I'm not sure if I understand your requirement correctly. Take a look at the demo
$(document).ready(function(){
$('#AddOT').click(function(){
var count = countColumn();
var insertedPosition = count-2;
$('#Table1').find('tr').each(function(index){
if (index == 0){
var colspan = $(this).attr('colspan') || "1";
$(this).attr('colspan',parseInt(colspan)+1);
}
else
{
$(this).find('td').eq(insertedPosition ).after('<td>cell 1a</td>');
}
});
});
});
function countColumn() {
var colCount = 0;
$('tr:nth-child(1) td').each(function () {
if ($(this).attr('colspan')) {
colCount += +$(this).attr('colspan');
} else {
colCount++;
}
});
return colCount;
}