Vaults are generated by the projection of the arch section on a plan, which is most of the time horizontal. A vault is an arch-shaped structure, usually of masonry, used as the ceiling of a room or other enclosed space, as the roof of a building, or as the support for a ceiling or roof. Masonry vaults are often composed of voussoirs, which are held in place, like the stones of an arch, by the pressure of the neighboring pieces. A temporary supporting structure must be erected within the vaulted area during construction, because a masonry vault does not become self-supporting until the central voussoirs or keystones are put in place. A number of different types of vaults are used architecturally. The simplest of these is the barrel, or tunnel vault, the roof of which is shaped like half a cylinder and is supported by straight walls. A groined vault is formed by the intersection of two vaults of the barrel type, usually at right angles to each other. The junctures at which the two vaults meet are elliptical ridges, called groins. A dome can be thought of as an arch which has been rotated around its vertical axis. As such, domes have a great deal of structural strength. A small dome can be constructed of ordinary masonry, held together by friction and compressive forces.
- Guna Tubes
Details and experiences
Building a vault with the Nubian technique
The back wall should be built first. It can have exactly the shape of the extrados of the vault or it can be quadrangular and the extrados of the vault will be drawn onto it. A template is needed to ensure the shape of the vault. It can advantageously be the future window frame on which are temporarily fixed some spacers to get the extrados shape of the vault. The template can also be made of welded Tor steel, which can be re-used afterwards for reinforced cement concrete.
It is necessary to create a net of string lines between the back wall and the template. Note that it is better to lay the net of string lines outside in the masonry. The reason is that any mistake in accuracy, with a block laid lower or slipping down, will not change the linearity of the string line.
In certain cases, it is sometimes necessary to lay the string lines below the masonry. It is then indispensable to work with a very high accuracy and to leave always 1 mm gap between the blocks and the string line.
Building a vault with the free spanning technique
We saw that this technique allows one to the lay courses horizontally and that it also uses vertical ones, like in the Nubian technique. What is presented here is only the particular details for laying the courses horizontally.
The binder varies when the vault rises. It starts with the same specification as for arches and progressively become more clayey. It is essential to check the balance of the portion of the vault which progressively corbels. Therefore, to ensure the height of the various courses their cord and span must be checked, to see if they are according to the calculations.
It is essential to compress the vertical joint very well and to keep it to the minimum, so as to reduce the shrinkage of the glue and cracks in the vault later on.
Acoustic of vaulted structures
Vaulted structures are characterised by two acoustic phenomena: echo and reverberation. Echo is a disturbing phenomenon which happens only in domes which are generated by the portion of a sphere. The dome might be built on a circular plan or on a quadrangular one (i.e. dome on pendentives) but it will always have an echo if it has the shape of the portion of a sphere or a shape close to the portion of a sphere. Hemispherical domes have the strongest echo. Pointed domes have rarely any echo but they can have a high reverberation and/or amplify the sounds. Domes which are generated by the intersection of vaults (i.e. cloister or groined dome) don’t have any echo at all.
Vaults and domes always manifest a high reverberation, which represents the time needed for the sound to fade away. This reverberation is due to 2 factors:
- The size of the volume created by the vaulted structure, which is generally larger than other ones.
- The shape of the structure, which tends to hold the sound within itself.
Echo and reverberation can be limited by acoustic correctors. These correctors are called single resonator absorbers. A German engineer, Helmholtz, elaborated some formulas to calculate them. The human voice ranges from 100 Hz to 2 kHz, with an average frequency at 400Hz. Correctors of various sizes can be installed to absorb the desired frequencies.
Composite Roofing with Brick Dome and Brick Vault
The roofing structure here is a composite work of vaults and domes. The Domes are used for the main enclosures. The beam on which it rests cantilevers the slab outward. This is done for rain water drainage for which a gutter is provided along the edge of the slab.
- China Mosaic
Details and experiences
Below is a wall section of the dome enclosure. Steel glass louvers from plinth level extend up to lintel level. Two reinforced concrete beams between which is a fixed glass opening.
Where in Auroville??
Vaulted Roofing with Steel
The building is a vaulted structure with its principle material being steel. The Vault rest on steel columns on its sides and further supported near the middle on either sides over steel columns which rest on rammed earth walls. Box steel section columns are welded to ‘I’ section beams that run across the vault. Over this re steel and wooden purlins upon which an aluminum sheet is used.
- Steel Sections
- Wooden Purlins
- Aluminum Clad Sheet
Details and experiences
Where in Auroville??
Roofing with "Guna Tubes"
Roofing with Guna Tubes is a variation of the vault and is achieved by substituting the standard material used to execute the curve, instead of voussoir type masonry. This method is unique since an element called ‘guna tube’ is used which consists of tapering conical, burnt clay pipes adapting them to come together, socket into one another and stacked in a curved form along the centering formwork. A series of such arches make a barrel vault capable of withstanding considerable loads. The top of the roof is given a plaster finish. After joints are filled and topped with plaster, the roof becomes rigid and waterproof, doing totally without steel or timber. Air inside the hollow-tiled roof protects from heat and cold. It is fabricated and ready for use within three days and requires no maintenance having a life span of more than fifty years. Being light in weight this variation of the vault roof is safe even in earthquake prone areas.
Due to all these advantages, this method has been used, by various architects in the Auroville experiment. This type is also called the ‘wardha roof’ incorporating a catenary arch profile, obtained via suspension due to gravitational forces, which is more efficient than a circle segment. Also alternate bows are laid in reverse direction to each other such that the tapering of the tumblers or tubes opposes the adjacent row, in order to reduce gaps to the minimum. Upon removal of the centering this roof may sink slightly, but only to enable the elements to socket into each other to the fullest extent. Having applied 1:4 cement plaster the finishing of this roof can be refined using a china mosaic with broken glazed tiles. Aesthetically and economically the viability of this roofing technique is rather impressive.
- guna tiles
Details and experiences
A series of arches made out of the guna tile make a barrel vault capable of withstanding considerable loads – upto 1 ton/m2. The top of the roof is given a plaster finish. After joints are filled and topped with plaster, the roof becomes rigid and waterproof, doing totally without steel or timber. The cost of this roof works out to Rs. 20/sq.ft. It has the following advantages: • Air inside the hollow-tiled roof protects from heat and cold. A 10o temperature difference is observed in slab roof and guna vault roof. • It has no under structure, yet can bear weight of 1000 kg/m2. • It is fabricated and ready for use within 3 days. • Requires no maintenance and has life span of more than 50 years. • It is not affected by rain, hail or wind. • Being light in weight (less than 12 kg/sq.ft). the vault roof is safe even in earthquakes. • Even if the mud walls collapse, the roof remains intact residing on pillars and beams. After construction of brick pillars, ring beams and gable walls, the M.S. trusses are placed in position above the opposite pillars held with guy ropes and timber poles. Now GI pipes (12 mm dia., 10 nos.) are placed on the trusses in the MS rings provided at specific points. They are supported by gable walls at both ends. Guna pipes are now laid dry on this skeleton shuttering in a plug-socket manner, completing the roof ring by ring. Lime/cement mortar is now poured on the roof, trowelling it into the gaps and leaving the top rough to receive the finishing coat. Care is taken to remove the skeleton support within 12 hours to allow natural setting.
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