On December 14, 2003 at 1955 eastern standard time, a Can Adair, CL-600-2B19 N445AW, registered to and operated by Air Wisconsin as flight 2146 from Miami, Florida, enrooted to Tampa, Florida, was struck by a pushback tug at Miami International Airport, Miami Florida. The instrument commercial flight was operated under the provisions of Title 14 CFR Part 121 with a flight plan filed. Visual weather conditions prevailed at the time of the accident. The airline transport rated pilot, crew and 40 revenue passengers were not injured. The accident occurred at 1955, on December 14, 2003. During the pushback from the gate, the tug driver attempted to stop the airplane on the ramp near spot 10. At the time of pushback the surface of the ramp was wet and dark. The tug operator applied the breaks and the tug began to skid. According to the tug operator, the aircraft slid to the right, and the tug was pushed to the left. The tow bar shear pin broke and the airplane turned to the right. The tug collided with the right side of the fuselage after the tug driver jumped from the tug. The passengers were deplaned on the ramp thru the air stair door. Examination of the tug operator's training records revealed the tug operator was certififed on 10/15/2003 in accordance with Aircraft Service International Group and Air Tran Airways standards and operating procedures. Examination of the airplane revealed 9 damaged ribs on the bottom side of the fuselage. The iris and four bolts were sent to the National Transportation Safety Board Materials Laboratory for further examination. The fracture analysis of the iris revealed that the iris was bent out of the tow bar and the four bolts broke in a shear/overstress action.