29 Khatri Towers and Foundations for Small Wind Systems

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

http://www.khatrinternational.com/

mailto:[email protected]

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


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


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


Piles with HSS HSS (Structural Tube

Sections) in lieu of rebar

Economical Install cost

No rebar in the pile

Drill, place steel, pour concrete


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


Micropile Analysis Computer Finite

Element Model

Post Tension of Micropile

Axial Compression with Post Tension of the Cap


Analysis of Structural Failures1. Wind Tower Structures

2. Structural Failures

3. Wind Power Economics

4. Structural Design Considerations

5. Improving Structural Design Practice


Wind Tower Structures1980s – 1990s:

Wind Towers < 40m Truss Structures

Turbine Sizes 250 kW – 500 kW


Why Taller Towers?Wind Energy BasicsPOWER = dW/dt = Energy (work)/time = Torque x Angular Velocity


Swept Rotor Area


Structural FailuresStructural Tubular Failures:

Diameter-thickness ratios are high

Buckling failure due to instability of the tower tube


Structural FailuresStructural Tubular

Failures:

E-stop load condition

Overspeed of the rotor


Foundation Failures


Tower Failure



Foundation Design Issues Overturning moment

Soil failure

Dynamic stiffness

Fatigue causing cracks in concrete

Rotational stiffness degradation

Soil-structure interaction


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


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


Thank You!Dilip Khatri,PhD, SE


Documents

29 Khatri Towers and Foundations for Small Wind Systems