FORT WORTH COLLAPSE: OUR CONTINUING LESSONS
On the evening of June 28, 1992, the Ft. Worth (TX) Fire Department responded to what it thought would be a routine apartment fire on the city’s east side. When the evening was over, three firefighters narrowly had escaped serious injury when the living room of apartment 2133 collapsed into the second and then the first floors and erupted into a ball of flame. The collapse occurred as the first-due company was beginning to stretch a handline to the second floor to start searching for the seat of the fire.
When investigators began sifting through the damaged apartments, they discovered the building was constructed with parallel chord wood truss floor/ceiling assemblies. An investigation was initiated to determine the exact sequence of events preceding the rapid collapse, centering on the possibility of building defects or code violations that may have contributed to the failure. The main floor/ceiling trusses ran east to west through the building. The trusses had a depth slightly in excess of 14 inches, which meant the maximum possible span without support would be limited to approximately 20 feet, based on analysis of the size of the truss by several truss manufacturers who visited the site. However, the span of each apartment was 34 ⅝ feet, meaning that two trusses were needed to span the distance. The trusses were supported at the apartment perimeter walls and a load-bearing wall between living areas and bedrooms.
The Havenwood Apartments, constructed in 1984-1985 and located at 6501 Boca Raton Blvd., is one of many apartment complexes situated in the Woodhaven area of East Ft. Worth, Texas, built in the prosperous 1980s.
The fire building, 6513 Melwood St., was a typical three-story woodframe/brick veneer garden apartment measuring 135 by 35 feet with a peaked wood truss roof and lightweight truss floor/ceiling assemblies. It contained 12 two-bedroom units (four apartments per floor). Each apartment had front and rear doors and front and rear balconies. The balconies were supported by a parallel chord lightweight wood truss assembly running perpendicular to the main floor/ceiling trusses. Each group of six apartments was served by wooden stairs at both the front and rear. The floor consisted of ¾ of an inch of lightweight concrete on a ’/flinch plywood decking.
There are two factors concerning these trusses that differ from usual construction practices today. The first deals with the material used for the webbing of the truss. Normal construction methods today call for the use of wood 2x4s for all of the trusses’ webbing material, including diagonal pieces. These in turn are fastened together with the top and bottom chord of the truss using gusset plates prior to leaving the manufacturer. While the truss arrangement for these apartments had wood 2x4s as the vertical pieces of the webbing, the diagonal components featured a diagonal metal bar with a gusset plate already attached to each end. These were fastened to the top and bottom chords of the truss and the vertical 2x4s using the gusset plates of each diagonal piece.
Second, the trusses were offset at the load-bearing wall; they did not form a single line from one end of the apartment to the other. Today’s practices would call for the trusses to butt directly against each other on the load-bearing wall. The advantage of this method is that a 2×4 header can be placed across the end of each truss so that plywood flooring laid for the floor above has a smooth surface at each end with which to secure the floor decking.
Neither of these differences in construction compromises the truss construction from an engineering standpoint. Both are specified here to enhance the understanding of truss construction and the differences that may be encountered throughout the city.
FIRE PROTECTION FEATURES
Each apartment unit was separated horizontally from adjacent units by a fire rated assembly of ⅝-inch gypsum board on both sides of the 2×4 framing. The truss floor/ceiling assembly appeared to have a minimum onehour fire rating. A gypsum board draft stop was provided at the midpoint in the attic and was in good physical condition with no major penetrations. Some gypsum board was provided in the truss space to act as a draft stop. This was generally done in line with the walls separating apartments.
The apartment building was not equipped with an automatic sprinkler system. The building did feature a manual fire alarm system with pull stations located at the center of each stair on each floor level (front and back). Audible horns also were provided. Each building in the complex had its own stand-alone system providing a local alarm to that particular building. Fire alarm control panels were mounted in the exterior brick veneer at the front corner of each building.
On the evening of the fire, the city of Ft. Worth experienced a violent thunderstorm with heavy rain, brief hail, and frequent lightning strikes. The fire department became overwhelmed with weather-related emergency incidents. During this frantic activity, a resident on Melwood Street placed a call via one of the nonemergency phone lines to the Ft. Worth Fire Department alarm office at 8:50 p.m., stating he had a faint smoke odor in his apartment. Before the fire alarm operator could question the caller and pinpoint the address, the phone line went dead. The caller walked out on his front balcony and saw people pointing at the chimney area. Seeing the heavy smoke coming from that area, the resident quickly evacuated with the rest of his family. Since the call had originated on one of the nonemergency lines, there was no way for the operator to trace it.
At 8:52 p.m., the first of several 911 calls was received reporting heavy smoke coming out of the chimney of 6513 Melwood, even though no fireplace was being used. About the same time, the resident of apartment 2133 noticed smoke issuing from his fireplace. Very soon thereafter he noticed that the living room floor felt “spongy,” at which point he quickly evacuated the apartment.
A security guard protecting the property also noticed the fire situation about this time and began pulling the manual fire alarms in the buildings to alert the occupants. When he pulled the fire alarm for the fire building, it did not function.
