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ENGINEERING DESIGN IN THE TIME OF THOMAS TELFORD
By Bill Addis
Thomas Telford, the Institution of Civil Engineers' first president, was born 250 years ago this year. His career spanned a half-century that saw some of the most remarkable changes in the way European engineers approached the design of buildings, bridges and machines. This paper reviews the development of engineering design, science and education during Telford's era, revealing that Britain was then far behind France, Germany and other European countries. Through the work of Telford and others, Britain's engineers embraced the practical experimental approach that scientists throughout Europe had developed in the eighteenth century to generate new engineering knowledge and understanding. By the early nineteenth century Britain was emerging as a leading engineering nation.
The era spanned by Thomas Telford's life (1757-1854) is usually characterised as being the period when wrought and cast iron began replacing the traditional materials of stone, bricks and timber for the construction of buildings, bridges and machines. However, these were not the only changes to the world of engineering.
The same half century saw many other developments that affected how engineers undertook design and construction. These developments were taking place all over continental Europe — from Scandinavia to Russia, Italy and Spain — and news of them usually travelled fast, either through open visits by leading engineers, via non-technical travellers, or by means of military and industrial espionage.
Telford was a gregarious man — indeed, promoting contact between engineers was one of his reasons for becoming president of the newly formed Institution of Civil Engineers in 1820 — and it is more than likely that he was aware of everything mentioned in the following review of the world of engineering design during his lifetime.
Building with new materials.
Telford's life covers the half-century when the use of concrete became firmly established in the construction industry. It had been widely used in France in marine engineering and for bridge foundations by the mid-eighteenth century.
John Smeaton (1724—1792) discovered hydraulic cement on a trip to the Netherlands and experimented to achieve the best mix design before using it for concrete (or beton as it was then called) in his Eddystone lighthouse (1756—1759) and in the foundations of Hexham Bridge (1777). By the early nineteenth century, it was being widely used in Britain in the construction of the docks and harbours, for example in London's docks, as foundations for the docks and buildings and for mass concrete walls.
Telford used a bed of concrete for the foundations of his St Katherine's docks (1826). Although the chemistry of concrete was established in the late eighteenth century, mainly by French scientists such as Vicat, the modern understanding of mix-design was not gained until the early twentieth century through the work of US engineering scientist Duff Abrams.
Wrought iron had been in widespread use in Europe since the late middle ages, both for military use — armour, cannon, other engines of war and shipbuilding, and for civil use — in tools and for load-bearing applications where timber was insufficiently strong, stiff, or durable. Notable uses were for tied masonry arches in many large churches, a practice dating from the sixth century in the Middle East, and the iron chains used in the dome of St Peter's in Rome (1550-1570) and by Christopher Wren at St Paul's cathedral in London (1670—1710). Most large timber roofs had iron straps to carry tension forces across joints and support long timber tie-beams. Wren used iron ties to support a mezzanine floor at Hampton Court near London, and to help support large, first-floor bookcases at Trinity College library in Cambridge. Making best use of Jean Tijou, his French iron master at St Paul's, Wren also used wrought iron in 1692—1693 to make columns to support the balcony in his refurbishment of the chapel of St Stephen at the House of Commons in Westminster.
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The use of cast iron in Europe also had its origins in military applications — notably the barrels of cannon in Germany from the mid-fifteenth century. The columns used to support the kitchen chimney in a monastery near Lisbon in 1752 were almost certainly an example of market diversification by the local cannon foundry. Smeaton was one of the first engineers in the 1750s to use cast iron consistently for the components of mills that were most subject to wear, and the first all-cast-iron machinery for a flour mill was built in 1784. The first cast-iron I-section rails were made by Jessop in the 1780s.
The iron bridge, completed in 1779 in Shropshire near the modern town of Telford, was built as a massive advertisement for new uses of cast iron, and a number of nearby churches featured the earliest use of cast iron in an architectural context, where columns were used to support balconies. Cast-iron columns in industrial buildings were used by William Strutt (1756-1830) in a number of buildings in the Derwent Valley, beginning with the cotton mill in Derby in 1792 — 1793. The first cast-iron floor beams were used by Thomas Bage in the flax mill at Shrewsbury in 1796-1797.
The manufacture of both wrought and cast iron improved during this period, both as a result of the direct practical experimentation in foundries and also using the results obtained by a number of scientists, notably by the French physicist Reaumur, whose book "The art of converting iron into steel and making cast iron softer" (more malleable), was published in 1722, and the Swedish metallurgist Т. О. Bergmann, who established the important effect of carbon content on the properties of alloys of iron in the 1760s. Riveted, wrought-iron boilers for steam engines were being made from the 1750s and flat sections and rods of wrought iron were being rolled (rather than hand-forged) in Sweden in the 1740s and in England from the early 1780s.
Making buildings more fireproof.
The increase in use of wrought and cast iron in buildings was largely a consequence of many fires in theatres and in multi-storey factories and warehouses that had often resulted in terrible loss of
life as well as the loss of buildings and the expensive manufacturing or theatrical stage machinery inside.
The French architect Jacques-Germain Soufflot was probably the first to take what would now be called a fire-engineering approach to the design of buildings for his theatre in Lyons, completed in 1754. He not only sought to avoid the use of flammable materials, he also installed water tanks and hosepipes above the stage, he created strict compartmentalisation between the stage, dressing rooms and auditorium, and the stairways were made entirely of stone and enclosed by sturdy fire doors.
