The most important bearing systems were developed in the mosques involving the widest spans. The bearing system is inseparable from the interior space. Designing the space implies conceiving how it will be structured and how it will be covered. In Sinan’s mosques, the bearing system is particularly clear and rational. The outer shell and the interior space reflect each other. The bearing elements do not turn into meaningless forms on the outer facade, neither are they used for mere decoration. Sinan of course borrowed elements from buildings erected before him, but he managed to develop new compositions and variations out of these elements. The spaces he designed are usually covered by domes, semidomes and trompes. At times, flat roofed cloister, but also cloister, groin and cross vaults are used.
The dome is the uppermost element. It consists of a semi-sphere ending with a circular base, which is placed over a cylindrical drum. It is supported by exterior buttresses or small buttressed arches called flying buttresses. Most often, the drum is not noticeable from the inside, although visible from the outside. It is usually also pierced all around with a quasi rhythmic sequence of filled and open windows. In the majority of Sinan’s mosques, the domes are between 11 and 14 m. in diameter.
The semidome functions as a sheltering element but also as a kind of prop supporting the arch which bears the dome.
The dome usually covers a square, sometimes a hexagonal or octagonal prism. The development of the polygon based dome plan in Islamic architecture owes much to Sinan. The transition from the square or polygonal structure to the dome circumference is provided through transitional elements. In the case of a square base, these elements will be either trompes (Haseki, Hadim Ibrahim) or pendentives (Uskudar Mihrimah, Edirnekapi Mihrimah, Sehzade, Suleymaniye, Kilic Ali, Zal Mahmud).
In hexagonal based systems (Kara Ahmet, Molla Celebi, Kadlrga Sokollu, Atik Valide), the transition to the dome’s circumference will be ensured by small pendentives, the main dome and the two trompes on each side of it covering a rectangular surface. The transitional elements from an octagonal based structure to the dome circumference (Rustem Pasha, Selimiye, Azapkapi Sokollu, Semsipasha,; Nisanci) is achieved through little pendentives, the dome and its neighbouring: trompes this time covering a square surface. Sinan has enriched these transitional elements with different variations. Trompes may be linked to wall corners through pendentives (Molla Celebi, Atik Valide) or their typical variant, stalactite vaultings or muqarnas (Kadirga Sokollu, Rustem Pasha). The trompes themselves may vary, some of them being fluted (Haseki, Hadim Ibrahim). Semidomes are linked to the bearing system with pendentives (Atik Valide, Azapkapi Sokollu) or trompes (Uskudar Mihrimah, Sehzade, Suleymaniye). Windows are placed at the bottom of the semidomes and trompes. If the span is very wide, it is reinforced with iron beams.
In Sinan’s works, this elaborate gradual rise from the external main walls up to the, main dome is an important development which reinforces the relationship between wall and dome from a structural point of view, and the relationship between prism and semi or quarter sphere from a visual point of view. On the exterior, the whole upper structure spreads out from the top like a pyramid until it reaches the outer walls. It spreads from the main dome flanked with its flying buttresses, the top part of the arches supporting it -which are often graded (Suleymaniye, Edirnekapi Mihrimah, Luleburgaz Sokollu) -and the stabilising turrets, down through the trompes and semidomes, all the way to the cupolas capping the; galleries and the latecomers porch. This gradation has the effect of emphasising the dominance of the dome without exaggerating it. Prisms, cylinders, and semi and ; quarter spheres fuse together in harmonious rhythm and symmetry. The whole weight of the structure is easily distributed onto the ground, without the use of discordant elements of support tiring the eye or confusing the mind.
Arches transfer the weight they receive from the dome, pendentives and walls above them to vertical bearing elements. In Sinan’s works, the dome lies on a prism formed by arches. The number of these arches is four, six or eight. Some semidomes and vaults seem to act like buttresses supporting the arches. Large arches have their bay filled in with a windowed wall, so that light can penetrate the interior. The Edirnekapi Mihrimah offers the most impressive example of the use of such windowed walls, followed by the Suleymaniye and the Selimiye. As the bay of the arch increases so does its soffit (in the Suleymaniye, the bay is of 21.60, and the soffit of 3.35 meters). Arches are interconnected with iron tie beams. On the exterior of the building, they are made particularly apparent, which rhythmically enhances the general modular system. Sinan mostly uses pointed arches in his works. They are double centred or double centred tangent arches, their centers situated at a distance corresponding to three fifths of their bay at spring line level. A study made on the windows in the arch-bay-walls shows that 60% of them are situated slightly below, 20% slightly above, and 20% t just on the springing line.
VERTICAL SUPPORT ELEMENTS
WALL AND PROPS
Only in narrow spanned mosques do we see Sinan use the walls alone to support the dome, and even here, the presence of arches can be traced on the inner wall surfaces. While the general bearing system consists of a masonry wall structure, the load distribution is quite clearly defined, from the covering elements downwards. This definition implies what may be called a “masonry carcass” system, in which the weight of the dome is transmitted to the large arches rising between their piers, and from there to the piers themselves, as well as to the pilasters and buttresses. A system which, in many places, I practically eliminates the need for bearing walls. Walls are reinforced by pilasters especially where the arches are based. Such props can be internal or external or situated on both sides. They can also be linked to each other by arches. Mihrab protrusions reinforce the piers on which the arches are based, as do the graded buttresses, which are more or less, hidden within the galleries and porticoes. From the Sehzade onward, Sinan brilliantly manages to cloak the buttresses with the galleries and arcades he designs on the lateral facades (also on the mihrab facade in the Selimiye). In the Selimiye the stairs too are situated inside the buttresses. In polygon based mosques especially, piers which are close to a wall are reinforced by a special arch linking them to the said wall. In Sinan’s mosques, the mihrab wall is thicker because of the niche (the thickest is found in the Suleymaniye, measuring 266 cm, the thinnest, at the Azapkapi Sokollu measuring 106 cm). In the side walls, the thickness is usually between 118 and 130 cm, reaching a maximum of 230 cm at the Suleymaniye, and 106 cm at the .Azapkapi Sokollu.
