Seismic retrofit of Sultan Selim Mosque redefines approaches to repair of historical masonry structures in Turkey
On top of the fifth of the seven hills of Istanbul, overlooking the Golden Horn, is the country’s second oldest existing imperial mosque, the Yavuz Sultan Selim Mosque. Completed in 1522, its single large brick masonry dome is flanked by two minarets -- a familiar and treasured landmark. However, a detailed investigation identified serious structural weaknesses that needed careful repair. BASF Construction Chemicals’ modern, technologically advanced solutions were essential to effect a restoration that met structural needs and, importantly, ensured the aesthetics of the 487 year old monument were retained.
Explains Engin Seyhan from BASF: “Over time, the quality of the materials used in original construction, natural environmental disasters and architectural elements of the design conspired to weaken the structure. A detailed investigation was thus carried out to assess the extent of the damage and what needed to be done to preserve the site’s structural integrity, ensuring the safety of visitors and safeguarding the mosque as a heritage site for future generations.
“As with the repair of all historical sites, there were two key requirements with regards to the products and approaches selected: they had to meet the requirements of the architects and engineers, and had to preserve the historical value - including the artistic and architectural uniqueness -- of the site.
“A challenge in this project was to find a solution that would not require the use of traditional strengthening techniques such as steel jacketing or increasing section dimensions which would conceal the original architecture and add to the load on the dome, also increasing earthquake loads.”
Says Oğuz Cem Çelik, “The masonry of Turkey’s historic mosques has demonstrated vulnerability to the earthquakes experienced in the country. Although the structural configurations are quite symmetric and have sufficient lateral rigidity due to larger wall thicknesses, these buildings are seismically vulnerable because of the many openings on the walls and the poor characteristics of the structural material used.”
The major structural problems of the Yavuz Selim Mosque originate from tensile stresses which occur at different points on the building. Structural cracks were concentrated at the lower zone of the main dome of the mosque, with meridional cracks occurring due to the circumferential tension.
The mosque layout
Yavuz Sultan Selim Mosque was commissioned by Süleyman I in honor of his late father Selim I. Today, only the mosque, hospices, Quranic School and royal tombs remain from the larger complex that also included a hostel for pilgrims, a madrasa, double baths and soup kitchen. The mosque has a rectangular, walled precinct. It is composed of a single-domed prayer hall with a diameter of 25 meters, preceded by a large, arcaded courtyard that is encircled by 18 columns and 22 domes, and flanked by square hospice wings.
There are two minarets with a single gallery each and rooms on either side of the mosque for the imam and müezzin. The enclosed cemetery is on the kiblah, or Mecca, side of the mosque
The main dome is 90cm thick at the top and at the support level. It is also supported by eight symmetrically located small-scale flying buttresses, again made of masonry members. The outer surfaces of all domes are covered with heavy lead layers as roofing to protect the building from natural elements.
The masonry walls support loads from the main dome and vary in thickness from 160cm to 260cm. Material used in these walls consists of shallow solid bricks and limestone, with equal thicknesses of crushed tiles and lime mortar for joints.
Understanding architectural stresses
To understand the crack formation, a seismic investigation and retrofitting project was conducted by Istanbul Technical University. Says Assistant Professor Dr. Oğuz Cem Çelik of Istanbul Technical University: “The University developed a 3D model of the dome in SAP 2000, specialized software for software analysis and design. Dynamic analysis was then conducted using ground motion records from the 17 August 1999 Kocaeli and 12 November 1999 Düzce earthquakes.
“The graphical 3D model was then retrofitted using cross-sections exposed to high tensile stresses. Galvanized steel rings were added to the support level of the dome and the tension zone on the inner face was strengthened with BASF Construction Chemicals’ MBrace Fibre C1-30 carbon fibre reinforced sheets. The same dynamic analysis was applied to the retrofitted model. The results showed that by applying these solutions, stresses could be significantly reduced – by up to 65% -- and more uniform stress distribution could be obtained over the dome.”
Explains BASF’s Talat Sivrioğlu: “The MBrace FRP Composite Strengthening System is a family of lightweight fibre reinforced polymeric (FRP) materials, externally bonded to the surface of structures. They comprise of either ready to use carbon laminate adhered directly to the surface or carbon, aramid or glass fibre sheets impregnated in-situ with a saturant resin system on the substrate surface. These systems provide very high tensile strength and are utilised for flexural and shear reinforcement and axial compression confinement of concrete, masonry and timber elements.”
BASF’s solutions were consequently selected for use on this project.
The cracks in the dome were first repaired with a puzolanic lime based injection mortar, BASF’s Albaria Iniezione 200, which is specially designed for historical masonry buildings. This restored the structural capacity of the dome.
BASF’s puzolanic lime based, high strength repair mortar Albaria Struttura, also designed for historical buildings, was then applied to the inner surface of the dome, making it stronger and smoother. Strengthening of the dome with the MBrace Fiber C1-30, carbon fibre reinforced polymeric sheets, then commenced.
The surface was primed with Mbrace Primer, and a layer of Mbrace saturant applied over the still tacky primer. The Mbrace Fibre was then applied and air removed by rolling with a ribbed roller. A second coat of saturant followed. Subsequent layers of Mbrace Fibre were then added.
Lightweight and practical, the MBrace Fiber System did not disturb architectural view of the dome, nor did it place any additional load on to the dome, which would increase its vulnerability in the case of another earthquake.
Says Oğuz Cem Çelik: “This seismic retrofit project of the brick masonry dome of Yavuz Sultan Selim Mosque is an important reference for other retrofit projects of historical buildings in Turkey.”
Engin Seyhan agrees: “The MBrace FRP systems are ideal for these projects as they can provide an increase in load-bearing capacity, a reduced deflection due to serviceability / working loads (increase in rigidity/stiffness) and an increase in structure fatigue strength. They also have the ability to limit or assist in crack control and an exact amount of reinforcement to be calculated and placed in relation to the performance required or the flow of stress. Perhaps most importantly, the use of Mbrace systems means less and faster maintenance, which means a reduction in long term, ongoing costs.
“BASF is pleased to have been able to play a role in a project as important as this one in terms of redefining approaches to preservation of historical sites.”