Doing hard work for others, the impact of building structure on fire, how much to understand

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From 2004 to 2006 , I received a master's degree in safety engineering, which included a course on occupational safety and health. As part of this course, I wrote a paper on occupational safety when I was a volunteer firefighter at the Oskar fire station.

In order to better demonstrate this topic, I studied many incidents that occurred in the United States when the firefighters were killed.

I noticed that most firefighters were killed because they fell from above the burning room.

I myself have never heard of a similar accident in Belgium or other neighboring countries. What is puzzling is why is the difference so great in the United States?

Later, I gradually realized that this was because their building structure was completely different from ours. In North America, many houses are made of wood, which is in stark contrast to our masonry.

In the past, large-size wood was mainly used for the construction of houses. The construction materials were also very expensive. A large wooden beam was more expensive than a wooden beam.

In the United States, the construction industry has switched from the original use of wooden beams to the use of truss beams, I-beams, and even small trusses. This particular form of construction is called light construction.

This new type of building has many advantages, first of all its low price and secondly its light material. Therefore, a large number of apartments in the United States tend to adopt a lightweight structure.

Another advantage is that due to the reduction in the amount of wood used, the damage to the environment is reduced.

However, in addition to numerous benefits, light buildings also have some deficiencies, one of which is poor fire performance (fire resistance).

The purpose of this paper is to discuss the shortcomings of the “wood structure, steel structure” of light buildings.

2. Risk

2.1 Stability

In the Netherlands, there is a term for structural engineers called "Structural Integrity" that is used to define the volume perimeter ratio.

More internationally used is the ratio of surface area to volume ( s/v ), which is in contrast to the above formula.

The volume of the beam is equal to the cross-sectional area of ​​the beam multiplied by its length. The surface area of ​​the beam is equal to the product of the perimeter and length of the section.

Therefore, the ratio of the volume to the surface area of ​​the beam is equal to the ratio of its cross-sectional area to its perimeter.

Comparing the overall structure of the I-beam and the beam, it is not difficult to find that the larger the cross-section of the beam, the more material is used, and the smaller its surface area.

The interior of the beam was better protected in the fire. The fire must burn the outer layer of the beam before it burns into the interior. During this period, the interior of the beam can still be normally loaded.

The web of the I-beam connects the upper and lower flanges. At low temperatures, I-beams can be as strong as wooden beams. Some wooden I-beams have a web thickness of no more than 7 mm .

However, in the event of a fire, the characteristics of the two types of architectural elements are completely different due to the higher temperatures. The longer perimeter of the I-beam means that more surface parts are exposed to fire.

Once the web breaks, the building will collapse. Obviously, the I-beam does not have fire resistance. The United States has conducted numerous studies in this area.

A large number of tests have shown that the fire resistance of wood flooring supported by lightweight wood structures is only 8 minutes.

This means that after a fire, the floor may collapse for a short period of time (probably causing firefighters to fall).

How long does it take to find a fire? How long does it take to call an alarm? How long does it take for the fire brigade to arrive? How long does it take to lay a water hose on the spot and start the attack?

Obviously, when the firefighters started the attack, the floor had become vulnerable. For basement fires, the extension of the hoseline by firefighters will further increase the load on the floor.

The total weight of two inside firefighters wearing air breathing apparatus and carrying related equipment often exceeds 200 kg .

This is why there are so many firefighters in the United States falling downstairs.

The same is true of the ceiling. When the firefighters extinguish fire in the fire floor, they must always be alert to the ceiling that may have fallen down.

The reason for the collapse in this case is even more complicated. It may be due to the collapse of the wooden joist in the ceiling, or to the loss of stress due to the load-bearing pillar or wall that supports the ceiling.

All in all, the vulnerability of light buildings in fire poses a great threat to the safety of firefighters.

2.2 The spread of fire and the spread of fire

The Belgian building code allows the use of wood lightweight structures in individual houses (contiguous houses are also permitted in the case of firewall partitions between adjacent buildings).

Fortunately, some "basic norms" have been written into the Royal Decree of Belgium.

This decree stipulates some requirements that must be satisfied when building apartments and office buildings, one of which is the fire resistance of building components. It requires that the building maintain its structural integrity for a period of time after the fire has occurred.

