What kind of semi trailer? Some can have lots more than half of 45K lb on them, and who is to say that even the nromal the 45K is evenly distributed. The trailer may be loaded so as to put more weight on the drivers and steer axles and less on the trailer axles. This is commonly done.
I suggest that you google 'landing gear' or 'trailer dollies' etc and find out what the maximum capacities that the manufacturers are offering. Then design your pad as if that load was on it.
Google 'mid america truck show' or 'great american trucking show' and if those work, find links to mfgr's.
rmw