Special construction methods

BMW HEADQUARTERS, MUNICH

The headquarters of BMW A.G. differs from conventional buildings to create an impressive corporate symbol in the form of a 100-m-high four-cylinder structure. The require-ments for appropriate office organization yielded a basic outline in the shape of a clover leaf. Stairways, elevators and sanitary areas are accommodated in the central core.

In this way, all the offices can be reached by the shortest possible route. Trendsetting methods were also used for the construction work.

A reinforced concrete version was chosen as the most economical solution. According to the design concept, the entire building with 18 office floors and a technical floor was to be suspended from a girder cross at the top of the roughly 100-m-high core via four central king posts. This is a modification of the outrigger truss (Section 3.2.2.1). The entire load of the building is transmitted to the founda-tions via the core as the central element; it also absorbs all wind forces. A mighty girder cross with a projection of 16 m is mounted at the top of the core.

The four king posts are secured to this central girder cross, each king post comprising 105 threaded steel bars with a load-bearing capacity equal to a suspended weight of 4,600 Mp. Small outer columns are additionally located between the floors. These outer columns are designed as compression columns above the technical floor (12th floor) and as king posts below.

Time and costs were the decisive reasons for choosing this innovative construction method. All 19 floors were successively produced at the foot of the shell and core; the first floors were even produced complete with facade and

glazing during construction of the supporting cross. The finished floors were then connected to the supporting cross via the king posts and raised one floor at a time every week with the aid of hoisting gear so that another floor could be produced in the space vacated at the foot of the core and then connected to the floor above (lift-slab method). Completion of the facade, glazing, installation and interior finishing proceeded on the suspended floors, unimpeded by the structural works and lifting operations.

In addition to reducing the construction time required, this method also eliminated the need for expensive tooling and assembly work.

LA GRANDE ARCHE, PARIS

This building, which has already been mentioned in Sec-tion 2, takes the form of a giant cube open on two sides with edge lengths of 110 m. It was completed at the end of 1989 on the 200th anniversary of the French Revolution and took 5 years to build (see photo on page 18).

The building has a weight of more than 300,000 Mp and is mounted on neoprene bearings, the loads being transmit-ted 30 m into the subsoil via twelve concrete pillars.

The cube’s main support is in the form of four prestressed upright reinforced concrete frames 21 m apart. They are complemented by horizontal members measuring roughly 70 m at ground and roof level. Each of these members is 9 m high, the equivalent of a 3-storey building. Since the two vertical sides of the cube would be without roof-level transverse bracing during construction, the required stabil-ity for that phase of the work was produced by means of horizontal steel truss reinforcements.

A total of 37 office floors are accommodated in the two 18-m-wide wings of the cube (each with an area of 42,000 m²).

Top: VIEW FROM THE HEADQUARTERS BUILDING Bottom: 32 HEADQUARTERS OF BMW A.G. IN MUNICH

Page 45 3 Technology of high-rise construction

3.2.2.3 Facade

The skeleton construction which has increasingly been used since the turn of the century has inevitably given rise to new possibilities for the facade. The size, shape and number of windows were no longer limited by structural requirements following the introduction of curtain facades, since the loads were now primarily transmitted by posts and columns.

PLANNING

Most facade designs today are still based on empirical know-how and are not tested until the design has been established in detail. The tests are carried out on true-to-scale models of individual facade elements in order to test adequate resistance to air and water, load-bearing capacity and the possibility of excessive deformation or glass breakage when subjected to corresponding loads, e.g. with the aid of firmly anchored aircraft engines.

DESIGN

Today’s modern facades are characterized by external wall elements equal to one floor in height and inserted

between the respective structural floors. Non-supporting metal facades suspended in front of the building have in-creasingly become established for economic reasons, par-ticularly in high-rise construction.

The scope for design is enlarged by coloured or mirrored window panels and linings of natural stone, ceramic tiles or brick. Almost any desired appearance can be pro-duced.

TECHNICAL PROPERTIES

Modern facades must meet complex requirements as re-gards construction technology, engineering design and construction physics. Thanks to its lightness and almost unlimited possibilities for profile design, aluminium has largely become the material of choice for the outer frame-work. The panes are made of high-grade glass filled with noble gases or with a surface coating that reflects infrared light. On the inside, modern facades are highly imperme-able to water and water vapour in order to prevent dam-age due to moisture.

Despite the large areas of glass, protection against the sun is more important than heat loss today due to good ther-mal insulation of modern facades. Even where sound-proofing and fire protection are concerned, glass and

metal facades are at least the equal of conventional con-structions.

Modern facades also require a sophisticated ventilation and cooling system. The air-conditioned or twin facade is a case in point. Here an additional facade of laminated glass is arranged in front of the conventional facade, thus creating a space through which air can circulate. More complex ventilation concepts for routing air into and out of the building may be realized by including additional vertical and horizontal bulkheads. Individually controlled ventilation flaps are capable of providing a more natural and far less complex exchange of air.

PRODUCTION AND ASSEMBLY

Due to the extensive know-how required with regard to material properties and construction physics and on ac-count of the great manufacturing depth, modern facades are only produced by specialized companies based on the architect’s design and in accordance with functional, as well as structural aspects before subsequently being as-sembled.

The degree of prefabrication in modern facades is consid-erable. The frames, glazing, parapet lining, sunshades and anti-glare finish, as well as thermal insulation and sealing are all assembled into single-storey facade elements in the manufacturer’s plant. In many cases, such technical equip-ment parts as radiators, air outlets and the ducting for electrical and electronic equipment are also already inte-grated at this stage.

In the meantime, fixing elements can be mounted on the shell of the high-rise building. These elements can usually be displaced in three planes to compensate the dimen-sional tolerances occurring in the shell. The facade elem-ents as such are fitted without the help of scaffolding, thus greatly reducing the time required for this work. The frame profiles are assembled with labyrinthine indenta-tions to compensate the deformation arising in the build-ing as a result of wind and live loads, as well as tempera-ture differences. Permanently elastic rubber profiles en-sure that the facade remains impermeable to air and water.

33 FACADE ASSEMBLY

3 Technology of high-rise construction Page 46

3.2.2.4 Roof

There are no fixed rules governing the roofs of high-rise buildings. The roof design depends only on the architect’s draft and on the purposes and functions to be fulfilled by the roof.

Most roofs are flat. The electromechanical drive system for the elevators is usually installed on the roof; in some cases, there is also a rail around the perimeter of the building to accommodate the equipment required for cleaning the facade, as well as the pertinent connections and facilities.

A heliport or parking space can also be set up on the flat roof of large high-rise buildings. It is sometimes even used in Japan for golfing practice.

Air-intake towers for air-conditioning systems, on the other hand, have become less common on modern high-rise buildings. Due to the great height of buildings, air-conditioning and heating systems are now decentralized and spread over several individual floors. Moreover, every installation and every superstructure on the roof means another opening in the intact roof skin and this can give rise to leakage problems, particularly on flat roofs. It is therefore advantageous to transfer such systems to lower floors.

Overhead glazing is another type of roof commonly found in high-rise buildings. Such roofs keep out the elements while at the same time creating spacious assembly areas, usually in the centre of the building. Atriums and conven-tion halls are two pertinent examples.

High-rise buildings with a sloping roof are usually round-ed off by an antenna system with appropriate lightning protection.