ESDU 74026 gives curves for estimating the r.m.s. stresses at four points within a box structure that is rectangular in section and in which the supporting shear spars and webs are orthogonal; it is initially unstressed. The four points are at the centres of the spars and ribs@ and at the centre of the spar and rib edges (or flanges) adjoining the cover plates. The method assumes coupled vibration of the cover plates and internal structure; the validity of that assumption is discussed and an alternative method suggested where it is no longer acceptable. The predictions are compared with the results drawn from the literature of over 50 experimental tests.