GetElementByID non trova elemento presente nella pagina

<asp:ScriptManager id="scriptManager" runat="server">
            <Scripts>
                <asp:ScriptReference Path="~/Scripts/FooterPlacer.js" />
            </Scripts>


Qui il file javascript:

window.onload = footerPlacer();

function footerPlacer()
{
    var layoutMainContainer = new Array(); //Holds the main container for the 2/3 cols layout
    layoutMainContainer[0] = "contentBox3COLS";
    layoutMainContainer[1] = "contentBox2COLS";
    
    var currentContainer;
    
    for(i=0; i< layoutMainContainer.length;i++)
    {
       currentContainer = document.getElementById('layoutMainContainer[i]');       
       if(currentContainer != null)
       {
            
       }
    }   
}

    var layoutMainContainer = new Array(); //Holds the main container for the 2/3 cols layout
    layoutMainContainer[0] = "contentBox3COLS";
    layoutMainContainer[1] = "contentBox2COLS";
    
    var currentContainer;
    
    for(i=0; i< layoutMainContainer.length;i++)
    {    
       currentContainer = document.getElementById(layoutMainContainer[i]);
         
       if(currentContainer != null)
       {
           //Prendi i figli del primo livello
    var children = new Array();
    for(k=0; k<currentContainer.childNodes.length;k++) 
   {
        if(currentContainer.childNodes[k].nodeType == 1) 
            {   children.append(child);      }
    }

       }
    }