A B727 CREW HAD A BROKEN MAIN GEAR SIDE STRUT BRACE.

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AUG/XA/99; FLT LGA-DCA. NORMAL RIVER VISUAL APCH TO RWY 18. NORMAL LNDG. IN BOX (APPROX 1000 FT TOUCHDOWN ZONE); UNEVENTFUL BRAKING; SLOW TURNOFF AT TXWY F. UPON EXITING RWY; CALLED FOR 'FLAPS UP; AFTER LNDG CHKLIST.' AS I PUT SPD BRAKE FORWARD; INTERMITTENT AURAL WARNING SOUNDED AND RED AND GREEN R MAIN LNDG GEAR INDICATORS WERE ALTERNATING FLASHING; ACCOMPANIED BY RHYTHMIC THUMPING SOUND IN PLANE IN SYNC WITH FLASHING FLIGHTS. CALLED GND CTL AND REQUESTED EMER VEHICLES AND STATED PROB. ASKED THEM TO PIN R MAIN LNDG GEAR AND CALL OUR COMPANY MECHS. TOLD THEM I WASN'T TAXIING UNTIL GEAR WAS PINNED. MECHS CAME OUT AND GND INTERPHONE COM WAS ESTABLISHED. MECHS STATED 'A SIDE STRUT ON R MAIN LNDG GEAR WAS BROKEN AND THEY COULDN'T PIN THE GEAR.' EMER DIRECTOR ON SCENE AND MYSELF DECIDED TO OFFLOAD PAX (39 ON BOARD) USING PORTABLE AIR STAIRS. UNLOADED PAX; 1 AT A TIME; SO AS NOT TO SHIFT WT IN PLANE. PAX TRANSPORTED TO TERMINAL; LOGBOOK ENTRY MADE AND ACFT SECURED TO MAINT. SUPPLEMENTAL INFO FROM ACN 447188: AFTER A SMOOTH LNDG ON RWY 18 AT DCA; WE TAXIED ON TO TXWY F WHERE WE GOT AN UNSAFE R MAIN GEAR LIGHT. STILL NOT TOTALLY OFF THE RWY; WE STOPPED TO GET MAINT FOR A TUG; AND DEPLANED THE PAX BY PORTABLE STAIRS. (REASON FOR LIGHT: SIDE STRUT BROKE OFF THE GEAR.)