I think this depends on the way the Beam is held.
If the beam is clamped at one side and free at the other side, you know that deformation at the end of the beam is given by:
x=F*l^3/(3*E*I)
And you know that F=K*x
So I would say that the K is given by:
K=(3*E*I)/l^3 with l: length of the beam
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