Towards the end of his life (1781) he was responsible for the first all-iron, fireproof roof truss, in the Louvre palace. This idea was used a few years later by the architect Victor Louis in his Theatre Francais in Paris, which had an iron roof trass spanning 22 m as well as a ceiling to the auditorium made using fireproof poteries — hollow clay pots and iron. News of this fireproof construction soon reached William Strult in Derbyshire, who used hollow clay pots in some of the jack arches in his fireproof mills and warehouses from 1792. Drury Lane theatre in London was the first to be fitted with an iron safety curtain in 1794.
Material strength and stiffness.
One characteristic of engineers is that they use calculations to raise their confidence that a proposed design will work. For load-bearing applications, the two key quantities are the strength and stiffness of materials. In Telford's time, these properties of materials were effectively embodied for common applications in well-known standard dimensions of, for example, timber floor beams or roof trusses of various spans. The dimensions of the elements of masonry buildings and retaining walls were also well known among the specialist dealing with these crafts; this tradition went back many centuries.
The use of iron, however, presented new challenges. Not only were its properties little known in the mid-eighteenth century, but there were no long-established standard designs and, most importantly, being a manufactured material, its properties varied significantly according to the source of the iron.
It was both the need for engineers to know materials properties and the general inquisitiveness of scientists that led to the growth of
29Q | Английский язык для студентов строительных специальностей
includmg one 15 m long at a scale of 1:20 - to establish the tension at the ends and to verify the shape of the chain under bridge-deck loads.
Drawing in three dimensions. ort ™e Pla"s' sections and perspective drawings were well established before the eighteenth century, these were of limited use m dealing with complex three-dimensional geometry. This problem attracted the growing interest of engineers in the early eighteenth century, especially for drawing large stones of irregular shape -me science of stereotomy. By drawing stones before construction began it was possible to prefabricate all the stones necessary to construct a vault, for example, and then assemble them quickly -very much more quickly than the traditional technique of cutting a stone only when the adjacent stones beneath and to the side had SfiS if*! Pl3Ced- Mthoueh this technique was well established in the whole of continental Europe by around 1740, it was little used m Britain and there is no evidence that Telford used it. о*ь T№°d ,of graphical representation familiar today as orthographic, or third-angle projection, was devised by the French engineer Gaspard Monge (1746-1818). Called "descriptive geometry by him, it was developed as a new means of setting out
япн I «!f г/" 7gg,ed terrain in order t0 balance the cut and fill ™ de,?Jh? Мсайоп - which means ensuring defending cannon could attack key surrounding areas and that the fort was not vuinerab e to attack by cannon mounted on nearby vantage points, l he whole process was extremely complex to calculate using three-dimensional co-ordinate geometry, highly prone to error, and could take several weeks.
iu™ginC t]f 8и.ф,"8е ofhis commanding officer when the young Monge completed this task in only a few days. The commandant's
Ц16?10?38 d,lsbelief but' on checking the work, this turned to wonder. Monge s technique was quickly declared a state secret and by around 1800, was taught in all the French military academies. Its use gradually escaped into civil projects and the public domain including England, by the mid-nineteenth century facilitating calculations. Jhe calculations that could be performed using drawings and geometrical constructions had been enhanced by the development ot orthographic projection by Monge.
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Generally speaking, though, it was the surveyors rather than draftsmen who were the most numerate among those involved in construction. Their work involved mapping the terrain and landscape, setting out new works such as bridges, roads and canals, as well as estimating the quantities of materials involved, including excavations, cut and fill, and construction materials ranging from brick, dressed stone, mortar, timber and iron to glass, lead and tiles for buildings. Land surveying required great accuracy that could be achieved only by hand calculations and the use of six-, eight- or sometimes ten-figure tables of logarithms, trigonometric ratios, squares, cubes and roots.
By the late eighteenth century there was a small but growing number of engineers, especially in continental Europe, who were highly trained in mathematics and were familiar with both complex analytical geometry and calculus. Belidor's book "Architecture Hydraulique" (1737—1753) was the first engineering book to use calculus, for calculations of water flow. Nevertheless, such sophisticated mathematics was beyond most engineers' abilities and
Most significant of all, this period saw engineers starting to use the slide rule for calculations that required no more than three-figure accuracy — that is most of their day-to-day calculations. The slide rule had been devised in the 1620s, very soon after the invention of logarithms, and was used mainly by astronomers and navigators. By about 1770 it was known to some engineers, but we can only guess at how widespread its use was.
The firm of Boulton & Watt recognised the usefulness of the slide rule and, from the mid-1770s, began making what came to be known as "Soho Scales" — named alter the company's Birmingham works — as a sideline to its steam-engine business. The first guidance in an engineering book on how to use a slide rule was in "A Treatise on the Steam Engine" by John Farey (1827), at a time when slide rules generally still did not have moving cursors to ease
Although Telford would have been aware of the use of contour lines to show depths of the sea, which were devised by the French geographer Ph. Buache in 1737, he did not live to see the rapid
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developments in the graphical representation of numerical data during the 1840s that increased the speed of performing engineering calculations. Charts devised by Leon Lalanne, the pioneer of this technique, showed, for example, how to represent an entire year's data for temperature and wind speeds in a single diagram. He also published charts for multiplication or division of numbers by graphical means.
Improving engineering education.
The specialist education of civil engineers grew out of various schools of military engineering in continental Europe, especially in France, in the late seventeenth and early eighteenth centuries. The first school of engineering open to the public was in Prague, now in the Czech Republic, in 1707 — a school that proudly celebrates its history back to 1344, when the Prague Public Engineering and Metallurgical School was founded.
