Historical Earthquake Damages to Domed Structures in Istanbul

1. Motivation Of The Research

Istanbul, the capital of Eastern Rome, Byzantine Empire and Ottoman Empire, has always been an important city, decorated with emblematic buildings. The seismicity of the city and the surrounding area, however, has been one of the most challenging points the designer of these daring historical structures had to face. Very strong tremors, recurring in every one and a half century in average, hit the city leaving a tragic mark in the history. The legendry dome of Hagia Sophia, the most important structure of the city, for instance, collapsed in 1509 due to a strong shaking. The dome of Beyazıt Mosque, commissioned by the Sultan Beyazıt, collapsed 3 years after its completion during the 1509 Earthquake as well. Fatih Mosque, commissioned by the conqueror of the city, Mehmet the 2nd, collapsed during the 1766 Earthquake to such an extent that the bearing system of the structure had to be redesigned during the reconstruction works. Atik Ali Paşa Mosque in Çemberlitaş experienced a severe damage during the 1766 Earthquake thus the load bearing system and the dome had to be repaired and even altered.

The main element of the religious buildings has been the main dome as far as Byzantine and Ottoman periods are concerned. Keeping the dome standing and constructing larger and larger domes were the main challenging issues the engineers of the old times had to face.

Istanbul has been the capital of three empires leading thus to a very rich collection of historical structures. The concentration of domed heritage structures is higher than in any other place. The main challenge, for the case of Istanbul, has always been the seismic safety of these structures because of the very active seismicity of the region, with more than 70 earthquakes with magnitudes 6 and above in the last 2 millennia (Figure 1).

Figure 1.

Known seismicity of the Marmara Region

(Modified from Ambraseys et al., 1991)

There are four structures considered in this Chapter, all placed within the old Constantinople, or the “Historical Peninsula” as the modern name calls, behind the 2^nd Theodosian Walls (Figure 4). The structures considered span from the 6^th century to the 18^th as far as their construction periods are concerned.

Hagia Sophia, Beyazit Mosque and Suleymaniye Mosque can be considered as continuity of each other. The most pronounced similarity among these three structures is the structural truss as four main piers, two semi domes settled on arches and two perpendicular arches.

1.1 Structural Features of Domed Mosque Structures

A classical Ottoman mosque, built in classical era of 15^th to 16^th century, presents several elements from Hagia Sophia in general. It consists of primary and secondary structural elements. Primary elements can be listed as the central dome, which is the most important architectural and structural aspect of the structure, four main arches perpendicular to each other, semi-domes (if any), secondary domes, thick outer walls and central piers. Secondary elements can be found as pendentives, central dome supporters (if any), weighing towers (if any), circular secondary columns (granite or marble) and circumferential belt of the central dome (Figure 2). Pendentives may be the most interesting parts of those kinds of structures and they can be defined as the extension of the main dome between the supporting arches as it has the shape of a curved equilateral triangle with its apex at the top of the main pier. Therefore, the arches have to carry not only the main dome but also most of the load acting on the pendentives directly.

Figure 2.

Components of a domed mosque structure

(Beyazıt Mosque in Istanbul is used as a model)