Windload on Ground PV Structures
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Transcript of Windload on Ground PV Structures
WIND LOAD CALCULATION
STRUCTURE DETAILS
Details ValueTilt angle 45
Height(feet) 3.33Length(feet) 10Width(feet) 4
Lower end clearance(feet) 0.5
STAGE 1WIND FORCE
F = Qd*A where,
Symbol Description ValueF Wind Force(pounds) 4996.144Qd Design Wind Pressure(psf) 124.9036A Area of PV structure(ft^2) 40
DESIGN WIND PRESSURE
Qd = Q*G*Cpi where,
Symbol Description ValueQd Design Wind Pressure(psf) 124.9036G Gust factor 0.85Cpi Internal Pressure Coefficient 4.7θ Tilt Angle 45
WIND PRESSURE
where,
Symbol Description ValueQ Wind Pressure (psf) 31.26Kz Velocity Pressure Coefficient 0.85Kzt Topographic Effect Factor 1
Q = 0.00256*Kz*Kzt*Kd*V2*I
Kd Wind Directionality Factor 1I Importance Factor 1.15V Design Wind Speed(mi/h) 111.9
VELOCITY PRESSURE COEFFICIENT
Kz = 2.01*(Z/Zg)^(2/α) or Kz = 2.01*(15/Zg)^(2/α)
To find the coefficient, please enter values below from sheet 2
Factor ValueExposure C
Height of Structure (feet) 3.33Zg 900α 9.5
Kz 0.85
TOPOGRAPHIC EFFECT FACTOR
This factor helps in adjusting wind pressure if our PV structure is located in hill stationsIf the PV structure is located in plains, we simply take this factor as 1I have not included calculation method for hilly areas because it is quite complicated. If need arises please let me know, I will provide a solution
Kzt 1
WIND DIRECTIONALITY FACTOR
This factor accounts for change in wind direction. This value varies from 0 to 1. We take the maximum value for maximum safety
Kd 1
IMPORTANCE FACTOR
Importance factor takes care of risk caused to human life if this structure fails because of wind force
Category Importance factor4 1.15
BASIC WIND SPEED
Zone (Colour Code) Wind Speed(m/s) Wind Speed(mi/h)LIGHT PINK 50 111.9
WIND LOAD CALCULATION
STAGE 2UPLIFT FORCE
In this force, we need to consider Lift coefficients from sheet 5-Design Wind PressureUse the table below to calculate uplift force
Symbol Description ValueF Wind Force(pounds) 3614.23Qd Design Wind Pressure(psf) 90.36A Area of PV structure(ft^2) 40
Tilt Angle Lift coefficient45 3.4
Total Downward Force = Weight of Structure
Item Value(Kg) Value(pounds)Weight of Structure(Kg) 3827.6912 8436.23Weight of 1 module(Kg) 120 264.48Number of modules(kg) 5 5.00Weight of Concrete(Kg) 250 551.00
Weight of metal frame(Kg) 300 661.20
Factor of Safety is 1.2
FOS 2.3 SAFE
SLIDING FORCE
If force acting on the structure is greater than downward(weight) of the structure then entire structure slides creating problem
Friction Force = Weight*Friction coefficient
Item Value(Kg) Value(pounds)Weight of Structure(Kg) 3827.69 8436.23
Friction coefficient 0.65 0.65Friction force 2488.00 5483.55
Factor of Safety is 1.2
FOS 1.1 NOT SAFE
OVERTURNING MOMENT
This force tries to overturn our structure
Overturning Moment = Force*Perpendicular Distance(height)
Overturning Moment 16629.303
Factor of Safety is 1.4
FOS 0.5 NOT SAFE
Importance factor takes care of risk caused to human life if this structure fails because of wind force
VELOCITY PRESSURE COEFFICIENT
NOTES:There are 2 factors which determine Velocity Pressure Coefficient, they are
1) Surface Roughness, and2) Ground Exposure type
What is Surface Roughness and how to calculate ?
Surface roughness denotes the terrain aroung our PV structures. This is again divided into three categories, they are,
Number Surface Roughness type Description1) Surface Roughness B This category represents urban areas which are closely packed, something like cities, towns etc2) Surface Roughness C This region has buildings less than 10m tall and distributed. This is in between region B and D. Something like villages fall under this category3) Surface Roughness D This region usually represents desert type terrain with no obstruction to wind. Very open space
What is Ground Exposure type and how to calculate ?