The famous Ecole des Ponts et Chaussees opened in Paris in
1747 under the directorship of the bridge engineer Jean-Rodolphe
Perronet. The Ecole des Mines followed in 1783 and the first of
many ecoles d'arts et metiers, which were rather less academic than
the Ecole des Ponls et Chaussees, though hardly less prestigious,
opened in 1780. The Ecole Centrale des Arts et Manufactures was
formed in 1829. Other schools were opened to address the technical
needs of craftsmen too, for instance the Ecole Royale Gratuite de
Dessin — the Royal Free School of Drawing — which was founded
The idea of polytechnical education, dedicated to harmonising theory and practice, spread through continental Europe with remarkable speed. Seven polytechnic schools were formed in Germany in the first 30 years of the nineteenth century. In Austria schools were formed in Prague (1806), Vienna (1815) and Cracow (1833). There were no similar schools in Britain during this period. The establishments that served anything like the role of the continental polytechnics were a number of military academies that trained engineers for the army and navy, the most famous of which was the Royal Military Academy at Woolwich in east London, established in 1741.
Edinburgh University was the first to offer lectures on applied mechanics in the 1790s, but such a course was not intended as part of a programme to produce academically trained engineers. Engineers in Britain were largely self-educated in their spare time until the mid-nineteenth century.
Publishing books and periodicals.
The first comprehensive books on civil engineering, which also served many of the needs of military engineers, were by the military engineer Bernard Forest de Belidor (1697-1761). His first, published in 1729, was "La Science des Ingenieurs". Despite its title, however, it contained relatively little engineering science as we know it (science meant body of knowledge).
Belidor's second book, "Architecture Hydraulique" (1737— 1753), in four volumes, was radically different. Its style and mathematical rigour followed that of a scientific text book. These books were followed by many more written by the engineers who gave courses at the growing number of polytechnics in continental Europe.
Both Smeaton and Telford had copies of Belidor's "Architecture Hydraulique" in their libraries and Telford owned many more of the French classic engineering texts published in his lifetime, including the encyclopaedic "Art of Building" by Jean Rondelet (1805-1810) and books describing bridge projects by Perronet (1788) and Wiebeking (1810) at a time when there was nothing equivalent in English. He also owned several German books — six by Jacob Leupold, dating from the 1720s and a book on hydraulic engineering by Wiebeking (1811-1813). Of the books in English in the libraries of Smeaton and Telford, the great majority were on mathematics and physics.
While a number of scientific academies, especially in France, published papers of interest and relevance to engineers during the eighteenth century, the papers of the Ecole Nationale des Ponts et Chausees were the first entirely devoted to civil engineering. The first periodical, published with the intention of keeping professional engineers informed of developments, was produced in 1797 by Johann Eytelwein (1764-1849), an engineer in Berlin's building department.
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For the whole of the eighteenth century and well into the nineteenth century, France led the world in terms of developing engineering science and trying to use it in engineering design. Nevetheless, Smeaton and Telford were among the world's leading engineers, and Britain saw many of the world's leading engineering projects. The marked contrast in the nature and level of engineers' education, and their approach to engineering design between Britain and continental Europe, remained well into the twentieth century. (New Civil Engineer International, May, 2008)
WINDY CITY WONDER by Jessica Rowson
Clever engineering has meant that North America's tallest
residential building will be solid as a rock despite its windy location.
Nestled among the forest of skyscrapers on the Chicago skyline,
the 92 storey Trump Tower is currently notching its way up to
become the city's second tallest building. The 415 m tower will be
completed in January 2009. The stepped concrete building has been
designed to reflect the height of nearby buildings by architect and
engineer for the project Skidmore, Owings & Merrill (SOM).
The first step aligns with the 130 m high Wrigley Building, the
second the 179 m high Marina City Towers, and the third the 212 m
high IBM Plaza, known as 330 North Wabash.
As important as these steps — also known as setbacks — are architecturally, they also have an important engineering role as they each contain an outrigger stability system. These 5.3 m deep by 1.7 m wide concrete monoliths transfer lateral loads between the perimeter columns and the central core. SOM associate partner Robert Sinn explains that the lateral shear resistance of the core and overturning resistance of the perimeter structure are mobilised by linking them at discrete levels using outrigger trusses or beams. He adds that this means just a few heavier vertical elements are
needed on the perimeter to keep the building stable, freeing up the facade.
The outrigger beams take up a storey height and are heavily reinforced. In some areas conventional bars are even replaced by an equivalent area of steel plate to ease congestion. Contractor Bovis Lend Lease is using self compacting concrete to penetrate densely reinforced areas. Surprisingly, the tall building does not require dampers to limit its movement. This is because of the stabilising effect of the heavy concrete core and columns and the setbacks. The asymmetric setbacks change the cross section of the building, so changing the frequency of wind passing it. This means that vortices, which would cause the building to move more, cannot build up.
Any massive building needs massive foundations. The building sits on 30 m long piles founded on bedrock. A permanent steel liner, which seals the excavation, cuts through 18 m of stiff clay and 12 m of boulders and fractured rock to form a socket in solid rock. On completion the Tramp Tower will hold the record for the world's highest residential building, but only for a year. After that it will be dwarfed by the 610 m, 150 storey Chicago Spire. Finite element analysis.
Engineers had to deal with the inherent problem of the uneven load distribution of a massive, asymmetrical building and its tendency to move sideways under its own weight. The solution was to carry out a time-based finite element analysis on the structure so that movements could be predicted and compensated for during construction. Bovis Lend Lease used these results to make millimetre adjustments at every storey to bring the building back to plumb.
Non-linear analysis predicted the short and long term displacement of Chicago's Trump Tower, which included the effects of creep and shrinkage. If no horizontal correction had been made during construction, the roof could have moved 300 mm out of line due to the combined effects of gravity, creep and shrinkage. Foundations.