Due to the weather-related incidents, several of the first-due fire companies were out of service on other calls when the Melwood Street fire was reported. To complicate matters, another apartment fire on the city’s extreme east side was being dispatched simultaneously with the Melwood fire. Thus, four of the five first-due companies on Melwood were out of service when the dispatch was made.
With such a large volume of calls being received by the four alarm operators on duty (prior to budget cuts there were five operators on duty each shift), the Melwood call was not dispatched until 8:58 p.m., almost six minutes after the first 911 call had been received. The initial dispatch consisted of a quint, two engines, a truck, and a battalion chief.
Battalion 4 arrived on the scene at 9:02 p.m. and observed heavy smoke issuing from the chimney at 6513 Melwood. The chief transmitted a working fire to the alarm office, which dispatched the air/light truck and the on-duty investigator at 9:04 p.m.
The chief ascended the front stairs to assess the situation. He observed smoke banking down from the ceiling in apartment 2133, although no fire was visible. The chief then descended to apartment 2123 directly below and encountered the same conditions. Despite the smoke obscuration at the second-floor ceiling level, the chief thought he could see the ceiling starting to sag in the middle. Although there was no visible fire on either floor, a crackling noise was heard emitting from the floor/ceiling assembly between the second and third floors near the chimney area. As the chief was returning to ground level to meet incoming units, he observed fire starting to eat its way through the gypsum board that enclosed the chimney at the point where the board intersected with the balcony above.
Quint 24, with its three-member crew, was the first to arrive (normal staffing for the quint is four personnel; budget constraints have made three personnel the usual complement, however). A team of three—battalion chief, captain, and firefighter—advanced a preconnect line to the second floor to pull the ceiling and locate the seat of the fire. Quint 24’s engineer recruited several civilians to help hand-lay a supply line to a hydrant west of the apparatus.
As the team arrived at the foot of the stairs, a loud crash of a collapse was heard from above. The windows on the second and third floors suddenly blew out and heavy fire erupted from both floors. After regrouping, the captain and firefighter used the stream to knock dow n what fire they could from their exterior position, then proceeded to the second and then the third floor landings of the exterior, unenclosed stairs, from where they quickly knocked down the majority of the remaining fire. The time of the collapse was 9:07 p.m.. 17 minutes after the first untraceable report of fire was made.
As soon as the collapse occurred, Battalion 4 radioed for a second alarm and a special-call battalion chief. The remaining units on the first alarm were directed to check for extension in all the void spaces. Firefighters opening walls on the west side of the bathrooms of apartment 2133 found fire spreading in the wall chase toward the attic. Heavy fire was found when the wood truss balcony of the third floor was opened near the chimney. Fire also was found in the ceiling of apartment 2124 near the chimney and along the southern border with apartment 2123.
Despite the heavy fire conditions that developed immediately following the collapse, the fire was contained quickly. Although six handlines were used on the fireground, Quint 24’s initial handline contained the fire. The last fire department unit left the fireground at 1:46 a m. on June 29th.
Based on the postfire investigation, a fire of electrical origin is believed to have ignited the combustible surface within the parallel wood chord trusses between the second and third floors. The space under apartment 2133’s living room is thought to be the specific point of origin. From this location, the fire developed slowly due to limited oxygen supply.
Because of the open trusses under apartment 2133, the fire spread throughout all areas underneath the apartment. There were two locations where the fire could spread vertically. One location was the bathroom chase at the far west end of apartment 2133. The other was the chimney chase at the apartment’s southeast corner.
Since the chimney chase was not separated from the truss space, the fire was able to enter the chase and attempt vertical travel, feeding on the plywood and wood 2x4s used in the construction of the chimney chase and attached balcony. Halfway up the chimney chase the fire was stopped by a metal collar placed around the chimney flue, which acted as a firestop (which was required by code). Although the firestopping performed its function of slowing down the fire’s advance, it eventually failed and the fire vented out the top of the chimney chase.
After the fire began venting through the top of the chimney, the first 911 call was received, at 8:52 p.m. For the next 15 minutes, a free-burning fire in the truss space near the chimney intensified until the entire living room of apartment 2133 finally collapsed at 9:07 p.m. The postfire investigation revealed that a baby grand piano had been in the living room of apartment 2133This concentrated load increased the force with which the third floor living room struck the second floor as it fell, which in turn caused the second floor to collapse onto the first floor.
Despite the collapse of all the trusses supporting the third-floor living room, all other trusses remained intact immediately following the fire. Upon further investigation, investigators learned that the floor of the northwest bedroom of apartment 2133 was sagging extensively. Although the floor remained intact while fire department units were on the scene, it collapsed into the second floor sometime during the night.
The precise nature of the truss failure is unknown. From observation, it appeared that the fire burned the chords in the southeast quadrant of the third floor’s living room to the point where the concentrated load near the room’s center caused the trusses to pull away from their end supports. The floor/ceiling assembly between the first and second floors was struck in the center by the falling piano. These trusses never severed completely, although the center of the floor/ceiling assembly came to rest only inches away from the floor slab of the ground floor. Trusses in this area had many of the gusset plates dislocated from the impact of the fall. Although several of these trusses were torn away from their end supports, enough remained in place to form a Vshaped collapse pattern, as opposed to a pancake collapse. None of the trusses in the floor/ceiling assembly between the first and second floors were damaged by the fire.
