"designed so that" is correct and usable in written English.
You can use it to indicate the purpose of something. For example, "The device was designed so that it would be easy to use."
Statutes are designed so that everyone knows what they are".
It was designed so that three dimensional motion of bubbles could be continually followed.
The stand is designed so that saws of all types and shapes can be tested.
The copolymer compositions were designed so that the thermally labile block would be the dispersed phase.
"It was designed so that people could get rid of unfit judges.
It is designed so that the tearing of one belt will not cause a catastrophic failure.
The motor is designed so that the pressures generated are nearly constant during burning.
Some new all-glass buildings are designed so that birds can easily detect them.
(The booth's louvred doors are designed so that store employees can look in).
The locks were designed so that each needed only one person for its operation.
Hoistway doors are designed so that they are always closed before the elevator can operate.
