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Foundations
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SMALL WIND CONFERENCE
JUNE 2011
Dilip Khatri, PhD, MBA, SEPrincipalKHATRI INTERNATIONAL, Inc.3579 E. Foothill Blvd., #736Pasadena, CA [email protected]
Khatri International Inc., www.khatrinternational.com
Wind Tower Foundations Spread Footings
Structural Cap with Piles: Drilled Piers
Pile Cap with Anchors
Pile Cap with Steel Piles
What are the key design factors?
1. Design Compliance + Constructability
2. Cost
3. Construction: Access, Materials, Value Engineering
4. SafetyKhatri International Inc., www.khatrinternational.com
Wind Tower Forces:1. Static Pressure2. Dynamic Forces3. Resonance/frequency loading4. OTM + V + P
Khatri International Inc., www.khatrinternational.com
Spread Footings Traditional foundation design solution
Relies on the bearing pressure of the soil
Cost = Excavation + Concrete + Rebar + Labor
As towers exceed 100 ft, the Spread Footing becomes excessively impractical for difficult site locations
Cost prohibitive as the foundation cost exceeds 20% of the tower installation
Khatri International Inc., www.khatrinternational.com
Pile Cap with Drilled Piers Cap Design with structural piles
Piles with reinforced concrete (drilled piers)
Cost = excavation + concrete + steel + labor
Cheaper on excavation, concrete, steel
Cost of drilling the pier and steel placement could be high
Khatri International Inc., www.khatrinternational.com
Piles with HSS HSS (Structural Tube
Sections) in lieu of rebar
Economical Install cost
No rebar in the pile
Drill, place steel, pour concrete
Khatri International Inc., www.khatrinternational.com
Micropiles Anchor Piles grouted in the drilling process
Hallow bar with grout injected into the pile hole
Micropiles are 12” or smaller
Axial Compression and Tension Load
Khatri International Inc., www.khatrinternational.com
Micropile Analysis Computer Finite
Element Model
Post Tension of Micropile
Axial Compression with Post Tension of the Cap
Khatri International Inc., www.khatrinternational.com
Analysis of Structural Failures1. Wind Tower Structures
2. Structural Failures
3. Wind Power Economics
4. Structural Design Considerations
5. Improving Structural Design Practice
Khatri International Inc., www.khatrinternational.com
Wind Tower Structures1980s – 1990s:
Wind Towers < 40m Truss Structures
Turbine Sizes 250 kW – 500 kW
Khatri International Inc., www.khatrinternational.com
Why Taller Towers?Wind Energy BasicsPOWER = dW/dt = Energy (work)/time = Torque x Angular Velocity
Khatri International Inc., www.khatrinternational.com
Swept Rotor Area
Khatri International Inc., www.khatrinternational.com
Structural FailuresStructural Tubular Failures:
Diameter-thickness ratios are high
Buckling failure due to instability of the tower tube
Khatri International Inc., www.khatrinternational.com
Structural FailuresStructural Tubular
Failures:
E-stop load condition
Overspeed of the rotor
Khatri International Inc., www.khatrinternational.com
Foundation Failures
Khatri International Inc., www.khatrinternational.com
Tower Failure
Khatri International Inc., www.khatrinternational.com
Khatri International Inc., www.khatrinternational.com
Foundation Design Issues Overturning moment
Soil failure
Dynamic stiffness
Fatigue causing cracks in concrete
Rotational stiffness degradation
Soil-structure interaction
Khatri International Inc., www.khatrinternational.com
Structural Failure/Collapse of Wind Towers 3 short videos of wind tower collapse
Tower design issues
E-stop loading
Rotor imbalance
Fatigue cracking
Buckling/stability failure
Khatri International Inc., www.khatrinternational.com
Conclusions AWEA, “Structural Failures of Wind Towers”, Khatri
Economical Design solutions are available
One size does NOT fit all; One foundation does not dictate all solutions for all soil sites; similarly one tower doesn’t necessarily work on all sites
Learn from our mistakes and implement new solutions
Structural Failures are linked to definable causes that have been experienced before and can be prevented
Khatri International Inc., www.khatrinternational.com
Thank You!Dilip Khatri,PhD, SE
Khatri International Inc., www.khatrinternational.com