check box validation

Hi

     I Want To Validate the Check box in grid view.I face One problem i m comparing total no of checked with textbox (entered data in textbox)  i m sending textbox through

 
function validates()
{

var ar = document.getElementById('<%=txtavailableseats.ClientID%>');

alert(ar); //this show null
for(j=0;j<grd_Cb.length;j++)
{
var obj = document.getElementById(grd_Cb[j]);if(obj.checked == Go to the complete details ...