CaixaForum Zaragoza

1. Carme Pinós

I like my buildings to be a structural expression. Even the designs of furniture we produce in the studio are pure structure. They minimise gestures based on the logic of strength, stability and support.

After Cube I, Caixa-Forum is one of my best examples of this.

In the case of Caixa-Forum, my first intuition was to convert the building into the gateway of the park that will be behind it. The scheme was not excessively complicated: two rooms of different size, conference rooms, a restaurant and an auditorium. Right from the start, the building/gateway idea involved suspending the two rooms, thus enabling the city to pass below them to enter the park.

I did not stop until I found a configuration which solved the scheme, and which, at the same time, permitted a formal play of structural attractiveness, since it is a special building with much perspective.

At the studio we started to play around with small models which gave us the logic of support and stability.

In several recent projects, we had solved the structure with concrete walls and steel trusses with their triangular shape shown on the façade. I did not want it to be the same on this occasion. The configuration was not hard to find: two overlapped squares whose intersection creates another forming the core and support area. A large truss crosses them diagonally, as the essential element from which the rest is generated.

We did not want the building to be perceived as something heavy. That is why we cut the concrete walls diagonally in their external parts to give the sensation of lightness.

We worked on all this with small models with a very clear vision in mind of how the scheme would look.

I remember that the request for us to participate in the restricted tender arrived two days before I had to leave for the United States for a week and we only had a month to produce it. In New York, without the resources I have in the studio, I was thinking about the above-mentioned points but without the means to check them. When I returned, I explained my ideas to the team. I gave them sketches and we knuckled down to work. The configuration we started out with was defined by these two rooms. They were going to give us the measurement for the rest of the scheme. I remember saying ‘if this configuration does not work, then let’s forget it. There’s no tender. I don’t have time in the three weeks that are left to develop another one.’ Like a small miracle, everything came together. One model after another gave us the final shape which became the almost completed project we see today.
Once we had the model that convinced us and which complied with the scheme, we had another test to pass: knowing that the structural configuration which we had intuitively produced was going to work. We still needed to pass Brufau’s test. But Brufau came, shook the model and said to us ‘it works. We’re going to be able to go ahead.’ As always, Brufau understood and pruned all our intuitions. In the remaining three weeks, we developed the storeys, fitting in the structural elements and the installations, and solving minor complications in the scheme, so that everything would fit together like clockwork.

Another aspect I was concerned about was the relationship between one room and another and the relationship between the user moving around the building and the city. I did not want moving from one exhibition to another to be through enclosed, compressed spaces, in which your mind cannot relax and prepare to see something different, which in the majority of cases has little to do with what you have already seen. For that reason, from the beginning, the exhibition rooms were not opposite each other or one on top of the other, but displaced at an angle, so that there is the possibility of leaving one exhibition and seeing the city below the other room given that they are so high up. I wanted to create spaces for decompression as opposed to the introspection required by looking at art.

This gesture provides us with the possibility of creating a terrace on the same level as the restaurant on the last floor, a magnificent vista point of the city and the river.

Essentially, these two concepts—a building/gateway for the park and the transition of one room into another having distant views—were the basis and soul of the design.

2. Robert Brufau

I have been collaborating non-stop for forty years now as a structural consultant in designs of not very conventional buildings, with complex schemes and considerable levels of difficulty. However, on very few occasions have I experienced an interaction between architecture and structure as intense and interactive as in the case of the Caixa-Forum Museum in Zaragoza. The building’s volumetric ideas and its integration into its surroundings were in Carme Pinós’ head right from the start of the tender. Yet, because of her forceful proposals, ongoing shoulder-to-shoulder contact was necessary between the architecture team, the structure team and the technical installation team.

From the beginning, she opted for a proposal that broke through the convention of this type of project, proposing a structure with a considerably cubic enclosure, which evoked a certain initial sensation of floating above the site. The presence of large cantilevers in all directions would emphasise this perception. One of the basic characteristics of the design is found in the relationship produced between the building’s interior space and the exterior space, since, as there are large external spaces sheltered from the rain, it is not easy to define its limits.

Below ground level, the site is almost completely covered, since it contains a large functional scheme which is rounded off with an auditorium for 250 people in one of the side wings, accessed from the first basement floor, and which does not coincide in plan with the building of the top museum structure. The floor area on these lower levels is slightly less than the floor area used by the museum facilities in the main top part.

For users accessing Caixa-Forum from the street network, this underground construction will not start to be noticeable until they have started to descend towards it, either from the main lobby, or from the side stepped entrance. It is surprising that the smaller construction in plan occurs precisely at the access level, devoid of exhibition content and only occupied by the main lobby—located in a prior appendix with its own structure and conceptually different from the rest—and by the staircase accessing the top section and leading to the underground auditorium.

