Fire Studies: Heavy Timber Buildings

Engine 29 (E-29) arrived to find medium smoke showing on the first floor of a six-story heavy timber building. (The 750-foot-long by 175-foot-deep building covered an entire city block.) The lieutenant ordered all of the companies that responded on the initial alarm to lead with 2½-inch hoselines. Battalion 3 (BC-3) arrived and completed the initial report and immediately struck the second alarm.

E-29 reported to BC-3 that it was on the interior and found a working fire on the first floor. It also reported that Engine 2 (E-2) was stretching its 2½-inch hoseline as backup and that, once water flowed in E-2’s line, E-29 would try to advance and would check that fire doors were closed.

The deputy chief (Deputy 1) arrived. After doing a 360-degree walkaround, he went through a transfer of command. BC-3’s report included its assessment that if the fire doors were closed, confinement to the first floor and the immediate area was possible. If it were the opposite case—the fire doors were open—containment was doubtful.

Deputy 1 shared with BC-3 that the conditions that he observed on his 360-degree walkaround left containment doubtful.

Deputy 1 assumed command. He assigned BC-3 to the role of Division 1 supervisor and assigned two additional engine companies to Division 1.

Battalion 4 (BC-4) arrived and was assigned to Division 2 to check conditions and for the possible upward spread of fire.

Conditions were deteriorating rapidly, and the smoke condition had increased. Window ventilation openings on the first floor of the building were starting to show heavy fire conditions.

Division 1 reported that the fire doors were blocked open, that the fire spread past the immediate fire area and that companies that were operating on Division 1 were being pulled out of the building.

Division 2 reported that fire conditions were becoming untenable on the second floor of the building and recommended withdrawal of all of the companies.

The incident commander (IC) struck the third and fourth alarms. Fire now showed on the top floor. BC-4 reported that it appeared that the fire doors were secured permanently in the open position, which allowed a rapid spread of fire through the second, third, fourth, fifth and sixth floors.

After pulling firefighters out of the building, the IC went to Plan B. He realized that the fast-moving fire, if not contained, would envelop the entire block and more than likely create firestorm conditions. His intentions were to attempt to contain the fire in the current section of the building.

The Alpha and Bravo sides of the fire building were very wide streets and posed minimal exposure problems. The Charlie and Delta sides were problematic, with narrow streets and numerous exposed structures. He had a two-part plan: attack the fire and protect the interior of the building and the exterior exposures.

A defensive attack that included master streams was set up in an attempt to slow the spread of fire and protect exposures, with emphasis on the Charlie side. The length and depth of the building would affect the actual amount of fire that could be knocked down.

(The stock that was in the building and the 20-foot-high ceilings allowed the fire to burn practically unabated.)

The benefit that was derived from the master streams on the Charlie side was that the penetration of fire was minimized through the windows, which provided some protection of the exposed structures.

Battalion 8, which responded on the second alarm, was assigned to take six companies to ensure that fire doors were closed on all floors at the first firewall’s location, in an effort to contain the fire to the section of the building that was involved. These companies were ordered to close the fire doors and to contain and extinguish any spread of fire past the fire doors.

It took eight alarms, but Plan B worked well. Once closed, the fire doors functioned as designed and contained the fire to the original section involved. The proactive steps to contain the fire to the one section saved 80 percent of the building.

Construction

Heavy timber buildings, which include mill buildings, are constructed with wooden timbers of large-dimensional lumber. Through proper design, heavy timber construction provides an excellent degree of fire resistance by requiring a minimum dimension for all load-carrying wooden members. This large-dimension lumber construction affords firefighters time to make an aggressive attack on a fire.

Wood burning produces a charcoal on the wood’s surface, which provides a protective coating that insulates the unburned wood and isolates it from the flame, which slows fire’s consumption of the wood. Hence, thick wood provides more structural strength than thinner wood.

Exterior walls of heavy timber buildings are of masonry construction and can be as tall as eight stories. Larger structures contain firewalls. A firewall in heavy timber construction is customarily a bearing wall of masonry materials. Openings in a firewall must be protected with the same fire rating as the wall that’s pierced. Door openings must contain self-closing fire doors.

If firewalls are located in a structure that has a combustible roof, they must extend above the roof, to prevent the spread of a roof fire. The roof can be supported by columns or fabricated of heavy timber truss. The common truss configurations are:

• flat and constructed of parallel chords
• triangular-shaped
• bowstring-shaped

Floors are built to carry heavy loads. This is accomplished through the installation of columns to support large floor timbers or a parallel chord truss that contains heavy timber components. The truss permits large open spaces with fewer columns.

