Hollywood’s behind-the-scenes crews have voted overwhelmingly to authorize a nationwide strike if they cannot reach agreement with producers on a new contract, their union announced on Monday. The International Alliance of Theatrical Stage Employees (IATSE), which represents some 60,000 off-screen workers, said that 90 percent of its members cast ballots and more than 98 percent of the votes returned were in favor of authorizing a strike. The vote strengthened the hand of IATSE leaders in stalled contract talks with the Alliance of Motion Picture and Television Producers (AMPTP). The alliance said it is the first time in IATSE’s 128-year history that union members have authorized a nationwide strike, and it comes amid disputes with studio producers about working conditions, pay, and schedules. Many members say working conditions are poor, workdays are long with few breaks, and hourly rates are below $18. “The members have spoken loud and clear,” the alliance’s …