Programmatically add site columns to a SharePoint List

SPSecurity.RunWithElevatedPrivileges(delegate()

                {

                    using (SPSite spSite = new SPSite(siteUrl))

                    {

                        using (SPWeb spWeb = spSite.OpenWeb())

                        {

                            spWeb.AllowUnsafeUpdates = true;

                            SPField fld = spWeb.AvailableFields.Cast<SPField>().FirstOrDefault(x => x.Title.ToString().ToLower() == existingSiteColumnName.ToString().ToLower());

                            projDocsList.Fields.Add(fld);

                            projDocsList.Update();

                            spWeb.AllowUnsafeUpdates = false;

                        }

                    }

                });