This steel is often used as a floor plate. I could not find the Brinell hardness rating.
I don't have current ASTM specs ( they are renewed every 5 years); However , it is very possible there is no hardness specified . There are tensile and yield strength and ductility requirements ; minimums ( usually in a spec like this, no maximums are specified). One can get a pretty good estimate of the ( minimum ) hardness from conversion charts. For example : a minimum tensile of 60 ksi corresponds to a Brinell of about 105 ; a minimum tensile of 100 ksi corresponds to a BHN of 200. The yield strengths do not correlate as well to hardness. Conversion tables are available on the net ( which will give better numbers than I took off a small chart).
I don't think there will be just one. From ASTM's web page:
This specification covers carbon, low-alloy, high-strength low-alloy, and alloy steel hot-rolled floor plates for flooring, stairways, transportation equipment, and general structural purposes. Steel materials shall undergo heat and product analysis. Steel specimens shall also undergo tensile tests and shall conform to the required values of yield point, yield strength, and tensile strength.
It sounds like the specification is for a family of products, not for one specific steel. So your ASTM A786 plate may be different from mine.