Hi

Structural engineering comes under Earth sciences in csir net no other paper comes close to your subject other than this. Physical sciences is pure physics  and it can't be chemical sciences,life sciences or mathematical sciences. In apset exam earth,atmospheric, ocean and planetary sciences ( geology ) comes close considering your postgraduation subject.

The below link has subjects and their syllabus in detail. Check it and choose closest to your specialization.

https://competition.careers360.com/exams/apset

Hope this helps.