Get an array of each block, in order, via script. Then something like this:

// C#
// Assumes you have "GameObject[] pieces;" already set up.

IEnumerator BridgeFall()
{
    for(int i = 0; i < pieces.Length; i++)
    {
        pieces[i].rigidbody.gravity = true;
        // or however you make them "fall".

        yield return new WaitForSeconds(1);
        // or however long you want to wait for.
    }
}

Then, just start the coroutine with StartCoroutine(BridgeFall()); whenever you wanted it to fall.