Amin Taha discusses two of his contemporary projects which explore an ancient construction method
It may be perceived as ‘unusual’ and ‘challenging’ by main contractors and ‘special’ by quantity surveyors (all of which implies ‘more expensive than conventional methods’), but the post and lintel structural system is a millennia-old standard for stone masons in mainland Europe. Used more recently by Fernand Pouillon and others as austerity construction in southern France after the Second World War, the very idea of a post and lintel method seems at first anti-modern and raises concerns of progressive collapse. The current and conventional alternative is complex: employ a steel or concrete frame superstructure, fire rate, insulate, water proof, fix stainless steel rails through the former back to the frame, tape and seal vapour barriers. Then fix stainless steel clamps to the back of stone cladding panels before carefully hanging them, with grout being unnecessary but commonplace, due to its allusion to the traditional load-bearing nature of blocks.
Late 19th and early 20th century innovations in steel and concrete frames no longer required the visible masonry to be load-bearing, leaving it free to become a self-supporting façade tied back to a frame, becoming thinner still and supported by shelves at all floor levels. These incremental innovations and requirements for better thermal, fire and waterproofing performance have gradually created an overly complex building system that has become normalised for buildings of all scales. Setting aside the extraordinary level of work required to achieve an unquestioned visual fakery of the architectonic origins of load-bearing masonry, three questions should arise. The first is rhetorical: if those materials and their properties are the vocabulary for all architectural languages, is such uncritical fakery really just illiterate gobbledygook, a reflection on the profession’s inability to teach and develop a better culture? The others are key design process questions: should one use that material at all in that context? And, if one does, then how to apply its inherent qualities and properties? These questions led us to analyse whether ‘contemporary conventional’ and more complex construction can be done with fewer materials and less labour, effort and cost – all of which will inherit a lower carbon footprint.
Methods that are unusual to the construction sector have to be proven as deliverable well in advance for a client to commit to planning approval, let alone tender to contractors. Having already worked with stone masons on private house projects to deliver self-supporting travertine helical staircases, Atelier Roméo and The Stone Masonry Company lent their early support to the bolder proposal at Clerkenwell Close, a five-storey office block in Islington. With this project, the volume and mass of the stone elements required only gravity and mortar to set them in position, with a steel fixing boss mortared into a carved pocket where column and lintel meet. Thermal isolation to the floor structure is provided by a 40mm-deep solid nylon bar before the fixing is bolted into its counterpart already fixed into the floor slab. Working with Webb Yates structural engineers we were able to determine the optimum column and lintel sizes that followed structural load paths across the exoskeleton, using the simplest fixing methods and logistics of delivering and erecting the stone on site.
Once the quarried stone samples are tested for strength, sizes are increased by about 15 per cent against a control structure such as unreinforced concrete columns to meet structural and fire criteria. The lessons learnt at Clerkenwell Close were used to improve the proposal for a 10-storey block of flats at Finchley Road. The prime one was the ability to reduce the level of reinforcement mitigating progressive collapse requirements by moving lintels by half a bay across the column grid. Logistical lessons were also brought forward to save time in sequencing the raising of floor slabs and stone exoskeleton. The technique will be employed at Finchley Road, where three differently scaled and separate stone buildings will be tied together with an open and naturally ventilated steel stair and lift core. Each building uses a solid stone exoskeleton of trabeated columns and lintels.
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More fundamentally for architects is the direct connection to the material itself. We, like, no doubt, a great many other practices, have a sample library stacked with small stone tiles, perhaps even a small block or two, each labelled on the back with a company name and the finish on its front surface. But neither these, nor CGIs, can provide a sense of mass of the stone structure. Initially, we made do with photographs describing ‘Natural Cleavage’ (stone split along the sedimentary layer), ‘Drilled Cleavage’ (split by hand drilling) and ‘Saw Cut’, with photographs also illustrating quartz and fossils visible on the surface. A visit to the quarry, however, circumvented the stone mason as interpreter for the quarry master. The latter is skilled in explaining the geological bed, its gentle curve or tilt, its 650 million year-old origins around the equator, the reasons why the first cuts out of the bed occurred, where and how that influences the block sizes extracted, the differing methods for extracting those blocks, and the potential visual/surface qualities that can be retained.
Usually the quarry master is only ever asked to deliver cleanly cut stone to mason’s yards and these are in turn only presented as naked blocks to architects waiting to be dressed in a multitude of finishes by the stonemasons to the architects’ preference. Therefore it was a relief, if not a delight, to the quarry master to hear we wanted to retain the fossilised coral, ammonite shells, quartz pockets and seams. Otherwise these would be cut off and crushed for road fill.