In this way, firefighters can work relatively safely within the building. The specification of fire resistance also helps to keep the fire within the fire room. The partition of the room can prevent the fire from spreading to the adjacent room for a certain period of time.

Walls and floors are usually made of concrete or bricks. In the fire, these walls and floors are powerful barriers against the spread of fire.

In this kind of building structure, when a fire broke out in Belgium, the collapse of buildings was rare.

If it is replaced with wooden walls and wooden floors, it is almost impossible to achieve the same effect. In order for the wooden wall to have a good fireproof effect, the wall must be protected by adding a protective covering such as a plasterboard.

In North America today, apartments are built almost exclusively in wood for cost reduction. As long as the price is cheap, there will always be people willing to do so.

Belgium requires that fire protection must be provided for walls and floors in order to achieve the necessary fire resistance, which virtually increases costs. However, the use of wood materials still appears from time to time.

On the Neuss Street in Brussels, a construction company is building an apartment building. Their idea is to add about 200 apartments on top of existing buildings .

If you want to use concrete to build, it will bring great challenges to the weight of the original building. If wood is used for construction, it will be able to build several more floors.

According to the plan, a protective layer should be added to the wood. However, during the construction process, fire has not yet occurred and the protective layer has been installed.

The Brussels Fire Department found that they were facing a huge fire because no measures had been taken to prevent the fire from spreading. The fire quickly spread to the entire building.

In the construction process (construction, renovation or demolition), lightweight timber construction represents a huge fire hazard. When the fire reaches a certain scale, it is very difficult for the fire department to start the rescue safely.

On the YouTube video site, there is a video about the fire in Houston, Texas. In the video, a construction worker was trapped on the balcony of a wooden apartment building under construction.

At the beginning of the video, we can clearly see that this person has escaped to the balcony. Obviously, he had no time to run down the stairs in the building.

This video clearly shows the speed at which the fire spreads. The fire spread almost instantaneously throughout the building.

2.3 Fire hazard

Lightweight buildings like this have a high fire risk. These buildings are like brickwork, and all electrical equipment (wires, sockets, ...) are placed in the wall.

The difference is that bricks are non-flammable. Electrical equipment is often a source of ignition. When the circuit fails, the temperature at the point of failure rises a lot.

Naturally, it is not a problem for a brick wall, but when it is replaced with a wooden wall, it will be smoldering, difficult to approach and gradually spread, and eventually evolve into a fire.

When the fire spreads to the building itself, it will be upgraded to a building fire. For the fire department, this is undoubtedly a huge challenge.

When the fire gradually developed and ignited the furniture near the wall in the room, another major risk factor was quietly formed. The flames spread to the top of the building and a ventilation-controlled fire was formed. At this point, protecting buildings becomes extremely difficult.

2.4 Evaluation of building structure

In Belgium, we are more inclined to build brick houses. As a Belgian proverb says: "Belgian has a brick in his stomach since birth."

The brick house is normal for us. At this point, social identity also has a great influence. People want to build a cheap house, but they are shy to let people see it.

Therefore, despite the increase in the number of wooden houses, it is not visible in the ordinary streets of Belgium. Most wooden houses have a brick wall as an exterior.

Therefore, in Belgium, it is difficult to distinguish between wooden buildings and strictly brick buildings only from the aspect of appearance.

Fires in such houses are likely to be taken lightly by fire fighters. After the fire brigade arrived, it was difficult to judge from the appearance that it was a wooden house.

In a ventilation-controlled fire, a large amount of flame rushes out of windows and doors, which means that the walls and ceiling are subjected to a lot of heat.

Experience tells firefighters that there is no problem with a brick concrete building. However, for wooden houses, the situation is different.

At night, when a fire broke out at the bottom of the building, the firefighters had to carry out search and rescue tasks upstairs. It is very likely that the fire has damaged the strength of the floor, causing search and rescue personnel to fall at any time. In this case, they may fall directly into the sea of ​​fire.

In a room consisting of brick walls and concrete floors, fire extinguishers can be fired by a group of firefighters. Another group of firefighters can start searching for trapped people in the bedrooms and other places.

Concrete slabs can guarantee the stability of the structure, but for wooden houses it is impossible.

