Building Construction and Fire

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.

Masonry trusses eliminate the need for header bricks and make it impossible to tell a solid from a veneer wall.

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.

Masonry piers are columns built into a wall to stiffen it and take concentrated loads, such as roof trusses.Cast iron fronts on two buildings. The one on the left is only 8 ½ feet wide.

Ordinary construction

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.

Differentiation difficult

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

Building Construction and Fire


Building Construction and Fire

This is the eighth in a series of articles on building construction and its effect on fire fighting operations and planning.

Buildings in which wood is a principal construction material can be classified as follows:

Post and frame, platform frame, balloon frame, plank and beam frame, brick and wood joisted (also called ordinary construction) and heavy timber (also called mill construction).

Just about every piece of wood in a wooden building is kindling, that is easily ignited, and concealed spaces in which fire can burn, shielded from hose streams, are inherent in wooden buildings, with some exceptions.

Post and frame construction has an identifiable frame or skeleton of structural members of large dimension. Smaller studs fill in between the larger members. This type is also known as braced frame. It is not often used for houses today, but is used for barns, churches and other large structures.

Most wooden buildings are balloon or platform frame construction. In both cases, the term “frame” is a misnomer since the building does not have a frame or skeleton, but rather is a bearing-wall building. Walls are built of the same size studs in rows.

Balloon construction

Balloon frame buildings present a serious fire problem because of the construction method. The studs are greeted the full height of the building from foundation to eaves. A ribbon board is nailed to the studs. The floor joists rest on the ribbon board and also are nailed to the studs. As a result, there is an interconnected series of air passages between the cellar, the side walls, the underfloor space and the attic. In a building with wood sheathing and wood lath, the stud channels are simply wooden chimneys. If we consult the “NFPA Fire Protection Handbook” or a current carpentry manual, we learn that it is recommended that all such areas be “firestopped” at the point of connection. Unfortunately, when most of the big multistory frame houses which we must deal with were built, firestopping was unknown. The word does not even appear in building manuals of the early 20th century.

Even when Firestopping is required, it may not be provided. One non-firestopped stud channel is quite sufficient to transmit fire initially from the cellar to the attic and then to the underfloor spaces.

Two types of firestopping

Firestopping can be considered to be of two types. Inherent firestopping comes about as an incidental part of a regular construction procedure, as is the case of stud channels in platform frame construction. It is reasonable to assume that inherent firestopping is present.

Legal firestopping serves no construction purpose and may even be a nuisance. But it is required by law. It is reasonable to assume that legal firestopping is not in place.

Suspect balloon frame construction in any old multistory frame building. Balloon frame is still recommended for walls with masonry veneer or stucco, due to less shrinkage, but the shortage of long studs makes balloon frame construction uncommon today.

Be particularly alert at basement fires. A ceiling may delay fire spread into joist channels for a time, so that on your arrival only the cellar fire is apparent. The fire suppression effort may drive the fire up through the walls and all of a sudden comes the alarming cry, “The attic is involved!” Cover all parts of the building with lines and examine walls for any sign of the passage of Fire. The practice in some fire departments of responding to residence fires with a light assignment is seriously deficient when the residence is in fact a “lumberyard” with literally hundreds of cubic feet of interconnected combustible void spaces.

Platform construction

In platform frame construction, a platform is built on the foundation. The stud walls for the first floor are erected. A platform is built to form the second floor. The second floor walls are erected on this platform. There is no continuous connection between the first and second floor stud channels. If the interior walls are noncombustible, such as wire lath and plaster or gypsum board, and if the sheathing is gypsum board (unfortunately not too common), fire spread through inner spaces is sharply restricted.

There is one important exception however. Many modern dwellings have a soffit to enclose the space above the kitchen cabinets. The soffit provides an unFirestopped space between the wall channels and the joist channels of the second floor (or attic in a onestory building). The tops of kitchen cabinets are made of thin wood or composition material, with minimal resistance to the passage of fire. A kitchen fire can suddenly involve the second floor or attic if it breaks through the top of a cabinet.

Split level houses are usually of platform frame. In addition to the possibility of Fire or smoke spread through the interior due to the open construction, some designs incorporate void spaces which may connect vertical and horizontal spaces.

Plank and beam construction is a modern type of construction for residences, as well as churches and other public buildings. Structural elements are of heavy timber, either sawn or laminated, and thus beams can be spaced several feet apart. Planks are used for flooring, and the exposed underside of the plank floor forms the ceiling of the floor below. To the extent that such construction eliminates the void spaces inherent in most wood structures, the building represents a superior fire risk. On the other hand, the finishes used on the wood may have very high flame spread characteristics and a fire may thus involve large surface areas in a short time. In one church fire, the actual fire in the auditorium was suppressed in about 10 minutes, but every square inch of surface, down to the floor line, was charred to a depth of ¼ inch or more because of the wood finish. In most cases, plank and beam construction is used for only part of the structure and the balance is of ordinary wood frame construction.