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With a nod to William Morris’s holistic ambitions regarding the skills of craftsmen, at Finchley Road and Clerkenwell Close it was important to avoid the irony of becoming super-bespoke. As part of an assembly of warrantied proprietary products, the optimised structural size of columns and lintels is therefore applied to standard extracted master blocks like a tailor’s stencil, from ground level upward and against a week-by-week extraction time – fewer cuts for larger pieces at ground level, more cuts for more pieces further up. We can determine very early on which pieces will reveal which finish but, like choosing wood grain for cabinetry work, its exactness won’t become evident until each column and lintel are laid out in the mason’s yard. There is a complexity that results from wanting to maintain a thread of the natural and brutally honest, if not primitive. Structural load paths will determine that there be larger, single-piece columns and lintels at ground level, with the whole gradually tapering to the top, with some increases here and there so that self-weight compensates for no load above. Once these sizes have been determined, they have to be applied to the standard blocks extracted from the quarry bed. Though the final appearance and methodology may at first appear ‘special’ or ‘different’, we aim to maintain a clear line of normal methodologies among the involved trades and professions to achieve this.
These are familiar themes fundamental to the literacy of architecture, yet they seem too rarely evident in all spectrums of buildings, principally because a lack of teaching and experience in these areas translates to a lack of professional confidence. Leaving quantity surveyors and contractors to direct design team groupthink, gradually layering one no doubt well-intended solution on another separates architects from their core skill, and diminishes architectural culture to buildings that more often than not simply narrate commercial expediency.
Amin Taha, director, Amin Taha + Groupwork
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Stone isn’t an unusual material to find on a construction site by any means. In fact, people respond to it well as a natural and innocuous material that embodies geological time and human history. Of late, it has been the unfortunate victim of a major flaw in the modern construction industry, which has relegated it to a solely decorative material, rather than one with its own inherent structural properties.
This first occurred to us when we began to collaborate with The Stonemasonry Company, 10 years ago. They found that their repertoire in stone staircases was becoming limited by only being able to build traditional cantilever stairs affixed to load-bearing walls. Freestanding stone structures were out of the question for them at the time, and so they asked us to collaborate on something much more complex, beginning what has become a long partnership in stone innovation.With The Stonemasonry Company we began looking into post-tensioned and reinforced stone as a concept and together built progressively more adventurous structures, culminating in the Formby Stair, a freestanding staircase which sweeps 320 degrees from one floor to the next. The philosophy behind these designs is to use a material as both the skin and the bone of a structure, creating something both rational and elegant. Through this collaboration the team has built up a range of techniques allowing us to analyse structures and test the limits of the material and, rather than stopping at stone staircases, we began applying these techniques to structures of a much larger scale.
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Although we recognise that stone is not going to replace concrete or steel, being able to use stone structurally on large buildings could have a huge impact by helping teams meet carbon targets. While concrete has a huge amount of embodied carbon, with cement taking particularly large amounts of energy to process, stone is simply quarried, cut to size and taken to site. According to Bath University’s Inventory of Carbon and Energy, there may be as much as a 90 per cent saving in carbon emissions by building out of raw stone rather than concrete – more if the quarry is local.
On 15 Clerkenwell Close, a five-storey office block in Islington, we were able to use some of the techniques we’d developed designing stairs. Consisting of reinforced concrete flat slabs and a stability core, the slabs are supported around the perimeter by what looks like a traditional concrete and steel frame clad in stone, but is in fact a much more rational system of solid stone load-bearing columns and lintels. A closer inspection of the eye-catching structure reveals ammonoids, drilled wedge holes and saw grooves, providing the clues to the true structural form.
In stark contrast to the huge number of false brick buildings being built today this method of construction is innovative, elegant and offers many efficiencies. At Clerkenwell Close the construction of the façade was faster and more economical than traditional building techniques.
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For a traditional concrete or steel frame clad in non-loadbearing stone or heavy carbon-intensive brick skins, the wastage rate and stone cutting costs would be much higher and the overall time on site increased. The structure would be constructed first, and the stone painstakingly hung from it. By instead making stone a primary structural element, the building is composed of fewer components meaning less trades on site and a faster construction programme.
The key to working in this way is to build good relationships with contractors and architects. Our relationship with both The Stonemasonry Company and Groupwork + Amin Taha has led to a number of successful projects using stone as a construction material, but this has involved each discipline stepping into the other’s domain, and building an understanding of how the structure and material can become the architecture of a space.
Steve Webb, director, Webb Yates Engineers
This article was published in the November issue of AJ Specification