Description
Modern industrial buildings are to fulfil a host of requirements, which the architect and the engineer working together have to translate into a suitable structure. These are primarily:
• Economic efficiency: industrial buildings are investments that have to be financed by making a profit on the market. Industrial buildings are therefore subject to a particularly stringent cost/benefit analysis.
• Flexibility of use: in times when product lifecycles are becoming shorter and shorter, and manufacturing methods are continually changing, flexibility of use is of central importance. Of great advantage are modular systems that can be extended in a variety of ways and allow conversion, and structures with large clear spans, i.e. interiors with a minimum of columns so that industrial operations are not obstructed.
• Fast construction times: the interval between the decision to begin production and the introduction on the market is often the deciding factor for the economic success of a product. Fast construction times are therefore of great significance. Therefore construction methods with standardized and prefabricated elements are preferred.
• Integration of the building services engineering: many industrial buildings require a high proportion of building services engineering. In typical production plants, about two thirds of the construction costs are for finishings and building services engineering, while only about a third goes on the load-bearing structure and construction. The structure must therefore be spacious, enabling supply routes that exceed the currently recognizable need, and should not impede the maintenance and repair of the building services engineering.
In addition, many production processes entail very specific requirements for the structure. Heavy machinery requires massive supporting structures with special foundations. The vibration they cause often has to be effectively isolated from the rest of the building. Some production processes require the tolerances of the structural deformations to be limited to precisely defined dimensions. Manufacturing facilities are therefore preferably designed as low-rise buildings extending over large areas.
Overall, these requirements in the construction of industrial buildings result in a strong preference for implementing above all modular factory building systems with a high repetition factor and individual building components that are prefabricated to a high degree.
These systems consist of vertical supports and horizontal trusses or girders. The emplacement of the columns is defined by a grid. Typical grid dimensions for industrial buildings of this type range from 14.40 m x 14.40 m to 24 m x 24 m. Larger grid dimensions are uneconomical, and are therefore only implemented in special cases, while smaller grid dimensions can quickly come into conflict with other requirements, in particular with regard to flexibility.
The spacing between the girders is one of the most important design parameters for industrial halls. It is determined in large part by the construction of the roofing. As a rule, the most economical solution is a trapezoidal sheet metal roofing, which can be easily installed and has a high load-bearing capacity while at the same time being lightweight. Spans of up to about 7 m are common for trapezoidal sheet metal. At greater spans, additional secondary girders are necessary for intermediate support. In buildings without thermal insulation, the roof can be sealed directly with the trapezoidal sheet metal. In this case it has to be laid in the direction of the downward slope, so that purlins or supplementary supports are necessary. Apart from passing on the weight of the roof, the trapezoidal sheet metal can be assigned other static functions. It can provide additional stabilization of columns and girders to prevent them from yielding laterally, as well as transmitting horizontal wind loads to the building’s stiffening components, if it takes the form of a shear-resistant panel, i.e. is framed on all four sides by a steel structure.
The spans that girders can attain depend on the materials used, the type of construction and the height of the cross-frame member. The materials used for girders primarily are concrete, steel, and very occasionally, wood.
Typical structural systems for industrial halls: in steel construction, two-pin frames are typical. In timber construction, the three-pin frame is often used. In concrete construction, the girders rest on restrained columns.
Concrete hall-type structures are typically made of prefabricated components. The individual elements of the load-bearing structure, ideally prefabricated, can be very rapidly fitted together on the building site with minimum labour. In the construction of simple industrial halls, standardized component dimensions are used as a rule. Special dimensions are possible, but give rise to additional costs. Prefabricated columns are either put into bucket foundations and then grouted, or concreted in one piece with foundations and delivered. In this way, structurally effective restraining at the base points can be easily achieved, enabling simple assembly without the columns having to be temporarily guyed. Longitudinal and lateral building stability is thereby achieved without implementing any other measures. The girders are laid on top of the columns; they require a forked support or other form of lateral securing so that they do not tip over. With untensioned girders, spans of up to 24 m are possible, and with pre-stressed concrete girders, spans of up to about 40 m. After assembly, the connections between the prefabricated parts are hinged. Although stiff connections between crossbars and columns may be achieved through casting with in-situ concrete afterwards, they are not usual because of the effort involved. In the construction of industrial buildings using prefabricated elements, restrained columns with pin-connected girders have met with wide acceptance as static system. Reinforced concrete structures have the advantage of being naturally fireproof. If the reinforcing is sufficiently covered with concrete, a fire resistance non-combustible/90 minute classification can be achieved without difficulty.
