225 Frames as Thickened Edges**

. . . assume that columns and beams are in and that you have marked the exact positions of the doors and windows with string or pencil marks - Natural Doors and Windows (221). You are ready to build the frames. Remember that a well made frame needs to be continuous with the surrounding wall, so that it helps the building structurally - Efficient Structure (206), Gradual Stiffening (208).

Any homogeneous membrane which has holes in it will tend to rupture at the holes, unless the edges of the holes are reinforced by thickening.

The most familiar example of this principle at work is in the human face itself. Both eyes and mouth are surrounded by extra bone and flesh. It is this thickening, around the eyes and mouth, which gives them their character and helps to make them such important parts of human physiognomy..

A building also has its eyes and mouth: the windows and the doors. And following the principle which we observe in nature, almost every building has its windows and doors elaborated, made more special, by just the kind of thickening we see in eyes and mouths.

The fact that openings in naturally occurring membranes are invariably thickened can be easily explained by considering how the lines of force in the membrane must flow around the hole.

The density of the lines represent increasing stress concentrations.

The increasing density of lines of force around the perimeter of the hole requires that additional material be generated there to prevent tearing.

Consider a soap film. When you prick the film, the tension pulls the film apart, and it disintegrates. But if you insert a ring of string into the film, the hole will hold, because the tensile forces which accumulate around the opening can be held by the thicker ring. This is in tension. The same is true for buckling and compression. When a thin plate is functioning in compression and a hole is made in it, the hole needs stiffening. It is important to recognize that this stiffening is not only supporting the opening itself against collapse, but it is taking care of the stresses in the membrane which would normally be distributed in that part of the membrane which is removed. Familiar examples of such stiffening in plates are the lips of steel around the portholes in a ship or in a locomotive cab.

 

A door frame as a thickening.

The same is true for doors and windows in a building. Where the walls are made of wood planks and lightweight concrete fill - ess Wall Membrames (218) - the thickened frames can be made from the same wood planks, placed to form a bulge, and then filled to be continuous with the wall. If other types of skin are used in the wall membranes, there will be other kinds of thickening: edges formed with chicken wire, burlap, and resin, filled with concrete; edges formed with chicken wire filled with rubble, and then mortar, plaster; edges formed with brick, filled, then plastered.

More general examples of frames as thickened edges exist all over the world. They include the thickening of the mud around the windows of a mud hut, the use of stone edges to the opening in a brick wall because the stone is stronger, the use of double studs around an opening in stud construction, the extra stone around the windows in a Gothic church, the extra weaving round the hole in any basket hut.

Therefore:

Do not consider door and window frames as separate rigid structures which are inserted into holes in walls. Think of them instead as thickenings of the very fabric of the wall itself, made to protect the wall against the concentrations of stress which develop around openings. In line with this conception, build the frames as thickenings of the wall material, continuous with the wall itself, made of the same materials, and poured, or built up, in a manner which is continuous with the structure of the wall.

In windows, splay the thickening, to create Deep Reveals (223); the form of doors and windows which will fill the frame, is given by the later patterns - Windows Which Open Wide (236), Solid Doors With Glass(237), Small Panes (239). . . .

A Pattern Language is published by Oxford University Press, Copyright Christopher Alexander, 1977.