VULNERABILITY OF HISTORICAL MINARETS; INVESTIGATION OF THEIR SEISMIC ASSESSMENT & RETROFITTING

Transcription

1 Emirates Journal for Engineering Research, 9 (2), (2004) (Regular Paper) VULNERABILITY OF HISTORICAL MINARETS; INVESTIGATION OF THEIR SEISMIC ASSESSMENT & RETROFITTING E. M. HIGAZY Department of Structural Engineering, Ain Shams University Cairo, Egypt تمثل المعالم التاريخية عناصر ملموسة للا رث الثقافي والفني للبشرية. وعلى الرغم من الحقيقة المجردة التي تسلم با ن سلامة والمحافظة على هذا الا رث هو مسو ولية جماعية لكافة التخصصات والمهن الا نسانية فا ن الهندسة الا نشاي ية تضطلع بدور بارز في هذا الخصوص بالمقارنة بالمهن الا خرى حيث ا نها تقود عملية حماية حالة هذه المعالم التي تضم بين جنباتها تفسير ا مادي ا للتاريخ. وتعد الما ذن عناصر ري يسية في فن الا نشاء للمساجد التاريخية والمعاصرة التي تشكل جوهر الفن الا سلامي في مصر. ا ن الطبيعة الا نشاي ية للما ذن من حيث كونها عناصر نحيفة وطويلة تتصرف ا نشاي ي ا ككوابيل تجعلها ع رضة للا ضرار عن التعرض لمخاطر الزلازل. يتناول هذا البحث بالدراسة استجابة المعالم التاريخية للزلازل بالقاهرة الا سلامية التي تمثل حقب مختلفة من التاريخ وعلى وجه التحديد خلال فترة الدولة العباسية/ الطولونية (من 827 ا لى 904) والدولة الفاطمية (من 969 ا لى 1171) والدولة الا يوبية (من 1171 ا لى 1250) والدولة المملوكية (من 1250 ا لى 1517) والدولة العثمانية (من 1517 ا لى 1848). ويتم في هذا البحث عرض تا ثير الشكل الا نشاي ي والمعماري والمواد المستخدمة في الا نشاء والتفاعل المتبادل بين الما ذنة وقواعدها الا نشاي ية. ويتم في النهاية مراجعة المقترح الخاص بتقوية الما ذن التي جرت عليها هذه الدراسة من وجهة نظر التصميم الذي يستند على الا داء بهدف تقليل مخاطر تلفها ا و تعرضها للانهيار الكامل. Historical monuments are the tangible elements of the cultural and artistic legacy of mankind. Despite the mere fact that their safety and preserveness is a collective responsibility of all disciplines of human professions, structural engineering, prominent among other professions, takes the lead in safeguarding the well being of those materialistic interpretation of history. Minarets are main elements of the art structure of historical and contemporary mosques which constitute the essence of the Islamic art in Egypt. The minaret structural nature of being slim and tall elements, with a cantilever performance, lends them to be most vulnerable when subjected to earthquake demands. This paper investigates the seismic response of historical monuments of Islamic Cairo representing different periods, namely, Abbasid/Tulunid (827 to 904), Fatimid (969 to 1171), Ayyubid (1171 to 1250), Mamluk (1250 to 1517) and Ottoman Turk (1517 to 1848). The effect of architectural and structural configuration, construction material and minaret/foundation interaction are explored. Finally, retrofitting proposal of the investigated minarets is revisited from a Performance-Based Design (PBD) standpoint aiming at reducing the risk of their prohibitive damage or catastrophic failure. INTRODUCTION In the context of investigating the seismic assessment and retrofitting of historical monuments; a comparative historical review of the difference in the architectural features and materials of construction of historical and contemporary minarets subject of the current study is deemed enriching to any further structural studies for monuments of similar eras. The construction aspect of the Islamic civilization is rich in variety stemming from influences of pre-islamic eras. This paper constitutes a structural treatment of the issue of conservation, which is retrofitting and/or repair per the structural terminology, of historical minarets elected to represent the history of Islamic architecture in Cairo. A history traced back to the Arab (Muslim) conquest of Egypt in 639 A.D. and the establishment of the city of Fustat in 641. Four primary periods are generally used to describe the ruling society and religious customs beginning in 827 and continuing until 1848: Abbasid or Tulunid (827 to 904). Fatimid (969 to 1171). Ayyubid (1171 to 1250). Ottoman Turk (1517 to 1848). SELECTED ERAS FOR MINARETS UNDER INVESTIGATION; ARCHITECTURAL/ STRUCTURAL HISTORY OF MINARETS The varying features, specifically, cross-sectional and longitudinal section configuration along with the types of materials used in the early times of construction of historical minarets are believed to be the governing factors in their expected seismic performance and hence in the election of the structural retrofitting/repair 59

