Building Construction and Fire
This is the ninth in a series of articles on building construction and its effect on fire fighting operations and planning.
The term “ordinary construction” describes an infinite variety of buildings. The chief common characteristic is that the walls are of masonry. Among the principal masonry construction terms we should be familiar with are:
Course: a horizontal line of masonry.
Stretcher course: bricks laid lengthwise.
Header course: bricks laid endwise, also called bond course.
Wythe: vertical section of a wall, one masonry unit thick.
Concrete masonry unit: a precast structural block made of cement, water, and aggregates. It may be hollow or solid.
Terra cotta: tiles made of clay and fine sand and fired in a kiln. Terra cotta is both structural (clay tiles) and decorative, such as for facings. Structural terra cotta has been replaced to a large extent by concrete block.
Rubble masonry: masonry composed of random stones.
Ashlar masonry: stone cut in rectangular units.
Solid masonry units: a unit whose net cross-sectional area in every plane parallel to the bearing surface is 75 percent or more of its gross cross-sectional area, measured in the same plane.
Hollow masonry units: a unit whose net cross-sectional area in every plane parallel to the bearing surface is less than 75 percent of its gross cross-sectional area, measured in the same plane.
Solid masonry walls: walls built of masonry units (either solid or hollow units) laid contiguously, with the joints filled with mortar.
Hollow masonry walls: walls built with two wythes of masonry with an air space between; the wythes are tied together (bonded) with masonry.
Cavity walls: hollow walls in which the wythes are tied together with steel ties or masonry trusses.
Hollow and cavity walls are used to limit rain penetration. Though there are no observed cases to demonstrate, it is at least possible that carbon monoxide from a fire could accumulate in the hollow space and explode disastrously. In the New London, Texas, school disaster on March 18, 1937, that killed 294 persons, unodorized natural gas accumulated in the voids in a masonry wall.
Plastic insulation used
It is becoming an accepted practice to place sheet plastic, or foamed-inplace plastic, insulation in hollow walls. The plastics used are of varying degrees of ignitability. Burning plastic produces large quantities of smoke. If the source of smoke cannot be found, it would be wise to check for plastic insulation in the walls.
Veneer wall: a wythe of masonry attached to the masonry bearing wall but not carrying any load but its own weight.
Composite wall: a wall of two different masonry materials, such as brick and concrete block, designed to react as one under load.
In recent years, the composite wall, using concrete block to the maximum to save on brick and labor, has been developed. When such walls were first developed, the conventional system of bonding the wall together by inserting brick headers according to various design practices was used.
Masonry wire truss
The common practice was to provide a row of headers every seventh course. Uneven settlement often caused the header bricks to crack. Therefore, the masonry wire truss was developed. This wire truss is bedded into the mortar in specified courses. As a result, the header course is no longer necessary and the appearance of a masonry bearing wall may be no different from that of a veneer wall (all stretchers).
In some veneer walls, bats (half bricks) were inserted to give the appearance of a bonded wall. Thus, it is impossible to tell a bearing wall from a veneer wall by external appearance alone.
Masonry walls are sometimes braced by masonry columns incorporated into the walls and called piers, buttresses and columns. These may be built inside or outside the building. Where visible, they tell us where the wall is strongest, often where the concentrated loads are applied, and where not to attempt to breach the wall.
Cross wall: Any wall at right angles to the wall in question. Such a wall also provides support.
Flying buttress: a masonry pier at a distance from the wall and connected to it. Such buttresses resist the outward thrust of arches. They are used in Gothic architecture.
Masonry height limited
There is an inherent limit to the height of masonry buildings. It is the necessity for increasing the thickness of the wall in relation to the height of the building. The usual rule is that the solid masonry walls shall be 12 inches thick for the uppermost 35 feet of height and increase 4 inches in thickness for each additional 35 feet or less in height. Hollow masonry walls are more restricted in height. Solid walls reinforced by masonry cross walls or piers can be thinner.
In recent years, high-rise brick buildings with no wall thicker than 12 inches and medium-rise brick buildings with no wall thicker than 8 inches have been developed, supplanting the traditional practice of ever-increasing wall thickness according to the building height.
There are several definitions of ordinary construction. Any specific building description intended for legal purposes should, of course, describe the building in the terms used in the applicable code.
Together with the wood construction, ordinary construction usually fits the term “vernacular construction,” construction which develops out of hand-me-down methods with little or no engineering. The opposite term to vernacular construction would be “engineered construction.”
