It doesn't seem excessive in my own experience. If in fact there were errors the installation crew may have had to delay until those errors were resolved... I don't think anyone in the forum would be able to answer whether or not they backcharges were justifiable without knowing exactly how your contract with the subcontractor was written; this is definately attorney type stuff ~ which may, in turn, lead to higher legal costs than the backcharges themselves... although it seems that the 1/3 your company was never notified about could be disputed in a business to business agreement... a hard situation any way you look at it!
I would request copies of documentation that justifies the backcharges.
To be reasonable, they shouldn't just throw a number at you, there should be a breakdown of what the problem was, the fix, and workhours spent in correction, etc. Expect costs for delays to be thrown in. Usually backcharges are negotiated against change orders.
I would also thoroughly review the contract specs and the AISC Code of Standard Practice (CSP) , if the contract invokes that spec.
Unfortunately, in most cases the fabricator is responsible for the detailing, even though drawings may have been subbed out. The owner's approval only means that, yes the drawings look somewhat like what one would expect, but, no, the actual dimensional details were not reviewed for accuracy. The term used is "general conformance" which means anything they want it to.
You could pass backcharges on to the detailer, but many are 1 or 2 person operations and you won't be able to get blood out of a turnip.
As I'm sure you have heard, there are times when erectors or GCs are losing money and backcharge over every little thing to recoup their losses. Sometimes, the backcharges reflect an "aggravation cost". You may also find that the backcharges are modest and reasonable.
Either way, paying a visit to the site while work in ongoing, talking with the erector and/or GC about problems and how to solve them, is time well spent. If there are detail problem make the detailer participate.
But for all jobs, know exactly what is required in the contract and specifications, and know exactly what you have provided as far as materials and quality and when it was shipped. Document, document, document everything. If you do, you will be taken seriously.
Chet Guilford