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Click to view map Coordinates: | Opened Oct 1960 Two extracts transcribed from an article published shortly after the Planetarium was completed and which relate to the design and construction of the building are provided below: 10. THE DOME The inner surface on to which the Planetarium sky is projected is a dome approximately 67 ft. (20.4 metres) in diameter. It is bounded on the lower side by the horizon which is on the level of the centre of the projector, at a height of almost ten feet (3.05 metres) above the floor. The highest point of the dome is therefore some 44 ft. (13.41 metres) above the floor and is thus sufficiently distant from the eye to create the illusion that the observer is looking at objects at an infinite distance when the stars are seen. The acoustic problems involved in such a hemispherical dome are, as is well known, very serious. If the inner surface of the dome were to be made of material which is a good reflector of sound waves, it would focus the waves reaching it from the narrator, and thus produce the well-known 'whispering gallery' effect, to the undoubted discomfort of some unfortunate member of the audience. To avoid this, therefore, the inner surface of the dome is now made of porous material through which some fraction of the impinging sound waves will pass into the space behind it, there to be dissipated by one means or another. In this building the inner surface is composed of 16-gauge sheet aluminum, with perforations 0.08 in. (2.03 millimetres) in diameter at 0.2 in. (5.08 millimetres) centres, making some thirty million in all. These holes are smaller than the smallest star image, so that there is no appreciable diminution of brightness if a star image passes across one of these holes. Behind the perforated inner dome a layer of fiber glass material has been placed to absorb any sound that penetrates the holes. The aluminum sheets themselves are attached by screws, numbering almost forty thousand, to a framework of wood which in turn is suspended from a network of several thousand steel bars of different lengths, connected in such a way that the ends of six rods are joined by screws holding washers which fit into slots in the ends of the rods. This geodesic type of framework rests on a concrete ring at horizon level and is otherwise entirely free and self-supporting. In fact it is of such strength that it provides a means of access to the top of the dome, since there is a gap of about two feet (60.96 centimetres) between the framework and the outer concrete dome which roofs the auditorium, about which Davis will have more to say later. Ref: Bleksley, Prof A.E.H. The Planetarium. The Certificated Engineer, July 1960: pg 314. DISCUSSION The Contractor, Mr. Desmond Davis provided a brief outline of some of the problems which where confronted during the construction of the building. At the outset it was fervently hoped that the planetarium would be ready for use during the Union Jubilee Festival in May, 1960. That this hope could not be realized was mainly due to the fact that the one of two full time erection teams allocated by Messrs Carl Zeiss could not, due to commitments in other parts of the world, arrive in Johannesburg before the beginning of December, 1959. At the commencement of the construction of the building and of the outer 5 in. (12.7 centimetres) concrete shell of the dome it was obvious that much careful planning would have to anticipate the sequence of work and the unusual provisions that would have to be made if the structure was to be ready for the erectors on their arrival. Delays due to weather and other uncontrollable factors inevitably occurred but progress was such that the final sections of concrete were poured with three days in hand. GENERAL CONSTRUCTION The hemispherical concrete dome approximately 72 ft. (21.95 metres) in diameter rests on a ring beam which is supported 18 ft. (5.49 metres) above floor level by the reinforced concrete wall carried by the framework over the corridors. It was constructed in two stages, the first of which terminates in an intermediate ring beam 35 ft. (10.67 metres) above floor level and the second is the final cap to a height of 51 ft. (15.54 metres). In order to simplify construction, the dome was divided into 24 equal segments so that the plan of any section is polygonal. (Figs. 1, 2, 3) The inner separate shell on to which is fixed the sheet aluminium projection screen, is truly hemispherical and rises to its apex from a projecting toe 9 ft. 10 in. (2.99 metres) above floor level. It consists of a geodesic lattice structure which is entirely self-supporting and is un-connected with the outer concrete shell. (Fig. 10) In order to make the concrete shell completely watertight it is covered with sheet copper which has the added advantages of resistance to corrosion from atmospheric causes and of requiring no maintenance. SPECIAL PROVISIONS The unusual shape and nature of the building called for a number of special provisions during various stages of construction in anticipation of subsequent operations, the chief of which were as follows:
