<script type="text/javascript">
    function revisaVariableSesion() {
        var sesionValida = '<%= Session["sesionValida"] %>';
        if(!sessionValida)
            window.location = "LogOut.aspx";
    }
    var interval = setInterval(
        function () {
        revisaVariableSesion();
    }, 5000);
</script>

protected void Page_Load(object sender, EventArgs e)
{
    try
    {
        Session.Abandon();
        FormsAuthentication.SignOut();
        Response.Cache.SetCacheability(HttpCacheability.NoCache);
        Response.ExpiresAbsolute = DateTime.Now.AddDays(-1d);
        Response.Expires = -1000;
        Response.CacheControl = "no-cache";

    catch (Exception ex)
    {
        Response.Write(ex.Message);
    }
    Response.Redirect("/Login.aspx");
}