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I want now to consider for a few moments what is involved architecturally in the choice of a structure. Most buildings consist of a set of flat floors, held up in space and forming the ceilings of the rooms below, which need to be supported at certain intervals by walls or columns or both. These supports can hold up a flat slab or one differentiated into deeper and shallower elements, this being usually a more economic solution. The elements that go to make up the slab will be either linear or planar. True planar elements can only be made up out of an isotropic material (one with the same properties in all directions): for instance, concrete, plywood or steel plate. In many structural systems there will be a hierarchy of differentiated members – boards on joists on beams, say, each successive group being laid counter to the one below.
The choice of a system will be influenced by a number of factors – eg, the requirements of the cellular subdivision of the building, and the impingement, if any, of the partitioning on’ the structure, and of the services. Should the architect wish to display the hierarchy of parts of which the structure consists, then he must decide whether or not the building is one where a linear, directional arrangement is appropriate or not. It may be so – say, in the case of offices ranged either side of a linear internal route. If, however, the architect decides that the spaces in the building, or the relationship of the building to its site, make a directional structure inappropriate, then it will be necessary to devise a system with equal, or at any rate similar, characteristics in two directions.
Curiously enough, we do not have, as far as I know, convenient accepted terms for describing these types of structural anatomy. I have battered my brains and my dictionary, also several dons and one classicist vicar, in an attempt to dig up an appropriate set of terms. The vicar jumped straight in and said ‘Quite simple: monothreptic and diathreptic – growing in one direction and growing in two directions’. Now I know engineers are well educated chaps, but I can just imagine saying down the telephone, ‘Well, actually I thought the beam arrangement ought to be more, well, kind of diathreptic, you know.’ So let us avoid being too fancy, and stick to Anglo Saxon three–letter words, and talk about one–way and two–way structures – and, I suppose, three–way and many way structures.
A one–way structure is one whose basic set of parts has certain characteristics in one direction and different ones in the other, and which therefore adds together with its sideways neighbours in a different way from a neighbour fore and aft. The two–way structure has a basic set whose characteristics are the same along the x and the y axis , though it need not actually be square; it can be distorted into a rectangle and still have the same characteristics, though not the same dimensions, along the two axes. It will abut similar neighbours in the same way on all sides.
‘Well, actually I thought the beam arrangement ought to be more, well, kind of diathreptic, you know.’
This will have little architectural significance if the structure will in due course disappear from view. Indeed, a one–way structure may be clad with a suspended ceiling of square (ie, two-way) acoustic tiles, which may relate to a two–way grid of moveable partitions. If the structural form is going to remain on show, however, it becomes the architectural form, and the choice of a one or two–way structure is an important decision. For instance, the building may be sited on a corner. A two–way anatomy goes round a corner, one might say, automatically, but when a one–way structure comes to a corner, all it can do is stop. You can then start next to it, running along the other street another one–way structure. They become neighbouring one-way structures, with or without a gap between them.
If the external anatomy of the building bears some relationship to the structural happenings within, it is likely that the external form of the building will bear a similar relationship to the two streets if there is a two–way structure: but probably a one-way structure will present, literally, a different face to the two streets. This may be entirely appropriate, since one street may be much less important than the other. But often it is entirely unjustified, and results in all those thoughtless blank–ended matchboxes we see at the junction of two equivalent thoroughfares. This corner problem illustrates how in our view the choice of a structural system relates even to the town–planning context of a building. While thinking about structural economy, the relationship of internal partitioning to downstanding beams, the relationship of cladding to the structure, and so on, you are taking decisions which affect the relationship of the anatomy of the building to its site and to its neighbours.
As well as choosing between a one-way and a two–way structural system, the architect must decide on the location of his columns in relation to the enclosing skin of the building. The columns can be outside the skin or in the line of the wall, or the whole column can be inside the building. Columns in the line of the wall cause all those well–known problems of cold bridges and tolerances, creating a sort of hiccough in the planning grid, and perhaps of ending up rather lost , if not entirely so, both from inside and out, because the thickness of a column and a wall these days are often not dissimilar. We have always had a sneaking regard for columns set within the space (ie, inside the enclosing skin), provided this can be done without sterilising useful space, because if the intention is to create an architecture whose form consists of structural enclosures, these become even more apparent and communicative if you are living with your support system; they create that room within a room effect, of which John Soane was the pastmaster.