The 9.81 m/s^2 of acceleration that we experience as gravity on the Earth's surface is not a centripetal acceleration, but, as the name implies, a gravitational acceleration, which is given by a = (k_g)*m/r^2, where k_g is the gravitational constant, m is the total mass of the attracting object (here the Earth) and r is the distance from the centre of the object (i.e. the Earth's radius). The formula for centripetal acceleration is not usable for that case, so it not giving out (something similar to) the equatorial rotational velocity is not surprising.
And I seem to get a velocity of about ~31.2 m/s = ~112.2 km/h for a ring with a radius of 100 m; could you post your calculations? I'm using v = sqrt(a*r) as given from the equation a=(v^2)/r where a = 9.81 m/s and r = 100 m.
EDIT: Important note: The force in your calculation above would /not/ be 9.81 Newton; it's given by F = m*a, where m is the mass of your human and a is the gravitational acceleration of 9.81 m/s^2.