2 E.M. Higazy scheme. In the following sections the architectural configuration of each historical era is discussed to a limited level of details, where sufficient data can be obtained elsewhere 1. All minarets subject of this investigation were selected to be ones that underwent severe seismic attacks in recent earthquakes and experienced some sort of damage. This criterion is believed to provide the necessary insight for the intended retrofitting/repair of similar type minarets. Tulunid Period The Islamic architecture of this period was influenced by the architecture Of the imperial capitals of Damascus, Baghdad, and Samarra. The mosque of Ahmed Ibn Tulun has a Samarran style, the only surviving example in Cairo. This monument is a very large congregational (or Friday) mosque with a large "Sahn". It is a flat-roofed hypostyle mosque constructed of well-fired red brick covered with stucco and fine decorative plaster. The interior central court is surmounted by a "Riwaq" on piers. The arches are pointed with slight horse-shoe shape. The decorative plaster is mainly on the face and soffit of the voussoirs. Each pier has an engaged column at each corner. It has three "Ziyadah" making the overall complex almost a perfect square. The outer walls of the "Ziyadah" have a pishtaq. The minaret was built as a separated architectural element, to reach the platform of the mosque there is an outer stair, then a circular stair surrounding the outer circumference of the minaret leads to the top of the medium part surmounted by the "Mabkhara". The minaret of Ibn Tulun mosque isn't completely circular therefore the Ottman Period third of its height has perpendicular sides surmounted by a cylindrical part. The construction material in this period was stone. From a structural stand point, the Tulunid minaret is of the least slenderness among other period minarets as it will be displayed in following sections. Fatimid period The design of the minarets also evolved during the Fatimid period. Most minarets had a square base and a shaft surmounted by a domed octagonal tower with arched openings and balcony sometimes referred to as "Mabkhara", or incense burner. In the Fatimid period, minarets were composed of 3 successive sections. The first section is a square with moderate height, the second section is cylindrical followed by an octagonal section surmounted by the "Mabkhara". The position of this minaret is usually above the entrance as at Giyushi mosque. The effect of the architectural design of the minaret of Kayrawan mosque is so clear while looking to Giyushi mosque on the top of the Mokattam Hill. This minaret is considered the oldest Egyptian minaret still keeping its complete form and shape. Although this minaret starts from the roof of the building, but its architectural design is similar to the Kayrawan' minaret design. Both has a square tower with chamfered corners, the tower width is less than the base width. The tower is surmounted by a pointed dome on an octagonal neck. It has been proved that this mosque built at this strategic position wasn't meant to be a mosque or a mausoleum as much as a tower for surveillance. Contemporary Period Tulunid Period Figure 1. Configurations of Historical Minarets investigated in the study. 60 Emirates Journal for Engineering Research, Vol.9, No.2, 2004

3 Vulnerability of Historical Minarets; Investigation of their Seismic Assessment & Retrofitting We can also notice the effect of the Kayrawan mosque in the design of the oldest remaining minarets from the Fatimid period, and this at El-Hakim Bi- Amrillah mosque at the corners of the front elevation (entrance Elevation). We can also see that the design of the north minaret is different from the west one. The north minaret has a square short base surmounted by a cylindrical high body, while the west minaret is formed by a square base surmounted by an octagonal tower which the section decreases in size when moving towards the top. Because of this difference between the two minarets design, each one of them was covered by a layer of stone that hides its lower part. A violent earthquake happened at 14 A.C. caused the loss of the top of the two minarets. Later, a new layer of stone was added to hide the rest of the shown part of the two minarets. This two layers were surmounted by a "Mabkhara". Ayyubid period The Ayyubids were the dynasty of "Salah Al-Din", the chivalrous opponent of the crusaders who reconquered Jerusalem, saved Cairo, and became sultan in1186. The "Sunni" sect of the Islamic faith was reestablished during this period. Since the Sunnis did not allow more than one large Friday or congregation mosque, no new large mosques were constructed. Instead they built "madrasas" to teach theology and law and "khanqahs", Usually for the "Sufi" order or dervishes. The mabkhara-type minaret was developed further with the addition of stalactites and increases in height making them appear more slender than earlier minarets. "Minaret of as-salih Negm ad-din Ayyub". This mabkhara- style minaret is about all that remains of the madrassa bearing the same name and is the only minaret of the Ayyubid period. There is a little lateral support at the base of the minaret since the madrasa is no longer present. The remnant unreinforced masonry walls are evident. The recent earthquake caused existing cracks to be significantly widened and extended. Some of the cracks may have formed as the madrasa progressively deteriorated and collapsed. An inhabited residential building behind the remnant madrasa also shows similar disrepair and signs of distress and should be evacuated. The minaret is also reported to be leaning. The upper portion of the minaret which has a smaller cross-section, is leaning observably more than the lower portion, which has a larger cross-section. Access to the interior or base of the minaret wasn't made available. Ottman period A new style of minarets appeared in this period, similar to the ottoman Turkish minarets. The minaret consist of a square base surmounted by a polygonal body. It ends with a terrace covered by a cone shape. The minaret location had also varied, it was usually at the corner of the building. Example of those minarets chosen for analysis, Mohamed Ali Mosque, Soliman Pasha Mosque and Sanan Pasha Mosque. SPECTRAL ANALYSIS OF HISTORICAL MINARETS In this section a three-dimensional spectral analysis was carried out for five different minarets, four of which represent different eras of Islamic civilization in Egypt, namely, Tulunid, Fatimid, Ayyubid and Ottoman along with a contemporary reinforced concrete 100 m high minaret. The main objective of the analysis was investigating the seismic performance of those minarets with respect to stress concentration locale along with the seismic displacement behavior. The stress concentration locale gives an insight of the potential retrofitting areas which in many cases turned to be at cross-section variation which lends the expected retrofitting process a higher level of complexity. The study of the deformational behavior guides the distribution of retrofitting materials over the minaret height. Figure 2. shows the total drift of the analyzed minarets at a common height of 40.00m. Figure 3. exhibits the deformed shape of the analyzed minarets. Figure 4. depicts the potential retrofitting zones (PRZ) based on the stress contours that allocate the locale of stress concentration and the absolute value of maximum stress. RETROFITTING CRITERION Stress results yielded from the spectral analysis are depicted in Figure 4. Potential retrofitting zones are defined as zones of high stress concentration most susceptible to plastic hinge formation. Depending on the minaret configuration one or more potential retrofitting zones are defined. In most cases, locales of section variation where the most eligible PRZ. Conteporary Fati mi d & Ottman Tulunid Ayyubid Deflection (cm) Figure 2. Total Drift at a common height of 40.00m for Analyzed Minarets Emirates Journal for Engineering Research, Vol.9, No.2,

