AN SF340 CAPT RPTED FATIGUE CONTRIBUTED TO HIS ALT UNDERSHOOT IN PIT'S AIRSPACE.

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ZOB DSNDED US TO 11000 FT AND HANDED US OFF TO PITTSBURG APCH. WHEN WE CHKED IN; PITTSBURG APCH ASKED US WHY WE WERE AT 11000 FT. WE REPLIED; 'IT'S THE LAST ALT WE WERE ASSIGNED.' AFTER A BRIEF PAUSE (DURING WHICH THE PIT APCH CTLR WAS APPARENTLY ON THE LAND LINE TO ZOB); THE PIT CTLR INFORMED US THAT WE WERE SUPPOSED TO BE AT 9000 FT. IT IS UNCLR WHETHER CLEVELAND FAILED TO CLR US TO 9000 FT OR WHETHER THEY CLRED US AND WE FORGOT TO DSND. MY FO COULD NOT VERIFY EITHER SCENARIO BECAUSE HE HAD BEEN OFF-FREQ GIVING A CABIN ANNOUNCEMENT; CALLING IN-RANGE; AND GETTING OUR GATE ASSIGNMENT ON COM #2 PER OUR COMPANY PROC. A CONTRIBUTING FACTOR IS THAT THE COMPANY WORKED ME NEAR FAA LIMITS DAY AFTER DAY PRECEDING THIS EVENT. TODAY I WAS SCHEDULED FOR ALMOST 8 HRS. THE PRIOR DAY I HAD TO BE REMOVED FROM 3 LEGS FOR BEING OVER 30 HRS IN 7 DAYS. THE FAA LIMITS (8 HRS FLYING; 16 HR DUTY; 30 HRS IN 7 DAYS; ETC) WORK SATISFACTORY FOR ANY 1 DAY; BUT I BELIEVE THEY ARE INSUFFICIENT TO PREVENT THE CUMULATIVE FATIGUE THAT OCCURS WHEN THE COMPANY USES PLTS TO AT/NEAR FAA LIMITS FOR SEVERAL CONSECUTIVE DAYS.