Codes and standards attempt to divide types of buildings into various classes, mutually exclusive. Unfortunately, many buildings with which the fire fighter must cope were built by people who used the material which seemed best suited to their purpose in a manner which seemed to do the job without any reference to the niceties of distinctions among formal classifications of construction types.
Masonry bearing walls
Ordinary construction is Main Street, U.S.A. The single unifying characteristic of ordinary construction is that the bearing walls are of some type of masonry, but even here we will make an exception and include buildings in which at least some of the bearing walls (usually only the front, or street, wall) are of cast iron. This is logical because the interiors are similar to masonry wall buildings, and there are only a few cast iron buildings left.
The simplest ordinary construction building has masonry bearing walls and wood joists as simple beams spanning from wall to wall. The joists are usually parallel to the front of the building. The roof may be similar to the floor in construction, or it may have a peak built with rafters or simple trusses.
The masonry bearing walls may be of brick, stone, concrete block, terra cotta tile, adobe, or cast-in-place concrete. The wall may be all of one material, different materials may be used in discrete areas, or different materials may be combined into composite-construction (expected to react together under load).
Wood beam limited
The simple wood beam floor is satisfactory for buildings up to a practical limit of about 25 feet in width. For a wider building or a building of irregular plan, some sort of interior column, girder and beam system must be provided. Every possible combination of building materials is used. Columns may be of wood, brick, stone, concrete block, steel or cast iron. Different materials may be used for columns in the same building. Interior bearing walls may take the place of columns. In light-floor-load buildings, interior ballon frame walls may provide intermediate support. Girders may be of wood or unprotected steel.
The connection systems by which the beams are attached to the girders and the girders are attached to the columns are of infinite variety, and it is in the weakness of connections that the principal collapse potential of the building during a fire is often found.
As in wood construction, void spaces are an inherent part of ordinary construction. Some fire protection measures, intended to prevent the extension of fire from the usable space to the void space (such as tin ceilings), prove to be barriers to the fire department’s efforts to reach the fire once the fire penetrates the void space. Modernization may make the building one big void space by eliminating windows.
Fire separation lacking
As a general rule, there is no effective fire separation within the building, either from floor to floor or within floors. Even where fire separations exist up through the regular floors of the building, they are often imperfect or nonexistent in attic spaces.
Buildings built in recent years may approach the pure definition of “noncombustible” construction. However, in many cases the difference from our paramount point of view, fire suppression, is minor, and thus such buildings are properly discussed here. A noncombustible void space can accumulate carbon monoxide gas as readily as a combustible void. A noncombustible void can contain combustible wiring and thermal insulation.
Metal roofs can provide self-sustaining fires because of the use of bituminous vapor seals, and a wall pushed out of line by an expanding, heated, steel truss, can be just as lethal as a wall pushed out of line by collapsing wood joists. In addition, the use of wooden eyebrows, cornices, canopies, colonial belfries, combustible interior wall and ceiling finish, and even wood veneer over masonry leaves us few truly noncombustible commercial or institutional buildings.
Similarly, the differentiation between ordinary construction and 19th century “fireproof” construction is not at all clear-cut, since building development is evolutionary. For instance, one early skyscraper consisted of seven stories of metal-framed construction, topped by four stories of masonry bearing wall construction.
Portions of an ordinary construction building may have been provided with some degree of fire resistance, either initially or as a result of later legal action. For instance, properly enclosed, fire-resistive stairways may have been installed in an old school, or a rated fire-resistive barrier may have been built around a special hazard, such as a boiler room. Rarely does this piecemeal provision of fire-resistant features alter the fundamental nature of the building.
The builder or alterations contractor may use fire-resistant components, or those similar in appearance, in some part of a building which by law need only be of ordinary construction. Be especially alert for this condition. It may lead to unwarranted assumptions as to fire resistance, which the building may not possess. For instance, the law may not require a finished ceiling in a store. The owner may choose to use a low flame spread, suspended ceiling for decorative effect in the sales area and omit it in the stockroom. Such a ceiling is similar in appearance to a listed fire-resistive roof and ceiling assembly, but there are vital differences.
Mill construction is a special type of masonry and timber framed building, developed to cope with the most pressing defects of ordinary construction. Many buildings were built with some features of mill construction but deficient in other important particulars, so they are classified as ordinary construction. In addition they possess the basic defect of all mill construction— the assembly of huge masses of fuel within the interior.
Copyright 1971 by Frank L. Brannigan