Answered: NEC3 ECC: How should PM deal with argument over who is responsible for elements of design?

I agree with everything that Glenn says below.

On the question of whether the Contractor should already know that they are responsible for designing the steel work or not, clause 20.1 says "The Contractor Provides the Works in accordance with the Works Information" and clause 21.1 is even more explicit in that it says "The Contractor designs the parts of the works which the Works Information states he is to design".

So for the Contractor to be responsible for designing the detailed fixing drawings for the steel, there is ideally an explicit statement in the original Works Informations information stating this. On the other hand, the statement could be something like "The Contractor is responsible for developing the all the existing design beyond that stated in the Employer's Works Information except for the structural steelwork, but excluding the detailed fixing drawings.

If there are ambiguous statements, then the PM still, as Glenn says, needs to change the Works Information to clarify which would be a compensation event. The question is then how is it assessed. Clause 63.8 is the contra preferentum rule on steroids : namely that ANY ambiguity - as opposed to balance of probability in terms of meaning is assessed against the party who created that ambiguity.