Constructing with pre-cast concrete elements only becomes economical when there is a high repetition factor of the building components, while for small and medium-sized buildings, steel is often a more advantageous material. Steel trusses also allow for generous installation routes and uncomplicated foundations, as they are very lightweight. The diagram shows the steel section usage for load-bearing structures for simple industrial halls with normal grid dimensions. Steel construction also has advantages with large spans, as the weight optimization of the supporting structure becomes more important.
Dimensions of steel sections in relation to typical modular dimensions of hall buildings
Steel trusses, like concrete girders, are often pin-connected to prefabricated concrete columns. If, however, steel columns are used at the same time, a stiff connection of column and girder can be created with little effort. In the construction of steel buildings, the typical structural system is therefore the frame. It consists of posts that on the edges are linked stiff with the cross-frame members. Restraining at the base points is feasible; however, it involves a lot of construction work on the foundation and the base point of the column, so that the two-hinged frame represents the most widespread static system in the construction of halls made of steel. Very large spans in excess of 100 m can be realized with cross-frame members and posts split up into lattices. For supporting elements with high bearing capacity, the failure of stability is an increasing risk, however. Through broadening the cross-section of the compression chord, like for example with a three-chord lattice girder, the stability of the supporting element can be assured, even with very large spans.
Construction of base points for different types of load-bearing structures: with steel and timber structures, the base points are hinged, while in reinforced concrete construction, the columns are restrained in bucket foundations
Corrosion protection has a significant influence on the cost of steel constructions. It is to be calculated according to the precise degree of corrosive exposure and environmental conditions as well as on the type of use and its duration. Three-layered coating systems with a life expectancy of fifteen to twenty years are usual.
For one-storey industrial and production plants of up to 1,200 m², no fire classification is necessary; however, for larger steel buildings, fire protection is to be considered in the early design stages, and to be established in discussion with the authorities responsible. Fire protection often represents the decisive factor for choosing a composite steel structure. With concrete-filled hollow steel columns or girders whose chambers are filled with concrete, the concrete cross-section remains after a fire. Composite steel structures are particularly advantageous when it comes to walkable ceilings, which can be built quickly and inexpensively using prefabricated systems.
Although wood is rarely used in the construction of industrial buildings, in the lower range of spans of up to 35 m, it is an entirely feasible alternative. Glue laminated timber can be easily shaped and is a material that can be employed in a statically determinate way. The manufacture of the supports, however, can only be automated to a lesser extent than the other materials already mentioned, so that a higher proportion of the costs goes on labor. Wooden buildings are assembled in the same way as buildings of prefabricated steel or concrete parts, by using prefabricated elements for the framing structure as much as possible so that assembly on the building site can be completed with minimum labor. Stiff connections can be easily manufactured in the plant, but are rather costly to do on site, so that assembly connections are hinged as a rule. Therefore, in construction with wood, the static system of the three-pin frame is to be found more often than in steel construction. In order to reduce the moment on the edges of the frame, wooden industrial halls often have corners moved inwards or rounded. Like concrete, wood offers natural fire protection; simply through oversizing it can achieve high fire resistance classifications. Thus, after a fire has burned for a certain length of time, a cross-section that is statically sufficient remains. As with steel, glue laminated timber plate girders can be split up into lattice girders to save weight. For wooden buildings to be long-lived, particular attention has to be devoted to preserve the timber by the structural design. By using construction methods appropriate to the building material, a lot of potential damage can be avoided right from the start.
