It is the beam, connecting the columns which creates a volume you can visualize, before it is complete; and when the columns are standing in the ground, you need the actual physical perimeter beam, to generate this volume before your eyes, to let you see the room as you are building it, and to tie the tops of the columns together, physically.
These reasons are conceptual. But of course, the conceptual simplicity and rightness of the beam around the room comes, in the end, from the more basic fact that this beam has a number of related structural functions, which make it an essential part of any room built as a natural structure. The perimeter beam has four structural functions:
1. It forms the natural thickening between the wall membrane and vault membrane, described in efficient structure (206).
2. It resists the horizontal thrust of the ceiling vault, wherever there are no outside external buttresses to do it, and no other vaults to lean against.
3. It functions as a lintel, wherever doors and windows pierce the wall membrane.
4. It transfers loads from columns in upper storys to the columns and the wall membrane below it, and spreads these loads out to distribute them evenly between the columns and the membrane.
CONSTRUCTION
These functions of the perimeter beam show that the beam must be as continuous as possible with walls and columns above, the walls and columns below, and with the floor. If we follow good materials (207), the beam must also be easy to make, and easy to cut to different lengths.
Available beams do not meet these requirements. Steel beams and precast or prestressed beams cannot easily be tied into the wall and floor to become continuous with these membranes. Far more important, they cannot easily be cut on site to conform to the exact dimensions of the different rooms which will occur in an organic plan.
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| A version of the perimeter beam consistent with the box column shown before. |
1020 217 PERIMETER beams
Of course, wood beams meet both requirements: they are easy to cut and can be tied along their lengths to wall and floor membranes. However, as we have said in good materials (207), wood is unavailable in many places, and even where it is available, it is becoming scarce and terribly expensive, especially in the large sizes needed for beams.
To avoid the use of wood, we have designed a perimeter beam—shown opposite—which is consistent with our box column, and designed to be used together with it. It is a beam made by first nailing up a channel made of wooden planks to the columns, before the wall membranes are made; then putting in reinforcing, and filling up with ultra-lightweight 60 pounds per cubic foot concrete, after the walls are made and filled. This beam is excellent for continuity. The wooden channel can first be made continuous with other skin elements by nailing, and the fill can then be made continuous by filling columns and beams and walls and vault in one continuous pour—see wall membranes (218)
and FLOOR-CEILING VAULTS (2 I 9).
Of course, there are many other ways of making a perimeter beam. First of all, there are several variants of our design: the U-shaped channel can be made of fiberboard, plywood, precast lightweight concrete, and, in every case, filled with lightweight concrete. Then there are various traditional perimeter beams— the Japanese version or the early American versions come to mind. And then there are a variety of structures which are not exactly even beams—but still act to spread vertical loads and counteract horizontal thrusts. A row of brick arches might function in this way, in a far fetched case so might a tension ring of jungle creeper.
Therefore:
Build a continuous perimeter beam around the room, strong enough to resist the horizontal thrust of the vault above, to spread the loads from upper stories onto columns, to tie the columns together, and to function as a lintel over openings in the wall. Make this beam continuous with columns, walls and floor above, and columns and walls below.
CONSTRUCTION
Remember to place reinforcing in such a way that the perimeter beam acts in a horizo?ital direction as well as vertical. When it forms the base for a floor-ceiling vault (219) it must be able to act as a ring beam to resist all those residual horizontal outward thrusts not contained by the vault. Strengthen the connection between the columns and the perimeter beam with diagonal braces where the columns are free standing—
COLUMN CONNECTION (227). . . .
1022 218 wall membrane*
. . . according to efficient structure (206) and final column distribution (213), the wall is a compressive load-bearing membrane, “stretched” between adjacent columns and continuous with them, the columns themselves placed at frequent intervals to act as stiffeners. The intervals vary from floor to floor, according to column height; and the wall thickness (membrane thickness) varies in a similar fashion. If the column stiffeners arc already in place according to box column (216), this pattern describes the way to stretch the membrane from column to column to form the walls.
* * *
In organic construction the walls must take their share of the loads. They must work continuously with the structure on all four of their sides; and act to resist shear and bending, and take loads in compression.
When walls are working like this, they arc essentially structural membranes: they are continuous in two dimensions; together with stiffeners and columns they resist loads in compression; and they create a continuous rigid connection between columns, beams, and floors, both above and below, to help resist shear and bending.
By contrast, curtain walls and walls which are essentially “infill,” do not act as membranes. They may function as walls in other respects—-they insulate, enclose, they define space—but they do not contribute to the overall structural solidity of the building. They let the frame do all the work; structurally they are wasted. | For the details of the argument that every part of the structure must cooperate to take loads, see efficient structure (206).]
A membrane, on the other hand, makes the wall an integral thing, working with the structure around it. How should we build such a wall membrane?
| CONSTRUCTION |
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| A version of an interior wall membrane which uses gypsum board as skin, and ultra-lightweight concretefor the fill. |
good materials (207) tells us that we should use hand cuttable, nailable, ecologically sound materials, which one can work with home tools, with the emphasis on earthen fill materials and sheet materials.
gradual stiffening (208) tells us that the process of building should be such that one can start with a flimsy structure and stiffen it during the course of construction, as materials are put in place, so that the process can be smooth and continuous.
218 wat.l membrane
An example of such a wall that we have built and tested uses gypboard for the inner skin, ship-lapped wooden boards for the outer skin and ultra-lightweight concrete for the fill. The wall is built by fixing nailing blocks to the sides of columns. We nail the skin to the nailing blocks, put chickenwire into the cavity to reinforce the concrete against shrinkage, and then pour the lightweight concrete into the cavity. The wall needs to be braced during pouring, and you can’t pour more than two or three feet at a time: the pressure gets too great. The last pour fills the perimeter beam and the top of the wall, and so makes them integral. The drawing opposite shows one way that we have made this particular kind of wall membrane.