WATCH THOSE WALLS!
—photo by Captain F. c. Barth, courtesy Milwaukee F. D.
Part 2: Signs of Deterioration
OCCASIONALLY walls give warning signs of deterioration or indicate the possibility that a collapse is near. These may be visual, audible, or both. Some of the visual signs include smoke and/or steam pushing out of the mortar joints under pressure; water flowing through mortar joints; cracks appearing or widening in a wall. When the movement of a wall or portion of a wall takes place, it is time for fire fighters to evacuate the immediate area at once.
Another indication of structural change in a wall occurs when the water of a fire stream turns to steam as the fire stream hits the wall. The walls are heated from the fire on the inside and cooled on the outside by the fire streams. This sudden cooling is indicated by the steam forming. Walls should be carefully watched if this sign is observed.
Another common sign, which indicates structural change is taking place, is bulging. This, of course, is usually caused by the expansion of the contents of the structure. Contents such as paper, corrugated cardboard, rags, felt, sawdust, milled feeds, etc., are the type of materials likely to cause trouble. Another sign is the sudden change in the color of smoke. This may indicate that the fire has reached a different type of combustibles, or that a back-draft condition has developed. The usual change of color, in the case of back-draft conditions occurring, is from the “normal” smoke color to a yellow or grayish yellow.
Deterioration of walls may also be recognized by audible signs. The sounds of dropping bits of mortar upon a fire fighter’s helmet or around him is an example. This might also be considered a visual sign if it is possible to see the mortar dropping. Other audible indications are the creaking of floors or walls.
Nonbearing walls may be panel walls, curtain walls, veneers or any other type of wall found in skeleton construction and supporting nothing other than its own weight. They are usually found to be thinner than similar (in height and area) bearing walls. Panel walls in buildings of skeleton construction are usually 8 to 12 inches thick for any of the common masonry materials. Other materials, such as glass, steel, aluminum, plastic or tile, may be used for the purpose of providing the curtain. In this case the walls may be much thinner.
The absence of a header or “tie-in” course, together with the absence of stars and plates which indicate the presence of tie rods and the lack of arches or heavy lintels over windows and doors are all indicative of nonbearing wall construction. However, modern remodeling and construction practices will many times conceal the stars, plates, arches and lintels behind a veneer or facing.
It must be remembered that in nonbearing wall construction all floor joists, roof rafters and/or trusses are supported by, and the loads are transmitted to, the foundations through a thoroughly connected framework. This framework usually consists of a series of girders and columns of steel, wood, or concrete and is relatively independent of the covering (walls). The enclosing walls, partitions, and floors are supported at each floor level by the structural framework.
Direction of collapse
Walls may fall out or they may fall in. Walls which fall in may sometimes kick out at that point w’here they break off, and as a result slide or fall out into the street.
Another type of wall collapse is called a “curtain fall,” that is, where the walls drop almost straight down. These occur particularly where there are large window areas, such as the front of a building. Bricks, debris and other materials fall almost straight down and may cover the sidewalk.
Walls usually fall out because heat expands them on the inside, and fire streams cool on the outside. This cooling of the wall, in turn, causes further straining and bulging, greatly adding to the danger of collapse. The failure of trusses or similar members supporting the roof may also push a wall out.
Panel or curtain walls have a tendency to bow in toward the fire or heat rather than out. Their relatively strong and rigid supports restrain them at the top, bottom and sides. The expansion caused by the heat of the fire is therefore directed inwards.
Dense smoke, steam, or water escaping under pressure through the mortar joints of a solid brick wall may indicate that trouble is not far off. When the mortar joints have been destroyed sufficiently to permit the passage of smoke, steam or water, the wall has become seriously weakened. The buildup of sufficient pressure to force the smoke out of the mortar joints also indicates the presence of conditions suitable for a back draft or smoke explosion.
Continued on next page
Wide World photo
Photo by Evanston Photographic Service courtesy Evanston, Ill., F. D.
—photo by Evanston Photographic Service courtesy Evanston, Ill., F. D.
A brick-faced hollow tile wall, or a hollow tile wall, will sometimes fail when water is thrown on the hot face of the wall, causing the bricks or tiles to crack off. Under the right conditions, the tile may seem to literally explode when cold water hits the hot wall. This is also a characteristic peculiar to almost all nonporous building materials. For additional informations refer to Chapter 22, The Fire Chief’s Handbook, entitled “Effects of Fire on Building Materials.”
Bearing walls are subject to collapse more frequently than nonbearing walls. When the additional load they are expected to carry is taken into consideration, the reason for this is quite apparent.
Many times bearing walls are the side walls of a structure and are supported by, or adjacent to, the walls of an adjoining structure. These walls of the adjoining structure provide additional support to the bearing walls of the building involved with fire. It is possible under this set of conditions for the end walls to be affected and even collapse. This particular collapse is the result of the thrust being transmitted along the bearing wall to the front or rear wall, or both.
Experience indicates that many breaks or cleavages occur between windows or other openings in a wall. This appears to be one of the weaker areas in a bearing wall. In fact, many buildings presently in use have the visible beginnings of failure.
In other cases, where a particularly strong belt course or sill course can be found, the break is likely to be along this structural member. As a masonry wall is built, its various elements and members are bonded together to give it strength as a single unit. When a belt course or a sill course is introduced into a wall, the homogeneity is destroyed. The bonding or materials to be bonded together change, and should forces be exerted which will ultimately destroy the wall, the area along a belt course or sill course is where the cleavage may be expected.
Hollow block walls
In addition to the foregoing characteristics, the reaction of the intense heat of a fire can dry out blocks until they begin to crumble. This tendency, of course, has an important effect upon the ability of the wall to carry any weight during a severe fire. When hollow-block walls collapse, they will usually lie out flat. They often pave an alley or the ground when they fall.
Veneered fronts with trussed roofs
Veneered fronts with trussed-type roofs create special structural problems for the fire fighter. These brickveneered fronts are usually held up by their weight alone or laid upon or against an unprotected steel beam. Although the first truss may be within inches of the veneered front, the wall contains few, if any, bonds to the roof or truss. As these veneered fronts generally extend several feet above the walls of the structure, there is no tie-in to the walls. Should the roof or front part of the building become seriously involved with fire, these veneered fronts are very likely to collapse.
The factors which may cause walls to collapse during a fire are numerous. It is not easy to predict accurately the exact point where a failure will first occur, even if many of the factors are known and recognized. Nor is it possible to predict the precise moment when the first movement will take place. However, careful observation and the recognition of changes occurring to a given structure during fire fighting operations, will greatly aid in preventing injury and loss of life should the building collapse.