A 3 m deep piled raft was poured continuously over a period of 22 hours. The concrete was poured using conveyor belts so that very few vibrators were needed; the temperature had to be carefully
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controlled as the concrete cured due to the raft thickness. If the differential temperature across the depth was too large, stresses would set up and lead to micro cracking.
(New Civil Engineer International, October, 2007)
"ROSSIA" WITH LOVE by Jessica Rowson
Europe's tallest skyscraper is being built in Moscow. If you're thinking of designing a tall building, make it at least 600 m or nobody will bat an eyelid. Moscow's latest addition to the 600 m plus club is the 612 m high, Rossia Tower, a cool 2 m higher than the Chicago Spire. Rossia's site is currently being cleared to make way for what will be Europe's tallest building. The skyscraper will incorporate retail and office space, a hotel and apartments on its 120 floors, three of them below ground level.
To the untrained eye, Rossia is an elongated pyramid, or rocket shaped structure, but on the inside, the structure tells a different story. At its base there are three colossal, high strength concrete abutments clamping the whole structure down. Each abutment forms the base of three wings of the building, from which columns radiate. The wings converge at a central spine, or concrete core, which runs the full height of the tower. Consultants Waterman International and Halvorson have designed the steel frame and composite floor structure. The plan and profile of the building take on the efficient geometry of a triangle to achieve maximum stability using the minimum amount of material.
Initially architect, Foster & Partners, designed the tower as three discrete blocks, arranged in a Y shape in plan. But this meant that each block was too slender, having a height to width ratio of 10:1. "Structural solutions were possible for this option of independent towers, but at these aspect ratios, the solutions would be inefficient," explains Waterman International project director Hugh Docherty. The decision was made to merge the blocks, so
they leaned into the central core. The sloping parallel columns could then brace the core laterally as well as carrying vertical loads. The result was a more efficient height to width ratio of 5:1. "So in terms of height to base, the building is squat," says Docherty. The design was starting to look like the familiar form of a cable stayed mast. However instead of tension cables, Rossia uses the sloping columns to act in compression — propping the central core and essentially acting like three dimensional arches.
The fan columns carry gravity load and wind overturning forces as direct axial loads. And as the weight of the building and its inhabitants exceeds the design wind load in the majority of the columns and core, there is little tension in the system. Piling contractor Soletanche is currently building a diaphragm wall on the site, but it will be at least six years before the 100 m tall mast crowns the building.
The tower's three wings comprise steel and concrete columns which fan out from the three massive abutments at the base. Visually, this gives the form of a tripod supporting the rest of the building — a structural form known for its efficiency. "Three legged stools are great. With four legs you start to bring in redundancy," says Waterman International project director Hugh Docherty. Having established the path for vertical and lateral loads, the remaining challenge was torsion. The facade of the wings is stiffened by a series of "reverse fan columns" which triangulate the facade. "The wings are designed as boxes with crossed bracing. These resist twisting," he explains. The rigid facade is further stiffened by steel chevron bracing up to the fourth floor on the outer edge of each wing. This provides sufficient torsional stiffness. But a structure with sloping columns causes other problems in the form of horizontal loads amassing at the base. "We used tension ties in the raft to stop the feet from spreading. We could have propped against diaphragm walls or relied on friction, but tension ties were the most controllable option," says Docherty. The construction sequence requires the fanning columns to be designed for erection loads. Later they will be encased in reinforced concrete to achieve the final strength for permanent loads.
(New Civil Engineer International, February, 2008)
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Building a super tall tower in Karachi calls for international know-how and an understanding of local subcontractors' capabilities.
In Karachi, Pakistan's second city, a tall building is characterised as anything over 10 storeys. The loftiest have reached 20. So it is no overstatement to say that the 78 storey Karachi Port Tower, construction of which is scheduled to start this year, will transform the city's skyline. Compared to the Burj Dubai, which will be the world's tallest building at 146 or more storeys, 78 storeys doesn't sound so remarkable. But Karachi Port Tower will be the tallest building on the Indian subcontinent. Building on this scale in Pakistan is a one-off and poses some interesting challenges. Nobody in the country has ever carried out a site investigation for a building of this size before. Exceptionally deep and large foundations are required but local batching plants are not equipped to produce concrete in the volumes and strength required. The specialist falsework, formwork, cranage and concrete pumping equipment needed for ultra-nigh buildings does not yet exist in Pakistan.
"Construction will require international know-how, but with local knowledge. Three joint ventures of foreign main contractors with local firms have been shortlisted," reveals Mott MacDonald director Steve Gregson, who is leading structural, facade, mechanical and electrical, and fire engineering. "But whichever of the three is selected, they will be heavily reliant on local subcontractors." Throughout the design process a close eye has been kept on buildability and making the structure suitable for local conditions and skills.
Client Karachi Port Trust is the port authority and operator and is also a major property and infrastructure owner. It is undertaking the project on a speculative basis. In addition to office space it also wants housing, a hotel and a conference centre, and it specified "something iconic". Mott MacDonald and architect Aedas won the design competition last year and are taking the design to "detailed
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concept" stage. Offices will occupy ground level up to floor 58, a hotel will take up floors 59 to 76 and the top two floors will be apartments and leisure facilities. The contractor will be appointed to deliver the $396 M plus scheme under a FIDIC design and build contract.
