alter database mydb
add file 
(name='my_db_data1',
filename='c:\program files\microsoft SQL server\mssql\data\my_db_data1.ndf',
size=3,maxsize=15,filegrowth=2)
go
alter database mydb
modify file
(name='my_db_log',
maxsize=20,filegrowth=2)
go