Safety Assessment of the Shah Mosque in Isfahan, Iran
The main aim of this thesis, by the author Giacomo Destro Bisol, is to assess the safety of the Shah Mosque in Isfahan, Iran.
Shah Mosque or Jameh Abbasi Mosque is a Mosque in Isfahan, standing in the south side of Naghsh-e Jahan Square. This Mosque has been built during the Safavid period, ordered by the Shah Abbas I (or Abbas the Great) of Persia. The Shah Mosque of Isfahan is one of the everlasting masterpieces of architecture in Iran. It is one of the finest Islamic edifices in the world, topped with an imposing bulbous double-shell dome with 22.65m span and 52m height.
History of the building
The building is part of a complex in Isfahan, planned to be the heart of the Safavid capital, with the square of Naqsh-e Jahān in the Center. This huge square is flanked on each side by four monumental buildings linked by a series of arcades of two stories: to the north, the Portico of Qeyssariyeh, to the south, the Royal Mosque, to the east, the Mosque of Sheykh Lotfollah and to the west the Pavilion of Ali Qapu a small Timurid Palace.
The construction of Shah Mosque started in 1611 and was completed in 1629. In 1979, it became together with the Naqhsh-e Jahān Meidān a World Heritage Site.
From the architectural viewpoint, the Mosque represents a classic Persian four eyvan plan (an eyvan is a rectangular hall or space, usually vaulted, walled on three sides, with one end entirely open) with a domed chamber over the mehrāb sactuary (the mehrāb is a semicircular niche in the wall of a Mosque that indicates the direction of Mecca and hence the direction that Muslims should face when praying). The principal façade of the Mosque had to remain flush with the southern side of the meydān, while at the same time orientating its mehrāb correctly.
Regarding the construction material, further to the use of brick as a main material, the discovery of a marble quarry in the vicinity of Isfahan provide the usage of the stone for covering only the inner walls, a local but scarce material.
Past structural intervention
During the recent centuries, there have been several restorations on Shah Mosque, mainly repairing mosaic faience and strengthening of the structure. The first conservation report is a Persian text containing all the planning and procedures, dated of 1932. The report points out a deep crack appeared in the south eyvan face to the dome, which resulted in a danger of collapse. Hossein Moarefi, as a maestro in restoration, proposed the following:
- Prepare a wooden scaffolding with 32 m height.
- Collect and code all the remaining tiles.
- Connect the two Minarets through a steel tie to reduce their displacement.
- Stabilize the eyvan with a net of three steel I-beams in the right and two others in the left side.
- Re-assemble old tiles with pins and new ones without pins.
- Apply I-beams steel rings with 90m perimeter on the extrados of the outer dome in the position of the traditional encircling wooden ties syste.
It is possible to say that this intervention of I-beams and cables was done in order to avoid the out of plane mechanism of the façade of the south eyvan. In addition, the foundation of the east dome has been reinforced, and the cracks restored as well as the roof of the Mosque, which was paved by bricks.
The Double Dome
In Islamic architecture, domes, whether as single domes or as part of a larger complex, have played an important role. They are considerably different in size and type.
The double-shell domes (dome whose two shells have noticeable distances) are the majority of Islamic domes and had gradually developed from the early Islamic epochs.
Constituent elements of the double dome
The most common prototype of double-shell domes consists of:
- The external shell has a longer diameter than the drum that it rests, and its height value usually surpasses its width.
- Internal shell, that covers the internal domical chamber and has a simple geometric shape compared to the external shell.
- Drum, used by the designers to make the building as high as possible. Its thickness must be sufficient to transfer and neutralize the vertical thrust of the external shell to the lower parts.
- Internal Stiffeners with or without wooden elements.
Most domes were constructed from quasi-brittle materials, such as stone, mud brick and baked brick. Sometimes, fragile ceramics were utilized not only to embellish the exterior faces of the external shell and drum, but also to prevent the penetrations of rain and snow.
The method for erecting the discontinuous double-shell can be seen on fig 2.9.
The double dome of the Shah Mosque
The Shah’ dome as a hybrid double dome (HDD) is a composite structure of brick double dome with radial stiffener walls, wooden ties arranged in radial, circumferential or diagonal positions adopted in the building construction and the structural system. It adopts the slightly bulbous shape for the outer dome and a pointed dome for the inner dome.
Other important components that must be mentioned are the various wooden elements present in the dome. Table 2.1 refers to the typology and function of those wooden elements.
Historical seismicity of the Isfahan region
The study of the seismicity of the area is of fundamental importance for:
- Better understand the actions suffered by the structure during its lifetime.
- Have an idea about the future actions that the structure will have to resist.
Seismic catalogues show that Isfahan was hit by earthquakes several times, with the loss of thousands of lives, but was never destroyed by an earthquake.
In 1844 AD, an earthquake shock was felt in Isfahan, which was originated from a damaging earthquake 120km north of the city. This shock caused no damage except a mall part of the Jameh Mosque (already structurally weakened) and caused cracks in the semi-dome of the eyvan of the Shah Mosque.
Seismic Hazard and risk
The Isfahan region is an area with low seismic hazard, but the risk is moderate to high, given the presence of vulnerable buildings of important cultural value, namely tall minarets and the imposing bulbous domes. Therefore, it seems appropriate to study the effect of seismic action in the safety of the Shah Mosque, in Isfahan.
Preliminary structural analysis with a provisional model
Structural analysis is a fundamental tool for safety evaluation, so it was decided to prepare a finite element model in order to obtain a first idea on the performance of the structure and to have a greater knowledge during the phase of in situ tests.
During this thesis, it was decided to exploit the geometric model on Midas FX+ by importing it onto the FEM DIANA software, where a new FE model was made.
