Photovoltaic array brackets are special brackets used in photovoltaic power generation systems to install and fix battery components. The foundation of photovoltaic support is a structure that transmits various forces that the support bears to the foundation. It has various forms and a large number of projects used. It is an important guarantee for the structural safety and normal operation of photovoltaic power stations.
There are many factors to be considered in the foundation design of ground photovoltaic power stations. According to the geological conditions of the project site, the corrosiveness of groundwater and soil, and other characteristics, the design strength and stiffness must meet the construction requirements, and the construction conditions should be combined to select an economical and applicable foundation. form, especially the basic form with relatively little disturbance to the ground surface. According to the cases of ground photovoltaic power station projects outside China, the most widely used foundation forms are: screw pile foundation, concrete cast-in-place pile foundation, pre-hole ground anchor pile foundation, prestressed concrete pipe pile foundation, concrete strip foundation and concrete independent foundation. foundation etc.
As the areas with better sunlight in the west have been gradually divided up, large-scale photovoltaic land is in short supply, and the industry is not optimistic about the situation of abandoning light and curtailing electricity in recent years, the transfer of photovoltaic power generation industry to the eastern region is imminent. However, the biggest problem facing the eastward shift of the photovoltaic power generation industry is land use. The development of surface photovoltaics has become an urgent need for the eastward shift of the photovoltaic power generation industry, and it is also a power generation method that the development of China’s photovoltaic power generation industry urgently needs to break through. According to the case of water surface photovoltaic power station projects outside China, the most widely used foundation forms are: prestressed concrete pipe pile foundation and floating foundation.
The support foundation form has a great influence on the entire photovoltaic power station project. Therefore, the topography, geology, water body and other conditions should be comprehensively considered, and the support foundation form should be selected reasonably.
The loads acting on the foundation of the photovoltaic module bracket mainly include: the self-weight of the bracket and the photovoltaic module, wind load, snow load, temperature load and earthquake load. Among them, the main control effect is wind load or snow load. Therefore, the stability of the foundation under the action of wind load or snow load should be ensured in the design. Especially under the action of wind load, the foundation may be pulled up, broken and other damage phenomena. The design should ensure that no foundation damage will occur under this force.
When calculating the support foundation, the constant load and live load should be combined according to the specification. Under the most unfavorable combination conditions, the base area and depth of the foundation should be determined according to the bearing capacity of the foundation, or the number of piles should be determined according to the bearing capacity of a single pile.
On the basis of each group of brackets, a plurality of component strings are generally arranged, and each component string is composed of dozens of photovoltaic components, and the photovoltaic components are arranged with a certain inclination angle. For independent foundations and strip foundations, the control standards are mainly the ratio of the detachment area of the bottom surface of the foundation, the bearing capacity of the foundation, the anti-overturning stability of the foundation, and the anti-slip of the foundation; under the action of wind loads, each group of support foundations may suffer. Overturning failure, and anti-overturning failure is the main controlling factor in the design of freestanding foundations and strip foundations. For the pile foundation, the stability is maintained by the use of soil embedding, and the control standards are mainly the vertical bearing capacity of a single pile, the pull-out resistance and the horizontal bearing capacity of the pile foundation. Because the vertical force of the support is very small, the bearing capacity of the foundation and the vertical bearing capacity of the single pile generally have little effect. Under the action of horizontal force such as wind load, the overturning load will be transformed into the uplift effect of the pile foundation, so the uplift resistance is an important control factor for the pile foundation.
(1) Anti-overturning standard
There are two forms of expression for the anti-overturning stability of the foundation. The first is Article 4.2.4 of “Code for Seismic Design of Buildings” (GB50011-2010): “High-rise buildings with a height-to-width ratio greater than 4 should not appear on the bottom of the foundation under the action of earthquakes. Breakaway zone (zero stress zone); for other buildings, the area of the breakaway zone (zero stress zone) between the bottom of the foundation and the foundation soil should not exceed 15% of the area of the bottom of the foundation.” The second is Article 6.7.5 of “Code for Design of Building Foundations” (GB50007-2011): “The control condition for retaining walls is that the anti-overturning moment/overturning moment is greater than or equal to 1.6”.
In engineering design practice, the “Code for Design of Building Foundations” (GB50007-2011) is generally used as the design control conditions for photovoltaic support foundations such as independent foundations and strip foundations, and the requirements and standards can be appropriately reduced. Combined with the characteristics of the actual site, considering the characteristics and economy of photovoltaic power generation, the anti-overturning standard of the photovoltaic support foundation can be appropriately reduced, that is, the ratio of anti-overturning moment/overturning moment greater than or equal to 1.2 can meet the requirements.
(2) Pull-out resistance standard
For the pile foundation, if the embedded fixation meets the requirements, the horizontal force and bending moment can be considered to be borne by the soil, and the uplift force plays the main control role. ) Determined in Article 5.4.5, that is, the pile foundation bearing the pull-out force shall not be larger than the sum of the characteristic value of the pull-out bearing capacity and the dead weight of the pile when the pile group foundation is non-integrally damaged. The formula is as follows (1):
where: Nk——pile pullout force calculated according to the standard combination of load effects;
Tuk——the standard value of ultimate ultimate bearing capacity of foundation piles when the pile group is non-integrative failure;
Gp – self-weight of foundation pile.
Since the self-weight of the foundation pile of the photovoltaic support is generally small, the self-weight of the foundation pile may not be considered in the design and verification process, and the formula can be further simplified, that is, the pull-out force of the foundation pile is not greater than the characteristic value of the pull-out bearing capacity, and the simplified formula is as follows (2):
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