The hanging weights model was developed over some ten years (roughly 1900 to 1910) and is actually the design of another church that was partially built -the crypt of the Colonia Guell in Santa Coloma de Cervello. It is a masterpiece in stone. You can see more about it on this web site.

Through this model, Gaudi studied the principles behind the design for the Sagrada Familia: i.e. the almost exclusive use of compression (and no tension) in the entire structure -this manifests itself in leaning and branching columns. Gaudi decided to "analytically" make the model for the Sagrada Familia, thus he relied on paper, pencil, and calculus versus on a physical model. Of course, this meant certain simplifications were necessary. These revolve primarily around the use of symmetry and collapsing the sections to nearly 2-dimensional bays (like the cross sections of the nave along the narrow axis).

Gaudi laid the entire floor plan out before his death, and he explicitly designed (via plaster models) the entire nave up to the crossing as well as the cloister and the sacristies. These models were destroyed during the civil war (mid 1930's) and have been reconstruced slowly since. The reconstructions are accurate because Gaudi used geometric shapes, formulas, and a consistent scheme of proportions for everything. Thus, for example, to reconstruct a vault, which is a hyperboloid, they only needed to determine the radius of the collar, and the asumptotic slopes of the hyperbola -two numbers which could fairly easily be determined from shards of model.

Probably the best book about all of this is Jordi Bonet's "The Essential Gaudi" published a couple of years ago. He's the head architect of the Sagrada Familia. The publisher is Ediciones Portic. You can probably find it at the gaudi2002 website (there is a link in the links section of the Gaudi Club). Another good one is Mark Burry's "The Expiatory Temple of the Sagrada Familia" published by Phaidon Press around 1993.