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420 CAN. J. CIV. ENG. VOL. 12, 1985
The authors agree with Ferahian that his discussion of the paper by S. M. Uzumeri et al. should have been referenced to provide a more complete picture of the history of the National Building Code of Canada (NBCC) seismic requirements up to 1977.
A knowledge of less than 100 years of instrumental recordings of earthquakes in Canada does not preclude relatively low probability estimates of future seismic ground motion. A knowledge of the larger historical earthquakes goes back at least 300 years in eastern Canada, although the history is shorter in other regions of the country. The important point is that the new zoning maps with a probability of exceedance of 10% in 50 years are not to be interpreted as an attempt to predict earthquake effects for the next 475 years (the return-period equivalent of this probability). They are moderate probability predictions of what can be ex- pected in the next 50 years, the typical lifetime of struc- tures designed to NBCC provisions. The assumption required is that the statistical distribution of earthquakes in the recent past, coupled with model assumptions about the earthquake source zones, allows a prediction into the near future.
Ferahian has argued that an uncertainty of approxi- mately a factor of 2 on these seismic ground motion estimates is very optimistic. We recognized that the uncertainty factor on the ground motion estimates is very difficult to quantify, and included a statement of uncertainty (e.g., approximately a factor of 2) solely to indicate to the user that these estimates are not exact. With respect to the merits of different probabilistic models, however, we believe that the sensitivity anal- yses in the references we cited (Weichert and Milne 1979; Basham et al. 1979) demonstrate that (a) the extreme-value method does not yield as robust an estimate as once thought, and (b) the new estimates are relatively robust against reasonable changes of model parameters down to probabilities of exceedance as low as per annum.
Ferahian questions our statement regarding the non-
code uses (and abuses) of the code. Yes, the code has been and will be misused despite warning and exclusion statements. Whenever the seismic requirements are used for cases where structural ductility is not com- parable with that found in buildings, there is the poten- tial for misuse. Specifically, if the force reductions that arise from ductile behaviour are not realized. then the forces specified by the building code are not'adequate for earthquake-resistant design.
The shortcomings of the 1970 zoning map have long been apparent. These have included inadequate sub- division in the higher-risk zones and an undue emphasis on peak acceleration as the governing ground motion parameter. The new zoning maps are seen as a sig- nificant improvement in these respects. It is slightly more complicated to use now that one needs two
but we are convinced that the engineering profession will not find this a major problem.
Finally, another impetus for updating the NBCC seis- mic provisions was provided by the model seismic code developed in 1978 by the Applied Technology Council (ATC-3) in the United States, which contains many novel and useful developments in seismic provisions, but not all of which are seen to be desirable or practical to implement. ATC-3 uses the 10% probability of ex- ceedance in 50 years as a basis for acceleration and velocity risk maps, and a version of the ATC-3 velocity map has been adopted by the American National Standards Institute as its zoning map for the United States. The changes to the 1985 NBCC will therefore put the zoning maps of Canada and the United States into closer harmony.
BASHAM, P. W., WEICHERT, D. H., and BERRY, M. J . 1979. Regional assessment of seismic risk in Eastern Canada. Bulletin of the Seismological Society of America, 69, pp. 1567- 1602.
WEICHERT, D. H. , and MILNE, W. G. 1979. On Canadian methodologies of probabilistic seismic risk estimation. Bulletin of the Seismological Society of America, 69, pp. 1549- 1566.
Fatigue investigation of a railway truss bridge:' Discussion
G. L. KULAK Department of Civil Engineering, University of Alberta, Edmonton, Alm., Canada T6G 2G7
Received November 23, 1984 Manuscript accepted January 30, 1985
Can. J . Civ. Eng. 12, 420-421 (1985)
'Paper by Z. L. Szeliski and 1. A. Elkholy. 1984. Canadian Journal of Civil Engineering, 11, pp. 625-63 I .
