function getWindowHeight() {
  var windowHeight = 0;
  if (typeof(window.innerHeight) == 'number') {
   windowHeight = window.innerHeight;
  }
  else {
   if (document.documentElement && document.documentElement.clientHeight) {
     windowHeight = document.documentElement.clientHeight;
    }
    else {
      if (document.body && document.body.clientHeight) {
        windowHeight = document.body.clientHeight;
      }
    }
  }
  return windowHeight;
}

function setFooter() {
  if (document.getElementById) {
    var windowHeight = getWindowHeight();
    if (windowHeight > 0) {
      var topHeight = document.getElementById('top').offsetHeight;
      var menuHeight = document.getElementById('menu').offsetHeight;
      var contHeight = document.getElementById('contenuti').offsetHeight;
      var totHeight = topHeight + menuHeight + contHeight;
      var footerElement = document.getElementById('piede');
      var footerHeight  = footerElement.offsetHeight;
      if (windowHeight - (totHeight + footerHeight + 5) >= 0) {
        footerElement.style.top = (windowHeight - totHeight - footerHeight) + 'px';
      }
      else {
        footerElement.style.top = 5 + 'px';
      }
/*alert(totHeight);*/
    }
  }
}

window.onload = function() {
  setFooter();
}

window.onresize = function() {
  setFooter();
}