Ground exposure denotes the type of terrain, wind travels before hitting our PV structures. This is divided into 3 categories
Number Ground Exposure Type Description1) Exposure B2) Exposure D3) Exposure C If condition does not satisfy both B and D, this is used
How to find Velocity Pressure Coefficient from above tables?
Steps:1) We need to find Exposure type2) We need to find height of structure
There are two different formulas depending on the height of PV structure
If height is greater than 15ft then use
Kz = 2.01*(Z/Zg)^(2/α)
If height is less than 15ft then use
Kz = 2.01*(15/Zg)^(2/α)
where, α and Zg are taken from the table below
Exposure Category Zg (feet) α
If SURFACE EXPOSURE B prevails for 800m or 20 times the height of PV structure (higher value among the 2).If SURFACE EXPOSURE D prevails for 1524m or 20 times the height of PV structure (higher value among the 2).
B 1200 7C 900 9.5D 700 11.5
Surface roughness denotes the terrain aroung our PV structures. This is again divided into three categories, they are,
DescriptionThis category represents urban areas which are closely packed, something like cities, towns etcThis region has buildings less than 10m tall and distributed. This is in between region B and D. Something like villages fall under this categoryThis region usually represents desert type terrain with no obstruction to wind. Very open space
Ground exposure denotes the type of terrain, wind travels before hitting our PV structures. This is divided into 3 categories
Description
If condition does not satisfy both B and D, this is used
prevails for 800m or 20 times the height of PV structure (higher value among the 2). prevails for 1524m or 20 times the height of PV structure (higher value among the 2).
DescriptionThis category represents urban areas which are closely packed, something like cities, towns etcThis region has buildings less than 10m tall and distributed. This is in between region B and D. Something like villages fall under this categoryThis region usually represents desert type terrain with no obstruction to wind. Very open space
Description
If condition does not satisfy both B and D, this is used
Importance Factor
Importance factor is divided into 4 categories. Please follow the table below and use this in sheet 1 to find wind force
Occupancy Category Nature of occupancy1 Temporary sheds, structures as those used during construction and those structures which possess low risk to human life2 Buildings and structures possessing low risk to human life and property in the event of failure. This does not include residential buildings3 All general buildings ,structures and residential buildings4 Important buildings such as hospitals, communication buildings, power plant structures etc.
Based on the occupancy category, select the Importance factor to find wind force. In sheet 1 just enter occupancy category, it will automatically collect data from this sheetI have included this table here just for your reference
Occupancy Category Importance factor1 0.872 13 1.154 1.15
Importance factor is divided into 4 categories. Please follow the table below and use this in sheet 1 to find wind force
Nature of occupancyTemporary sheds, structures as those used during construction and those structures which possess low risk to human lifeBuildings and structures possessing low risk to human life and property in the event of failure. This does not include residential buildingsAll general buildings ,structures and residential buildingsImportant buildings such as hospitals, communication buildings, power plant structures etc.
Based on the occupancy category, select the Importance factor to find wind force. In sheet 1 just enter occupancy category, it will automatically collect data from this sheet
Basic Wind Speed
Please use this map to find the appropriate wind speed for any location.I have included 6 colour codes to make it clear. Use the drop down list box in page 1 to input basic wind speed
I have included 6 colour codes to make it clear. Use the drop down list box in page 1 to input basic wind speed
DARK PINK
LIGHT PINKGREEN
LIGHT BLUE
YELLOW
DARK BLUE
Design Wind Pressure
This is the pressure that is actually acting on our PV structures. We make a small assumption here. Our PV structures are considered to be completely open and henceexternal pressure coefficients are zero and we have only internal presure coefficients
In the following table, I have mentioned pressure coefficients for various tilt angles. Please use the closest possible tilt angle to calculate design wind pressure
Roof Angle Pressure Coefficients Lift Pressure coefficients0 1.5 1.5
7.5 2.4 1.615 2.9 2.2
22.5 3.5 3.130 4.2 3.6
37.5 4.3 3.645 4.7 3.4
Gust factor:
This factor accounts for loading effect of wind turbulence on structures. This is different for rigid and flexible structures. PV structures are rigid and henceGust factor is taken as a constant value of 0.85 always
This is the pressure that is actually acting on our PV structures. We make a small assumption here. Our PV structures are considered to be completely open and henceexternal pressure coefficients are zero and we have only internal presure coefficients
In the following table, I have mentioned pressure coefficients for various tilt angles. Please use the closest possible tilt angle to calculate design wind pressure
This factor accounts for loading effect of wind turbulence on structures. This is different for rigid and flexible structures. PV structures are rigid and hence