One, the use of structural steel in the building for lightning protection and grounding
In the "Building Lightning Protection Design Code" (GB50057-94), mentioned several times in the lightning protection design, priority should be given to the use of the building's own structural reinforcement or steel structure and other natural metals, as part of the lightning protection device, so that the safety and reliability of the premise can take into account the economy. Therefore, how to use the building's metal conductor is an important issue in lightning protection design.
1, roof structure and flashover
Modern architectural art in addition to the pursuit of colorful lines on the fa?ade, the top of the building modeling is also striving for change. Due to the novel thin shell, hyperbolic net frame and other large number of applications, the roof can no longer be simply divided into flat and sloped roof, which brings certain difficulties to the lightning protection design. In addition to the design should be in accordance with the "Building Lightning Protection Design Code" (GB50057-94) in Appendix II requirements in the outer edge of the roof and the protruding parts of the lightning susceptible to lightning strikes such as the setting of lightning straps, direct structural reinforcement of the roof as part of the lightning net is also very necessary.
For waterproofing and anti-cracking considerations, the roof structure is generally cast-in-place concrete slab, its reinforcement by the upper reinforcement and the lower reinforcement, denser reinforcement, more connection points, and the plate reinforcement and beam reinforcement tie connection to form a pathway. Towers and stairwells protruding from the roof are also connected to the substructure through reinforced concrete columns or structural columns. Therefore, when the use of the building itself as a flashover reinforcement, structural reinforcement connections in the key areas such as column reinforcement and beam reinforcement binding points for welding, can meet the requirements of the formation of electrical pathways, that is, GB50057-94 Article 3.3.5 of the article description pointed out that: "in the path of the lightning current flow through the path of the lightning current, there are a number of parallel bonding points when it is safe." The note also states: "The use of roof reinforcement as a flashover is based on the premise that it is permissible for some fragments of concrete to be dislodged and a small piece of waterproofing and insulation to be damaged when the roof is struck by lightning". This does not cause much damage to the roof structure and does not affect the safety of the building.
There are some noteworthy, protruding roof metal objects such as metal frame, billboards, flagpoles, solar water heaters, cold water towers, aviation obstruction lights, etc., in addition to its size should be consistent with the GB50057-94 Article 4.4.1 and 4.1.2 provisions of the article, due to the above metal objects are usually fixed through the expansion bolts in the roof panels, or fixed to the vegetative concrete foundation, it is necessary to The electrical path is formed by reliable electrical connections. Protruding from the roof of the non-metallic objects, according to GB50057-94 Article 3.3.2 should be installed flashover and connected to the roof lightning protection device.
2, the use of concrete columns, wall reinforcement as a lightning lead
Different structural forms of various types of buildings are equipped with a certain number of reinforced concrete columns, such as masonry structure set up in the structural columns, set up in the concrete structure of the frame columns, shear walls, etc., the diameter of reinforcement in the columns according to the "seismic design code for buildings," GB50011-2001 article 7.3.2 It is stipulated that the minimum longitudinal reinforcement of structural columns in brick structure is 4φ12, and the reinforcement of frame columns in frame structure usually adopts Φ14 or above threaded steel bars, which can meet the requirements of Article 3.3.5 and 4.2.1 of GB50057-94. The connection form of reinforcement in the column usually adopts binding connection, welding and mechanical connection. According to the "Code for Construction and Acceptance of Grounding Device of Electrical Installation Project" (GB50169-92), the connection of the lightning lead wire is lap welding, and the lap length is 6 times of the diameter of round steel, so it is not permitted to use rebar to replace the round steel as the lap reinforcement. In addition, as the lead wire of the main steel in the civil construction, if it is used in the butt welding (in the project commonly used welding form of flash butt welding and electroslag pressure welding, are butt welding), should be in the butt welding in accordance with the specifications of complementary welding lap welding round steel.
3, the use of foundation beams as a grounding device
The form of building foundations can be divided into unreinforced extended foundation, extended foundation, columns under the bar foundation, raft, box foundation, pile foundation and composite foundation. According to GB50057-94 Article 3.2.4, Article 3.3.5, Article 3.4.3, Article 4.4.3, the grounding device should be 50cm below the ground, Article 4.3.5 also provides that: anti-direct lightning artificial grounding from the building entrances and exits or sidewalks should not be less than 3m, when less than 3m horizontal grounding body local depth should not be less than 1m or take insulation protection measures. Building foundation depth is usually determined by the height of the foundation itself, the height of the buried pipeline under the ground and the depth of frost and corrosion protection, generally greater than 0.5m. However, in the masonry structure shown in Figure 1, the wall under the bar foundation due to the building waterproofing requirements, the foundation ring beam is usually set at an elevation of -0.060 in lieu of moisture-proof layer, and therefore can not be used as a grounding device. And columns under the bar foundation and raft, box-shaped foundation in the foundation floor with rib beams, columns under the independent foundation and various types of pile foundation are equipped with foundation beams or bearing beams, all of the above can meet the requirements as the foundation grounding body device.
