hidden variable value is empty on code behind

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VB

ASP.NET

hi all

Problem is I have hidden variable and stored a value to hidden variable through javascript
while retrieving hidden varaible from code behind it is retrieved as empty value

javascript code is

function CalculateTotalAmountWithLoadingCharge() {
var loadingValue = parseFloat(document.getElementById('<%= txtLoadingcharge.ClientID %>').value)
if (loadingValue >= 0) {
document.getElementById('<%= hdnLoadingCharge.ClientID %>').value = loadingValue
document.getElementById('<%= hdnLoadingChargeTemp.ClientID %>').value = "valuechanged";

}

<asp:UpdatePanel ID="upHiddenField" runat="server" >
<ContentTemplate>
<asp:HiddenField ID="hdnQuantity" runat="server" />
<asp:HiddenField ID="hdnLoadingCharge" runat="server" />
</ContentTemplate>
</asp:UpdatePanel>

<asp:UpdatePanel ID="upFreightType" runat="server"  UpdateMode="Conditional" >
        <ContentTemplate>
        
                <asp:TextBox ID="txtQuantity"  runat="server" CssClass="textBox" Text="1" onchange="onchangeQuantity();" Width="40px" />
             <
                <asp:TextBox ID="txtBasicAmount" runat="server" onchange="BasicAmountChange()" Text="0"  CssClass="textBox" Width="160px" />
            </div>
      
        
        <%--</td>
        </tr>
             <tr align="left">
           <td  style="width:100%;">--%>
                <asp:Label ID="lblLoadingCharge" runat="server" Text="Loading Charge:" CssClass="label" />
                 <asp:TextBox ID="txtLoadingcharge" runat="server" CssClass="textBox" Text="0" onchange="CalculateTotalAmountWithLoadingCharge()" Width="160px" />
                 </div>
             
                 <asp:Label ID="lblTotalAmount" runat="server" Text="Total Amount:" CssClass="label" />
                <asp:TextBox ID="txtTotalAmount"  runat="server" Text="0" CssClass="textBox"  Width="160px" />
             
      </ContentTemplate>
        <Triggers>
        <asp:AsyncPostBackTrigger ControlID="ddlFreightType" EventName="SelectedIndexChanged" />
        </Triggers>
        </asp:UpdatePanel>
        </td>
        </tr>
 


<asp:Button ID="btnSave" runat="server" Text="Book" CssClass="button" Width="160px" OnClientClick="CalculateTotalAmountWithLoadingCharge();" />

in code behind

Dim loadingcharge As Integer=hdnLoadingCharge.Value

I have update panel to get value to retrieve the value on dropdown selected index changed that works fine after retrieving the value i process and store it in hidden variable while trying
to access it in my code behind the value of hidden variable is empty string (i.e) "".

How can i retrieve the hidden variable value from code behind


Thanks in advance . what have i done wrong