Is internal structural repairs covered under society expense

Structural audit and repair of beams, columns etc. involve active role of Managing Committee of the Society which has to be for entire premises and not for sole flat.

If the MC do not cooperate and take any action immediately call for Special General Body Meeting as per rules and replace MC with new members 

They will not reimburse you , it is better you issue a legal notice to them and if they still failed then file for inunction sui.

Though nothing has been written in the acts and rules either of a cooperative housing society or apartment owners association but I understand that structural work of an apartment in any building of any cooperative hoising society is to carried out by the MC since structural stability of the whole building comes into question.

Since you have already carried out the job (you didn't say any words about the type of structural repairs carried out by you) which is within your apartment the MC will play games not to pay. 

Further, when you found that the MC is not taking any action instead keeping silent it was desirable from your part to approach to a Chartered Structural Engineer/firmz who is an advocate and is in the panel of a court. Doing so, he/firm would have inspected the site, gravity of the job needed and impact / potential danger if not done,  gave you the estimate to be incurred and even would have sent a letter to the MC for and on your behalf.  

It is difficult to guide you further unless and until the nature of structural repairs done by you and what made you to get it done even without the written permission of the MC.