Working with Tables in Table Designer (Visual Database Tools)
The Table Designer allows you to change an existing database by changing, adding, duplicating, and deleting tables.
If the table is published for replication, you must make schema changes using the Transact-SQL statement ALTER TABLE or SQL Server Management Objects (SMO). When schema changes are made using the Table Designer or the Database Diagram Designer, it attempts to drop and recreate the table. You cannot drop published objects, therefore the schema change will fail.