How can steel structure buildings be protected against lightning?
May 23, 2022
Whenever the plum rain season is approaching, the sky in the middle and lower reaches of the Yangtze River in my country will be gloomy for days, with continuous rainfall, sometimes large and small, and occasionally accompanied by thunder and lightning. Then, the lightning protection problem of buildings needs to be taken seriously.
In recent years, light steel structure buildings have been widely used in various industrial plants, warehouses, gymnasiums and other places due to their beautiful appearance, short construction period and low engineering cost. However, the metal plate roof is prone to lightning strikes, so it is different from the lightning protection design of ordinary brick-concrete structure and frame structure buildings. Today, let us combine an example of a steel structure workshop to discuss the related issues of lightning protection design of metal roofs!
1. Project overview
A steel structure workshop is an enterprise integrating power generation, electrolytic aluminum and carbon. Among them, the casting workshop is mainly responsible for processing aluminum wire rods and aluminum ingot products. The whole building adopts light steel structure, and the actual size of the building is a rectangle, L=182m, W=93m, H=18.5m (from the floor to the bottom of the roof ventilator). It is reported that the annual average thunderstorm day here is 22.0d/a. Since the workshop is located in the factory area and there are many buildings around it, it is not an isolated building. Therefore, the correction factor is taken as 1 for calculation of general buildings.
2. Determination of lightning protection level of buildings
Calculate the annual average density of lightning strikes on the earth based on known conditions:
Equivalent area for intercepting the same number of lightning strikes as the building:
+πH (200-H)] · 10-6
Finally, the estimated number of lightning strikes in the building per year:
In the formula: Td—annual average thunderstorm days (d/a);
Ng—the annual average density of lightning strikes in the area where the building is located (times/km2.a);
Ae—equivalent area (m2) that intercepts the same number of lightning strikes as the building;
L, W, H—respectively the length, width and height of the building (m);
The number of lightning strikes is expected to be 0.3 times/a≥N≥0.06 times/a, so it is set according to the third type of lightning protection.
3. Lightning Termination
According to light steel structure buildings, the use of lightning rods is cumbersome and inconsistent with the appearance of the entire building, while the use of metal roofs as lightning receptors must meet four requirements.
According to the specification requirements "when there is no flammable material under the metal plate, its thickness should not be less than 0.5mm". The main workshop building envelope (roof) of the workshop is made of hot-dip galvanized steel sheet, the thickness of the substrate is 0.53mm (including the galvanized layer), and a glass wool pad with a thickness of 75mm is added under the roof panel, which is wrapped with aluminum foil and φ1.5 Non-embroidered steel mesh layer support and the upper edge of the purlin. Glass wool is a class A building material with small thermal conductivity, flame retardant, non-toxic and stable chemical properties, which fully meet the requirements of the specification for non-flammability. At the same time, according to the introduction of civil engineering, the construction of the enclosure system of light steel structure buildings adopts occlusion and overlap, and the length of the tower connection must reach at least one wave crest or wave trough, and more than 100m can fully meet the requirements of the specification.
In addition, the hot-dip galvanized steel sheet used on the roof is a good conductor, and the thin paint protective layer applied on the surface is not an insulating coating according to the specification notes, so it meets the specification requirements.
To sum up, in this project, the metal panel of the roof light steel structure can be used as an air-termination device.
To do a good job in the design of the metal plate roof lightning protection system, in addition to choosing a good lightning protection form, choose a good lightning receptor, but also do a good job of down-conducting and grounding.
4. Lead down
For light steel structure buildings, there are many types of color profiled panels, which are divided into single panels and composite panels. The workshop uses composite panels. During the construction, the civil engineering major and the electrical major are required to cooperate to ensure the reliable connection of the roof metal profiled plate, roof truss, purlins and steel columns. Therefore, steel columns are used as down conductors during construction.
5. Grounding device
In the current design, the basic steel bar is often used as the natural grounding body, and the 40X4 galvanized flat steel is used to connect it, and the general equipotential bonding is implemented. In this design, 40X4 galvanized flat steel is not strictly required, but the strip foundation between the steel columns and the independent foundation of the steel column are used for reliable welding to form an electrical path. For buildings with light steel structure, the foundation bolts should be pre-buried in the construction of the independent foundation of the steel column, so that when the steel column is in place, the anchor bolt, nut and the steel column are connected together. It should be noted that the anchor bolts and the steel column foundation reinforcement are not connected underground! Therefore, electrical designers should require civil engineering majors to indicate in the design instructions that the basic steel bars and grounding bolts should be welded reliably with no less than φ10 steel bars or round steel during construction, and anti-corrosion treatment should be performed at the welding places for grounding purposes. In this way, a reliable connection is formed from the roof to the steel column, and then to the foundation, ensuring that the lightning current has a complete discharge channel.
In addition, don't forget to reserve a grounding connection plate at an appropriate outdoor location to prevent the construction unit from connecting the artificial grounding device and testing the grounding resistance when the grounding resistance does not meet the requirements during construction.
Lightning protection of metal roof buildings is a difficult project to handle, but in such buildings, as long as the roof sandwich panel (or profiled steel plate) of light steel structure buildings with a thickness of 0.5mm can be used as a lightning receptor, the use of steel The column is used as the down-conductor, and the basic steel bar is used as the natural grounding body and is reliably connected by galvanized flat steel or ground ring beam and is equipped with an equipotential connection.