Some Innovative Steel Structures

SOME INNOVATIVE STEEL STRUCTURES Victor Gioncu Politechnica University of Timisoara, Romania Abstract The paper presents some innovative steel structures designed in the last time by the author: corrugated hypar shells, double-layer space trusses, single-layer reticulated shells and tensioned hybrid membranes. 1. Introduction The visible progress in the development of steel structures is extremely significant emphasizing continuous achievement of creative designers, engineers and architects alike. Over the last period engineers have been called upon to built larger and larger structures, all of them with less material and less cost. So, the best engineers matured under the discipline of extreme economy and their ideas developed under competitive cost control. Architectural concepts of aesthetics are changed all the time, leading to the introduction of new structural systems. During the last decades, designers agree that for large span structures, conventional planar beam and truss solutions are uneconomical. So they looked to the innovative solutions, as the space structures. The advantages of these structures have been known for many years. The second half of XX century could be characterized as the era of space structures, being the result of cooperation between architects and structural engineers or the perfect harmony between image and technology. Architects started to experiment with new shapes and some years ago reinforced concrete shells were in fashion. The visual beauty of these structures appealed to architects. However, the reinforced concrete is no the ideal structural material. Some progressive engineers realized the limitation of reinforced concrete shells (long construction time, elaborate scaffolding and framework) and turned their attention to a new structure type, the steel space structures. For many years the engineers appreciated their ability to cover large spans with minimum weight, but the difficulty of the complex analysis of these systems originally contributed to their limited use. The introduction of electronic computers changed this situation and produced an important impact in development of steel space structures. In this area the progress is almost unbelievable and the use of light-weight structures greatly changed the engineering approach to design. After this progress in space structure design, a new stage is created by the rapid improvement in structural textiles. This lead within the last decade to the dramatic development of a new structural system, the membrane structures. Rarely attempted three decades ago, this new form is now accepted by the progressive designers as a best solution to cover large span buildings. But the membrane systems are kinematically undetermined and they can be rigid only by using the pre-stressing and some rigid elements. Therefore, during the last decade a new technique has been developed, the hybrid membrane structures, by assembling a flexible membrane fabric with a rigid steel structure. Keeping pace with this progress, some innovative steel structures were designed by some teams from Timisoara Technical University, Building Research Institute and Design Office. The author wish to thank their colleagues from these teams, dr. Nicolae Balut, Dorin Porumb, Nicolae Rennon and Victor Vacaru, the co-authors of many of the following designed structures.

2. Corrugated hypar shells Over the last decades considerable interest has been shown in the practical utilization of hypars. The use of this shell type has become popular due to their elegant aspect and the very simple forming. The hypars are the creation of the reinforced concrete era, but in the recent years the steel hypars begin to be used. To built steel hypars is possible due to the property of corrugated sheet to be distorted. So, from a plane sheet one can obtain a double curved surface (Fig. 1a). The steel hypar is a very good solution for covering medium spans, because the span is limited by the dimensions of delivered corrugated sheet. The roof of a petrol station was designed (Fig. 1b). The sheets are fixed on discontinue plates by self-drilling screws. This procedure allows the water draining from the roof in the ogee gutters.

Fig 1a

Fig 1 (continued) b

3. Double-layer space trusses The interest in double-layer space trusses by progressive designers is growing constantly and the increasing number of such structures built in steel shows that these systems often compete very successfully with the more conventional structures and, at the same time, proving the architects with more impressive forms. The designing of joint system constitute the most important part of structure designing. There are three main types: welded, bolted or mixed joints. Fig. 2 shows a double-layer trusses for a restaurant where the joints were welded spheres, being obtained from a pipe stub by pressing in a semi-spherical mould. Fig. 3 shows a dismountable structure for a marked, composed of square base pyramids, square frames and contour bars, all these elements being made of thin-walled cold formed sections. All component units are shop welded and bolt connected at site. In Fig. 4 it is presented a double-layer truss erected for a custom house with mixed joints, the top and bottom faces being welded, while the diagonals are bolted.

Fig. 2 a

Fig. 2 b

Fig. 3 a Fig. 3 b

Fig. 4 a Fig. 4 b

4. Single-layer reticulated shells The popularity of reticulated shells results from their possibility to be built with a small number of identical components, bars and joints. The braced barrel vaults are regarded by architects as the best example of prefabricated single-layer reticulated shells. Fig. 5 shows a sport hall with low capacity (up to 500 seats) composed by two stiffening 32 span trussed arches, two end walls, three-bay triangulated barrel vault carried out of plane prefabricated trusses and longitudinal edge beams. An other very popular form of reticulated shells is the hypar roof. A latticed shell consisting of multiple hypars, presented in Fig. 6, was used for covering a market. The component unit of a roof is composed by the latticed shell made of straight tubes, disposed along the two families of generatrices, triangular latticed edge beams, the r.c. columns sustaining the shell unit at the four corners and the joints, obtained by tube flattening. Fig 5 a

Fig 5 b

Fig 5 (continued) c

Fig 5 (continued) d

Fig 6 a

Fig 6 b

Fig 6 (continued) c

Fig 6 (continued) d

5. Tensioned hybrid membranes Tension membrane structures give for architects and structural engineers the possibility to create new exciting solutions, in which the architectural expression and the structure form are in a perfect concordance. Because the textile material is very light-weight, these structure types are very efficient and economically. But there are also some disadvantages, as complex surface formation, large deformations, intricate anchorage, etc., which give many problems in designing. A very good solution is the tensioned hybrid membranes, obtained by the combination of a flexible membrane with a rigid supporting structure, composed by steel trusses, arches, frames, etc. Therefore, these were the reasons for which to cover a market was selected such structure type. The steel structure is composed by (Fig. 7) transversal frames (made by a laced arches, tubular built-up columns and external pre-stressed ties), longitudinal stiffening systems (composed by transversal pre-stressed cables and longitudinal pre-stressed ties), intermediary tension frames, placed at the half of building and at one extremity (given the possibility of a future extension) and end tension system (composed by inclined columns and ties). The roof is composed by the middle tension membranes in the form of saddle hypars supported on steel arches and end membrane, supported on end steel arch and tensioned inclined end columns. The steel structure received the ECCS Steel Design Award in 1997.

Fig 7 a

Fig 7 (continued) b

Fig 7 (continued) c

Fig 7 (continued) d 6. Conclusions The paper presents some steel structures designed by the author framing in the field of innovative solutions, showing the evolution of concepts in progressive designing. 7. References Makowski, Z.S. (1993): Space structures. A review of the development within the last decade. Space Structures 4 (eds. G.A.R. Parke, C.M. Howard), 4th International Conference on Space Structures, Guildford, 5-10 September 1993, Thomas Telford, London, Vol. 1, 1-8 Makowski, Z.S. (1995): Light-weight structures. Their development and impact upon modern engineering. In Light-weight Structures in Civil Engineering (ed. J.B. Obrebski), Warsaw, 25-29 September, 1995, 43-50