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DISC
The paper by Szeliski and Elkholy is a useful con- tribution to the subject of fatigue evaluation of existing steel structures. The topic, as any engineer employed by a railway system well appreciates, will be of in- creasing concern as the steel bridges constructed during the major railway expansion that took place in the early 1900's reach a theoretical limit of usefulness.
The writer would like to comment specifically on just one area, that described by the authors under the heading "Fatigue detail category." As correctly identi- fied by the authors, AASHTO and AREA specifications currently allow a riveted connection to be placed in category C or in category D. The distinction is that category C is applicable when it can be verified that the rivets are tight and that a normal level of clamping force exists in the rivet. Otherwise, category D must be se- lected. The selection of the "proper" category is an important decision; it can have a significant influence on the conclusions reached. For example, the authors' Fig. 8 (and the associated text) show the life remaining for the case of cars over 60 tons (535 kN). If the fatigue category selected is C, as used by the authors, there is a remaining fatigue life. If, however, category D is selected, the fatigue life of the structure has apparently already been exhausted.
It appears that the selection of fatigue category C in this case was made on the basis that ". . . the rivet holes were of good quality . . ." There is certainly no argument that in a structure of this kind it is most likely that the rivet holes will be the location of potential fatigue cracks. Likewise, the writer can accept that the holes in this structure were of good quality. Assuming that the rivet holes in any structure are of reasonable quality and do not therefore present an unusual fatigue condition, it is mainly the presence or absence of a preload in the rivet filling the hole that has the effect of changing the fatigue category from D to C, not the quality of the hole. The writer cannot accept that there is any reason- able way to ascertain whether there is a preload in the rivet, and if so, what its level might be. This comment applies to new work as well as to old work. The clamping force induced when a hot-formed rivet cools and shrinks is known to vary from joint to joint, and even among the rivets within a group in a particular joint (Carter et al. 1955). Baker and Kulak (1985) have summarized the studies available on the effect of
'USSIONS 42 1
clamping force on fatigue strength and have put forward their opinion that (a) clamping force is probably the predominant feature in explaining the rather large scatter in the fatigue life of riveted laboratory speci- mens, and (b) the amount of clamping force is known to be highly variable in new work. On this basis, they suggest that category D of the AASHTO or AREA specifications be used for riveted connections in exist- ing steel structures. (The writer recognizes that the ref- erence Baker and Kulak was not available to the authors at the time their paper was published. However, the source material upon which these opinions were based was available.) If the authors have any other basis upon which they have selected category C, for example mea- surement of clamping forces in the rivets of the struc- ture being examined, they will of course identify this in their reply to the discussion. It would also be helpful to know whether the holes were punched or were sub- punched and reamed. The method of hole formation can also potentially influence fatigue life. Likewise, nicks or gouges around holes, as a result of drifting during erection, would be damaging insofar as fatigue strength is concerned. Clearly, the selection of category D over category C can have significant economic implications.
Readers of the paper would also have gained from some reference to and discussion of the work by Reemsnyder (1975) on the benefits of replacing se- lected rivets at critical locations by high-strength bolts. Finally, it seems appropriate that the source of the data contained in Fig. 2 of the paper be acknowledged. The writer believes that it should be attributed to Fisher and Daniels (1976).
BAKER, K. A,, and KULAK, G. L. 1985. Fatigue of riveted connections. Canadian Journal of Civil Engineering, 12, pp. 184-191.
CARTER, J. W. , LENZEN, K. H., and WYLY, L. T. 1955. Fatigue in riveted and bolted single lap joints. Transactions of the American Society of Civil Engineering, 120, pp. 1353- 1380.
FISHER, J. W. , and DANIELS, J. H. 1976. An investigation of the estimated fatigue damage in members of the 380-ft. main span, Fraser River Bridge. American Railway En- gineering Association, Proceedings, 77, pp. 577-597.
REEMSNYDER, H. S. 1975. Fatigue life extension of riveted connections. ASCE Journal of the Structural Division, 101(ST12), pp. 2591 -2608.
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