Here we have asteroid with width of 44 meters and height of 66 meters impacting ISD's bridge at 500 m/s (values estimations are eyeballed but I don't think real values are off by too large margin).
Volume of asteroid should be 100 355 m^3 or 1.0355x10e11 cm^3; density should range between 2.2 and 7.7 grams per cubic centimeter. So mass is between 2.2x10e11 and 7.73x10e11 grams or 2.2x10e8 and 7.73x10e8 kilograms, which gives us value of 2.75x10e13 and 9.66x10e13 J (27.5 and 96.6 TJ). That is 6.57 and 23 kilotons. Bridge is completely obliterated, as seen in above screenshots.
EDIT: I would like to thank Darkstar since I used first two screenshots from his page (I have movie on my computer but I don't have any program to capture screenshots - best thing I can do is "PrtScr" and then paste it in Photoshop. I did it with "Generations".
