t. Walls.— Exterior walls in fire-resisting buildings may be of three types; (1) those which support a load in addition to their own weight, or "load-supporting," (2) those which support merely themselves or "self-supporting" and (3) those which are dependent for sup port upon a steel framework, sometimes called walls." Prior to the skeleton-con structed buildings exterior walls were built of masonry, without the support of metal columns and this is still feasible in buildings of mod erate height. In the earlier skeleton-constructed buildings self-supporting walls were employed, the loads being borne by metal columns within the walls. The modern fireproof building has entailed the curtain wall, which is hung upon the steel frame like a masonry or concrete en velope; in reality each story has its own four walls.
As regards fire-resistance the walls may be viewed from two standpoints, the extent of fire damage and cost of reconstruction. It has been conclusively demonstrated that the self-support ing masonry wall suffers less from fire damage than the curtain wall. In the latter the wall carrying beams are frequently so near the sur face as to permit expansion and bending, thus wrecking the wall. From the standpoint of re construction cost, however, the curtain wall is superior, assuming always that it is sufficient for the protection of the enclosed steel frame. Damage to a portion of a self-supporting wall often involves the removal of large unaffected areas, while the load-bearing function renders this type of wall liable to complete collapse. Parts of the curtain wall, however, may be easily removed and replaced.
The most satisfactory wall materials are brick, terra-cotta and concrete. Iron and stone are sometimes employed but the latter cannot be relied upon as a protection for the steel framework. Stone work should be absolutely free from any load-bearing function as it suffers severely under high temperatures. The use of brick as a fire-resisting material has already been discussed. Structural terra-cotta may be used for walls where the building contains no large amount of combustible material and has no severe exposure. Concrete walls strengthened by reinforcing iron rods are also very satisfac tory, being made either solid or double with an enclosed air space.
As stated, exterior walls frequently contain iron or steel columns. The protection of these is not a serious problem usually, due to the pro tection and additional strength derived from the wall itself. The problem of the protection
of interior columns is more difficult.
2. Column Protection.— The most important load-supporting members of a building are the interior columns and yet these stand isolated and exposed to fire on all sides, so that their protection becomes a primary consideration in the fire-resisting building. The requirements for an efficient column protection are (a) re sistance to fire and water, so that the column may be protected through a long fire; (b) non conductivity of heat so that the metal column will not expand, bend or buckle from heat; (c) tenacity of position so as not to crack and fall; (d) adequate thickness; (e) good construction. It was formerly considered advisable in column protection its leave an air space between the col umn and ts protecting coat to act as heat in sulation. The testimony of investigators of th( San Francisco fire, however, proves this method to be inferior to the solid covering. The types of column protection principally used are as follows: (a) Metal Lath and Plaster.— This consists in wrapping the column with wire netting to which plaster is applied. If wrapped too tightly the plaster does not take hold and falls quickly under fire. Furring strips are therefore used to permit the proper application of plaster. This method is improved by a double applica tion of lath and plaster, with a space between. All such protections, however, were shown at Baltimore and San Francisco to be insufficient to endure severe conditions and must he con sidered merely as suited to withstand moderate temperatures.
(b) Plaster Blocks.— These are entirely un reliable, due to disintegration by heat and water.
(c) Terra-cotta.— This method consists of surrounding the column with terra-cotta blocks, either solid or hollow, shaped to conform to the column. Any space intervening between the column and the protection should be filled with concrete or additional terra-cotta. It is difficult to estimate the efficiency of the terra-cotta pro tection from actual experience because of the poor quality of construction in the past, which has already made the method insufficient in practice, however good theoretically. In the San Francisco and Baltimore fires this material did not equal others, partly from this cause.