4 E.M. Higazy Owing to the historic nature of the minarets of interest, the election of retrofitting scheme should consider the respect of the historical fabric and doesn t allow in most cases for adding surface bonded retrofitting materials such as CFRP laminates or jackets and certainly prohibits the use of secondary retrofitting structures such as jackets or steel brackets. The technique seen most appropriate for retrofitting of historical minarets is epoxy injection 4 or introduction of joint reinforcement. Treating the potential retrofitting process from a performance engineering stand point introduces the need for studying of the deformational performance of the retrofitted minarets which is expected to dictate a different pattern for PRZ than the one based on the stress analysis. Figure 3. Minarets Deformed Shape-Deformation Analysis 62 Emirates Journal for Engineering Research, Vol.9, No.2, 2004

5 Vulnerability of Historical Minarets; Investigation of their Seismic Assessment & Retrofitting Figure 4. Potenial retrofitting zones for minarets of different eras. Stress Contours/Stress based analysis of minarets. Emirates Journal for Engineering Research, Vol.9, No.2,

6 E.M. Higazy CONCLUSIONS This research investigates the seismic performance of historical minarets with the objective of defining a criterion for their seismic retrofit/repair aiming at reducing the risk of their prohibitive damage or catastrophic failure. The spectral analysis of five full scale minarets, four of which represents different historical eras of the Islamic civilization in Egypt in addition to a single contemporary minaret, yielded the following conclusions; 1. The stress analysis of all minarets manifested an agreement in the stress performance of all minarets of different eras, hence, different materials of construction, with respect to yielding potential retrofitting/repair zones at cross section change locale notwithstanding the shape of the cross section, the abruptness of variation or the material used. 2. The concentration of the PRZ at the locales of section variation adds to the complexity of the retrofitting/repair process of historical minarets which is in essence a conservation process where the historical fabric is as important as the structural soundness. 3. Deformational response of the investigated minarets showed the seismic deformation of all historical minarets of average height m to be within an acceptable limit that doesn t pose a real threat nor complications to any intended retrofitting/repair process and makes the treatment of the retrofitting of historical minarets within a PBD frame work a feasible option. REFERENCES 1. Sykora, D., Look, D., Croci, G and Karaesmen, E, Reconnaissance Report of Damage to Hitroic Monuments in Cairo, Egypt Following the October 12, 1992 Dahshur Earthquake, NCEER Technical Report , State University of New York at Buffalo, Elgamal, A.W., Amer, M., Adalier, K. and Abul-Fadl, A., Engineering Aspects of the October 12, 1992 Egyptian Earthquake, NCEER Technical Report , State University of New York at Buffalo, Higazy, E.M. and Adham, S.A., Guidelines for Strengthening and Conservation of Historic Monuments in Cairo, Egypt, A Technical Proposal Submitted to the American Research Center in Egypt and the Egyptian Supreme Council for Antiquities, El-Nawawy, O., Ahmed, E.Y., Higazy, E.M. and Fawzy, S.A., Out-of-Plane Hystertic Performance of Mansonry Walls Retrofiotted by CFRP Laminates, Presented and the International Structural Engineering Conference, Aswan, Egypt SEAOC Vision 2000 Performance Based Engineering of Buildings, A vision 2000 committee report, SEAOC, April Emirates Journal for Engineering Research, Vol.9, No.2, 2004