. P-DELTA (P-∆) EFFECT: The ratio (θx) of the secondary moment at ith storey (Mxs) resulting from P-∆ effect to the primary moment at ith storey (Mxp) is estimated by the following equation θx = Mxs/ Mxp = Px ∆x/VxHx where M xs is the secondary moment at xth storey, M xp is the primary moment at xth storey, Px is the total weight at xth storey and above, Hx is the height of xth storey. QAMARUDDIN and AL-HARTHY 30 Figure 3. Seismicity in Oman since 1980. According to the provision of the code, when θx at each storey level of the building is less than 0.10, there is no need to account for the P-∆ effect. DIAPHRAGM DESIGN FORCE: The code requires that horizontal diaphragms (floors and roofs) be designed to resist the following force given by: in which Wpx is the weight of the diaphragm and attached parts of the building at the ith storey. The code states that Fpx need not exceed 0.75ZI Wpx, but it shall not be less than 0.35ZI Wpx.       += ∑∑ == N 1 i i N x i pxitpx W/W)FF(F EARTHQUAKE RESPONSE OF A HISTORICAL CASTLE 31 Data for Seismic Response In the absence of appropriate data for seismic response study, it is assumed that the Nizwa castle is located in a seismic zone equivalent to seismic zone 3 (UBC-94) and hence seismic zone factor Z = 0.3. Seismic zone 3 has been selected in view of the historical importance of the Nizwa castle. Further, the foundation soil of the castle is considered such that the site coefficient S = 1.2. The structural factor (Rw), the occupancy importance factor (I) and the time coefficient factor (Ct) are selected as 6.0, 1.0 and 0.02, respectively, in accordance with the appropriate building material and building system of the Nizwa castle. Seismic Weight Determination Seismic weight of the three storied Nizwa masonry castle has been determined employing its appropriate material and mechanical properties. Details of the seismic weight determination are reported elsewhere (Al-Shariqi, 1998). The results of the seismic weight computation of the masonry building are shown in Table 2. Stiffness Determination of Shear Walls Determination of the shear wall stiffness is reported in this section. A new methodology (Qamaruddin, 1999) has been employed in determining the wall stiffness. In this method, the wall is divided into three elements: namely pier, top spandrel and bottom spandrel. Then the three non- dimensional parameters (pier aspect ratio, top and bottom spandrel aspect ratio) are evaluated for different walls. Employing the appropriate tables (Qamaruddin, 1999), the stiffness of the walls has been determined. In the present study, the stiffness of the walls parallel to transverse (T) and longitudinal (L) directions of the building has been computed separately. This has been done in view of the assumption that the walls located parallel to the transverse direction of the building would resist any expected ground motion occurring parallel to the transverse direction of the structure. Similar is the condition for the walls located in the longitudinal direction of the building, for the expected earthquake motion in the longitudinal direction of the building. The results for the story-wise lumped stiffness of the shear walls located parallel to the transverse and longitudinal directions of the building are also tabulated in Table 2. Details of the stiffness determination of the shear walls are reported elsewhere (Al-Shariqi, 1998). Seismic Response of the Castle Earthquake response has been determined using the methodology of UBC-94 for the masonry building subjected to two orthogonal horizontal ground motions separately, taking them one at a time. The results of such response computation have been shown in Tables 3 to 7. Table 2: Data for Seismic Response Computation Storey level Storey height (m) Seismic weight (kN) Storey stiffness (kN/m) for T-shear walls* Storey stiffness (kN/m) for L-shear walls+ 3 9 87245 3502139 6931218 2 4 884063 85230520 57793623 1 4 2178666 38868586 28148832 * T-shear walls parallel to earthquake motion in transverse direction of the castle. + L-shear walls parallel to earthquake motion in longitudinal direction of the castle. Results and Discussions The Nizwa castle is subjected to earthquake ground motion in two orthogonal directions, i.e., longitudinal and transverse separately. The discussion of the results thus obtained (Tables 3 to 7) is described in the following sub-sections. QAMARUDDIN and AL-HARTHY 32 LATERAL FORCES COMPARISON: Table 3: Lateral Force and Shear Force Storey level Lateral force, Fx (kN) for T-direction* L-direction+ Shear force, Vx (kN) for T-direction L-direction 3 30028 25970 30028 25970 2 143186 123837 173214 149807 1 176433 152591 349647 302398 * Earthquake ground motion in transverse direction of the castle. + Earthquake ground motion in longitudinal direction of the castle. The results from Table 3 show that the lateral forces developed in the structure subjected to ground shaking in the T-direction are about 13.5 % larger than the corresponding lateral forces values obtained for the ground motion in the L-direction in all the stories of the castle. COMPARISON OF BASE SHEAR FORCES: It can be seen from Table 3 that the values of the shear forces in the respective stories of the structure subjected to ground motion in the T-direction are also about 13.5 % higher than the corresponding values obtained for the earthquake ground motion in the L-direction. COMPARISON OF STOREY DRIFTS: Table 4: Storey Drift and Lateral Displacement Storey level Storey Drift (m) for T - direction L- direction Lateral Displacement (m) for T - direction L- direction 3 0.008 0.0037 0.019 0.017 2 0.002 0.0026 0.011 0.013 1 0.009 0.01 0.009 0.01 From Table 4, it is seen that the values of the storey drift show different variation trends in the building as obtained by the static method. For example, the storey drift values obtained in stories 1 and 2 are respectively about 11% and 30% higher when the structure is subjected to the L-direction of the earthquake ground shaking in comparison with the corresponding values as obtained for T-direction of the ground motion. Unlike this trend, the storey drift value obtained in third floor