I have left my hometown Istanbul over ten years ago.
Occasionally, I find myself longing for the breath-taking sight of its undulating
sea of domes and sharply rising minarets. When I started writing this blog, I
knew I would one day write about its monuments.
Figure 1 – A view of
the Sehzade mosque from east (date unknown, retrieved from [1])
In November last year, I visited Sehzade Mosque, one of
Sinan’s countless masterpieces. It was built between the years 1544-1548, in the
memory of Sehzade (Prince) Mehmed, Sultan Suleyman’s much beloved son. The
architectural customs of the empire would have dictated Mehmed’s mausoleum to
be located in Bursa, the empire’s previous capital. It was customary for the
deceased prince’s funds to be used to finance the building of a mausoleum, and
associated charitable and religions institutions. However, breaking with
tradition, Sultan Suleyman supported the building of a grand complex in the
capital Istanbul, equal in rank to imperial mosques built in the name of his
forebearers (Fatih, Bayezid and Selim). The complex was thus accorded with
architectural features reserved to imperial mosques. These included two
elegantly decorated minarets with two galleries and a wonderfully elaborate
structural plan [2] (see Figure 1). No other monument built for an Ottoman
prince thereafter was accorded such privileges, and in my opinion, no other
mosque thereafter executed with such decorative brilliance.
This monument was only made possible with careful planning
and engineering. For the remainder of this post, I will focus on a few engineering
aspects that I have noted during my visit and research.
Figure 2 – A
schematic of the Sehzade mosque with an illustration of the flow of loads.
Retrieved from [3]
The rational
structural plan
The design of the Sehzade Mosque is underpinned by a
rational structural plan that reflects the flow of forces. This is illustrated
by a schematic of the mosque in Figure 2. The mosque is placed on a north-south
axis, and a 19m diameter semi-circular dome carves an unimpeded central space.
The brick dome sits on the crown of four thick stone arches. The central dome
exerts lateral pressure which is transmitted to these arches, and to the brick
pendentives. While the pendentives have little lateral load carrying capacity,
diagonally placed flying buttresses and weighing towers, enhance the lateral
load resistance in this direction (see Figure 3). The lateral loads which are
transmitted to the arch and pendentive are then distributed downwards. A
portion of these loads is transmitted via the piers to its foundations. The
remaining portion (in the N-S and E-W directions) is transmitted to four
semi-domes, which then transfer the lateral thrust to the perimeter buttress
walls. From here, the forces flow to the foundations from the chunky buttress
walls, which are skilfully hidden behind a domical arcade (see Figure 1). It is
reasonable to think that the structure was constructed bottom up, starting with
perimeter buttress walls and ending with the central dome. This rational
arrangement of structural elements results in a graceful and dynamic exterior
(see Figure 3) and a unified interior space with a gradually receding ceiling
(see Figure 4).
While evaluating the plan and dimensions, it is useful to
quantify the distribution of lateral loads in the system. The maximum thrust
that can be carried by the side domes under self-weight is proportional to the
square of the diameter of the dome. Therefore due to the 1:2.5 diameter ratio
between central and higher tier side domes, each side dome can only carry a
maximum of 1/6th of the lateral thrust exerted by the central dome.
This demonstrates that the piers under the arches carry a significant portion
of the lateral load, while the side domes provide stability against foundation
movements. This explains why the piers are massive, with a 4.5m thickness.
Figure 3 – A photo of
the Sehzade mosque. Note the diagonally placed flying buttresses on the drum of
the mosque and the weighing towers placed in front. (Retrieved from [8])
Further information can be obtained by investigating the
side cross-section view of the central semi-circular dome of the Sehzade mosque
in Figure 5. When viewed from outside (e.g. Figure 3), it appears as if the
area under the dome is an upright drum, with window openings to let the light
in. However, seen from inside, the windows openings appear curved (see Figure 4).
Looking closely at the plane section in Figure 5, it becomes obvious that at
angle of embrace of approx. 110 degrees, the dome section becomes thicker, to
become flat on the outside. According to simple membrane analyses [5], below this
is the height where meridional cracks would appear due to emerging tensile hoop
stresses for solid domes. However, having a thicker section here is likely to
have made the dome more resilient towards this sort of damage.
Figure 4 – A photo of
the interior of Sehzade mosque, looking north towards the mihrab.
Figure 5 – A cross
section to-scale drawing of the Sehzade mosque (Retrieved from [4])
The elusive earthquake
resistance
The Sehzade mosque has been stable for centuries, and is
unlikely to experience large static movements which may threaten its stability.
However, dynamic loading is a major threat, since Istanbul is in a zone of
seismic activity. During its 500 year lifetime, Sehzade mosque has survived
several major earthquakes, including the 1766 and 1894 earthquakes, which have
caused damage in nearby historic buildings (the dome of Fatih mosque collapsed
in 1766).
It is difficult to ascertain if the Sehzade mosque (or other
similar mosques) will survive potentially more intense future earthquakes. We
have limited information (or visual record) of the damage sustained by historic
mosques during earthquakes. However, previous experience and research have
highlighted that that these structures are particularly vulnerable. Their first
translation and torsional vibration modes of mosques are typically in the high
frequency range (0.2-0.5s in both directions), which will result in an
amplification of the applied dynamic forces on the structure.