- Model Geometry
- Element Type
- Mesh definition
- Material Property
- Material Model
IN SITU TESTS AND CALIBRATION OF THE MODEL
A fundamental step in the safety assessment of existing structures, in order to obtain reliable results, is the calibration of the model through experimental data gathered directly from the real structure. With this aim, it was decided to perform non-destructive tests on the Shah Mosque in Isfahan.
In Situ Tests
The choice of tests to be performed in the Mosque is essentially based on two requirements.
- Giving the fact that the Shah Mosque is a building of significant cultural value, destructive or intrusive tests are excluded.
- The logistical point of view plays an important role, as the great distance certainly does not allow the transportation of bulky equipment.
The choice felt on:
- Sonic tests, aimed at the characterization of the elastic properties of the material.
- Ambient vibration tests, aimed at understanding the overall behavior of the structure.
Operation Modal Analysis
The modal Estimation was carried out using ARTeMIS software, which allows analyzing the results from all tests setups simultaneously.
FINAL STRUCTURAL ANALYSIS
The Final numerical model exploits the setups used for the preliminary analysis, although some modify are necessary. The differences applied to the model are:
- The material properties are changed after the characterization of the elastic properties through the sonic tests.
- The boundary condition is changed as a result of the calibration model.
1. Material Property
The Final material properties are summarized in table 5.2.
2. Element Type
The element is based on linear interpolation. By default, Diana applies a 3 point integration scheme.
Incremental Vertical Analysis
After calibrating the numerical model and defining the settings for the analysis, it was possible to perform a vertical incremental structural analysis.
Incremental Horizontal Analysis
To assess the response of a structure under seismic actions, several methods exist. In this thesis, a pushover analysis with load profile proportional to the mass is used.
1. Pushover +Y direction
2. Pushover -Y direction
During the development of this thesis, it was attempted to reach the highest possible level of knowledge on the Shah Mosque. For this reason, a careful study of the building as a whole was made.
The first step regarded the state-of-the-art, aimed at collecting all available information about the building. In this phase, particular attention was paid to the historical research, as it leads us to understand better the structural performance of the building. This research was focused on typological and historical geneses of the building, in terms of construction methods, interventions made on the building and actions suffered by the structure during its life.
The second step was a preliminary structural analysis made using the data collected in the literature through the state-of-the-art process. This phase was a fundamental tool in order to understand the possible behavior of the structure. The preliminary structural analysis was specially useful to reach a high knowledge level of the structure. In fact, given the distance between the building and Portugal, having a great knowledge before the survey campaign was important. For this purpose, before the in-situ tests were performed:
- Detailed analysis of the geometry was made, to understand what was necessary to check with more accuracy.
- Eigenvalue analysis was made, to prepare an adequate ambient vibration test plan. In fact, the knowledge of the dynamic behavior of the structure can help in: (a) understanding the best positions for the accelerometers in order to obtain good records; (b) check, during the test, if the results are consistent with the preliminary numerical model.
The third step was the in-situ tests and the subsequent data process. During this phase, dynamic identification and sonic tests were performed in situ. The distance to the building and the limited time available required a precise organization. A good action plan made day by day and excellent teamwork were a winning strategy to obtain reliable experimental data. Subsequently, the processing of the data stressed the importance of the numerical model in the analysis of the experimental results. As mentioned above, the numerical and the experimental model must go hand by hand to reach good information. To finalize this phase, the model was calibrated. This path had as a target the assessment of the static condition of the mosque.
The last step performed was the structural analysis of the Shah Mosque. The results obtained show a reasonable condition of the mosque under gravitational loads. The structure can withstand more than two times its self-weight. Regarding the response of the structure under seismic actions, even if not enough studied in this thesis, the earliest failure mechanism found is the out of plane mechanism of the eyvan with a base shear factor around 0,2.
As a result of the previous works and in order to understand deeply the static behavior of the Shah Mosque, the following recommendations for further studies were proposed:
- Geometry, the numerical model can be updated with the elements neglected in this study. The main parts do not consider in this thesis are the minarets, which could be the weakest elements especially under seismic action, and the adjoining structure, considered in this thesis only as a boundary condition.
- Material, the mechanical properties of the masonry constituent the Shah Mosque are not precise enough, because the properties in terms of strength are missing.
- Soil, the specific geotechnical characterization for foundation of the mosque is only partly present. The information about the soil is the one related to the Metro project in Isfahan, which describes the soil in a zone close to the mosque. For this reason, a geotechnical survey of the foundation of the mosque is useful.
- In order to understand the real behavior of the structure, proper boundary conditions are essential. To this end, the updating of the model with the soil property is required and more careful process of the dynamic identification data obtained.
- Modern strengthening intervention, the response of the structure can be reliable only if all the structural elements are characterized. For this reason, the implementation of the model with the strengthening elements is suggested.
Seismic hazard, the knowledge of the probability that an earthquake will occur in a given geographic area, within a given window of time, and with ground motion intensity exceeding a given threshold is necessary to assess the performance of the structure. A study of the seismic hazard in the Isfahan region seems fundamental.
- The seismic analysis of this type of buildings needs particular attention. For this reason, the following analyses are suggested: (a) pushover analysis in the X and Y direction (positive and negative) – for this type of analysis also, the use of at least two load profiles is suggested; (b) dynamic non-linear analysis with a real or spectrum-compatible accelerogram.
- Seismic assessment, an adequate comparison between the capacity of the structure and the demand required from the possible earthquake is necessary, in order to establish the safety level of the structure.
- Strengthening proposal, the evaluation of the past strengthening design and the proposal of new strengthening interventions on the structure (either if needed or only conceptually for research’ sake) is interesting.