DH8A CREW IGNORED THE GPWS 'SINK RATE' WARNING WHEN LNDG IN SEVERE TURB AND XWIND. A HARD LNDG DAMAGED THE ACFT.

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ON A VISUAL APCH TO RWY 3 IN ZZZ; THE ACFT SUSTAINED DAMAGE AFTER A HARD LNDG. THE FO WAS THE PF AND HAD THE ACFT ON A STABILIZED APCH SLIGHTLY ABOVE THE ILS GS. MODERATE TURB BEGAN ON OUR DSCNT AT ABOUT 8000 FT MSL AND CONTINUED THROUGHOUT THE APCH. EARLY IN OUR DSCNT; WHILE WITH POTOMAC APCH; AN ACFT WHO HAD JUST TAKEN OFF RPTED MODERATE TO SEVERE TURB (TYPE UNKNOWN). A FEW MI OUT; THE TWR GAVE US THE WIND SPD AT 20 KTS WITH HIGHER GUSTS RPTED PREVIOUSLY. THE FO WAS HOLDING A 25 DEG L WIND CORRECTION TO MAINTAIN ON THE LOC. AT ABOUT 500 FT; TWR GAVE US A WIND CHK AT 25 KTS. WE HAD BRIEFED A VREF OF 113 KTS TO ACCOUNT FOR THE GUSTY CONDITIONS. FO HAD THE PWR FAIRLY STEADY AT ABOUT 30% TORQUE. AT 500 FT LNDG CHKLIST WAS COMPLETE AND TURB BECAME SEVERE. I LOOKED OUTSIDE THE REST OF THE WAY DOWN. INDICATED AIRSPD WAS FAIRLY STEADY AT 130-135 KTS PRIOR TO 500 FT. AFTER 500 FT; THE TURB BECAME SO BAD I COULD NO LONGER READ THE INDICATOR; NOR WAS I TRYING TO. I WAS LOOKING OUTSIDE. I POSITIONED MY HAND NEAR THE PWR LEVERS TO BACK UP THE FO. WE WERE CLOSE TO THE THRESHOLD WHEN I HEARD A 'SINK RATE' FROM THE GPWS AND HEARD TORQUE INCREASE; THOUGH VISUALLY WE DID NOT APPEAR TO BE LOW AND ULTIMATELY; WE DID NOT LAND SHORTER THAN NORMAL. I WAS NOT ALARMED BY ANY EXCESSIVE SINK RATE. I WAS MORE ALARMED BY HOW VIOLENTLY THE ACFT WAS SHAKING. WE WERE CLOSE TO THE GND WHEN I HEARD THE GPWS 'SINK RATE' AND MY SPONTANEOUS THOUGHT PROCESS WAS TO MAINTAIN THE EXTRA AIRSPD WE HAD AND GET THE ACFT ON THE GND. I NEVER CONSIDERED A GAR BECAUSE: 1) SINK RATE DID NOT APPEAR EXCESSIVE VISUALLY. 2) WE WERE CLOSE TO THE GND AND IT DID NOT SEEM LIKE A GOOD OPTION. 3) IT DID NOT APPEAR NECESSARY. THE FO MAINTAINED THE AIRSPD UNTIL WE WERE LOW OVER THE RWY AND SHARPLY BROUGHT THE PWR TO FLT IDLE. IT FELT LIKE SOMEONE PICKED UP THE AIRPLANE AND THREW IT INTO THE GND. THE ACFT TOUCHED DOWN VIOLENTLY; HARD ON THE MAIN GEAR. AFTER WE CLRED THE RWY; TWR SAID THE WIND WENT TO 30 KTS AS WE CROSSED THE THRESHOLD. I TAXIED TO THE GATE AND CALLED MAINT CTL WITH A HARD LNDG WRITE-UP. AT THE TIME; I FELT IT WAS LIKELY SOMETHING MAY HAVE BEEN BENT OR OUT OF PLACE; BUT I HAD NO IDEA WE HAD DONE SERIOUS DAMAGE TO THE ACFT. FIRST; I WAS SURPRISED BY HOW HARD WE HIT; THEN I WAS SURPRISED TO HEAR HOW MUCH DAMAGE IT HAD CAUSED. I DID NOT LEARN OF THE SERIOUSNESS OF THE DAMAGE UNTIL LATE THE NEXT NIGHT WHEN I WENT TO THE HANGAR TO SEE THE ACFT. WHEN I SUBMITTED AN INCIDENT RPT TO THE COMPANY; I DID NOT REALIZE THE ACFT WAS DAMAGED.