Castles in the sky: how new York city's colossal skyscrapers were constructed

High above the streets of Manhattan, the teams of riveters worked to a set routine. As an enormous crane hoisted each steel beam into place on the growing Empire State Building, a workman called the "heater" warmed each rivet in a portable furnace until it glowed cherry-red, removed it with tongs and tossed it to a "catcher", perched precariously on the very edge of nothing. Usually he caught it in his "catching can", but sometimes he missed. Using tongs, the catcher knocked off the cinders and lodged the rivet in the prepared hole. Another workmate held it firmly with the aid of a heavy steel bar, while a third smashed the rivet into place with a compressed air hammer.

It took 60 000 tons of steel to build the Empire State. The beams and girders were cast in Pittsburgh, and within a day or two of being made, each numbered piece had been transported to Manhattan, hoisted into position and riveted into place. There was little storage space available on site, so elaborate charts and timetables were used to monitor progress and to ensure that deliveries kept precise pace with the erectors' and riveters' schedules.

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Careful organisation builds a giant.

The charts listed every lorry due to arrive, what it would carry, who would be responsible for it and where it ought to go. Each beam was hoisted by crane to the appropriate floor, then transported to wherever it was required on a miniature railway system. This methodical approach worked exceptionally well and on occasions the building rose by more than a storey in a single day.

The Empire State 102 storeys were finished in record time. It took just six months to complete the 381 m building instead of the anticipated 18 months, feat that set new standards of efficiency for the construction industry. But it was 1931, in the early years of the Great Depression, and much of the space remained unlet. The building was dubbed "The Empire State Building". It had cost $24 million to construct, which was cheap at the time, but for the first few years a major source of income used by the developer to pay property taxes was ticket sales for the observatories on the 86th and 102nd floors. From the top on a clear day, it is possible to see 80 km away. A skyscraper made of steel.

The steel skeleton of the riveted structure means that it is immensely strong — the building sways less than 6 mm on the 85th floor in a strong wind. In July 1945 an off-course US Air Force bomber, travelling at a speed of 400 km/h in fog and rain, crashed into the 78th and 79th floors. The three man crew and 11 people in the building were killed, but the structure suffered no permanent damage. Survivors recall that the building simply rocked a couple of times.

Manhattan's distinctive skyline started to take shape when steel began to be used for tall buildings. Earlier buildings had been made from a variety of materials, including stone, brick, wood and cast iron. But a masonry building taller than about ten storeys would have required supporting walls so thick at the base that there would be hardly any floor space on the ground floor and, before lifts were invented, building height was limited to the number of steps people were prepared to climb. One of Manhattan's most striking early skyscrapers, built between 1901 and 1903 was Flatiron building. It owes its unique shape to the narrow triangular site it occupies at the junction of Broadway and 5th Avenue at 23rd Street.

Twenty storeys high, its riveted steel frame is clad in French Renaissance-style stonework.

Taller and taller.

The Flatiron may also have been the first building to create strange aerodynamic effects in the surrounding streets. Even today, Manhattan's tall buildings create unusual wind currents, causing snowflakes to float upwards. Before long, the Flatiron was dwarfed by other skyscrapers, including the Woolworlh Building, completed in 1913. The architect, Cass Gilbert, chose the Gothic style for the 60-storey tower. The structure itself was made from steel and the exterior completely clad in terracotta. It could house 14000 workers, serviced by 19 lifts and 2800 telephones — an astonishing number for the time.

When the building was finished it won immediate praise from the public — but some architectural purists were aghast, their sensibilities offended by Gilbert's use of Gothic detail purely for decorative effect, rather than for structural purposes. This was contrary to the modernist stricture that "form should follow function".

(Reader's Digest: How Was It Done? The Story of Human Ingenuity Through the Ages, 2000)

^TEXT 12

HONG KONG — BASTION OF BAMBOO SCAFFOLDING by Muthukaruppan Ramanathan

Hong Kong's skyline is dominated by some of the world's tallest buildings. Nevertheless, the city still uses bamboo scaffolding for much of its construction work — a traditional skill passed down over 5000 years. Bamboo is sustainable, lightweight and cheap and, as long as it remains fairly dry, a good construction material with significant mechanical properties. Researchers, engineers, environmentalists and bureaucrats have taken an increasing interest in the craft, such that regulations and practice continue to be improved and refined. However, to alleviate remaining design and safety concerns a structural design code is needed.

