Each ordinance passed by the Council shall be published at least once within fifteen days after the adoption of the ordinance by the Council. All ordinances of the City shall become effective immediately upon the publication thereof, unless a date upon which an ordinance shall become effective which is subsequent to the date of the publication thereof is specifically provided in the ordinance itself. The publication of any ordinance in full after its final passage as part of the published proceedings of the Council shall constitute publication of such ordinance as required herein. An ordinance which is declared therein to be immediately necessary for the preservation of the public peace, health or safety may be given effect before publication as otherwise required by this Charter, if preliminary publication is accomplished by posting copies thereof in conspicuous locations in five public places in the City; and the Clerk shall, immediately after such posting, enter in the Ordinance Book under the record of the ordinance, a certificate under his hand stating the time and place of such publication by posting, which certificate shall be prima-facie evidence of such publication by posting; however, such ordinance shall also be published in the manner required for other ordinances within fifteen days after its adoption.