I would recommend using jQuery with this function:
$(document).ready(function(){
$('#about').addClass('expand');
});
This will add the expand class to an element with id of about when the dom is ready on page load.
This should work:
window.onload = function() {
document.getElementById('about').className = 'expand';
};
Or if you're using jQuery:
$(function() {
$('#about').addClass('expand');
});