Roof of the Assembly arid Entrance Halls of Wohlen High School

Santiago Calatrava

Subject | The architect and engineer Santiago Calatrava has won recognition with his extravagant architectural constructions, in which statics and design form surprising symbioses. His dual studies of architecture in Valencia and civil engineering at the ETH in Zurich were already predictive of that. Although it dealt with the engineering subject "The Foldability of Frameworks" his thesis was written at the architectural faculty of the ETH Zurich. This desire to combine creative and structural expressions in buildings is fully visible in Calatrava's sketches. He thus follows in the shoes of the 60 year older Pier Luigi Nervi, who in his wide-span constructions made manifest the flow of forces through the use of shaped concrete elements. Unlike Nervi, however, Calatrava does not restrict himself to reinforced concrete, but tries his hand with structures of steel and wood and with composite systems involving the use of various materials.

Four years after founding his architectural and building engineering office in Zurich he was commissioned to design the roofs for the assembly and entrance halls of the High School in Wohlen, built by the architects Burkhard, Meyer and Steiger. It is interesting how Calatrava went about the task, proceeding not step by step from one single basic idea but courageously developing completely new concepts. The pleasure he derives from the realization of a particular structural idea is clearly visible. A good example of this are the preliminary studies for the round entrance hall.

In one design, which he called the "Circus Roof", the columns and bars of wood and steel were to carry a romantically capricious roof with shaped wooden ribs, translucent marble slabs and a large convex steel mirror over the central lantern. Already here the unusual three-dimensional effect is achieved by the use of clear-cut compression and tension elements. This approach was then abandoned, among other things because of the columns that rose from the floor and restricted movement in the entrance hall. Another design was based on the umbrella principle with struts and stress-bearing ribs; yet another was a flat dome carried by shaped wooden ribs springing from a central open compression ring. At last, after 15 months, came the final decision: a stellular, open, folded-plate dome in


Aargauische Kantonschule, Allmendstraße 26-26, 5610 Wohlen, Switzerland

Design and Construction of Roofs

Calatrava Vails SA, Zurich/Paris

Design of School U. Burkhard, A. Meyer, M. Steiger Architekten, Baden, Switzerland

Timber Construction Wey Elementbau AG, Villmergen

Chief Carpenter Hans Meier

Date of Completion


For the entrance hall cupola, for which 1000 man-hours were required, the production costs, excluding planning and other incidentals, came to 200 000 Swiss Francs. For the roof of the assembly hall, including the stage roof, which was of much simpler design, the manhours needed amounted to 2500, the costs to approx. 350 000 Swiss Francs.

Wohlen High School Roofs And Hall

5 | Longitudinal section and underside of one of the entrance hall trusses, scale 1:50. At top, at insulating layer level, the 100 x 160 mm compression beam, at bottom the 100 x 160 mm edge beam. At the bottom tip of the truss the steel piece for fixing the tieback, fashioned from three 100 x 400 mm chromed steel flats. The compression rod of the laminated fir has a diameter of 60-180 mm.

6 | Various truss cross-sec-tions, scale 1:50. Construction of roof floor: 40 x 120 mm facing boards planed on one side as truss top side, vapour barrier, 140-mm-high squared timbers, in spaces between these a 100 mm mineral wool heat insulation layer and a 40 mm air space, 22-30 mm roof boarding, standing seam roof cladding of titanium-zinc sheeting. The V-shaped undersides are of 60-mm-thick three-layer laminated wood.

7 | Cross-section of truss at ridge, scale 1:50. Floor construction: 13 mm open-joint acoustic boarding, black vapour barrier, 120 x 180 mm longitudinal beam, in between 100 mm heat insu lating matting, 50 x 50 and 50 x 100 mm counter-lath-ing on side connections, 27 mm tongue-and-groove boarding. The bottom 200 x 330 mm arched girder is shaped for seating the strut and for greater elegance.

wood. Each of the previous ideas was in itself something out of the ordinary, both in design and execution, but the final idea is sophisticated yet simple in a very special way and at the same time of exceptional lightness and elegance.

Equally original is the roof over the assembly hall, which in this particular case makes use of special struts to lift the roof from the surrounding walls, thus creating a row of windows between the two parts of the building. The supporting structure consists of triangular arched trusses with unique spatial effect, marred slightly by the simplicity of the stage opening in the end wall.

The school was completed and inaugurated in 1988.

Structure Entrance Hall | The architect was faced with the task of covering a space some 12 m in diameter. The solution developed by Calatrava was a star-shaped strutted frame of triangular box-type trusses held at the centre by a compression ring 2 m in diameter of 60 mm iron tubing and supported at the circumference by the concrete ring of the floor. The triangular trusses are approx. 1.20 m high at the outer supports and approx. 30 cm high at the inner ring. They also diminish in width from 200 to 30 cm in line with the ground plan geometry. The V-shaped underneath surfaces are of 60-mm-thick three-layer glued pine. The upper surface is of 40 x 120 mm tongue-and-groove boards planed on the visible side only and serving as a deck for the roof structure. The system of supports for the circular strutted frame is divided into an upper support for the vertical loads and a 1 m lower tieback of three plates of 10 x 40 mm chromium-plated iron to absorb the horizontal forces and join the bottom ends of the V-shaped elements. Thus increased to 2.60 m, the difference in height between the two pressure points reduces the horizontal forces arising in the strutted frame. To prevent any twisting of the top tubular iron compression ring the top boarding carries a 100 x 160 mm compression beam, thus creating a lever some 200 mm long which stabilises the upper support system.

