What is the design responsibility of the Contractor? Will the Contractor be liable for defective design?

Well, these questions are really depend on what type of building contract does the Contractor and the Employer go into? Is it a design and build contract? Or it is a conventional construction contract where the Contractor will only construct a structure based on the Consultant Designer's design. It is quite clear that if the contract is design and build contract, the Contractor is responsible for the design. But the next question will arise, to what extent the Contractor liable to the design?

The answer is the Contractor will be wholly liable to that. Any defective design will be the fault of the design and build contractor.

For conventional construction contract, I would rather refer it to non-design-and-build contract, the Contractor is basically responsible only to develop shop drawings based on tender drawings designed by the Consultant Designer (the Engineer and Architect).

The question of whether they are liable for defective design depends on whether or not they are aware that the design of the Consultant Designer is defective.

If the Contractor is aware of the design defect, they should write letter informing the same to the Designer or Employer. The Contractor has no responsibility to propose an alternative design replacing the defective design. Proper notice to the consultant and Employer will release them from being liable to defective design. However, failure in doing so may cause the Contractor to be partially liable for the defective design.

Yes of course, the manufacturer is liable when designer. It is very important to ensure for this action.

If the Design as per Contract is carried out by the Employer then the Contractor is not liable for defective design.

Actually It depens on the terms and conditions of the contract itself, base on that it will be decided if the design responsibility of the Contractor or not

the design approved before the execution of work if employer approved the design then no responsibility of the contractor to change the design.

This depends on the scope of the work and the related terms in the construction contract. If the contract is an EPC contract the contractor may be held responsible for defective design.

But designs are generally approved by the project owner before the same is executed !!