Timoshenko has derived for the large deflection in the case of beam fixed at one end and free at the other end (image attached). Where in the length of the buckled beam is found by integrating tangent of the deflected profile to the vertical from 0 (fixed end) to alpha (free end). He also shows how we can find the final profile of the buckled beam.

When we try to apply the same method for the case of beam where both the ends are fixed, the angle made by the tangent at both the fixed ends and at the center of the beam is zero.

Is there a method to find the shape of the buckled beam, where in both the ends are fixed?

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