This minimisation of site coverage on the ground floor, continued on the mezzanine floor, conditions the image of the building, since the limits of its coverage increase as the building rises. However, the museum section coverage focuses especially on the quadrants of the southeast and northwest zones, leaving the exterior corners of the other two quadrants empty. The first floor coverage is only in the centre and in the back quadrant, whilst the coverage on the second floor is complete. The third and fourth floors contain the centre and the southeast zone quadrant, with the building disappearing in the northwest zone.

This increase in the height of the storey must be resolved with the same resistant vertical elements emerging from the level of the base of the access floor, i.e. with the two elongated columns located along the main backbone and with the two L-shaped walls which define the concavity of the top storeys. The presence of cores for the stairwell and all three lifts end up defining the vertical structure.

The attached diagram shows the growth criterion for the four main load-bearing elements, which define the theoretical approach to the whole. The oval column of the main backbone increases its dimensions as it goes up, starting with dimensions of 2.30 x 1 m on the ground floor and ending up with 4.80 x 0.90 m at the roof of the second storey. The external vertex of this column deliberately tapers so that onlookers view it as a very slender element. Yet it is a particularly important element, given that its resistant function is to receive the majority of the weight of the prior large cantilever with its three structural floors supported by two large Warren trusses, which can be seen in the previous section. That is the reason why it has a reinforced web with powerful embedded steel reinforcement.

The two L-shaped walls, which taper at the ends, start with arms whose exterior fair-faced areas are
3.20 x 1.90 m on the ground floor, with the short arms ending with 10.60 x 1.90/0.90 at the floor of storey 1 and the long arms with 15 x 1.90/0.90 m at the floor of storey 2. They will then stop sloping and continue rising with a free vertical edge. A very effective formal resource is used in both walls, as its exterior vertex has a central recess to make it appear more slender. This is achieved with the formwork as a result of the shadow line forming the recess.

The photographs show the rising geometry of these L-shaped walls, as well as the relative arrangement in height of the long arms compared with the short arms. Passersby see the concave side of the two L-shaped walls, with the only exception of the steel emergency staircase, located in the southwest quadrant.

The balance between the two sides is broken up, since the cantilever in the northwest quadrant section is smaller than its opposite section, which is also at a lower position in height. The organisation of these two walls and their geometrical realisation thus become the most significant aspect of the building, as their unquestionable structural purpose merges with their powerful architectural expression.

The L-shaped walls are hollow inside. They are formed by two external faces with a thickness of 30 cm. The width of the hollowing is 130 cm where the walls intersect, and 30 cm at the end of the walls.

The two faces are joined by means of connections arranged at regular intervals in plan, with each wall forming a whole that is very rigid, and thus capable of offering a good reaction to wind stresses.

The most special point of the structure of the Caixa-Forum building in Zaragoza is, however, the two cubic blocks for the museum exhibition, which, sited in the southeast and northwest quadrants, intersect in the central area of each storey. Their steel structures, in both cases designed as a large cantilever volume, cause observers some concern as a result of the powerful floating image of both cubes, on the one hand, and the apparent lack of a balancing counterweight, on the other.

What is the key aspect for deciphering this special and daring volumetric composition without having to make use of vertical supports at the end vertices of the two large cubes? The explanation lies in the two trusses on each side (one on a higher level than the other), which, starting out embedded in the vertical M edge of each L-shaped wall, define the two façades of each volume, apparently working as a cantilever supporting the majority of the cube they belong to. This is not the case, since these trusses are resting on the ends of truss girders VA and VB (orthogonal to each other and supported by the oval column) and on the end of truss girder VD, which has a diagonal alignment, at 45°, crossing the entire storey, supported by the two central columns and between them joining the two vertices of the large cantilevers, although at a different level.

Consequently, the presence of these trusses VA, VB and VD, which actually leave the oval column as a cantilever, is the key aspect for interpreting this structure. Together these façade trusses define the basic frame which receives all the internal purlins. The large diagonal truss VD is arranged, as shown in the cross-section, as a Warren truss with a height which, to adapt to the definitive form of the volume, varies between 5 metres at the end and 9 metres at the highest point. A structural floor cuts it at mid height in the central area of the building.

To finish describing this structure, we need to mention the vertical elements which close the façade of the large museum cubes. Coinciding with the ends of the main structural elements, these vertical members join the two structures (the top with the bottom one) so that, besides providing support for the façade substructure, they make it possible for the overall movements of both levels to be compatible.

Print edition ISSN: 2341-0531 / Digital edition ISSN: 2387-0346. Copyright © 2016 ZARCH. All Rights Reserved