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 placeContainer(){
	var height=getWindowHeight();
	if(height<box_height){
		document.getElementById(divId).style.top=0;
		document.getElementById(divId).style.marginTop=0;
	}else{
		document.getElementById(divId).style.top="50%";
		document.getElementById(divId).style.marginTop="-"+(box_height/2)+"px";
	}
}

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