Replacing Input Name With Variables

i'm trying to figure out how to access my input text with a variable:

Code working:
function 123(param){
var x = document.form1;
total = 0;
if(x.txSecurApp.checked){
total = parseFloat(x.tAdulteA3.value) + txSecur;
}
else{
total = parseFloat(x.tAdulteA3.value);
}}

Trying to :
function 123(param){
// param is tAdulteA
var str = param + Ɖ'
var x = document.form1;
total = 0;
if(x.txSecurApp.checked){
total = parseFloat(x.str.value) + txSecur;
}
else{
total = parseFloat(x.str.value);
}}

the var str is the problem, can i access my input this way.