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Hong Kong continues its long-standing tradition of using bamboo scaffolding for new construction, renovation, repair work and signage. The city stands alone from the rest of the world in recognizing the sustainability of bamboo over steel and aluminium and has progressively raised training and safety standards of bamboo scaffolders. Bamboo scaffolding had previously been used in many parts of southeast Asia and mainland China but has mostly now been replaced by metal scaffolding. Except, that is, in Hong Kong, home to five of the world's 25 tallest buildings. Its bamboo scaffblders remain unperturbed by the ever-increasing heights at which they weave their bamboo webs. Working with giant grass.

According to Chinese legend the craft dates back 5000 years, when Yau Chao-Shi, a mythological character whose birthday is still celebrated, taught his people how to construct nest-like bamboo shelters in trees. Bamboo grows up to 30 m tall. It reaches its full height in one year and persists for several years without growing taller or wider, making it technically a grass rather than a tree. It is nevertheless the world's fastest growing woody plant and, as such, is particularly useful for stabilising riverbanks and preventing slope erosion. Bamboo stems can be harvested after three years and are self-renewing, with new shoots produced from the roots without replanting. The circular hollow stems make light and, when seasoned, tough construction poles that can be used without further processing or finishing.

Some 1250 species and 150 traditional applications have been identified for bamboo. Millions of people live in houses made of bamboo in parts of central and north America and Asia. It provides floor decking, wall panels, rafters, ceilings, roofs, doors and windows. Bamboo is also used for building fences and light traffic bridges in south Asia. Traces of bamboo scaffolding are still seen in south China, but in Hong Kong it continues to be extensively used, researched and improved.

The traditional art of bamboo scaffolding has been passed on from one generation to the next with little written information. However, with increasing interest from the construction industry, educational institutions, statutory departments and promoters of

sustainable resources, there has been a steady growth in the number of written regulations, guidelines, codes, conferences and publica­tions on the subject. The focus in Hong Kong is on the locally available bamboo types, namely kao jue and mao jue, the nominal external diameters of which at base are 40 mm and 75 mm respectively.

The Code of Practice for Scaffolding Safely was first published in 1995 by the Hong Kong Labour Department. The code provided practical guidelines to the construction and maintenance of both bamboo and metal scaffolds. It gave broad material specification and minimum requirements for the configuration of commonly used scaffold types. Loads on working platforms supported by the scaffolds, however, were provided only for metal scaffolds. A separate code for bamboos, entitled Code of Practice for Bamboo Scaffolding Safety, was published in 2001. This included the minimum imposed loads on working platforms and a performance specification.

In 2006, the Hong Kong Buildings Department published Guidelines on the design and construction of bamboo scaffolds to supplement the Labour Department code. The guidelines provide recommended practice for the design, erection, maintenance and dismantling of bamboo scaffolds. If scaffold layouts need to deviate from the guidelines, the Buildings Department recommends a performance-based design by a corporate civil or structural member of the Hong Kong Institution of Engineers. After gauging industry reaction, the Buildings Department will consider publishing a code of practice in the future.

Forms of bamboo scaffolding.

Bamboo scaffolding is used in its various forms according to the utility. Double-layered bamboo scaffolds are most commonly used in new construction and also in major renovation. External works such as rendering, painting, wall tiles and plumbing are carried out from a continuous working platform laid between the inner and outer scaffolds. The inner layer is erected at about 200 mm from the building edge, and the outer layer at about 600 mm from the inner layer. There are many variations to form, articulation, types of bamboos and spacing based on specific site requirements and the craftsmen's training and preference.

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For the outer layer, large-diameter (75 mm) mao jue poles are used as the main vertical posts spaced at about 1 — 3 m centres. They rest on firm ground at ground level and on steel brackets securely fixed to the structural members of the building at higher levels. Buildings Department guidelines require these steel brackets to be provided at 15 m vertical intervals or every fifth floor. Each post should rest on one steel bracket, and the horizontal spacing between the brackets should not exceed 1—3 m. Smaller 40 mm diameter kao jue poles are fixed as standards (uprights) between each pair of mao jue posts.

The posts and standards are connected by mao jue ledgers (horizontals), with a vertical spacing of about 600-750 mm. The scaffolders sit and put their leg over the ledgers and fix the tier above it. The upright standards and posts are also lapped by them sitting in this position. The 600—750 mm gap spacing is just convenient working distance which is the height between the scaffolder's hip and shoulder. This way, the joints and members are load-tested during the erection stage itself by the scaffolder's own weight. Cross-bracing is provided by using kao jue poles inclined at an angle of 45—60°. For the inner layer, kao jue poles are used for main posts and ledgers; the intermediate standards are generally not necessary. The main posts of both layers are supported by the same set of triangular steel brackets.