Steel construction is rare in Pakistan, so Karachi Port Tower will be built from concrete. It will consist of a cylindrical core ringed by columns at the building perimeter. Structurally, square cores are stiffer, Gregson notes. The cylinder was specified for architectural reasons and to achieve spatial efficiency within the circular footprint of the tower. But lack of stiffness has been more than made up for by increasing the diameter of the core to 31.5 m and tying in the ring of perimeter columns.
The core size and other aspects of the structural design were dictated by the post-9/11 rethink of fire evacuation from tall buildings, driven by Mott MacDonald's fire specialists. "You used to be told if there's a fire, evacuate using the stairs," says Justin Garman, one of Mott MacDonald's fire engineers. "But the World Trade Center disaster showed that stair capacity wasn't enough, and that some people were physically incapable of descending tens of storeys by stair.
"So now, for very tall buildings, lifts are being looked on as integral to the fire evacuation strategy." Lift capacity has been designed for an office population density of one person per 11 m2, so there will be a lot of them. Karachi Port Tower will be equipped with a combination of express and local lifts. High-speed lifts, moving people over large numbers of floors, will be double dickers. Passengers will then catch local lifts from transition zones to their destined floor.
Over the height of the tower there will be three transition zones. Structurally these are very different to the tower's typical open plan floors. Floor slabs throughout the tower will be 260 mm thick post-tensioned concrete, stiffened by a 400 mm deep edge beam. Columns will be tied into the circular ring by an 850 mm-deep downstand. But the two-storey transition zones will be of far heavier construction, with thicker floor slabs and heavily reinforced concrete outrigger shear walls running from the core to the building
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perimeter columns. Each zone will house a technical floor dedicated to building services, and a fire-proofed refuge. It is to these refuges that people will be led if fire breaks out. They will be speeded to ground level in express lifts.
The transition zone shear walls play an important role in linking the core and columns. Gregson says that at the lowest of the technical floors the stiffening effect of the outrigger walls is minimal. "When we modelled the structure we found we don't need outrigger walls there, so we've omitted them and gained a significant cost saving." Design has had to deal with the old problem of axial shortening between core and columns under dead load. This occurs when a structural member is squashed by the weight of the structure above. The taller the column, the greater the degree of potential shortening. Sized purely for structural efficiency, columns would have shortened by more than 75 mm, Gregson notes. "You can allow for a degree of axial shortening by introducing a slight camber into the floor slab. That camber comes down as you build the structure up, and the floor ends up level." But a greater than 75 mm correction was at the edge of technical feasibility. Columns have therefore been sized to reduce stress and shortening. In plan they are elongated triangles with rounded corners, measuring 2 m wide by 3 m deep. Column sizes diminish as they rise up the building — first in width, then in depth.
Gregson says that lower down the tower, axial shortening could have been reduced by specifying very high-strength concrete. "But we want to keep the concrete mix within the realms of what is feasible in Pakistan." Achieving C100 would require the use of exotic additives and precise mix control. C65 concrete will be easier to batch and more forgiving in construction.
Concessions to the local construction market have also been made in the arrangement of columns and in the tower's foundations. "We initially looked at following the spiral with the columns, so they would have been raking," recalls Gregson. However, "to make them work it would have required very heavy reinforcement and precise steel fixing. Because there's no precedent for a building of this height in Pakistan, we felt it sensible not to add avoidable complexity." Though columns are oriented to the curvature of the
facade, an alternative way of expressing the spiral was found, says Gregson. "The spiral is achieved by cantilevering the floorplate by just over 3 m on opposing sides of the tower. As you go up the tower, the cantilever moves around a few degrees."
(New Civil Engineer International, May, 2008)
Careful adaption of an existing two-storey basement in Poland's capital has meant that it can take the increased load from the new 54-storey Zlota 44 residential tower.
Warsaw, Poland's capital, is something of a surprise. The city was almost destroyed in the Second World War and fewer than a fifth of its buildings were left standing. Its redevelopment under the country's post-war communist government was, for the most part, relatively modest, with the notable exception of the impeccably reconstructed old town.
Elsewhere, stark, system-based construction produced a hard-to-love, Modernist architecture known as social realism. In recent years the city has embraced the obligatory glass-and-steel look of the modern city. But that too is set to change with an increasing number of iconic landmark buildings by high-profile international architects in the pipeline.
A new movement.
Zlota 44 is a 192 m-tall residential tower designed by US-based architect Daniel Libeskind, who was born in Poland to parents who had survived the Holocaust. The building is part of a new movement that is, according to Libeskind, redefining Warsaw "through culture, fashion and an unrivalled approach to living".
This may be so, but the project could also play an important role in redefining Poland's fledgling geotechnical community. Foundation design on the project is by Amp, whose Polish geotechnical group is led by Mariusz Leszczynski. Leszczynski, who cut his teeth working as a geotechnical designer for Buro Happold in the
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UK, says Poland's geotechnical sector is largely underdeveloped. Most geotechnical and foundation design work, he says, is carried out by structural engineers who are "geotechnical by experience". A few university professors ran consultancy practices as a sideline and they are deemed custodians of the nation's geotechnical knowledge.
For Arap, it has been something of an uphill struggle to develop a market for top-end consultancy based around geotechnical expertise within a commercial private business. But through projects such as Zlota 44, Amp's geotechnical capability is beginning to be recognised among Poland's wider engineering community and, more importantly, the development and investment community. Amp's foundation design has taken practical Polish geotechnical engineering to new heights of sophistication, as well as saving the client money. Developer Oreo Group's gleaming new 54-storey edifice will occupy a central Warsaw location across the road from the imposing Palace of Culture and Science, an architectural showcase from the communist era.
