Strife has erupted at the Boston Athenaeum. As the private library has courted younger members and improved fund-raising, it has been rocked by internal divisions and staff departures: Nearly half of the Athenaeum’s roughly 55 employees have left in the past 3.5 years, a striking turn at an institution where tenure is often measured by the decade. Current and former employees, board members, and longtime supporters described an institution in turmoil, as director Elizabeth Barker seeks to modernize the tradition-bound library while being accused of disregarding its essential character and expert staff. Senior staff describe a “hostile” and “ruthless” administration that undermined their expertise. Beloved colleagues have been fired on the spot and marched out of the building. Others have retired in disgust, giving little or no notice.