One line of evidence supporting the rubble pile hypothesis comes from the measured rotation rates of 688 main belt and near-Earth asteroids. They indicate that no asteroid is currently spinning fast enough to be in a state of tension (Harris 1996). A subset of this information is even more revealing; the rotation period distribution of the smallest asteroids (i.e., 107 asteroids smaller than 10 km), abruptly truncates at P = 2.27 h. Interestingly, this is exactly where rubble pile bodies would begin to fly apart from centrifugal forces. Since solid objects should conceivably be able to spin at nearly any speed, the data suggests that most km-sized bodies lack tensile strength.
Figure 1: Histogram of rotation frequencies for 107 asteroids with diameters
smaller than 10 km. The plotted line is a Maxwellian distribution normalized
to the RMS spin rate, 5.54 cycles/day (P = 4.33 hours). Note the
distribution truncates at P = 2.27 hours, the same spin rate where rubble
pile asteroids would begin to fly apart (Figure from Harris 1996).