Columns must be a minimum of eight inches thick. Girders that span the distance between the columns must be a minimum of six inches thick. The thickness of the floor of heavy timber buildings must be a minimum of three inches. This often is installed using one-inch tongue-and-groove planks that are cross-laid.

Interior walls and ceilings usually aren’t finished. As a result, this leaves exposed masonry walls and creates few concealed spaces to encounter. This type of construction utilizes the underside of the exposed wood floors as the ceiling on the floor below.

Firefighting in mill buildings

Fire in heavy timber buildings can present unique challenges.

The fire service doesn’t encounter the number of large-building fires that it did in the past. Although the reduction in these fires is a positive for the community, it makes it difficult for firefighters to keep sharp the necessary skills that are required to fight these types of fires.

Mill buildings present a tremendous fire problem because of the contents, the methods of stock storage and the large amount of exposed wood that are associated with them. Wooden floors might have absorbed oil or other contaminants over the years, which could add to the fuel load. Once ignited, these buildings generate massive amounts of heat. Fires that occur in mill buildings are difficult to control and can threaten exposed buildings severely.

Since there are few concealed spaces in unmodified mill buildings, firefighters can mount an offensive attack and control a fire in the early stages of development. The lack of concealed spaces permits hose streams to reach the fire. However, once a fire gets past the incipient stage, it can spread rapidly and be difficult to control.

Fire attack

Once the location of the fire in a heavy timber building is known, a hoseline should be stretched to attack the fire. This attack shouldn’t drive the fire into other areas of the building. When possible, the fire should be fought from the unburned side.

Because these buildings can be of tremendous size, a decision on which direction to fight the fire must consider access, firewall locations, and minimizing both contents and building loss.

A fire that’s above the first floor initially is fought from the stairways. The IC should designate at which stairway to fight the fire. Should multiple stairways be used, close coordination is a must, to prevent opposing hoselines. If the fire threatens the floor above, a hoseline should be stretched immediately to that location.

Hoselines must be stretched to back up the initial line as well as the hoseline that’s stretched to the floor above. Backup lines must be either the same size or larger than the initial hoseline. Strong consideration must be given to the utilization of 2½-inch hoselines in this circumstance. Although smaller hoselines can be deployed much more quickly than can 2½-inch hoselines, they don’t have the knockdown and cooling capability that the latter have.

These buildings often have floor openings that accommodate large tanks (such as that which are found in large breweries, dye works, etc.) or conveyor belts that service multiple floors. Floor openings work against containment efforts. Hoselines must be stretched to the floor above the floor opening to check for fire extension.

The large area of heavy timber buildings along with their high ceilings permit an accumulation of heat at the ceiling level that might not be recognized by firefighters who are operating an offensive attack. Remember, as the fire expands, it can flash over a large area and threaten firefighters who are operating in the building.

Compartmentation

Compartmentation, which is accomplished through the use of firewalls and properly functioning fire doors, assists in confinement efforts. This also can be accomplished in a large building where a defensive attack is underway by the fire department.

The IC should decide where a fire can be stopped. This consideration depends heavily on resources and water supply.

The defensive attack occurs on one section of the fire building while other units are situated on the interior to protect other areas with an offensive attack. Companies that are deployed on the building’s interior should be located at the appropriate firewall location and ensure that the fire doors are closed and that hoselines are in place to handle any minor extension of fire that occurs. In multistoried heavy timber buildings, this effort takes coordination of and communication between companies from the ground floor up to and including the roof.

Division or group supervisors must ensure that units that are on all levels of the building operate at the same firewall location on every level. The goal of these companies isn’t to attack the fire in the areas in which a defensive attack is occurring but to prevent fire extension past those areas.

The sturdiness of construction of heavy timber buildings allows the fire department flexibility. A well-involved fire in these buildings can be attacked initially with exterior streams. This “blitz attack” can knock down large amounts of fire while interior hoselines are stretched. After handlines are in place and the fire is knocked down, the exterior lines can be shut down, and an offensive attack can be initiated.

The success of any offensive attack depends greatly on the implementation of proper ventilation. Creating openings in advance of the hoselines is a must. Without adequate, proper ventilation, the hoselines will be unable to push into the fire area, and the attack that’s waged will be useless.

Pluses and minuses

The sturdiness of construction of heavy timber buildings provides an excellent degree of fire resistance and affords firefighters the time that’s required to make an aggressive attack on the fire.

Naturally, the best method of protection for heavy timber buildings is the installation and maintenance of a sprinkler system. Yet even with protective systems, high piled stock and poor housekeeping can degrade the sprinkler system’s effectiveness.

Safe fireground operations can be enhanced by the utilization of a preplan, of on-site inspections and of familiarization tours in the area.