Ceiling fans have become increasingly popular in today's homes and businesses, such as restaurants.  In the past, ceiling boxes for supporting the fans were tested under static loads only.  With the increasing popularity of ceiling fans came the concern that the dynamic loads of an unbalanced ceiling fan could cause the mounting flanges of the ceiling boxes to flex and eventually break off.  This concern lead to a new 1987 N.E.C. code provision (N.E.C. 370-17C) that required ceiling boxes intended to be used as the sole support of ceiling fans to be tested and listed for that use.  Underwriters Laboratories (U.L.) responded with a new test which no standard ceiling box has yet been able to pass. 
The prior art ceiling boxes are made of metal and have a pair of mounting flanges with a threaded mounting hole extending through each flange.  In essence, these flanges are small ears formed integrally with the wall of the box and bent inwardly.  The ceiling fan is attached to the ceiling box by a pair of mounting screws which are threadedly received in the mounting holes of the pair of flanges. 
The problem with these prior art ceiling boxes is that the unbalanced dynamic loads cause the fan to rock back and forth, which in turn flexes and ultimately fatigues the flanges of the metal ceiling box until they break off. 
Also, it is common practice to secure a ceiling box to a mounting bracket and then to mount the mounting bracket to a joist or other structural support.  However, such mounting brackets are typically spot welded to the ceiling boxes and the cyclic loading of unbalanced ceiling fans causing the spot welds to fatigue and break. 
Examples of same prior disclosures of electrical boxes having various flange constructions are shown in the following U.S. Pat.  Nos.  3,075,487 to Appleton; 3,630,406 to Hammes; 4,057,164 to Maier, and 4,176,758 to Glick. 
Accordingly, there exists the need for a fixture box which resists the fatigue induced by the dynamic loads of an unbalanced ceiling fan.  Also, there exists the need for a fixture box and mounting bracket securing arrangement which resists fatigue induced by the dynamic loads of an unbalanced ceiling fan.