The walls erected within the bays of the arches or between the buttresses are usually pierced with numerous windows, their function being practically reduced to that of partition walls. Sinan was particularly skillful at composing the window layout between the arches. Such walls were also very skillfully used to hide the thickness of arches or buttresses.
PIERS AND COLUMNS
Domes are most often supported by piers in Sinan’s mosques. These piers are in turn linked to the walls, pilasters or buttresses through arches. As the number of piers increases, their cross-section gets narrower. Only in a few buildings (the Kara Ahmet Pasha and Atik Valide mosques, and the Kanuni and Selim II tombs), are domes supported by columns.
The larger polygon shaped piers or pillars (7.5 x 7.5 m. at the Suleymaniye) may have their corners indented or a mihrab niche lodged into them, or else be profiled, in order to be given a slimmer appearance.
ST ABILISING TURRETS
Sometimes piers may rise higher than the covering elements, and form stabilising turrets which are usually crowned with a fluted dome. Weight towers were erected to counteract the possible destabilising effects of lateral forces on the piers.
Ottoman minarets are of an elegance rarely found in other Islamic countries, soaring like lances towards the sky. They are decked with one to three balconies, which are reached by inner staircases. The steps of these Staircases are shaped out of a single stone, with one of their sides forming a kind of ; vertical core cylinder as they pile up in the axis of the minaret. The steps are held together with tenons on the centre side which forms the core cylinder while their other side is clamped to the minaret wall, the leaden binders used e to reinforce the joints also serving to soften the horizontal strain caused by wind or earthquakes. In the Selimiye, and for the first time, Sinan built three separate staircases which reach each of the three balconies of the four minarets without ever intersecting. The minaret’s conical roof or spire is made of timber rafters covered with roof boarding and sheet of lead, the spire cover rest on a wooden pole planted in the central axis of the minaret.
BUILDING TECHNIQUES AND MATERIALS USED
The construction technique of the period basically perpetuated pre-existing traditional stone and brick masonry practices.
Domes and vaults: They are made of special bricks and their thickness increases with their span. The thickness of the dome moreover gradually increases as we near its bottom. Iron bands surround and reinforce the dome at the level of the windows situated in its lower part, where they counteract its tendency to widen. The outer surface of the dome is plastered with earth and then covered with leaden sheets.
Transitional elements: Pendentives and trompes are made of stone, bricks or alternate rows of each brick surfaces are covered with plaster or ceramic tiles. Stone, bricks or plaster is used for the muqarnas or stalactite vaultings.
Arches: Wide spanned arches are made of hewn stones, sometimes in alternate rows of different colour. Arches with smaller bays are made of alternate rows of stones and bricks or only bricks. Arches are linked to each other and on the other sides with iron beams attached at the top of their supporting columns or piers. Window arches have their outer side filled in with marble or stone panels or geometrical lattices carved out of marble, while the inside is decorated with geometrical motifs made of plastered bricks or a combination of stone, brick and plaster, or else with tile panels or gypsum framed stained glass.
Eaves: In some buildings, very large eave overhangs supported by long iron props can be seen in entrance porches.
Columns and piers: Columns used in porticoes and galleries are usually made of a single piece of granite, serpantin, porphyry, pudding or breccia stone, often brought from antique buildings. Newly made columns are made of Marmara marble or Kestanbol granite. The ratio of their diameter to their length is approximately 5-7.7 in the galleries, 6-7.7 in latecomers’ porches, 6-7 in courtyards porticoes and 6-9 in external porticoes. Their capital and base are decorated with a variety of diamond or stalactite (mukarnas) motifs. A rich diversity of these motifs are readily used in a single portico’s row of columns. Capitals in late comers’ porches are usually of the stalactite kind. The typical height of a capital corresponds to 1.2-1.5 times the diameter of its column. A bronze ring links the capital and base to the body of the columns. In 40% of the cases, columns have no base.36 Certain base profiles resemble antique motifs while others are of the stalactite type. As for piers, they are of a round or polygonal shape, and made of quality hewn stones.
Walls: In large and quality buildings, both sides of the walls are made of hewn stones, with the gap left in between filled with mortared rubble stone. The stones of a facade may reach 40 cm in height. In buildings other than large mosques, alternate rows of stones and bricks are sometimes used. The most common recurring order consists of two rows of bricks, followed by one row of stones and then three rows of bricks. Coarse Limestone, called kufeki or Bakirkoy stone was mostly used, with lime mortar mixed with powdered brick serving as a binder. Stones are also tied together with iron clamps. Horizontal reinforcing beams are mostly wooden, with iron beams also used at times. A green colored tuff is used for the foundations, which are widened towards the bottom. When the soil is poor or loose, wooden stakes are planted and linked together with a system of wooden beams, which is covered by a layer of mortar and then by the foundations proper. Although it is built on solid ground, the Suleymaniye is supported by such a grid of wooden beams.
Floors: Marble slabs cover exposed surfaces like courtyards. These slabs do not usually form regular geometrical designs. The material is parsimoniously used, chipped slab corners being frequently cased with correspondingly shaped neighbouring slabs (casing technique). Inside the buildings, hexagonal, square or rectangular shaped terra-cotta tiles are used.