The pilot was entering the pattern with the intent to land when the wind direction shifted and the air traffic controller requested the pilot land on the opposite runway. According to the pilot on short final at approximately 20 feet above ground level, the airplane experienced a wind shear and the plane landed hard on the runway. Subsequently the airplane turned sharply left causing the right lower wing to contact the runway, damaging the main spar. The pilot reported that there were no preimpact mechanical malfunctions or failures with the airframe and engine that would have precluded normal flight.