Oreo's site was previously occupied by a seven-storey office development constmcted in 1989, with two basement levels all founded on a raft. When built, it was the first application of diaphragm walls in the city. A major challenge is the fact that the site is constrained on all sides, with neighbouring buildings just 600 mm from the basement wall. The original intention to provide four or five levels of underground car parking would have meant not only a major demolition job to remove the existing thick basement raft, but also a phenomenal amount of monitoring and a very costly legal undertaking — even assuming constmction went exactly to plan.
Armed with this knowledge, Oreo rethought and redesigned the building's lower floors, making use of just two basement levels and allowing a combination of commercial and car parking to extend up to the eighth level. Structural loads at foundation level were of the order of 600KPa-800KPa, but rather than providing a deep-piled solution Amp investigated whether it could be designed as a piled raft system.
These would be designed similar to those used in Frankfurt, where piles are designed purely to limit settlement, rather than to carry loads. As a local twist to this approach, Amp proposed using
single diaphragm wall panels as barrettes. From a constmction point of view this meant the original diaphragm wall could be left in place, the old raft shaved off, and a new raft cast on top of it. The new raft would be 2 m thick below the tower area, reducing to a thickness of 1.5 m elsewhere.
This approach would produce a much less expensive foundation. Leszczynski realised this approach required investigation using 3D finite element analysis to determine whether it would work as a true piled raft and also to determine how the connection between the barrettes and the raft affected the behaviour of the foundation. An accurate determination of the length of the barrettes was also needed.
For Leszczynski, it was vital that the analysis accounted for soil-stmcture interaction and that a more sophisticated soil model than Mohr Coulomb was used. The approach meant Amp had to commission a much more comprehensive ground investigation than is typical in Poland, on the basis that there would be little point in doing a complex finite element analysis if it were not confident it had correctly identified the ground conditions.
The investigation included eight boreholes and eight cone penetration tests, with shear-wave velocity measurements to determine strain ratios. By taking samples and reconstituting these at their insitu density, Amp was able to correlate the ground stiffness to its grading and density. This confirmed Leszczynski's hunch supported by observed settlements in existing buildings in Warsaw that the ground was much stiffer than allowed for in conventional analysis.
In profile, the ground at the site comprised the thick and ubiquitous blanket of made ground, present throughout Warsaw as an uncomfortable reminder of the war. Below this, the natural ground is made up of two layers of boulder clay, the result of two glaciations.
This clay, says Leszczynski, is "a very good material, very stiff, not susceptible to swelling and has a high content of gravel and boulders". The boulder clay horizons are underlain by a very dense interglacial gravel and sand and, below this, a stiff Tertiary Clay, similar to London Clay at about 40 m. Amp used MIDAS
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GTS finite element software. This indicated that the foundation barrettes did not need to extend to the deep clay and could be shortened from 20 m to 17.5 m.
It also showed that a pinned, rather than a fixed, connection between the raft and the barrettes was better in terms of bending moments, although this created the need to "beef up" the steel in the raft locally. The analysis also showed that, perhaps counter-intuitively, the load is actually carried by the barrettes, resulting is very little strain at raft level. Leszczynski feels Arap's design has certainly led to a better understanding of the ground and foundation behaviour.
Foundation work was carried out by contractor Inkom, which installed the 17.5 m long, 0.8 m by 2.8 m barrettes through slots cut in the foundation raft. Leszczynski was concerned that creating these slots and exposing the underside of the original raft could lead to a softening below it. As a precaution, Inkom injected grout at low pressure below the raft around the slot locations. In addition it prestressed the barrettes by base-grouting them, although this was primarily to mitigate against potential construction errors, rather than a necessary part of the design.
Foundation work was complete when NCEI visited the site and the structure was three storeys above ground. Provided Poland's residential market rides through the credit crunch, general contractor Besix should be completing the project by autumn next year.
(New Civil Engineer International, January, 2009)
DOWNTOWN by Adrian Greeman
Birmingham's latest development includes a 44 storey-tower with a five storey basement. Bachy Soletanche is just finishing the foundations.
Birmingham, England has long had a reputation as a windswept concrete jungle, the result of road focused re-development in the
1960s. But a wave of new development is modernising the city centre with friendlier mixed use schemes. One of the biggest is transforming a bleak space close to the Snow Hill station, the city's second central railway station. For years the area has been mainly rough ground, used for car parking alongside a main road, with railway lines nearby and assorted 1960s concrete multistorey car parks.
Now steel frame blocks are rising on a three part site being developed by Ballymore for mixed use, with offices, retail space and hotel floors above. Largest will be a development with two towers on the square site at the end, one of these a future landmark with 44 floors, the city's highest building. Landscaped space will also be over a five-storey basement car park filling the whole 96,000 m2 space. To create this large volume and tower foundations, groundwork specialist Bachy Soletanche has been installing a deep contiguous piled wall around the site this summer. In recent weeks, as the large excavation inside got underway inside, it has been back on site to install a line of ground anchors in the wall.
"These are for temporary support of the wall during the basement construction," explains contracts manager Steve Mallinson. "Once the concreted base slab and floors are in place they will provide all the structural support needed and the anchors will be cut through." The tendons will remain in the ground afterwards. "We also had to do ten plunge columns for the site approach ramp within the main wall," says Mallinson. These hefty steel H-section columns, surrounded by pea gravel inside their pile casings, are gradually being exposed again as the site excavation proceeds.
Contractor PC Harrington is doing the excavation and base concreting at present. But until recently Bachy has unusually had the site to itself. "We were effectively a main contractor," says Mallinson, "installing security and site welfare, arranging spoil disposal and concrete deliveries."
