Wind gusts, along with an older style of building construction that might have given the fire an easy path upward, probably contributed to the devastating nine-alarm fire that killed two firefighters in the Back Bay on Wednesday afternoon, according to specialists.
The cause and precise path of the fire at 298 Beacon St. remains under investigation, but specialists said winds that gusted to about 45 miles per hour almost certainly created rapidly-changing and unpredictable conditions. A US study recently found that winds as low as 10 miles per hour can dangerously alter the ventilation flow that shapes a fire’s path. With full protective gear, a firefighter who found himself in that path would only be able to tolerate the conditions for 30 seconds.
“I don’t know what happened in the incident yesterday, but just fighting a fire on such a windy day is enormously dangerous,” said Kathy A. Notarianni, head of the fire protection engineering department at Worcester Polytechnic Institute. “We can assume that what scientists see at 10 mile-per-hour winds is only exacerbated at 40-mile-per-hour winds.”
Two characteristics made the fire particularly challenging, Notarianni said. The fire started in the basement, adding to the danger because there are usually limited paths to escape in an emergency. The wind gusting off the Charles River would have then added danger by creating rapidly changing conditions, probably playing a major role in transforming an ordinary fire into a big disaster.
Wind fuels fires in several ways. Just as a person blows on a campfire to help it, the influx of fresh oxygen from wind that increased air flow through the house accelerated the fire. Wind can also cause mixing of layers that are usually distinct. For example, people are told to stay close to the floor when they are trying to escape a fire, because there is a cooler layer of air that is less smoky. Wind can mix up the layers and create “untenable conditions, floor to ceiling,” Notarianni said.
Lastly, wind can change the fire dynamics by altering the ventilation flow. The changes can push the flames to move fast in unexpected directions.
The internal structure of the building also could have created a conduit for the fire to spread upward. The four-story home was built in the late 19th century, according to the city and a Back Bay Houses website that tracks architectural history. Specialists said it was probably an older style of construction, called balloon frame construction, in which there are unobstructed chaseways that connect the basement to the attic. That could have given the fire a vertical path.
“The fire may start in a small area and be confined into a hidden space in the building and spread until it reaches a point that it breaks into an open area. At the point it does, it has quite a bit of intensity,” said Ken Willette, of the public fire protection division at the National Fire Protection Association.
Susan Prindle, chairwoman of the architecture committee for the Neighborhood Association of the Back Bay, said it was common in the Back Bay for floors to be connected by an air shaft, but she did not know the layout of 298 Beacon St.
The explosion that many people recalled hearing during the fire was probably caused by black smoke collected in a closed room, where the particles in the smoke became fuel as they were intensely heated and mixed with fresh air.
“The fire actually gets so hot that when you have very little flame but very high temperature and when fresh air is introduced into that, there is a rapid expansion of flame and a pressure wave,” Willette said. The wave can cause windows to blow out and create backdraft.
The smoke that engulfed the neighborhood and hung low might have been an indication of just how large a role the weather played in the fire’s development. Willette said that wind and temperature play a role when smoke stays low. On cool days, he said, smoke does not rise as quickly.
What saved the blaze from spreading, however, were firewalls — 8 inches of brick on each side, according to Prindle.
The fire will be intensively scrutinized through studies and computer simulations that will guide the design of future firefighting procedures and tools.Carolyn Y. Johnson can be reached tcjohnson@globe. com.