San Francisco, Oct. 20—The dirigible Shenandoah arrived over San Francisco at 3:05 P. M., today and sailed over the business section for a half hour. As far as the city was concerned, she was first spoken off Point Bonita, six miles to the north, at 2:40 P. M., for an hour before the great envelope, steel grey against the sky, could be seen by thousands of eager watchers on the roofs of buildings and in the streets.