Construction of frame corners for different types of load-bearing structures: in steel and timber construction, the frame corners are flexurally rigid, while in reinforced concrete construction, the girder is pin-connected to the column
Unlike the roofs of one-storey buildings, floors of multi-storey buildings are charged by live loads, having to carry significantly heavier loads and satisfy considerably higher requirements for the deformation and vibration behavior. For this reason, smaller grid dimensions are typical, for example, 7 x 7 m or 7 x 14 m. For floors with regular column grids, the semi-finished component systems supplied by different manufacturers have long been accepted. They function as the formwork for in-situ concrete toppings, and as reinforcing, they are at the same time an integral component of the finished floor. With multi-storey buildings, reinforcing through frames, i.e. through stiff connections between support and girder, is usually uneconomical. Instead, compact cores are used for bracing, or individual fields are cross-braced.
The described simple systems are optimized in many different ways and can be variously configured. The aim is not only to reduce weight and costs, but also to develop supporting systems that can be implemented to affect shape and design.
The main stress on girders and frames is from bending. The first step to optimizing the load-bearing structure is to distribute the bending force through tie bars and struts, i.e. trusses. If the height of the structural member is adjusted to the moment distribution, the force in the upper chord and the bottom chord remains constant. The bottom chord subject to tension can thus be a continuous cable with a constant cross-section. The 120 m wide Hall 4 of the Hannover Trade Fair is spanned column-free by such girders.
The arch and the suspended roof are supporting systems that transmit the external loads mainly with pressure or tensile force. A priori, they are much lighter than systems subject to bending, and are indispensable for large spans. The CargoLifter Hangar in Brand impressively demonstrates the effectiveness of an arched load-bearing structure. With a span of 210 m, it is one of the largest industrial halls in the world at the moment. In general, in the construction of industrial buildings, the potential of the arch can only rarely be exploited to its full extent because of the required clearance profile. Occasionally, arched girders are used for bulk commodity warehouses, as in this case, the geometry of the load-bearing structure adjusts to the angle of repose, for instance in the case of the CADYL Grain Silo.
Constructions that are subject exclusively to tension are not endangered by failure of stability, i.e. buckling or denting. The roof structure is therefore even lighter than an arched one, but requires correspondingly strong anchoring on the edges and large foundations in order to accept tensile forces. Suspension roofs were repeatedly used in the construction of industrial buildings, some of them with very large spans. In Trade Fair Hall 26 of the Hannover Messe, steel bands running under the roof membrane support the roof covering. They have a cross-section of 30 x 400 mm, and span with a sag of approximately 7 m over 55 m. Because of the small amount of flexural rigidity and the low weight, with suspension roofs, particular attention should always be paid to the wind suction force and to vibration caused by wind. In Trade Fair Hall 26, pre-stressed façades and additional spring elements stabilize the roof.
Building commissions that cannot be adapted to a hierarchized, level load-bearing structure can be executed by using space trusses that distribute the external loads over a cambered surface. This way, optimized structures with minimum structural weight can be realized.
The concrete shell construction method was developed in the thirties of the last century and first used in industrial construction. With the exception of cooling towers and silos, it has almost died out in the meantime, because of the high degree of complexity involved in planning and realization. Although concrete, by its very nature adapting to any shape, appears to be more suitable, double-cambered structures nowadays are almost always built as shells using steel construction, such as Hall 3A in Frankfurt, which spans 165 m with five double-cambered arched trusses.
In space trusses subject to tension, the variety of shapes ranges from mechanically braced, anticlastic cambered surfaces to pneumatically pre-stressed, synclastic shapes. Because they are lightweight, membranes enable wide-spanned, light-permeable roofing. The primary structure necessary to brace the membrane consists of a conventional system, however. Membranes subject to tension are advantageous as building envelopes above all when they serve as wide-spanned weather protection and have no further physical requirements to fulfil, as for example in the carport for the German Bureau for Waste Management or with the Märker Cement Factory. Textile roofing for air-conditioned spaces consists of a double layer, with transparent thermal insulation between the layers. Pneumatically supported cushions of transparent ETFE foils are also frequently used. Currently the Eden Project in Cornwall has the largest roof of this kind.
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Originally published in: Jürgen Adam, Katharina Hausmann, Frank Jüttner, Industrial Buildings: A Design Manual, Birkhäuser, 2004.