Creep Problems in Structural Members

Abstract

to fatlure too large). Thls effect may or nay not be of mportance te pract~cal declsxon-mkrng, depending on cmrcmstances. As conceived by Fukuzono 119851, plots of Inverse rate agalnst tlize, nearly Ilnear for 4 nearly 2, m y be usefully employed for forecasts. Cancave-up curvatures occur for a<2, and seem more dlfflcuht ta accurately extraplate by graphical means. Equatlon [?I may however often be used to advantage. Faslure occurs when the lnverse rate value of fif is obtained; thls 1s not precisely the pornt of Inter-sectmon of the inverse rate curve wrth the t~lnne abscls-sa. Predlctlons may be updated wlth acqulsltron of new data. Accuracy o f the m t h d 1s cond~tlaned by precision and frequency of observatsons and the segularrty of slope lsadxng condltlons. Varlable stress hlstorles wxll produce rate changes and variation In predicted trme to faxlure. With varlataon of stress, uslng the sim-pllfied case wlih neglect of bf : dbia la/oajn (1-t/tf) ldl-a where & is rate attached to an arbitrary aa, and m is a constant. An equatlon may also be wrltten for the general case. Increase rn stress thus produces an lm-&late Increase In rate, a process that Largely accounts for "klnksW [sharp bends) often observed in rate-time curves. Norra~alzxed displacements X* m y be written for the smple case as rate = 4 = i j a i , ~ / ~) ~. The general case may be described rn analogous fashion. With varzable stress, time to farlure 1s governed by a life fraction law where ti is time over which ui is applied, and ti is rupture t i e corresponding to constant stress ui. Because slope mventents generally include such complex factors as stress change requiring consideration, the experience of the interpreter remains an important factor. The method seems to apply well to diverse catttgorles of landslides An rock and soil, including first-time and reactivated slxdes, pit slope failures, toppling, and waste pile failures. With data sets that display strong seasonal changes the data can bar considered in t w ways : (1) long-term forecast (months to years) (2) short-tern forecaerts (days to months) In (I), the data set over a period of years is considered. Because preclprtation and snowmelt varies from year to year, slope loading and resistance conditions from one year to the next are not constant-that is, not only i s the variation seasonal, but each year the relative magnitude differs and 1s uncertain. The data may st111 J x used to suggest an approximate failure date, but the t ~ m e 'window' of possible failure remains large, owing both to the uncertainty in parameter variation and to the unknown, specific failure threshold. With seasonal variability, if slope escapes failure during the period of high-rates in a given year, the 'wilhdw' m y enlarge to encompass the following year. High precision in log-term forecasting i s thus impossible , though the results are still accurate enough for useful ntanagment decision-making. Fukuzono, T., Proc. IV Int. B. 1962. Complex stress, creep relaxation and fracture of metallic alloys. Edinburgh : H.M. Stationary Office. 032 the other hand, withrn a gxven year, there is an excellent chance that failure could be anticipated in advance in the short-term, say a week or more in advance. Mount Toe, in Italy, could have been anticipated (by the method described above) well over a week in advance, had the method been available. The same may apply to irapendlng failure at La Clapihre in France.