Ground-Mount Solar Array Foundations

Transcription

1 Ground-Mount Solar Array Foundations Thomas E. Billups, PE Principal, GZA GeoEnvironmental, Inc. Providence, RI Foundation Support Ground Mount Solar Systems Types of Foundations Used: Precast Concrete Footings (Ballast Blocks) Cast-in Place Concrete Footings Drilled Piers Piles (helical piles, light wide flange sections) 1

2 Solar Panel Loads Dead Weight Snow Load Seismic (Lateral Load) Wind (Lateral Load) Dead and Live (Snow) Loads Dead Load Supplied by Manufacturer Typically Around 4 PSF. Typical Size of the PV Arrays Varies Depending on Manufacturer. 4 Panels by 4 Panels 300 SF Around 1,200 Pounds. 3 Panels by 4 Panels 225 SF Around 900 Pounds. Snow Load Ground Snow Load According To Local Building Code Provisions. ACSE Allows Reduction Cold Roof Slope Factor With an Unobstructed Slippery Slope. 2

3 (Snow Load Continued) Typical Snow Load Factors: Importance Factor 0.8 Exposure Category Cold Roof Thermal Factor 1.2 Unobstructed Slippery Slope Factor 0.73 for Panels inclined at 30 Degrees From Horizontal Design Snow Load = 0.7 X Ground Snow Load for Panels inclined at 30 Degrees. Seismic and Wind Loads Wind Loads Are Larger Than Seismic Loads Seismic Loads Are typically a Percentage of the Dead Weight, Which is Small Compared to loads generated from Wind Speeds of 90 to 120 mph. Wind Load Determined Using ASCE Design Wind Loads on Open Buildings With Monoslope/Pitched Roofs P = q h x G x C N q h = Velocity Pressure (Next Slide) G = Gust Factor = 0.85 (Rigid Structure) C N = Net Pressure Coefficient (Open Roof Structure, ASCE 7 Figure 16A) For 30 Degee inclination: Produces Lift With Wind in One Primary Direction CN = 1.8 Produces Down Force in the Opposite Direction CN = 2.1 3

4 Wind Perpendicular to Project Area Wind Load (Continued) q h = Velocity Pressure = x V 2 x K z x K zt x K d x I V = the Basic Wind Speed Building Code K z = Velocity Exposure Coefficient (0.85) K zt = Topographic Factor (1) K d = Wind Directionality Factor (0.87) I = Importance Factor (0.87) Wind Produces Horizontal and Uplift loading in One Direction, And Horizontal and Downward Load in the Opposite Direction. Manufacturers May Have Wind Tunnel Test Results That Result in Lower Coefficients for C N. 4

5 Load Combinations Chapter 2 - ASCE 7-05 Can Use Factored Loads or Working Stress Applicable Working Stress Load Combinations: Dead + Live (Snow) Dead + Wind Dead Wind Live 0.6 Dead + Wind Typical Combined Loads on 4 X 4 Array Total Downward Load 10 to 14 kips Total Horizontal Load 3 to 5 kips Total Uplift Load 2 to 4 kips 5

6 Frame Supported by 4 Drilled Piers Solar Installation Haverhill, MA Unirac System On Drilled Piers Pier Depths Determined Using Brohm s Method Advantages/Features Drilled Piers Installed with Auger Mounted on a Backhoe; Good for Applications Above Groundwater; Can Preassemble Reinforcing Cages; Terracing or Grading of Site not Necessary; Simple Design Using Brohm s Method; Uplift resistance Using Dead Weight of Concrete; Not suitable on Capped Landfill Sites. 6

7 Cast In Place Concrete Footings Cast-In-Place Concrete Footings (Ballast Blocks) Haverhill, MA Features Advantages - Cast-In-Place Footings/Ballast Blocks Simple to form and construct, but more labor intensive on site that pre-cast concrete footings; Can Pre-Assemble Reinforcing Cages; Good for Flat Sites Regrading/Terracing is Required; Simple Footing Design Procedures for Bearing, Overturning and Sliding. 7

8 Completed Installation Precast Concrete Ballast Blocks A-Frame Solar Racks Everett, MA 8

9 Leveling/Adjustment Hardware Should be set mid height to allow for adjustment Base Course Preparation? 9

10 Features/Advantages Precast Footings Concrete Cast in a Controlled Environment; Construction Schedule May be Accelerated; Other Advantages and Features Same as for Cast-in- Place Concrete Footings; Commonly used on Flat Portions of Landfill Sites. Helical Piles 10

11 Piles (Helical, W Sections) Features and Advantages Solar Arrays (4 X 4, 3 X 4) Are Supported on Pairs of Piles. L-Pile Software Useful for Determining Embedment Depths, Bending Moment and Deflections for the Load Combinations. Combined Axial/Bending Stresses Need to Be Checked. Manufacturer s Indicate that Deflections of 2 to 3 Inches are Ok under worst case wind loading. Fast Installation - Schedule 40 Pipe Sections With Auger Tips are Screwed Into the Ground; Axial Uplift and Download Capacity Based on Calibrated Torque; Torque Equipment is Mounted on an Excavator; Regrading of Site Is not Required. Not Suitable for Landfill Sites Piles - Disadvantages Obstructions Can Limit Pile Embedments. Not Suitable for Dense Soils With Cobbles and Boulders (Glacial Till). Top of Pile Has Tight Tolerance So That Rack Attachment Hardware Will Fit (2 inches). Cobbles and Boulders Will Deflect Pile Tips and Tops. Helical Piles: The Larger Diameters of Helical Flights Can Leave Gaps Around the Smaller Diameter Piles. 11

12 L-Pile Method of Analysis Deflection Bending Questions? 12