for(var i=20; i<40; i++){
  var s = i*8,
      m = s/2;

  $('<div class="ring ring-'+i+'"></div>')
    .appendTo( $('.ring').last() )
    .css({
      width : s+'px',
      height : s+'px',
      margin : '-'+m+'px 0 0 -'+m+'px',
      borderWidth: m/2+'px'
    });
}