SF34 UNDER TOW FROM GATE AT IAD STRIKES TERMINAL WITH L WINGTIP. ABSENCE OF VOICE COMS WITH GND PERSONNEL RESULTED IN FLT CREW INABILITY TO WARN THEM.

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WE WERE COMMENCING PUSHBACK. THE #2 ENG WAS SPINNING. THE TUG INITIALLY PULLED THE ACFT FORWARD. HE THEN PUSHED BACK ABOUT THE SAME DISTANCE (APPROX 5-10 FT). AFTER THAT HE BEGAN TO PULL FORWARD AGAIN. THE CAPT AND I SAW OUR ACFT APCHING THE TERMINAL AT AN ANGLE; THEREFORE THE L WINGTIP AREA WAS COMING DANGEROUSLY CLOSE TO THE BUILDING. WE TRIED TO GIVE APPROPRIATE HAND SIGNALS TO STOP; USE OF HYD PUMP TO CREATE NOISE; AND TAXI AND LNDG LIGHTS; NONE OF WHICH WERE SUCCESSFUL IN PREVENTING THE COLLISION. THE L WINGTIP STRUCK THE BUILDING CAUSING SIGNIFICANT DAMAGE TO BOTH WING AND BUILDING. ONCE WE REALIZED THE ACFT HAD BEEN PULLED FORWARD BEYOND THE SAAB340 PARKING STOP MARKING; WE TRIED GIVING ALL OF THE ABOVE SIGNALS TO PREVENT THE ACFT FROM BEING MOVED ANY FURTHER. BECAUSE THE ACFT WAS MOVING FORWARD; EMER BRAKING WAS NOT APPLIED DUE TO NOSE GEAR STRUCTURAL CONSIDERATION. HAD THE NOSE GEAR STRUCTURAL LIMITATIONS BEEN EXCEEDED THE NOSE GEAR COULD HAVE COLLAPSED CAUSING SIGNIFICANTLY MORE DAMAGE.