In a wooden house, the wall may be too hot and the fire will spread to the entire building. In particular, the floor above the fire floor and the floor in the attic floor are likely to be surrounded by fires. Once the fire spreads to the attic or the roof, it is hard to control.

In the case of a bricked appearance, the fire brigade may not initially consider the danger of a rapid fire spread.

When a fire occurs in a room in a brick building, firefighters will quickly extinguish the fire during the development phase, and this will basically solve the problem.

But if this happens in a wooden house, firefighters must confirm whether the fire has spread to the entire building.

3. Battle cases

3.1 Houston , TX : Roof Collapse

May 31, 2013, about 12:18 or so, the Houston Fire Department received a report that a restaurant attic fire.

The Houston Fire Department is a professional fire station with 3,800 firefighters. It protects more than 2 million people within a 1,600- square-kilometer area , and its population is about double that of Brussels, and its area is ten times that.

No. 51 water tanker team arrived at the scene first, and on the way to the scene, they could see that the smoke had started to gather.

12:12, rescue forces reach the scene, and expand in the attack immediately. 12:15, thermometer display ceiling temperature reached 84 ℃.

The intruder entered the building about 3 meters away from the building and began extinguishing the fire with a 70 mm hose. They reported that the temperature inside the fire is not high, but the visibility is zero.

12:18, because there may be a problem with the water supply, in the attack group withdrew from the scene.

Two minutes later, the No. 51 water tanker team again initiated the internal attack. No. 68 water tanker team also came to the fire to reinforce and laid the second inline main line into the fire.

12:00 about 23 minutes, part of the building collapses, four firefighters sacrificed on the spot and 15 others seriously injured, this time, from 51 units arrived at the scene just over 12 minutes.

In our current buildings, trusses are commonly used in the steel industry and sometimes in large span rooms.

The disadvantage of this building element is that it is very fragile. Because the overall structure of the truss is low, it can easily collapse in fires.

Most importantly, special attention is required when using wooden cables to connect wooden truss panels. Chords and webs are usually connected by tooth plates or nail plates. These building elements are merely thin steel sheets covered with gridded nails.

The connection of these plates to the string is to maintain the structure of the entire truss with this architectural configuration. What is dangerous is that they deform under heat, which can easily cause them to fall off.

Once the truss panel is no longer fully connected with the string and web, collapse occurs.

The restaurant in Houston was refurbished and was initially covered only by the roof. Later, concrete roofs were added to the roof. It was precisely because of this additional burden that the building collapsed earlier than expected.

Afterwards, the survey showed that restaurant employees had smelled a burnt smell three hours before the fire broke out . After several quick checks, no fire was found.

Perhaps, somewhere in the building has begun to burn. Only when the flames burned out of the building to the outside, did people find that there was a fire. Someone immediately reported a fire.

At this point, the fire had been smoldering in the building for three hours and spread across the wooden walls.

3.2 Coleraine, Ohio: Firefighters Fall from Upstairs

April 4, 2008, Clariant town fire brigade received a report that a fire broke out in a separate house.

This particular fire brigade consisting of 60 professional firefighters and 150 volunteer firefighters, and the protection of the 60,000 residents in the area of 117 km2.

6:11 am, the house fire alarm automatic fire alarm, the police immediately firefighters.

6:20, to confirm the fire control room.

Three minutes later, the first team of water tankers arrived at the scene and learned from the owner that the fire was in the basement and no one was trapped. Two firefighters (a squadron commander and a combatant) started the attack with a 45 mm hose line.

The investigation found that there was some smog in the door, and it took some time for the hose to advance in the building. The third firefighter also entered the building to help the two insiders.

Later, the attackers entered the basement downstairs, and the third firefighter took the water outside the house. When he returned to the door, the captain ordered him to stay outside. At this time, the in-house attacker had an accident.

6:37, internal and rescue personnel launched the attack to work. At this point, it was only 14 minutes before the first team of water tankers arrived .

Rescue workers noticed that the ground floor of the building had collapsed and the two firefighters in the attack team were killed.

Afterwards, the investigation revealed that within 16 minutes after the fire alarm sounded , the intruder entered the building. At that time, the fire only burned for a maximum of 16 minutes.