Scaffolds are tied to buildings using 6 mm diameter horizontal mild-steel wires at vertical and horizontal spacings not exceeding 6.6 m and 3 m respectively. These are referred to as "putlogs" though, unlike putlogs in metal scaffolding systems, which transmit vertical platform loads into the building wall, putlogs in bamboo scaffolding provide lateral restraint and wind resistance. They are provided at closer spacing higher up, where wind forces can be significant.

Working platforms for carrying out external works are normally vertically spaced at 2 m. The maximum imposed load on working platforms varies between 0—75 kPa (very light duty) to 3 kPa (very heavy duty) in the Labour Department's code. The code requires that no more than two working platforms in the former case and no more than one working platform in the latter case should be in use in any bay, that is the space between two adjacent standards along

the face of the scaffold. In the double-layered system, kao jue transom, poles spanning between the ledgers are used to support the working platforms, and are therefore provided at every platform level and with a maximum horizontal spacing of about 1.5—2.4 m.

Single-layered bamboo scaffolding is generally used to provide protective cover to renovation works on existing buildings where working platforms are not necessary. It is also used for some new construction, for example where external curtain glass walls are designed to be installed as large panels. Such scaffolds are also used in demolition, where all demolition works are carried out from inside the building including removal of external features. It is mandatory to fit tarpaulin sheets to contain demolition dust and debris, but these need to be removed during typhoons. Fixed external rather than internal ledgers facilitate the removal of tarpaulins.

However, single-layered scaffolds are less safe than double-layered scaffolds and usage is discouraged. They are not included in either the Labour Department's code or the Buildings Department guidelines. For minor repair works, including external plumbing, air-conditioning unit replacements and concrete or rendering repair, truss-out bamboo scaffolds are commonly used. This is light-duty, short-duration work and small-diameter kao jue poles are used. The standards, ledgers and raking poles are all supported by triangular steel brackets, which in turn are securely fixed to the structural elements of the building. Putlog ties restrain the top of the standards.

Bamboo scaffolding is also used to erect many of the large cantilevered advertising signs frequently seen over Chinese streets. In Hong Kong, the maximum allowed length to height ratio is 4:3 and the scaffold should be erected in a manner not to obstruct the traffic flow below. Such scaffolds are usually supported by steel wires or hang-poles securely fixed to the structural elements of the building. The lower level is usually decked out with wooden planks to act as a working platform.

Lashing connections.

Joints in bamboo construction have always been tricky. The poles are neither perfectly straight nor round, they have nodes at irregular spacings and their thickness varies. Despite such

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limitations, practitioners have developed many connection details over the years. Lashing poles with soaked bamboo strips was the preferred method for a long time, but the strips decayed and needed constant attention and replacement.

By the late 1970s, nylon strips had replaced bamboo strips. The new strips made erection faster and they also lasted longer, but connections still tended to weaken over time due to weathering. This led to Hong Kong researchers specifying engineered connections through experimental investigation. The recommended value for basic characteristic resistance for each lashing is 1.1 kN with a partial safety factor of 1.1-1.25.

The nylon strips are well-specified, with a minimum ultimate strength of 0.5 kN, a width of 5.5 — 6 mm and a thickness of 0.85 —

1 mm. The overlap of two lashed bamboo poles should be 1.5—

2 m, and the distance between the lashings should not be greater than 300 mm.

Other materials such as putlogs, anchor bolts and steel brackets are specified in the Buildings Department guidelines. The established specifications for bamboo poles, however, were given in the Labour Department's earlier codes. The poles should be 3—5 years old and air-dried in vertical positions under indoor conditions for at least

3 months before use; they should be free from cracks, irregular
knots and worm-eaten spots.

Despite bamboo and lashing materials having been specified, the effort of codifying the design data is still ongoing.

Engineering characteristics.

One of the disadvantages in designing bamboo structures is the lack of structural design data and established mechanical properties. Building codes over the globe have yet to embrace bamboo, though a draft code on bamboo structural design is under review. Basic mechanical properties have been dealt with by many authors but, unlike timber, bamboo properties do not relate to species because of the dependency on other factors such as geographical location and age.

