The Senate does not ratify treaties. The Constitution says that the President of the United States "shall have Power, by and with the Advice and Consent of the Senate, to make Treaties, provided two-thirds of the Senators present concur."
Although two-thirds of the Senate must agree to the treaty, ratification is what happens between the nations involved when the treaty is actually signed. The treaty does not get to the point of ratification, however, without the approval of two-thirds of the Senate. The Senate typically consents to treaties on which the body votes. The Treaty of Versailles is one of several exceptions. It was rejected twice by the Senate. Often, the Senate just does not vote on a treaty to which it does not intend to agree.