Exterior walls, in general, are of five kinds:

1. Masonry walls of brick or stone, supporting their own weight and the adjacent floor and roof loads.

2. Masonry walls supporting their own weight, but no floor or roof loads.

3. Masonry walls not self-supporting.

4. Walls of iron, copper or other metai.

5. Walls of concrete.

Load=bearing Walls. Walls of the first class will be readily understood as regards their general characteristics, and will be treated more in detail under the heading "Building Laws and Specifications."

Self-supporting Walls. Walls of the second class are generally of brick or stone, and have contained in them steel elements carrying the floor and roof loads. These elements consist of vertical members spaced at intervals in the wall and called the wall columns, and, between them, horizontal members, generally at the floor levels and also over all openings. These members at the floor levels are called the wall girders; and those over the openings, the lintels. The wall girders carry the floor and roof loads to the columns, and so to the foundations. The lintels, in this class of wall, rest on the masonry and sometimes are omitted entirely, depending on the necessity of supporting the stone lintels, on the impracticability of turning brick arches, or on the necessity of relieving such arches of part of the load.

Fig. 1 shows a construction of this type. The particular form of section of the wall girders and of the lintels varies, of course, with the conditions; but the essential feature to be noted is that all loads are kept off the walls, except the weight of the masonry itself.

Curtain Walls. Walls of the third class differ from the preceding in that they themselves must be supported on the steel framework. The walls themselves may consist of brick, or of brick with stone or terra cotta trimmings or facings. The steel elements are the wall columns and wall girders, as before, and the horizontal members over the openings. These latter, instead of being called lintels, however, are called spandrel beams, since, instead of simply spanning the opening, they take all the load of the wall between the wall girders and the head of the opening, and carry this load to the columns. The wall girders, too, besides the floor load, generally carry the load of the wall for the story above. In some cases this wall i carried through several stories to heavy girders below, but such construction is not common.

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Fig 1

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In buildings where this class of wall is used, the framework, in addition to carrying the loads, must furnish a portion of the lateral stiffness to resist wind and other strains. This feature will be more particularly discussed under "High-Building: Construction."

Figs. 2 and 3 show types of construction in this class.

Metal Walls. Walls of the fourth class are not commonly met with in what is termed fireproof construction, but are more generally used in buildings having their floors and roofs framed in whole or in part with wood. When they do occur, however, they come, Structurally, into the previous class, as far as the elements of the framework necessary for the support of the floor and roof loads and their own weight are concerned.

The chief difference is in adapting the spandrel beams to the support of the particular covering used. Fig. 4 illustrates such construction. As before, the section of the wall girders varies in each case with the conditions, and the spandrel section varies even more.

In both of the two classes just described (curtain walls and

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Fig. 3 metal walls), no form of construction can be called standard.

The only principle which the student should observe and follow is that the wall girders and spandrel beams must be so arranged and designed as to carry properly all floor and roof loads, to support and carry properiy each part of the wall itself, and, further, to provide necessary stiffness to the building.

Concrete Walls. Walls of the fifth class are rarely met with at present, except in mills or manufacturing plants, and discussion of their features is accordingly reserved for the discussion on "Mill Buildings."