METHOD OF CONSTRUCTION 1st stage. The construction of the concrete base wall, 6 ft. 3 in high (1.91 metres), above the corridor slab up to the level of the lower ring beam, followed normal practice and no unusual difficulties were experienced. The individual casings for the reinforced concrete ribs above the ring beam were fabricated on the ground and the reinforcing rods were fixed accurately within them before they were hoisted into position. A light steel framework of scaffold tubing was built to support each box and was anchored to the short lengths of tubing already cast into the ring beam. The boxes could then be plumbed accurately and held firmly in position while the shuttering for the shell of the dome was being completed and while the concrete was being poured. (Figs. 1, 4 and 5). In order to simplify the procedure of construction, shoulders were incorporated in the casings for the ribs which could then be simply supported by the tubular framework, as can been seen in Fig. 1. This method of erection proved to be both accurate and satisfactory. Had an attempt been made to build the castings in mid-air, difficulties would have been experienced and grave errors in location could have occurred due to the difficulty of using plumb-lines accurately in the prevailing season of high winds. Even in the location of complete units some delays were experienced from the effects of wind. The intermediate ring beam was then cast as a series of chords connecting the upper ends of the ribs. (Figs. 2 and 3). 2nd Stage. The dome had then to be completed as an un-ribbed shell from the intermediate ring beam to the apex slab, which is raised 2 ft. above the level of the hemisphere. For this purpose:
These trusses were bent to the correct curvature and held in this form by tension wires and bracing as can be clearly seen in Fig. 6. Since the initial load of wet concrete when poured would cause an appreciable deflection in the trusses, a pair of them was erected at ground level and loaded artificially in order to determine this deflection. The initial curvature was adjusted so that the curvature under the load of wet concrete would be correct. Units, consisting of a pair of trusses with shuttering and reinforcing assembled between them were then hoisted into position in the dome, as is shown in Fig. 6. The 60 ft. (18.29 metres) castor-mounted derrick mast with a short horizontal jib had served admirably up to this stage of construction. It had proved to be sufficiently mobile to deal with lifts to any part of the dome. As the final trusses were placed in position, however, its circumferential movement became more and more restricted until eventually it could not be moved at all and an opening had to be left around it in the shell through which concrete could be raised and finally the derrick could be dismantled and lowered to the ground. Special steel eyes were cast into the inner surface of the dome from which tackle for lowering both the derrick and the central tower could later be hung. The pattern left by the trusses and the actual lowering eyes can be seen clearly in Fig. 10. Concrete had to be poured while the form work remained within the working radius of the jib of the derrick as can be seen in Fig. 7. The dome cap presented no particular problem. Outer surface. 18 gauge annealed copper sheets with welted seams covers the outer surface of the dome. It is fixed to dovetailed wooden battens screeded into the concrete. (Fig. 8). ALUMINIUM PROJECTION SCREEN A widespread impression exists that the framework supporting the perforated aluminium projection dome is attached to the outer concrete dome. This impression is incorrect for there is no physical connection whatsoever between these two domes. There is, in fact, sufficient space between them for a man of average height to stand upright. (Fig. 10.) The steel framework supporting the screen originates from a 5 in. (12.7 centimetres) thick concrete toe circling the auditorium at approximately horizon height. (Fig. 3.) The lattice framework was made and erected by Messrs Dyckerhoff and Wickman for Carl Zeiss. For its erection they bought with them a mobile bridge scaffold, mounted on wheels which ran on a circular track on the floor of the auditorium. From appropriate platforms on this rotating scaffold, access to any portion of the dome was readily possible. A portion of this equipment can be seen in (Fig. 9.) Briefly, the triangular lattice framework or Geodesic framework, as it is referred to, consists of several thousand flat steel bars interconnected by means of steel bolts. As all these bars are in compression all four corners of each bar are notched and are simply located between two large washers at either end of the securing bolts. A portion of this equipment can be seen in (Fig. 10.) Generally six, and in some cases five, bars come together at each bolt. The lengths of the bars vary to give a perfect hemisphere when the whole of the framework is connected. A portion of this equipment can be seen in (Fig. 9.) From each bolt and projecting into the auditorium, is a flat steel bar or hanger 12 in. (30.48 centimetres) long to which timber battens are fixed. To these battens are screwed the perforated aluminium sheets. (Fig. 10.) The erection of the complete Geodesic framework and the screen occupied a period of two and a half months. Screws were used to fasten individual sheets to the battens. To eliminate the 'whispering gallery' effect mentioned by Professor Bleksley, the aluminium sheets were perforated by approximately 30 million holes, each smaller than the smallest star image. In addition, a double layer of fibre glass wool supported by netting was wrapped round the outer surface of the framework. (Fig. 10.) After the screen had been completed it was found that the disconcerting echoes, which were experienced from the outer concrete dome, had been entirely eliminated. In conclusion I wish to record my thanks first of all to the architect, Mr. E. N. FINSEN, for his whole-hearted co-operation during the construction and for his accurate photographic record of the work, some of which has been presented to you tonight, and also to Mr. C. A. Rigby, consulting engineer, and members of his staff, particularly Mr. Pokorny. Ref: Bleksley, Prof A.E.H. The Planetarium. The Certificated Engineer, July 1960: pp 315 - 321. Copy of article provided by Dr. Claire Flanagan, the Director of the Johannesburg Planetarium. Submitted by William MARTINSON All truncated references not fully cited below are those of Joanna Walker's original text and cited in full in the 'Bibliography' entry of the Lexicon. |
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