SOME INNOVATIVE STEEL STRUCTURES

Victor Gioncu

Politechnica University of Timisoara, Romania

AbstractThe 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 tensionedhybrid membranes.

1. Introduction

The visible progress in the development of steel structures is extremely significant emphasizingcontinuous achievement of creative designers, engineers and architects alike. Over the last periodengineers have been called upon to built larger and larger structures, all of them with less materialand less cost. So, the best engineers matured under the discipline of extreme economy and theirideas developed under competitive cost control. Architectural concepts of aesthetics are changedall the time, leading to the introduction of new structural systems. During the last decades, designers agree that for large span structures, conventional planarbeam and truss solutions are uneconomical. So they looked to the innovative solutions, as thespace structures. The advantages of these structures have been known for many years. The secondhalf of XX century could be characterized as the era of space structures, being the result ofcooperation between architects and structural engineers or the perfect harmony between imageand technology. Architects started to experiment with new shapes and some years ago reinforcedconcrete shells were in fashion. The visual beauty of these structures appealed to architects. However, the reinforced concrete is no the ideal structural material. Some progressiveengineers realized the limitation of reinforced concrete shells (long construction time, elaboratescaffolding and framework) and turned their attention to a new structure type, the steel spacestructures. For many years the engineers appreciated their ability to cover large spans withminimum weight, but the difficulty of the complex analysis of these systems originally contributedto their limited use. The introduction of electronic computers changed this situation and producedan important impact in development of steel space structures. In this area the progress is almostunbelievable and the use of light-weight structures greatly changed the engineering approach todesign. After this progress in space structure design, a new stage is created by the rapid improvementin structural textiles. This lead within the last decade to the dramatic development of a newstructural system, the membrane structures. Rarely attempted three decades ago, this new formis now accepted by the progressive designers as a best solution to cover large span buildings. Butthe membrane systems are kinematically undetermined and they can be rigid only by using thepre-stressing and some rigid elements. Therefore, during the last decade a new technique hasbeen developed, the hybrid membrane structures, by assembling a flexible membrane fabricwith a rigid steel structure. Keeping pace with this progress, some innovative steel structures were designed by someteams from Timisoara Technical University, Building Research Institute and Design Office. Theauthor wish to thank their colleagues from these teams, dr. Nicolae Balut, Dorin Porumb, NicolaeRennon 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 simpleforming. The hypars are the creation of the reinforced concrete era, but in the recent years thesteel hypars begin to be used. To built steel hypars is possible due to theproperty of corrugated sheet to be distorted. So, from a plane sheet one can obtain a doublecurved 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 petrolstation 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

3. Double-layer space trusses

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

Fig 1 (continued) b

Fig. 2 b

Fig. 2 a

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 numberof identical components, bars and joints. The braced barrel vaults are regarded by architects asthe best example of prefabricated single-layer reticulated shells. Fig. 5 shows a sport hall withlow capacity (up to 500 seats) composed by two stiffening 32 span trussed arches, two endwalls, three-bay triangulated barrel vault carried out of plane prefabricated trusses and longitudinaledge beams. An other very popular form of reticulated shells is the hypar roof. A latticed shellconsisting of multiple hypars, presented in Fig. 6, was used for covering a market. The componentunit of a roof is composed by the latticed shell made ofstraight tubes, disposed along the two families ofgeneratrices, triangular latticed edge beams, the r.c.columns sustaining the shell unit at the four cornersand the joints, obtained by tube flattening.

Fig 5 b

Fig 5 a

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 createnew exciting solutions, in which the architectural expression and the structure form are in aperfect concordance. Because the textile material is very light-weight, these structure types arevery efficient and economically. But there are also some disadvantages, as complex surfaceformation, 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 aflexible 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 structuretype. 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 (composedby 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 themiddle tension membranes in the form of saddle hypars supported on steel arches and endmembrane, supported on end steel arch and tensioned inclined end columns. The steel structurereceived the ECCS Steel Design Award in 1997.

Fig 7 a

Fig 7 (continued) c

Fig 7 (continued) b

6. Conclusions

The paper presents some steel structures designed by the author framing in the field of innovativesolutions, 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 SpaceStructures, Guildford, 5-10 September 1993, Thomas Telford, London, Vol. 1, 1-8

Makowski, Z.S. (1995): Light-weight structures. Their development and impact upon modernengineering. In Light-weight Structures in Civil Engineering (ed. J.B. Obrebski), Warsaw, 25-29September, 1995, 43-50

Fig 7 (continued) d