A better solution to solve your issue is to define your own 'sorter' function for the columns. Please refer to the code below:
<th data-options="field:'f1',width:80,sortable:true,sorter:function(a,b){return parseInt(a) > parseInt(b) ? 1 : -1;}">C1</th>
<th data-options="field:'f2',width:100,sortable:true">C2</th>