Snowflakes can also kill people? Which buildings are the most vulnerable to snow? You will know after reading this!

Under the combined influence of cold and warm air, a new round of rain and snow will occur in central and eastern my country from January 25 to 29. Although snowflakes are very light, not all buildings can withstand them. So, which buildings are the most "weak to snow"? Want to know the answer? You will know after reading this.

Written by reporter Zhao Tianyu Edited by Ding Lin

Interview experts:

Li Yan (Senior Engineer, Public Meteorological Service Center, China Meteorological Administration)

Di Xiaotan (Chief Engineer of National Construction Engineering Quality Inspection Center)

10 mm of "snowfall" equals 10 cm of snow depth

As we all know, snowflakes are very light. The total weight of 5,000 to 10,000 snowflakes is only 1 gram. However, even snowflakes that are "lighter than a feather" can generate a huge weight that is enough to crush a building when they are gathered together.

To know how heavy the snow is, we must first understand the concepts of "snowfall" and "snow depth".

Li Yan, senior engineer at the China Meteorological Administration's Public Service Center, said that "snowfall" in meteorology refers to a value measured by meteorological observers after melting the snow collected within 24 hours into water using a standard container.

Snowfall is measured in millimeters. Generally speaking, more than 10 millimeters is considered a blizzard, and more than 20 millimeters is considered a severe blizzard.

▲The amount of snowfall in meteorology actually measures the amount of "snow water". 10 mm of snowfall can reach blizzard level.

The depth of snow accumulation varies between the north and the south. The snow in the north is relatively fluffy, and 1 mm of snowfall corresponds to a snow depth of about 8 to 10 mm. "To put it simply, 1 mm of snowfall corresponds to 1 cm of snow accumulation," said Li Yan.

In contrast, in the southern snowfall areas south of the Qinling Mountains and Huaihe River and north of the Nanling Mountains, especially in the middle and lower reaches of the Yangtze River, the snowfall has a higher moisture content because of the humid climate. Therefore, with the same amount of snowfall, the snow accumulation is relatively not as thick as in the north: "1 mm of snowfall can accumulate about 6 to 8 mm of snow."

▲Ranking of snow depth in provincial capital cities in my country (Source: China Weather)

In the eastern part of Northeast my country, northeastern Inner Mongolia, northern Xinjiang and parts of Tibet, the annual snow accumulation can reach more than 30 cm, and in some areas it can reach 50 cm. At the same time, the annual snow accumulation in the southern regions, especially in the Yangtze River Basin, where the snow accumulation efficiency is not as high as that in the north, is not ambiguous at all. In particular, the annual maximum snow depth in Jiangsu, Anhui and Zhejiang can reach 30 cm, which is higher than many areas in North China and Northeast China. This is closely related to the climate characteristics of these regions.

Snowflakes are light, but they cannot be underestimated.

Although 10,000 snowflakes weigh only 1 gram, snowfall is a continuous weather process, and even small amounts can add up to a large number. One cubic meter of new snow is made up of 6 to 8 billion snowflakes, and its specific weight is difficult to calculate in grams.

We all know that the density of pure water at room temperature is 1 gram per cubic centimeter, so 1 cubic meter of water is equivalent to 1 ton.

However, due to factors such as temperature, the specific density of snow is difficult to accurately define, and its approximate range is between 0.05 and 0.3 grams per cubic centimeter. Therefore, we cannot accurately calculate the weight of snow using "density × volume".

However, since the "snowfall" can directly correspond to the weight of "snow water", we can use this concept to make an estimate of the weight of snow. "The weight of 1 cm of snow (1 mm of snowfall) on 1 square meter is 1 kg, and 10 kg on 10 square meters." Li Yan explained.

Take the snowstorm in Hefei on January 4, 2018 as an example. How much snow did it snow on that day? The exact number was 30 mm, which exceeded the single-day snowfall during the 2008 ice and snow disaster and almost reached the level of a heavy snowstorm.

The actual snow depth reached 27 centimeters. Li Yan made a simple calculation: "If this much snow is spread on a 100-square-meter flat roof, it would weigh about 2.7 tons."

▲On January 4, 2018, Hefei, Anhui Province, was hit by heavy snow. Many BRT bus stops collapsed on Wangjiang Road in Hefei City.

Judging from the pictures, the area of ​​the collapsed platform roof in Hefei is at least 10 square meters, so the weight of the snow it bears is no less than 270 kilograms. In other words, when the platform roof collapsed, there were equivalent to 3 to 4 adult men standing on it.

If it were an ordinary house, it would be no problem for the top to bear 3 to 4 adult men. However, the steel-aluminum structure platform shed that collapsed in the accident had a weak ability to withstand pressure.

Steel frame structure is unsafe

Are building factors also an important cause of snowfall collapse? To this end, the reporter contacted Di Xiaotan, chief engineer of the National Construction Engineering Quality Inspection Center.

Why can snowfall crush buildings? Di Xiaotan believes that three reasons have contributed to this situation. "First, the design reference for the maximum snowfall during house design is biased, and the 'basic snow pressure' is too small; second, the 'load coefficient' of the house is too small, which is still a little different from the European and American standards; in addition, some buildings are relatively light, and the 'self-weight coefficient' is too low."

Di Xiaotan explained that the snow pressure resistance of buildings in our country is usually designed according to the maximum snow pressure that has occurred in the past 50 years. However, in the actual statistical process, due to incomplete data on the density of snow accumulation, the snow pressure value can only be estimated based on the average density, which leads to the situation that the basic snow pressure value is smaller than the actual snow pressure value. This basic snow pressure value is only 0.2kN/m2, but in reality, the snow pressure in many places can reach 0.4-0.5kN/m2, or even higher.

▲For a large-span structure like a gymnasium, if a lightweight metal roof is used, the compressive strength is weak and it is more likely to collapse when encountering a large amount of snow.

Secondly, in terms of building load coefficient, my country's buildings are also significantly lower than those in Europe and the United States. Di Xiaotan emphasized, "my country's building load coefficient has always been 1.4, while this coefficient is 1.5 in Europe and 1.6 in the United States. Their buildings are much stronger than ours." For this reason, the newly revised "Uniform Standard for Reliability Design of Building Structures" has adjusted the building load coefficient to 1.5.

But the most important of the three building factors is the material properties of the building itself. Light steel structures are light in weight, so most of the buildings that collapsed or overturned due to snowfall are made of light steel or metal (such as platforms, simple sheds, factories, etc.). In addition, some buildings with large span structures (such as gymnasiums that also use light metal as roofs) are also prone to collapse. "In comparison, reinforced concrete structures are much stronger." Di Xiaotan told reporters.

▲A Nordic town, the roofs are all pointed, which can better resist snow pressure

The reporter learned that many buildings in areas with heavy snowfall will consciously make some designs to improve the house's snow pressure resistance. In places such as Northern Europe and Eastern Europe where it often snows in winter, most buildings have pointed roofs and a certain slope. Snow can easily slide off the roof, reducing the pressure on the roof structure, protecting the roof from damage by snow, and protecting the safety of the building. In addition, when the snowfall exceeds a certain level, people will also promptly use a variety of methods to remove the snow on the roof (in terms of snow removal, the northern region of my country is generally more mature than the southern region).

Because the pressure resistance of a building involves so many factors, truly improving the house's ability to resist snow pressure is not something that can be achieved overnight.

Produced by: Science Central Kitchen

Produced by: Beijing Science and Technology News | Beijing Science and Technology Media

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