Two, the electrical pipeline buried and structural arrangement
The laying of electrical pipeline is divided into two kinds of open and dark laying. Open laying is the pipeline installed in the wall, ceiling surface, the structure has little impact, while the dark laying is completely different. Concealed laying, electrical piping buried is an important part of the building installation project. Electrical pre-embedded piping is characterized by the number of roots, complex layout, especially in the wall of the vertical pre-embedded piping and horizontal pre-embedded piping in the slab due to weakening the structural component cross-section of the structure of a certain impact, the following will be discussed in several ways
1, vertical pre-embedded piping in the structure of the wall laying
When the vertical pre-embedded piping is buried in the reinforced concrete columns Or reinforced concrete shear wall, laying method is relatively simple, only need to change the line casing to steel pipe, and with the structural reinforcement tied fixed, to prevent deviation in the pouring and vibration of concrete. Due to the small diameter of the electrical pipeline, the concrete walls and columns have little impact, and can be flexibly arranged according to need. However, when the pipeline is buried vertically in the masonry wall, the burial method is relatively complex, which is also the place where the electrical installation project and civil engineering contradict each other more. The form of structural walls are mainly load-bearing walls in the masonry structure and non-load-bearing infill walls in the concrete structure, the following two cases are discussed separately
(1) Buried in the load-bearing walls of the masonry structure
Masonry structure includes brick masonry, concrete block masonry, stone masonry and so on. First of all, in the masonry structure does not allow the opening of horizontal and diagonal through grooves, horizontal pre-embedded piping is usually buried in each layer of the ring beam. Vertical pre-embedded pipes buried in the wall were previously laid directly in the wall grooves, but this practice will cause damage to the structural wall, especially when more pipes are buried side by side, the bearing capacity of the entire wall section has an impact. According to the "masonry structure design code" (GB50003-2001) in article 6.2.14 "should not be in the section of the long side of the load-bearing wall less than 500mm, independent columns buried pipeline; should not be threaded through the wall in the dark line or reserved, cut trench, can not be avoided, it should be necessary to take the necessary measures or weakened according to the cross-section of the wall after the calculation of the carrying capacity of the wall. ". Currently feasible method is to leave 120 deep notch in the masonry brick wall, the width can be used according to the number of juxtaposed pipelines a brick or half a brick, to be pre-buried after the pipeline using C20 fine gravel concrete filling.
When the use of hollow brick or concrete hollow block, there is also a way to use the hole in the masonry buried pipeline, according to GB50003-2001 in the 6.2.14 note "on the force of the smaller or not potted masonry block masonry, allowed in the vertical holes in the wall to set up the pipeline". But in fact, the commonly used KP1-type porous brick aperture of about 20mm, DM1-type porous brick aperture of about 18mm, are smaller, and heavier blocks, group masonry requires staggered mortar joints, so this method of construction is inconvenient.
When the wall is a half-brick wall, in accordance with the norms, in the half-brick wall is not allowed to concealed pipelines, such as unavoidable, it is used in the form of local addition of concrete structural columns, will be buried in the columns of the pipeline.
(2) in the concrete structure of the infill wall buried
Concrete structure of the infill wall only bear the self-weight of the wall itself, commonly used aerated concrete blocks, fly ash concrete hollow block, etc., such materials are characterized by low strength, light weight, even if the destruction of the main structure also has no effect. Therefore, the pre-buried in the infill wall just need to consider the crack resistance, sound insulation and other factors, in the infill wall should not be more than half of the thickness of the wall groove.
2. Horizontal embedded pipelines in the structural floor buried
Structural floor cover mainly prefabricated assembly floor cover, cast-in-place concrete floor cover, as well as no-beam floor cover, ribbed plate floor cover, laminated plate floor cover, etc., due to the first two forms of the more common, here only need to pin the two forms of prefabricated assembly and cast-in-place concrete floor cover to be discussed.
(1) Horizontal embedded pipe in precast assembled floor cover
Precast assembled floor cover includes precast two-way prestressed slab and precast prestressed hollow core slab, usually using prestressed concrete hollow core slab. Although according to the type of reinforcement of the plate there are cold-drawn low carbon steel wire, cold-rolled ribbed steel bars and other differences, but the plate cross-section form and the form of reinforcement layout is basically the same. In the prefabricated plate floor cover in the arrangement of the pipeline need to be pre-structured to the professional understanding of the layout of the prefabricated plate so that the pipeline along the prefabricated plate in the circular hole or plate joints arrangement. It should be noted that, when arranging the pipeline in the round hole, the lead hole should be avoided to avoid the location of the plate stress tendons. The cross-section of a common circular hole plate and the location of the force reinforcement is shown in Figure 3. When the pipeline is arranged along the plate seam, because usually the width of the plate seam is 20-30mm, the pre-buried pipeline will lead to grouting difficult to dense, can be discussed with the structural professionals to take 40-50mm plate seam, in the plate seam with an additional φ12 rebar to be resolved.
(2) Horizontal embedded pipelines in the cast-in-place concrete floor cover
With the change of concrete materials and the progress of construction technology, the cost of cast-in-place concrete floor cover has been gradually reduced, the construction process has been gradually simplified, and the scope of application is becoming more and more extensive. Electrical pipelines in the cast-in-place slab in the plane layout is more flexible, but should pay attention to the pipeline should not be cross in the cast-in-place slab, can not be arranged side by side, and at the same time, according to the "national civil building engineering design technical measures" electrical part of Article 5.1.9 pointed out that: laid in the reinforced concrete cast-in-place slab of the maximum outside diameter of the wire should not be more than the plate thickness of 1 / 3. This is due to the cast-in-place plate plate thickness of 80-150mm, pipelines on the concrete section of the concrete. 150mm, the pipeline on the concrete cross-section of the weakening is relatively large, and through the length of the pipeline will cause a weak zone in the concrete slab, careless handling will cause cracking of the concrete slab, or leave the project hidden trouble.
The horizontal embedded pipe laid in the cast-in-place slab should also be taken to prevent mechanical damage measures, buried in the cast-in-place slab pipe bending radius is not less than 10 times the outside diameter of the pipe.
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