One code violation in particular added to the early collapse of the third-floor living room. Section 1706 of the 1982 edition of the Uniform Building Code (UBC) requires a onehour fire rated separation for the chimney chase. Since this separation was not provided between the truss space and the chimney chase, the fire quickly moved into the chase and began impinging on the metal collar serving as a firestop. Once the metal collar failed, the fire became freeburning in the truss area near the chimney. The failure of the living room trusses after approximately 15 to 17 minutes of free-burning is consistent with expectations.
A second code violation involved UBC Section 25l6.f.4.A.ii, which required draft stops to be installed in floor/ceiling assemblies in line with walls separating apartment units. Although there was compliance in most areas, draft stopping was not provided in the floor/ceiling assembly between the second and third floors in line with the walls separating apartments 2123 and 2124. This in turn allowed the fire to spread horizontally without difficulty.
Several factors delayed firefighters from reaching the scene of the fire. The first delay involved the alarm office’s inability to dispatch the call immediately, due to the volume of calls being received for weather-related incidents. The weather, a malfunctioning gate at the complex’s west entrance, and a blocked fire lane also contributed to the slower response time. Without a doubt, these factors contributed to the firefighters’ being outside of the collapse zone when the third floor fell. If the response times of Quint 24 and Battalion 4 had been one minute faster, the three firefighters would have been in the second floor living room pulling ceilings when the third floor collapsed.
Although the parallel chord wood trusses performed as expected in this particular fire, they still pose a severe danger to firefighters, especially when fires are ignited and travel within the truss spaces. Truss manufacturers will cite statistics showing the relatively low number of fires that originate in void spaces. Unfortunately, when these fires do occur, they carry the potential for devastating losses.
There are only two effective methods of preventing firefighter injury due to the collapse of parallel chord wood truss assemblies. The most effective method is to provide complete automatic sprinkler protection throughout all spaces of the building, including the truss spaces. In this situation, however, the third floor still would have collapsed even if a working NFPA 13R residential sprinkler system had been present. A 13R system allows sprinklers to be deleted in nonaccessible void spaces even if they are combustible. Although the 13R system provides adequate sprinkler coverage for the livable areas representing the highest probability of fire ignition (i.e., kitchen, living room, etc.), it does not cover the ignition sources that cause the most danger to firefighters (i.e,, fires in the truss space that lead to structural collapse). These ignition sources range from electrical fires that start within the truss space to fires that spread into the truss space from areas such as concealed vertical chases (usually caused by plumbers’ torches) and wood balconies (usually caused by barbecue grills).
Since politics and financial concerns make it fairly unlikely that complete automatic sprinkler systems will be installed in apartment buildings containing trusses, firefighters must rely on the second method of protecting themselves from injury due to truss collapse: effective prefire planning. While prefire planning is accomplished more easily when an apartment complex is under construction, it can be done with respect to existing buildings. There are almost always some kinds of openings in the floor/ ceiling assembly that allow for visual inspection. If firefighters had been aware of the free-burning fire in the truss space at the Havenwood Apartments, they certainly would not have proceeded toward the second floor living room to pull ceilings.
LESSONS LEARNED AND REINFORCED
- Effective prefire planning is the only reliable way for firefighters to keep track of buildings that feature parallel wood chord truss floor/ceiling assemblies. Awareness of the presence of these assemblies is crucial to proper fireground operations that limit firefighter exposure to potentially dangerous structural collapse.
- A properly installed automatic sprinkler system protecting all areas of a building containing parallel wood chord trusses greatly reduces the chance of fire and structural collapse.
- Anytime heavy smoke is visible from the chimney area of an apartment building containing parallel wood chord trusses, firefighters automatically should assume, until proven otherwise, that the fire is free-burning.
- Anytime a fire is believed to be In the floor/ceiling assembly between two floors, firefighters automatically should assume, until proven otherwise, that the fire involves parallel wood chord trusses.
- Building inspectors must strictly enforce the one-hour enclosure for vertical chimney chases. In particular, the enclosure must be maintained where the chase comes in contact with the truss space. Inspectors also must enforce all draft stopping requirements, especially those required in truss spaces.
- Wood trusses do not always collapse suddenly and without warning; they may show signs of sagging similar to those displayed by steel trusses. While this should not be used as the only guide for determining the stability of floor/ceiling assemblies, it can be used to determine the imminent failure of a truss.
- Staff shortages in alarm offices during budget reductions must be amended for situations involving severe weather. Due to the numerous calls that can be anticipated during this period, provisions for providing additional personnel must be made to reduce dispatching delays.
- Fire alarm systems should be checked annually by fire prevention personnel or a qualified fire alarm installation company. In this case, other systems in other buildings in the complex also were not operational. Shortly after the fire, the complex’s management was given notice to repair all fire alarm systems to working order.