Element height help

hi there
I am trying to get the following elements to display a height that is the same as the parent container.

var divHeight = ((document.getElementById(‘content’).offsetHeight) + ‘px’)
alert(divHeight);
document.getElementById(‘col1’).style.height = divHeight;
document.getElementById(‘col2’).style.height = divHeight;
document.getElementById(‘col3’).style.height = divHeight;
</script>

unfortunately divHeight has no value. it relies on the content that is inside it.

if that makes sense??

any help much apreciated.

cheers