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General Information
The present project is the pre-dimensioning of a photovoltaic structure to be quoted by Eucomsa from Cox-Abengoa group.
The site is in Slovakia on the outskirts of the city of Zilina in the north of the country. Slovakia is one of the European countries adopting the Eurocode as a unique standard for structural design, so the whole design process will be based on this standard through the CYPE program.
Eurocodes in Europe
Geometry
Structure geometry is based on 5 lines of 8 "Y" shaped gantries. In order to increase the open space on the surface under the structure, the even-numbered gantries do not have pillars, so they are braced transversely with a lattice to the odd-numbered gantries to transmit the load.
The "Y" gantries have two wings 7.5 meters long above the horizontal, with a slope of 11º, together with a free height of pillars of 4 meters from the ground to meet the triangular truss that supports these wings and the transversal lattice. These lattices have a maximum edge width of 2 meters.
The roof purlins that support the modules are supported on the upper chord of the triangular truss and will have a Z-section. We will have 8 roof purlins for each wing of the "Y" gantry, since each one of them supports 4 rows of photovoltaic modules, 2 roof purlins per module.
Structure will also have roof bracings at the roof purling bottom plane between the two lines of lateral "Y" gantries to support lateral wind and seismic loads.
A distinction has been made between exterior and interior "Y" gantries, since the exterior ones support greater loads than the interior ones. Exterior frames are considered to be those located at the beginning and end of the line of gantries together with their adjoining ones. It has been decided to make this distinction so as not to penalize the entire structure.
A same distinction is made between lateral and central pillars. Central pillars are considered to be those that are in lines of gantries flanked on both sides by other lines of these gantries.
Material used in the structure profiles is steel S275 for the hot-rolled steel profiles and steel S235 for the cold-formed profiles.
Loads
Loads to be taken into account will be the structure self-weight, dead loads from weight of the photovoltaic panels, front and lateral wind, snow and earthquake in accordance with the Eurocode combination hypotheses.
These loads will be applied principally to the roof purlins, since they are the first structural element on which the main loads to be considered, transmitted by the surface of the photovoltaic panels.
Basic wind speed ( v_b ) for this site is 24m/s. (Data provided by Eucomsa Cox-Abengoa).
Basic wind speed map (vb) for Slovakia
Wind pressure according to Eurocode ( q_{ref} ), can be calculated as:
w = q_{ref} · C_e(z) · C_p
Being:
q_{ref} = \large \frac{1}{2} \normalsize \rho v_b^2
With \rho as wind density at 1.25kg/m³.
A structure with a maximum height of 7.5 meters and a terrain type II has been considered in the calculation of C_e (z) .
Zilina sorroundings. Terrain tipe II
With these values, the parameter C_e (z) is 2.17.
The coefficients C_p will be taken from the case of an isolated troughed free roof, multiplied by the factors for multispan constructions:
The snow load to be applied at the site ( s_k ) is 1.09kN/m² (data provided by Eucomsa Cox-Abengoa).
Snow load distribution (sk) for Slovakia
In addition, this value must be increased by the accumulation coefficient ( \mu ).
The values of ( \mu_1 ) and ( \mu_3 ) are 0.8 and 1.09 respectively for an 11º slope roof.
Therefore, snow has as a trapezoidal load whose lowest height is 872N/m² and whose highest height is 1,192N/m².
Basic earthquake acceleration of the area is 0.63m/s². (Data provided by Eucomsa Cox-Abengoa).
Acceleration factor is therefore 0.63/9.81 = 0.06. To give a reference, it is one of the lowest seismographic data.
Other parameters to be taken into account:
Parameters for seismic loading
Taking into account the aforementioned parameters, we obtain the design spectrum for seismic loads.
Design spectrum for seismic loads
Dead loads will be equivalent to the weight of the photovoltaic panels.
A unit weight per module of 30kg will be considered, the nominal load to be applied will be 150N/m2 which will be distributed between two purlins for each row of modules.
Since each purlin covers a section of approximately 1m, each purlin will support a load of 0.15kN/m2.
Photovoltaic panel
Results
Load envelope for foundation design is shown below.
It is important to remember that the foundation design requires a prior geotechnical study to determine the soil conditions. In this analysis, we only provide the loads that come from the Eurocode standard extrapolated to the foundation elements so that they can be considered together with the geotechnical study for the foundation design of the calculated structure.
Structure beams have been verified for compliance with all ultimate limit states as required by the Eurocode design standard.
Ultimate checks
Main modes of vibration
The designed structure complies with the ultimate limit states provided by the loads defined in the Eurocode for the site where it is located.
The only rigid nodes considered in the structure are those at the base of the pillars. Therefore, these areas must be reinforced with gussets on the two axes of rotation to achieve this rigid node so that the structure approaches the simulation.
At the point of the pillar where the transversal lattices start, it must also be stiffened, at least in the direction of the transversal lattice, to avoid the translationality of all the "Y" gantries against lateral wind and seismic loads.