Balloon construction, showing ends of floor joists at top resting on ribbon board.

As a building material, wood has many evident advantages, but it is combustible, yielding about 8,000 Btu per pound. It varies greatly in strength and modulus of elasticity from species to species and within a group of samples. It is subject to destructive attack from moisture, insects and other vermin, and time. Wood which has been thus attacked is weakened and may fail earlier than would be expected in a fire.

Fire-retardant coatings, such as those which intumesce or swell up when exposed to fire, can reduce the fire hazard (flame spread, fuel contributed, smoke generated) characteristics of wood if applied as specified. Application to an exposed surface only may be of little significance if a concealed, unprotected surface is attacked by the fire.

Unwarranted claims

The difficulty of igniting large timbers, due to their high mass-tosurface ratio, leads to unwarranted claims for resistance to fire on behalf of heavy wooden structural members. The term “slow burning” that is often applied to heavy timber is deserving of the same fate as “inflammable” because it is misleading. Wood may be slow to ignite under certain circumstances, but it is not “slow burning.” Heavy timbers may take an appreciable time to be consumed, but this is no measure of their structural stability. The key to the stability of a structure with heavy timbers may well rest in the fire resistance of the connectors, or in other words, how the elements are joined.

In true mill constructed buildings, special provisions were made to provide adequate connections, but many buildings incorporating heavy timber members have weak connections from the fire point of view. It does not matter that a heavy timber truss is of massive wood. The truss will fail when the steel straps fail. In old wood-girdered buildings with cast iron columns, examine closely how the load of the girder is transferred to the column. In some cases, a column is set on the one below. The end of a huge timber girder is carved out in a semicircle to accommodate half the column. The girder on the other side is likewise cut away. The two girder ends rest on a plate cast into the column. The design can stand normal loads for a century, but let a fire consume but a small portion of the “massive timber” where it is thin at the column and the girder will collapse.

Collapse Danger of Roofs With Light Wood Trusses

Lightweight wood trusses, assembled of 2X4 lumber and using metal gusset plates—or even glue—to connect the members to one another, are rapidly replacing conventional wood rafters. Roofs supported by such trusses are extremely dangerous, as they do not have even the little fire resistance possessed by 2X10 or 2X12 rafters.

The following rule is offered: If the fire situation requires that a lightweight wood truss roof be ventilated, it should be done by men who are not standing on the roof.

No wooden structural member has ever been granted any fire-resistance rating. In many codes, there is a requirement that where another material (such as steel, for instance) is substituted for heavy timber, it must be provided with 1-hour fire resistance. This does not mean that heavy timber is the equivalent of 1-hour fire-resistant construction.

Beware of alterations

On the other hand, we should be especially wary of alterations which have been made in a building built of heavy timber. In a church, a 6X8 timber column supporting 18 tons was removed and a steel tube truss was substituted. The article describing the alteration was silent as to any fire resistance provided for the truss. If none was provided, the fire department may be taken unawares, possibly fatally.

“Heavy timber” beams and columns in restaurants, hotels and other public buildings should be carefully examined. Many are false. In some cases, an unprotected steel member is concealed in a wooden box. In other cases, the “beam” is completely false. It is not structurally significant, but it is a menace to safety. Some “beams” are made of molded polyurethane, the solid which burns like gasoline, and these are finding their way into buildings which will provide this generation’s Cocoanut Grove.

Another popular decorating idea is the suspended wooden ceiling. The resistance of the suspension system to fire is questionable. It is up where the heat is high. Its presence may go undetected in smoke and many which I have seen are heavy enough to cause serious injury.

Wood ornamentation

The whole country is “raising its eyebrows.” All across the country, useless wooden cornices of lumber, plywood and shingles are being installed on buildings, old and new. If it were not so sad, it would be funny to watch the establishment provide the fuel for the destructionist.

Often such cornices run for hundreds of feet without a firestop. Lightly constructed, they are a collapse hazard. Depending upon construction, the cornice may be independent of the building, sealed off from the interior by a masonry wall. On the other hand, it may provide rapid access to the void space above the ceiling or a path of communication of fire or smoke from one store to another.

Be aware of loopholes in your local building code. Even where a fire-rated exterior wall is required, there may be permission for combustible ornamentation. If there is not an area limit, your city may have the first fully shingled high-rise apartment house.

Plywood is composed of an odd number of layers of wood with the grain of adjacent layers running at right angles. Depending upon the glue used, and some imported plywood veneers had very poor glues, the plywood delaminates, that is, separates into layers, increasing the surface area. Even without delamination, the thinness of the commonly used wood panels makes them burn readily. If installed on furring strips, the air space behind increases the hazard. If an adhesive is used to secure the panels to a wall, it is often highly flammable.

Even the most naive are aware that forests and lumberyards make serious fires. Why should we expect any less serious problem when we move a lumberyard into a building?

Copyright 1971 by Frank L. Brannigan