construction workers are working on beams.
construction workers on duty
If the workers are working on beams then it is implied they are performing their job in construction, which is the same as saying they are on duty.
exists e1 x1 x2 x3.(on(e1,x3) & n_beam(x3) & Actor(e1,x1) & v_work(e1) & n_worker(x1) & of(x1,x2) & n_construction(x2))
exists x1 x2 x3.(a_topic(x1) & on(x1,x3) & n_duty(x3) & n_worker(x1) & of(x1,x2) & n_construction(x2))