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Tensioned stone is a high-performance composite construction material: stone held in compression with tension elements. The tension elements can be connected to the outside of the stone, but more typically tendons are threaded internally through a drilled duct.
Tensioned stone can consist of a single block of stone, though drill limitations and other considerations mean it is typically an assembly of multiple blocks with grout between pieces. Tensioned stone has been used in both vertical columns (posts), and in horizontal beams (lintels). It has also been used in more unusual stonemasonry applications: arch stabilization, foot bridges, granite flag posts, cantilevered sculptures, a space frame, and staircases.
Tensioned stone has an affiliation with massive precut stone, which is a central technique of modern load-bearing stonemasonry. It is also aligned with Engineered wood and straw structural insulated panels (SSIPs), which are all reconfigurations of traditional materials for modern construction that involve some pre-fabrication.
The most common form of tensioned stone is post-tensioned stone, which also has the longest history. A second method, developed in the early 2020s, is pre-tensioned stone.
As with pre-stressed concrete, the pre- and post-tensioned methods can be used in different contexts: pre-tensioned stone may be more appropriate for prefabrication, while post-tensioning may be more suitable for on-site assembly.
For concrete, this problem has been long solved: in addition to conventional tensile reinforcement, engineers developed prestressed concrete methods starting around 1888. Such tension-reinforced concrete applications combine compressive strength with pre-stressed tensile compression for combined strength much greater than either of the individual components, and have been in wide use for decades. One of the early concrete engineers Eugène Freyssinet improved concrete pre-stressing methods, and it is claimed that he also applied post-tensioned concrete methods to stone. As for concrete, post-tensioning maintains stone in compression, thereby increasing its strength.
In 2020, post-tensioned stone was featured prominently in "The New Stone Age", an exhibition at The Building Centre.
Architect James Simpson writes:
Post-tensioned stone footbridges with spans up to 40 m have been built in Japan, Switzerland, Germany, and Spain, and are sold commercially in spans of up to 20 m by Kusser Granitwerke.
Seismic post-tensioning of brick and stone masonry buildings with steel bolts dates from at latest the 19th century, including following the Charleston earthquake of 1883, and on buildings in seismic regions of Italy. Seismic post-tensioning of masonry is done with considerably lower tension than pre-stressed concrete or modern tensioned stone.
In the mid-20th century, the Sydney Opera House shells were constructed from pre-cast concrete masonry beams that were assembled into a pointed arch vault using post-tensioning. By 1982, post-tensioned masonry was sufficiently widespread to fill a book published by the Institution of Civil Engineers, though this was brick and precast concrete masonry. In 1985 and 1986, structural engineer Remo Pedreschi and others published studies of post-tensioned brick.
In the 1960s–80s, Kluesner Engineering developed post-tensioned stone for use as external panels.
"Early experiments with posttensioned Indiana limestone units were sponsored by the Building Stone Institute in 1967 and by the Indiana Limestone Institute in 1970. In these programs, several posttensioned beams and slabs were fabricated and tested… The advantages of posttensioned stone are much the same as for concrete. It permits the stone to carry larger loads over longer spans than would be possible with conventional units. The stone units can be plant-fabricated in much larger units to span column to column in the building… A few structural applications have been built using beams for such building features as porticoes, where the live loads have been limited to roof loads and wind loads."
In the 1980s, the Rock of Ages Corporation developed Accu-Tensioned Granite Press Rolls for use in the paper industry. A column of granite was lathed and then drilled along its length, before the placement and tensioning of steel rods.
In the 1980s, the General Post Office of Sydney underwent a restoration that used reinforcement by post-tensioning the sandstone clock tower. As a result of the seismic reinforcement, the Sydney GPO has been claimed as the first post-tensioned stone building. The structural engineering was led by Colin Crisp of McBean and Crisp. "This technique of post tensioning an existing building is a world first and has raised international interest." As retrofitted seismic bolts were previously used, it appears likely that the GPO's priority claim relates to how the structural calculations guided the tendon placement and increased tension forces.
"A more than one hundred year old sandstone masonry building, … the GPO Tower will be strengthened with four vertical post-tensioning tendons, 19 diameter 0.5" strands each, and a number of horizontal prestressing bars diameter 35mm at floor levels. ... Special steel chairs will be used to anchor the tendons and spread the anchorage forces of 1,771 kN (400 kips). The anchorages of the unbonded tendons allow for monitoring and adjustment of the tendon forces to compensate volume changes of the sandstone, if necessary."