Calatrava makes this extravagant construction even more extravagant. He cuts away the underneath edge of each V-shaped wooden member, thus making it arcuate, and replaces the missing bottom edge with a compression rod of glued laminated fir. To counteract buckling these 4.20-m-long compression rods are 180 mm thick in the middle and 60 mm at each end. The lower edges of the V-trusses are replaced by welded and chromed angle bars, which, fixed to the wood surfaces by means of M6 hex-head wood screws, take up the shearing forces of the compression rods and transmit them to the bottom tiebacks. When the structure had been completed, the architect disliked the look of the joints between the boards of the roof floor and had it covered with a smooth plywood surface painted white. This, indeed, creates a much quieter effect.

Execution of the work proved particularly difficult for the timber construction firm. The V-shaped members were produced at the factory. On the site an assembly tower was erected in the middle of the room, on which the compression ring was placed. Then the V-members were linked to it by means of metal straps, which were welded to the ring. The outer support ring carries 45-cm-high steel brackets, each with a 30 cm slot for screwing up the angle steels of the V-mem-bers. The rest of the roof was then built on top of the top boarding. First, squared 140 x 160 mm timbers were screwed on to hold the V-members together and also the above-mentioned 100 x 160 mm compression beam. Then came a bottom 100 x 160 mm boundary beam with 40 x 160 mm cleats for the roof overhang and an 80 x 140 mm intermediate timber to shorten the span of the boarding over the lower area. Finally, on top of the vapour barrier and the 100 mm heat insulation came the 30 mm roof boards, roofing felt and roof skin of titanium-zinc sheeting as a standing seam roof covering. Not until the bottom tiebacks had been fitted

8 | Erection of entrance hall roof. V-shaped underside of truss at factory.

9 | The first truss is fitted. On right, the central compression ring with fixing rings welded in position.

10 | The steel component between compression rod and steel flats of tieback, which are bolted to the straps.

11 | Three trusses with tiebacks in position and supports.

and tensioned was it possible to lower and dismantle the assembly tower. There was little horizontal displacement of the structure and the slot connections in the vertical supports proved fully adequate. This was due to the exceptionally accurate way in which the timber construction company had performed the work.

Structure Assembly Hall | The assembly hall is also covered using triangular trusses, but these arch over the hall in the usual way. The overall span is 10.60 m and the truss spacing and thus individual component width 4 m.The structure could be regarded as an elliptical three-hinge arched girder construction, which carries the intermediate floor-supporting transverse arches by way of slanting strutting. This slant gives the whole structure the necessary longitudinal rigidity. From the production point of view they are triangular trusses, since each curved beam, from the support to the apex hinge, the fan-like strutting, the transverse half-arches and the upper roof floor construction form a complete triangular component that could only be constructed at the factory with the necessary precision. The supports are shaped, 2.70-m-high concrete columns, which absorb the skew of the arches at the point of support and thus easily transmit the normal forces from arch to ground.

The arched beams measure 200 x 330 mm and have notches on each side serving as seatings for the struts and chamfered side surfaces at their bottom ends. The fan-like struts vary in size from 80 x 80 to 80 x 120 mm depending on their increasing length. The ten longest struts are also interconnected with steel rods to prevent buckling. The top transverse arches measure 80 x 140 mm and are held together by 4-m-long 120 x 180 mm longitudinal girders 80 cm apart. Between these is a i.og-m-thick heat-insulating layer, on this is placed black plastic sheeting as a vapour barrier and seepage protection and 13 mm acoustic boarding with open joints. Attached to the longitudinal girders is a 50 x 50 mm counter-lathing system for the ventilation, to which the 27 mm roof boards with titanium-zinc skin are fixed. The triangular part of the truss from the point of support to the vertex hinge, together with the roof superstructure and overhang, were produced in the factory on specially designed iron-frame templates as prefabricated components. Great attention had to be paid to ensuring the neat appearance of the strutting. Their connection to the arch, for instance, had to be invisible. To achieve this the strut was first screwed to the arch through the back of it, then the strut opposite was fixed using a glued wood dowel. With the next strut the process was reversed, so that on each side of the arch there was an alternation of screwed and dowelled connections, and no connecting element was visible. Particular care was, of course, necessary to ensure a uniform arrangement of the struts, each of which had a different joint angle depending on its position in the arch. Through good preparatory work and applying a fixed-cycle work method only two days were required for the assembly of each component. For five arches, or ten components, that makes 20 days production time.

Transporting the long prefabricated arch components to the building site proved highly problematic, since they measured 4 x 5.5 x 8 m and weighed 2.5 tonnes each. With the help of the police a suitable - much longer - route was worked out to get them to the site, which, in fact, was only 3 km away. Transportation and assembly using a pneumatic crane took only a day.

After being placed in position by the crane the ten arch components were bolted together with steel plates with Compriband between the arches. To seat these on the concrete columns special steel shoes were installed.

14I Erection of assembly hall roof. Fitting of struts in factory.

15 I An arch component is lifted into position at building site.

16 I The shaped concrete columns serve as arch supports.

1 I Ground plan of stadium, scale i:iooo.

2 | Cross-section through stadium, scale 1:500. The tilted main arches rest on inclined concrete foundations, which duplicate as abutments for the boundary arches. These form the lateral limits of the specta tor seating.

3 I Representation of structural system. The curved tie beams transmit the weight of the roof as normal tensile loads fj. The boundary arches "b" absorb these as pressure forces, since they act almost at ground level, and collect them in the arch supports as £fj.The main arches "a" take up the vertical fraction V,- of the tensile force fi as pressure force, which is added to J\Z\ at the supports, and the horizontal fraction hi, which is cancelled out via the coupling beams through the sameness of the two arch systems.

4 | View of the ceiling.

5 | Interior showing main beams and suspended shells.

6 | View of stadium with entrance.
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