More recent research on bamboo scaffolding carried out at Hong Kong Polytechnic University with the support of the International Network for Bamboo and Rattan has thus concentrated on the locally

available kao jue and mao jue varieties. The studies indicated that despite large variations in diameter, wall thickness and moisture content, representative values of mechanical properties could be arrived at. Two failure modes, namely end bearing and splitting, were identified in compression tests. End-bearing failures were mainly due to high moisture content. For bending tests, splitting and local crashing were identified as reasons for failure.

Based on a systematic experimental investigation on column-buckling behaviour of bamboo members, researchers found that load reduction due to column buckling is significant and accordingly developed a limit-state design method. Two failure modes, namely overall buckling and local buckling, were identified respectively in mao jue (long column-wet) and kao jue (short column-wet) members in most of the cases.

The engineering characteristics of structural bamboos are similar to those of timber and codification along similar lines to timber should be the next logical step. If the Hong Kong Buildings Department publishes a code of practice for bamboo scaffold as it has suggested, this would lead to promotion and wider acceptance of bamboo as a building material worldwide. Training and safety.

In the past, the skills required for erecting bamboo scaffolding were taught by a master scaffolder through a traditional apprentice system that would last for 3 years. However, apprentices can now pick up the skill within a year through on-job training or by attending a Construction Industry Training Authority training

course.

After working in industry for at least 4 years, a scaffolder can take the CITA trade test, which consists of a 0.5 h written examination followed by a 6 h practical test in which the candidate

is required to:

- inspect an erected scaffold and rectify the defects;

- dismantle the scaffold safely;

- re-erect the scaffold.

In 2006, 268 candidates applied for the test and 62% passed. Apart from the extent of workcompleted and quality of workmanship, candidates are also assessed on their safety-consciousness.

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АН scaffolders in Hong Kong are required to wear safety harnesses, fall arrestors and independent lifelines. Scaffolds must also be inspected by a competent person — a CITA trade test holder with at least 10 years' experience — every 2 weeks and more often during tropical cyclones and strong monsoons. The main contractor's full-time supervisor should also inspect the integrity of the scaffolds regularly and record findings in stipulated forms.

Despite the training and skill of Hong Kong's bamboo scaffolders and the government's associated code and guidelines, serious accidents continue to occur, mostly in truss-out bamboo scaffolds. The overall accident rate in Hong Kong's construction industry in the past few years is around 60 for every 1000 workers. A total of 3835 accident cases were recorded in 2004, 3548 in 2005 and 3129 (projected) in 2006. The numbers of fatal accidents during these years are 17, 25 and 17 respectively. Repair and maintenance projects account for the majority of the accidents. Nearly half of the fatal accidents are grouped under "fall of person from height" involving truss-out bamboo scaffolds, unfenced edges, ladders and so on.

Many of the accidents in truss-out bamboo scaffolds have resulted from failures of the support bracket anchor bolts. The difficulty in fixing the anchor bolts from inside the building is cited as a major cause. Inadequacy of secure fixtures of lifelines to anchorage points has also been noted in many cases. Industry, institutions and government departments are working together in addressing the problem.

Metal versus bamboo.

Unlike metal scaffold poles, bamboo poles do not need oiling, painting or covered storage. They are much lighter and easier to handle, leading to fast erection: one worker can erect 75-100 m² of double-layered bamboo scaffolding a day, some 6—8 times faster than for a similar metal scaffold. Also, the cost of bamboo poles is only about 6% of the cost of steel poles.

Bamboo scaffolds are generally split into 15 m tall frames with those above ground level supported by steel brackets fixed to the main structure. The heavier weight of metal scaffolding means that vertical loads are usually taken all the way down to the base. This is

workable for medium-rise buildings but for a multi-storey building of, say, 124 m height, it is estimated that standards will be required in groups of three at base with double at intermediate levels. Intermediate steel brackets, if designed, would also be massive. In Hong Kong, where high-rise buildings are a norm and many urban sites are bounded by busy streets, shopping arcades and pedestrian bridges, treble and double metal scaffolds would not be practical.

Bamboo also has a much lower carbon footprint than metal. For Hong Kong it is cultivated in the neighbouring Guangxi province in abundance, and then transported to Hong Kong along the Pearl River simply by lashing the poles together and floating them downstream. Depending on the type of scaffolding, bamboo poles can be reused three to five times.

A city of old and new.

Hong Kong is a forward-looking city but one which also values its traditions and customs. For example, century-old tramcars pass slowly through the heart of the central district while state-of-the-art mass-transit trains speed through tunnels below and traditional dim-sum restaurants sit alongside the latest fast-food outlets.

(New Civil Engineer International, March, 2009)

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