Designed and constructed between 1991 and 2004, the arches of a 6000 sqm Sanctuary of Saint Pio of Pietrelcina by architect Renzo Piano and structural engineering led by Maurizio Milan of Favero & Milan engineers and Arup Group.
Built in 1994, the 34 m-span Inachus footbridge in Oita, Japan uses post-tensioned stone, designed by engineer Mamoru Kawaguchi. "The bridge has a lenticular shape with an arched upper chord and a suspended lower chord, connected to each other by means of web members consisting of steel tubes arranged to form inverted pyramids."
Completed in 1995, Queen's Building at Emmanuel College, Cambridge by Hopkins Architects and Buro Happold with Ove Arup and Partners. "The columns were also used to provide lateral stability to the building and were post-tensioned using stainless steel rods attached directly to the foundations."
Completed in 1999, Punt da SuransunsPunt da Suransuns in Switzerland, is a 40 m footbridge designed by civil engineers Jürg Conzett and Gianfranco Bronzini.
"Punt da Suransuns is a stress-ribbon bridge with a span of 40 m … constructed with slabs of Andeer granite, which are prestressed over rectangular steel bars … When traversing the bridge the vertical oscillation can be felt, but pedestrians have commented that the bridge is not as flexible as it looks."
Erected in 1999, Southwark Gateway Needle by Eric Parry is made of 25 blocks of Portland stone held 16m high by post-tensioning.
In 2002, 30 Finsbury Square was completed by Eric Parry Architects and Whitbybird engineers. "The columns were constructed conventionally and left to set for 7 days prior to post-tensioning."
In 2004, Kusser Graniteworks started manufacturing post-tensioned granite–diorite flagpoles.
In 2005, a prototype of the first post-tensioned spiral stone staircase was made, called "Escalier Ridolfi". It was designed by Claudio D'Amato and Giuseppe Fallacara.
In 2006, the full-scale Escalier Ridolfi staircase was presented at the Venice Biennale of Architecture, constructed by Ateliers Romeo stonemasons
In 2007, Kusser Granitwerke constructed their first tensioned stone footbridge, in Rosenheim.
Completed in 2009, a building on Southampton Row, designed by Sheppard Robson architects, used post-tensioned stone.
In 2013, Giuseppe Fallacara and Marco Stigliano demonstrated a "tensegrilithic" prototype, combining stone with steel rods and cables into a tensegrity structure.
From 2019, Webb Yates engineers used extended horizontal stone lintels in residential construction, and proof-of-principle stone flooring panels.
A 330˚ helix staircase was designed by Foster and Partners for the 2019-completed Dolunay Villa in Turkey. and constructed by the Stonemasonry Company with Webb Yates.
In 2022, Jürg Conzett and Gianfranco Bronzini were awarded the Swiss Grand Award for Art (Architecture) to recognize their body of work, which includes a number of tensioned-stone footbridges, including Punt da Suransuns (1999), the Waterfall bridge along the Trutg dil Flem trail, and Orrido di Cavaglia (2021). Many of their tensioned-stone projects use gneiss, a stone widely quarried in Switzerland.
In 2024, the Royal Academy of Arts summer exhibition displayed a stone–steel space frame (3D truss) made from cored, tensioned cylinders of limestone and steel joints.
"Imagine crane masts, bridges or space frames like the Eden Project and Stadium Australia being formed with stone elements instead of steel. With a world-saving 75 per cent carbon reduction, inherent durability and fire resistance, we can put waste stone to use and make some really pretty structures." —Steve Webb
Due for completion in 2026, the Sagrada Familia cathedral under construction in Barcelona in partnership with the Arup Group uses post-tensioned stone assemblies. "Jordi Faulí, the architect in charge of La Sagrada Familia in Barcelona, has stated that they will implement this technique for 800 panels that form part of the central towers of the basilica; in fact, prestressing will allow for a higher resistance to winds with less weight." Using post-tensioning allows the construction to avoid using stone-cladded concrete and speeds up construction. Structural engineer Tristram Carfrae of Arup is leading the engineering effort.
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