to add a new column append a TD to each TR (or change the colspan of a TD in each row you didn't modify).
you were doing this
<tr><td id="col">first<td>second</td></td></tr>
when you probably want to be doing something like...
<tr id="row_1"><td>first</td><td>second</td></tr>
$('tr#row_1').append('<td>second</td>');