The width of the beam that supports the underlying wooden floor was 5 cm and the height was 25 cm . However, due to the burning of the fire, the strength decreased, and eventually the collapse occurred.

The investigators concluded that because of the high temperature of the basement, the internal attack team was forced to decide to evacuate through the room from the back door.

The total weight of the team members inside the attack group is too heavy for the beams that have been severely weakened, causing the ground floor to collapse and the inside attack team members to fall into the burning basement from the bottom.

In this short period of 16 minutes, 5 cm wide wooden beams were weakened and eventually collapsed.

Imagine if it is supported by beams 7 mm thick web of words, what would happen?

3.3 Accident in Uccle, Belgium

In Belgium, at least in this accident, light buildings have had a serious impact on fire fighting.

On August 30, two firefighters were killed in a fire in Brussels in Ukraine.

The building on fire is made up of several different parts. The location where the firefighters sacrificed was made up of a lightweight wooden structure. In this accident, the extreme fire characteristic was a direct fuse, and the explosion of the smoke directly led to it. The collapse of the building.

The question is, if the building is made of concrete, then the same accident will happen? Can the smoke spread rapidly in different parts of the building? Will a concrete building collapse due to the explosion of smoke?

4. Countermeasures "Please check"

4.1 Statistics of Building Structure

Do a good job in building structure statistics. The content of the statistics includes the building structure of the building and its underlying structure.

Once a fire occurs in a building, the command center can keep abreast of the type of building through building registration.

In the rush to the scene, firefighters can get the structure of the fire building through the command center for the first time. In this way, firefighters can determine in advance whether they are facing a light building.

When necessary, fire extinguishing strategies can be adjusted in a timely manner. This is the only way to prevent the accident from happening.

Of course, this is a large-scale project, but it is also of great benefit to other areas.

Many industrial buildings use steel structures. Steel is a building material with poor fire resistance without additional protection.

In these circumstances, it is useful for firefighters to obtain this information before the fire is saved.

4.2 Conduct learning and training on building structure and collapse principles

If the fire department is adequately trained, firefighters will be able to accurately identify the type of building and determine the risks that different building configurations may pose.

In this series of articles, Orange Rescue will next describe the collapse of the building and increase firefighters' initial attempts to understand the different architectural configurations. Knowledge about building methods will often come in handy for building fire suppression.

4.3 Huge influence

If you really turn to the trend of building wooden houses, then this will have a huge impact on firefighting.

The efficient management of Sydney fires allowed the first tanker team to reach the site within 5 to 10 minutes after receiving the alarm .

If you want to protect your home, you must control the fire within the next 5 minutes. If the big fire spreads within the building, then the whole house will surely be buried in the sea of ​​fire.

In order to achieve this, a large number of professional firefighters are required.

Each fire station is equipped with a tank fire truck and four firefighters. In this way, a close fire network can be formed in the area. The area of ​​each fire station is not large enough to reach the site in the shortest possible time.

The same organizational structure can also be found in the United States and Canada. Here, professional firefighters protect the suburbs so that they can quickly extinguish fires caused by wood.

In Belgium, volunteer firefighters form the basis of suburban fire services. Volunteer firefighters usually have a few minutes longer than professional firefighters. After all, they have to drive by themselves to the fire station.

The most important thing is that there are few fire stations in Belgium, but they are all on a large scale, which also prolongs the duration of the police. Both of these factors have caused the fire brigade to often not be able to quickly reach the scene and promptly extinguish fires in wooden houses.

In the future, the fire brigade will likely face more of the wooden houses that were completely burned on the fire.

5. I still have a dream

In 2013, I was in an article about the collapse of the accident cited Martin Luther King's words:.. "I have a dream."

This is the call for action, calling for learning from accidents in Belgium. In most countries, every serious accident has been systematically investigated and a detailed investigation report has been issued so that people can understand as much as possible where the problem is.

Sometimes we spend a lot of time analyzing the accident and giving detailed accident reports, which can to a certain extent prevent similar accidents from happening again.

We must carry out these tasks irrespective of losses, and we must not let the firefighters who died in vain sacrifice ...

I still have a dream ...

Author Karel Lambert, Translation: Orange Rescue.

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