is about 53% higher for the T-direction of the ground motion compared to the L-direction one. The above trend of storey drift variation may be attributed mainly to the variation of the storey stiffness (Table 2). According to the UBC-94 code, the maximum permissible storey drift for the castle is determined as 0.020 m. From Table 4, it is established that the storey drifts determined in all the stories of the castle are less than the maximum permissible value of the storey drift. In view of this, the serviceability requirement of the historical castle is met VARIATION OF LATERAL DISPLACEMENTS: It can be observed from Table 4 that the story- wise lateral displacements of the building under the L-direction of earthquake force are about 11% and 18% higher in the first and second stories respectively in comparison with the corresponding values obtained for the T-direction of the ground motion. In contrast to this observation, the lateral displacement of the third storey in the L- direction of earthquake motion is about 10.5% lower than that obtained for the T-direction. Table 5: Overturning and Torsional Moment Store y level Overturning Moment (kN.m) for T-direction L-direction Torsional Moment (kN.m) for T-direction L-direction 3 - - 51648 70119 2 270252 233730 297928 404479 1 963108 832958 601393 816475 Base 2361696 2042550 - - EARTHQUAKE RESPONSE OF A HISTORICAL CASTLE 33 COMPARISON OF OVERTURNING AND TORSIONAL MOMENTS: From Table 5, it is observed that the overturning moment in all the stories of the structure subjected to the T-direction of the ground shaking are about 13.5% larger than those obtained for the ground motion in the L-direction. This trend is true because the lateral forces developed in the structure under the T-direction of the ground motion are more than their corresponding values for the L-direction of the earthquake shaking. But, the torsional moment values in all the stories of the masonry castle are about 26.3% higher when the structure is subjected to the ground shaking in the L-direction in comparison with the response obtained for the T- direction of the ground motion. VARIATION OF SECONDARY MOMENTS: It is observed from Table 6 that there is a different trend in the story-wise development of the secondary moment. For example, the Table 6 shows greater values of the secondary moments in the first and second stories of the masonry castle subjected to the T- direction of the ground motion than the corresponding values in the L-direction of the earthquake force. But, the secondary moment in third storey of the structure for the L-direction of earthquake is greater than the corresponding secondary moment in the T-direction. Table 6: Primary (Mxp) and Secondary (Mxs) Moment Storey level Mxp (kN.m) for T-direction L-direction Mxs (kN.m) for T-direction L-direction Ratio of Mxs/ Mxp for T-direction L-direction 3 270252 233730 698 323 0.026 0.0014 2 692856 599228 1943 2526 0.0028 0.0042 1 1398588 1209592 28350 31499 0.02 0.026 It is seen from Table 6 that the ratios (θx) of the secondary moment (Mxs) resulting from P-∆ effect to the primary moment (Mxp) in all the stories of the masonry castle are much less than 0.10. This statement is true for the castle subjected to the T- as well as L-directions of earthquake ground motion. Therefore, there is no need to account for P-∆ effect in the design of the castle because according to the provision of the UBC-94 code, θx at each storey level of the building should be less than 0.10. DIAPHRAGM FORCES VARIATION: Table 7: Diaphragm Forces (Fpx) Storey level Minimum Fpx (kN) Computed Fpx (kN) for T-direction L-direction Maximum Fpx (kN) 3 14398 47195 40817 30853 2 44822 76126 65839 96048 1 138522 146437 126649 296833 Table 7 shows that the diaphragm forces developed in the roofing/flooring systems of the castle subject to ground shaking in the T-direction are higher than the corresponding values for the earthquake motion in the L-direction. It can also be seen from the table that the diaphragm forces developed in the third storey are more than the maximum diaphragm force permitted by the UBC-94 code. In view of this observation, it is established that the roofing system of the castle may fail in any future earthquake consistent with basic data considered in the seismic response determination of the Nizwa masonry castle. Conclusions Firstly, the seismic activity in the Sultanate of Oman has been studied through available literature. Then, the seismic response of the Nizwa castle subjected to the expected earthquake ground shaking has been determined. Based on the present study, the following conclusions can be drawn: 1. Since the Sultanate of Oman is situated within the seismic belt, contrary to past belief, there is a possibility of occurrence of earthquakes, especially in Batinah Coastal plain. 2. The earthquake response analysis of the three storied Nizwa castle show that there is no definite trend in the variation of different seismic responses. 3. The seismic response results may be used in the evaluation of seismic capability of the structural elements of the existing Nizwa castle. If required, strengthening measures will have to be undertaken to guard against possible failure of this structure during any future earthquake. QAMARUDDIN and AL-HARTHY 34 4. The results of the present investigation show that some retrofitting measures with respect to the roofing system has to be undertaken in order that the existing historical building may survive in any possible future damaging earthquakes. References AL-SHARIQI, A. 1998. Seismic Analysis of a Historical Building in Oman. Project Report, Department of Civil Engineering, Sultan Qaboos University, Oman. DICKSON, P. 1986. Preliminary Assessment of the Earthquake Hazard in the Sultanate of Oman, UNDP Consultancy Mission Report. INTERNATIONAL CONFERENCE OF BUILDING OFFICIALS, 1994. Uniform Building Code (UBC). Whittier, CA. PAZ, M. 1997. Structural Dynamics Theory and Computation, (4th Edition). Chapman & Hall, New York. QAMARUDDIN, M. 1999. In-Plane Stiffness of Shear Walls with Openings. J. of Building and Environment, 24: 109-127. Received 31 May 1999 Accepted 11 October 1999