Resolvido !!!
Utilizei assim :
define( '_JEXEC', 1 );
define( 'DS', DIRECTORY_SEPARATOR );
define('JPATH_BASE', 'path_site');
require_once ( JPATH_BASE .DS.'includes'.DS.'defines.php' );
require_once ( JPATH_BASE .DS.'includes'.DS.'framework.php' );
$mainframe =& JFactory::getApplication('site');
$mainframe->initialise();