Tech Weekly | Yingjianke Photovoltaic Mounting System Design Common Issues Series - 1

(Source: Yingjianke)

Questions and Answers

Question 1

Can the weight of the photovoltaic modules be automatically calculated when entering the photovoltaic module information?

Answer:

Yes. The software user manual also contains calculation examples, located in the software installation directory - [Docs folder] - [User Manual].

The specific calculation for the module load is as follows:

The weight of the module is 25kg, and the module area is 2.128*1.048=2.230144m2.

The module load is

0.245/2.230144=0.11kN/m2.

The load width is 2128/2=1064mm.

The load for the intermediate span area is 0.111.064cos(12°)=0.12kN/m.

There are cantilever sections of purlins in the edge span area, with a longitudinal cantilever length of the module of 666mm and a longitudinal cantilever length of the purlin of 720mm.

0.11666/720cos(12°)=0.11kN/m.

This is consistent with the software output.

Question 2

How should the wind vibration coefficient parameter be filled out? If the wind vibration coefficient calculated automatically by the software is sometimes very large, why is that?

Answer:

According to the requirements of Article 4.6.5 of the “General Specifications,” the wind vibration coefficient should not be less than 1.2.

The software calculates the wind vibration coefficient according to Article 8.4.3 of the “Load Code” and can execute the provision in Article 4.6.5-1 of the “General Engineering Structure Code” (GB55001-2021) that the wind load amplification factor should not be less than 1.2. When the parameter [Execute “General Engineering Structure Code” (GB55001-2021)] is checked, the program evaluates the calculated value of the wind vibration coefficient. If it is less than 1.2, the program uses a value of 1.2.

“Load Code” Article 8.4.3

Question 3

How is the wind pressure height variation coefficient calculated in the photovoltaic bracket software?

Answer:

Click [Calculate Wind Pressure Coefficients] to pop up the wind load parameter automatic calculation dialog box, select information such as ground roughness category, terrain conditions, bracket height, and correction coefficients, then click calculate. The program will display the calculated results for the wind vibration coefficient and wind pressure height variation coefficient in the lower right corner.

The wind pressure height variation coefficient is calculated based on Article 8.2.1 of the “Building Structure Load Code” (GB 50009). First, determine it according to the ground roughness category (A, B, C, D) and the height of the bracket above ground relative to its own height. Then, the program will automatically execute corrections based on terrain conditions: for sloped or peak terrains, the terrain correction coefficient η will be automatically considered according to Article 8.2.2 of the specification; for other special terrains such as mountain passes or valleys, users can manually input the corresponding correction values in [Other Terrain Correction Coefficients].

Furthermore, if the terrain correction coefficient for distant offshore sea and island terrain specified in Article 8.2.3 of the specification needs to be considered, users can check the option [Consider Distance from Coast Correction Coefficient] and input the corresponding distance value to complete the correction.

8.2.2 For buildings in mountainous areas, in addition to determining the wind pressure height variation coefficient according to the roughness category for flat ground from Table 8.2.1 of this specification, terrain condition corrections should also be considered. The correction coefficient η should be adopted as follows:

1. For peaks and slopes, the correction coefficient should be calculated as follows:

1. The correction coefficient at point B at the top can be calculated using the following formula:

Where:

tanα——The slope on the windward side of the peak or slope; when tanα is greater than 0.3, take 0.3;

k——Coefficient, take 2.2 for peaks and 1.4 for slopes;

H——Total height of the peak or slope (m)

z——Height from the calculation position of the building to the ground (m); when z＞2.5H, take z＝2.5H.

The verification process is as follows:

ηB$1{1+ktana(1-z/2.5H)}2

$1{1+1.4x0.1x(1-10/250)}2

$11.13442

$11.28686336

Considering the correction for slope or peak using formula 8.2.2-2, interpolation is performed.

Z=10m, H=100m

(10/100)x(1.2868-1)+1=1.0286, consistent with the software output of 1.0286.

Next, verify when the height z of the bracket relative to the base of the slope is 20m:

Z=20m, H=100m

(20/100)x(1.2868-1)+1=1.05736, consistent with the software output of 1.05737.

Question 4

How should the wind vibration coefficient for purlins be filled out?

Answer:

In the design of purlins, the program enhances the internal forces of the purlins under wind load conditions by calculating the ratio of the [Purlin Wind Vibration Coefficient] to the overall [Wind Vibration Coefficient], simulating the local shape coefficient of purlins specified in Table 4.1.3-2 of the “Photovoltaic Bracket Structure Design Code.” Generally, the inputted purlin wind vibration coefficient should not be less than the overall wind vibration coefficient.

It is important to note that the automatically generated wind load displayed in the modeling load is the load value transmitted to the main structure (beams, columns) and is not affected by the purlin wind vibration coefficient.

Note: The new version (version 7.0.0 and later) has added [Local Shape Coefficient].

Question 5

How should the shape reduction coefficient be filled out?

Answer:

This coefficient is used to consider the shape coefficient reduction for wind load specified in Note 2 of Table 4.1.3-1 of the “Photovoltaic Bracket Structure Design Code,” applicable to bracket structures that meet specific conditions in the photovoltaic array. Users can input the corresponding reduction coefficient based on the actual engineering situation.

“Photovoltaic Bracket Structure Design Code” Table 4.1.3-1 Note 2: When the number of rows in the photovoltaic panel array exceeds 7, the shape coefficients of the brackets within the second column and the fourth row at both ends can be reduced. The reduction coefficient can be taken as 0.85; the average value of the positive pressure shape coefficient after reduction should not be less than 0.6. The average value of the negative pressure shape coefficient should not exceed -0.9.

Everyone is welcome to discuss and exchange questions~ Thanks to all engineers for their valuable suggestions!

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