It was a change, he says, not having to interleave between other work, though with two support cranes, two Bauer BG 22 piling rigs, spoil heaps, reinforcement deliveries and site accommodation to deal with, the site became full enough. As the 241 piles in the perimeter wall were installed he even had to block off two of three site entrances, which meant some careful logistics were needed.
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For the 220 m length of the main wall, project design consultant WSP had opted for contiguous piles, "which is the right choice," says Mallinson, "because the ground is dry and you don't need any interlocking." Piles are 750 mm diameter.
The wall will hold back the ground which comprises a few metres of fill, then a 3—5 m thick sand layer which becomes weathered sandstone further down and gradually more competent rock. "The bedrock layer slopes from 2 m to 14 m down across the site and the piles must be up to 17.5 m deep," says site engineer Mathew Brown, "though they average out a little less."
To get through this fairly soft ground should be relatively straightforward. Bachy hoped to work with continuous flight piling mainly, which is quick and economical. But there is always a but. On this site it was obstacles in the ground, remnants of the 1960s, including various road underpasses and subways. "A lot of it was grubbed out in a preparatory contract," says Brown. "But there was some left where it would have caused undermining of the highway."
The obstacles were mainly several metres down and up to 3 m thick. To get through meant using the full strength of the Bauer rigs in straight boring mode — the dual purpose rigs could be converted for such work in about 24 hours and then drove through the hard material with tungsten carbide boring heads. "We had site investigation data but did further probe piles at various locations around the perimeter to work out what we could do with the CFA and what would take the harder cased bored work," says Mallinson. In the end about 30% bored piling was needed, somewhat less than Bachy had estimated, which meant it came out ahead.
But there is often another but. The sandstone and sand caused difficulties with both types of piling "because porous ground tends to suck the moisture out of the concrete," says Mallinson. "That made it stiffer and harder to get the pre-made reinforcement cages in after the augur was withdrawn." Bachy switched to a more fluid mix and a highly disciplined pile procedure where cages were positioned within a minute of the augur being withdrawn.
For the ten top-down piles Bachy installed a basic bored pile with casing and then used its special plunge column rig to achieve the 5 mm accuracy needed for positioning the I section steel columns.
A steel frame sitting on the casings had three sets of hydraulic rams for precision adjustment of the central steel while it was fixed with around 5 m of concrete at the pile base. Pea gravel fills the casing. The 12 weeks' schedule met, Bachy retired for a month while the excavation began, returning in late October to begin anchoring. Some 70 anchors go in, a row of one every three piles. Each is 15 m long and 178 mm diameter, driven by a Casagrande M6 articulating rig.
Five strand reinforcement bundles from Diwidag are grouted into the bottom 6 m or so of the anchor which runs at a 45° incline into the sandstone. That too has gone to schedule and the site is now almost ready for the main works by contractor Altius. The Snow Hill development as it will look. Snow Hill development includes 56,000 m2 of office space, a five-star hotel and 332 luxury apartments in a 44-storey tower, five major new public spaces which — it is hoped — will create a new core to Birmingham's commercial heartland Kier Group is the main contractor with Amp heading up the mechanical and electrical engineering contract, while Alan Baxter Associates is the structures and highways consultant. Ballymore Properties is the developer of the Snow Hill project. It has worked on 22 city centre projects in Liverpool, Luton, Bristol and London. In London's Docklands, current schemes include Pan Peninsula, Ontario Tower and Leamouth Peninsula.
(New Civil Engineer International, February, 2008)
OPEN PLAN SURGERY by Andrew Mylius
Opening up the basement of St Pancras Station's Midland Grand Hotel has called for radical re-engineering of its foundations.
Getting miners, their excavation equipment and construction materials into the tight spaces beneath the Midland Grand Hotel fronting London's St Pancras Station was like playing sardines, says Claire Carr. She is overseeing a surgical operation to remove
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walls in the old hotel basement to create a direct link between King's Cross St Pancras Underground station in front of the Midland Grand Hotel, and St Pancras International railway terminus, which is immediately behind it.
"Eurostar will start using the station in November. We're creating easy through-access for passengers moving between the London Underground and high speed trains," says Carr. She is section manager for CORBER, a joint venture between Costain, Laing O'Rourke, Bachy Soletanche and Emcor Rail, which is carrying out the rejuvenation of St Pancras station. It is doing the work for London & Continental Railways, owner and operator of High Speed 1, formerly known as the Channel Tunnel Rail Link.
Carr says that to support the hotel's seven storeys of neo-gothic brickwork, walls in the basement chambers were up to 1.5 m thick, carrying point loads of 500 kN. The space was divided into four rooms, roughly 7 m square, two either side of a 3 m wide corridor. Doors giving access to the corridor, and from the corridor into each of the rooms, were between 800 mm and 900 mm wide. "Space within the chambers was limited, and the doorways formed extremely tight bottlenecks on movements of people and materials," Carr says. Opening the basement up to create space for free-flowing passenger movement follows a 60-point method statement. "We've arrived at the point where we've got a large open plan area dotted with columns — we've come a long way," Carr summarises.
Alongside working in confined spaces, one of CORBER's key challenges was to limit settlement. "Our work strategy has been governed by the requirement to keep settlement to under 5 mm," says Carr. Instrumentation has been installed on the upper floors of the hotel to keep tabs on the building response to changes being carried out to its footings.
Work started 17 months ago with the excavation of 3 m by 2 m pits to locate the hotel corbelled brick foundations. These were found 6 m down, bearing onto London Clay. A team of miners employed by Costain carried out the excavation work, using timber props and shoring to support the sides of the holes. "Because of the
conditions in which we're working, we've gone back to very traditional methods and materials," Carr notes. "Timber's far easier to use than steel in tight spaces like this."