From previous observations of earthquake damage, several
critical elements can be identified:
- Foundations under the piers which carry lateral loads: A significant portion of the induced lateral loads due to self-weight are transmitted to the base of the piers. Base shears and moments due to lateral earthquake loading would similarly be expected to concentrate in this area. Potential settlements here during dynamic loading can be detrimental to the stability of the dome.
- Pendentives: The arches of the mosque represent the two load carrying frames in the orthogonal directions. Due to earthquake directionality effects, ground motion in orthogonal directions can be different, leading to differential movement and tensile stresses in the pendentive area that connects the orthogonal arches. Damage in this area could lead to support relief for the dome and partial collapse. Such damage has been observed in the past for the dome of Hagia Sophia.
- Dome windows: Due to the presence of window openings on the dome, the lateral loading needs to be transmitted through the reduced solid sections of the dome in between windows. These represent likely locations for strain concentration, and may get damaged due to lateral loads.
Nonetheless, our understanding of the dynamic behaviour of
these structures remain rudimentary. In order to ensure that these structures
survive for another 500 years, we need to have an improved understanding of
their dynamic behaviour. One way to achieve this would be to use new sensing
techniques to measure the dynamic response of these structures, to advance our
understanding of their behaviour.
Acoustic
characteristics
An imam situated in the south (mihrab) area of the mosque
leads prayer in mosques. Occasionally, the muezzin, who would be located in the
southeast corner of the Sehzade mosque, reads prayers aloud. The congregation
responds to the prayers recited by the imam and muezzin. Therefore, optimising
the acoustics of mosques, especially from the perspective of speech intelligibility,
was an important engineering problem in Sinan’s period.
Within this context, it is desirable to achieve a homogenous
dissipation of sound energy inside the mosque, to prevent directional sound
effects. It is equally important to ensure sufficiently quick absorption of
sound by the structure, to ensure that consecutive sounds are not mixed. The
domed structure of the mosque presented a challenging environment to achieve these
objectives. The curved surface of the dome reflects the sound energy in different
directions with little dissipation and much time delay, resulting in the sound
to dissipate over a long time. This causes a long reverberation time, and it is
not acoustically desirable.
There were numerous established acoustic solutions to deal
with the aforementioned issues. For instance, using plasters with strong
absorption characteristics to cover the dome surface enabled a more diffuse
sound field. In a recent investigation of the nearby Suleymaniye mosque, it was
concluded that the restoration of dome decorations with cement based plasters,
has led to increased reverberation times. The original lime based plaster
absorbed the sound more effectively, decreasing the reverberation times.
Figure 6 – (left) Schematic for a Helmholtz (cavity) resonator
from Sultan Ahmed mosque and (right) a photo of a filled-in cavity (Photos
retrieved from [7])
Another solution for acoustic improvement is Helmholtz
resonators. This is a time-honoured method of absorbing a narrow frequency band
from broadband sound waves. It is achieved in a fascinatingly simple manner. A
cavity is opened on highly sound-reflective surfaces inside the mosque. From
inside the mosque, these cavities look like simple cylindirical holes with
diameters up to 10cm. However, inside the hole, the narrow cavity neck gets
wider, and can protrude 0.5m deep into the dome, providing a large cavity
volume (see Figure 6 for similar resonators from another mosque). The incoming
sound waves at a particular frequency are affected by the springiness of the
air inside the cavity. The incoming sound tries to squeeze the air in the
cavity, but the air resists, this interaction causes a range of sound waves to
behave like a mass on a spring, whose motion is dissipated (absorbed) over time.
By changing the dimensions of the cavity neck and backing volume, the absorption
frequency band can be modulated.
During a restoration of Sehzade mosque during the 1990s, 144
cavity (Helmholtz) resonators were found [7]. Most of these were located on the
dome. The original sizes of these resonators were such that they were designed
to filter low frequency noise, smaller than 250 Hz. Unfortunately, many were
filled in during previous restoration works (see Figure 6), and were not
functioning as intended. Since measurements had demonstrated that reverberation
time for low frequencies is unusually high inside the mosque, it was suggested
that the destructive restoration works have affected the mosque acoustics negatively.
However, the influence of this erroneous intervention may not be felt strongly
by congregations today, imams and muezzins often use microphones to lead
prayer.
Despite these small changes, Sehzade Mosque is a
well-preserved and wonderful monument, that makes one wonder what life in
Istanbul in the 16th century would have been like, under that sea of
domes and sharply rising minarets.
REFERENCES
[2] Neciopglu, Gulru. The Age of Sinan. Reaktion Books,
2011.
[3] Karaesmen, E., et al. "Seismic behaviour of old
masonry structures." Proceedings of the tenth World Conference on
earthquake engineering. 1992.
[4] Kuban, Doğan, and Cemal Emden. Osmanlı mimarisi.
Yapi-Endustri Merkezi, 2007.
[5] Heyman, Jacques. The stone skeleton: structural
engineering of masonry architecture. Cambridge University Press, 1997
[6] Gül, Zühre Sü, and Mehmet Çalışkan. "A DISCUSSION
ON THE ACOUSTICS OF SÜLEYMANİYE MOSQUE FOR ITS ORIGINAL STATE."
[7] Kayili, Mutbul.
"Acoustic solutions in classic ottoman architecture." Foundation for Science technology
and Civilization (2005).
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