With footing levels established, ground was taken down to the same level throughout the basement area. Powered wheelbarrows and a small conveyor were used to remove spoil as two mini-diggers toiled away. Next, lmwide, 4.5 m deep reinforced concrete strip foundations were cast either side of the walls to take temporary works loading. "We needed very substantial foundations to take propping forces when it came to opening up the walls," Carr explains. Opening up the walls involved taking cores at high level, where they met the edges of vaults making up the basement jack arch ceiling. Subcontractor Shepley inserted I-section needles through these holes supporting them on propped I-beams running flush with and either side of the walls.
"We were strictly prohibited from opening up more than 25% of the wall at once, so we had to install the needle using a hit one, miss three, hit one pattern. Once we'd been around all the walls once, we went back and did the same again and again." Grout was used to fill cavities in the brickwork of the topmost section of wall, sandwiched between the longitudinal I-beams. The grout also flooded the void between the wall and the web and inner flanges of the I-beams, creating a composite steel-masonry-steel sandwich. Only when the grout had achieved full design strength were props supporting the I-beams jacked imperceptibly, relieving the walls of load. This enabled slots to be cut in the walls. Reinforced concrete saddles were cast, bridging between the strip foundations, on which new cast iron columns were positioned. With all of the columns in place it was finally possible to cut out the remaining brickwork. Floor level between the strip foundations was raised to the same height by placing mass concrete.
(New Civil Engineer International, February, 2007)
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BASEMENT BUILD UP by Jessica Rowson
Chicago's horizon is skyscraper heaven, but soon one building will stand head and shoulders above the rest. Jessica Rowson reports from the Windy City.
When built, the 150-storey Chicago Spire will be 610 m tall. Compared to the proposed 54 m tall Freedom Tower on Ground Zero in New York and the current tallest building in the United States — the 110-storey Sears tower in Chicago which stands 442 m tall — this will be a real skyscraper among tall buildings. It is designed by Spanish engineer architect Satiago Calatrava. Not only will the height make it stand out from the crowd, but it also has a rather unusual shape as it twists into a spiral which soars skywards. The floor plate is based on a circle but the edge is pinched into cantilever points at even spaces around the outside, giving it the appearance of a wide toothed cog. The cantilevers will be rotated to give the facade the appearance of a very elegant helter skelter.
"It looks complicated, but there's high repetition which means less cost," says D. McLean vice president of the Spire's structural consultant Thornton Tomasetti. "The [concrete] core remains in the same position, but the floor plates appear to slowly rotate in plan with each change in floor elevation. However the columns and the inner floor plates are repeated at each level and the edges of the floor slab rotate." All great things must start somewhere and this project begins with some heavy duty ground work. Twenty, 3 m diameter rock caisson piles will support the building central circular core and there are seven outer columns at ground level with a further pair of the 3 m diameter rock caissons beneath each of these.
The rock caisson piles are large diameter concrete piles installed with a permanent casing typically used when very high loads need to be supported. These huge piles pass through the eight-storey basement and socket into the bedrock to an average depth of 3 m. Workers have already installed all of the 3 m caisson piles as well as
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smaller intermediate bell or under-reamed piles. These under reamed piles support the columns that will in turn support floor slabs in the eight-storey basement.
There are two types of basement column, both are formed by driving 25 m long, 910 mm diameter tubular casings into the ground. The bottom of these casings forms piles below lowest basement slab. Most are filled with reinforced concrete up to ground level. Where the loads are greatest the casings are only filled with reinforced concrete up to basement slab level. Steel drop-in columns are then placed on top of them to form supports for the basement floors. The casings are later removed before basement excavation work begins. "To construct the steel drop-in columns we terminate the concrete caisson at the bottom basement level and leave an empty shaft," says McLean.
A steel column is hung into the shaft just above the installed caisson. The base plate area of the steel column is concreted in place and left to cure. Later the shaft is filled with sand or weak slurry to prevent the clay collapsing when the steel casing is removed." It is important to prevent the ground around the steel columns collapsing after the casings are removed, because there is a risk that underground voids could make the areas around the columns dangerous to work in.
The basement will be constructed top down from the ground floor. The finished ground floor slab will brace the walls. The outriggers usually run through the core walls leaving space for lifts, stairs and services. But the Spire's circular shape and circular core meant that the designers had to find a different solution. "Normally cores are rectangular and the main structural elements can be installed through the core," says McLean. "With a circular core one cannot do that as the diagonal elements intersect at the core centre making it very difficult to fit anything in the triangular spaces."
"We decided on a ring system around the core walls connected at two floor levels, which would not interfere with the inner core layout. The ring elements are horizontal steel trasses which encircle the core wall." In addition, it was decided to include the outer column transfers within these outrigger systems. These outrigger/ transfer levels were situated at levels 35 to 40, 72 to 74,109 to 111 and 142 to 144 and designated as space for the plant rooms.
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The building floors are framed in composite steel. "We needed concrete for a robust core, but an all concrete construction would have resulted in very large columns in the lower part of the building," says McLean. "With steel we can use smaller columns." McLean says the one very important issue in composite construction is the differential movement between the concrete core and the steel columns. Concrete shrinks and creeps over time whereas steel shortens under loading, but at a different rate. This is not such a problem with low rise buildings or buildings which are all steel or all concrete, as the columns and cores shrink at the same rate. The core of the Spire is being constructed above its final elevation so that there is a slight fall in the level of the slab from the core outwards. As the concrete core shrinks, the floor level should approach a flat surface The main basement work has yet to start but tenders for a general contractor have been invited.
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