COLUMN-BASE PLATE JOINTS UNDER MONOTONIC LOADS: THEORETICAL AND EXPERIMENTAL
5. ALTERNATIVE MODEL FOR EVALUATING THE INITIAL STIFFNESS The accuracy provided by Eurocode 3 in predicting the column-base joint
stiffness can be generally considered satisfactory provided that the attention is focused mainly on the ultimate behaviour. Conversely, when structural deformability is mainly of concern, a more accurate prediction of the base joint rotational stiffness can be useful. To this scope, some simplifications introduced by Eurocode 3 for an easier application, can be removed. In particular, the definition of an effective width to be specifically applied in stiffness calculation and, therefore, different from that used by Eurocode 3 for resistance evaluation could allow a further improvement of the stiffness prediction.
Regarding the component “concrete in compression”, according to the approach suggested by Steenhuis et al. (2008), the extension of the concrete zone subjected to compression in elastic range can be evaluated considering an equivalence between the behaviour, in terms of maximum deformation, of a flexible plate supported by springs with that of a rigid plate. From this analysis, the spreading of the action transmitted by the compressed column flange, which defines the effective width and length of the concrete in compression (Figure 1), provides a value of the parameter c in elastic range equal to 1.25 tbp.
In addition, with reference to the base plate in tension, according to an approach already suggested for beam-to-column joints (Faella et al., 1999), two different issues need to be considered: the first one regards the definition of the effective width in elastic range which can be defined assuming a 45° spreading of the concentrated bolt action starting from the bolt head edge, the second one regards the influence of bolt preloading, due to tightening, which can be accounted for by means of the following relationships: − ⋅ ψ ⎛ ⎞ = = ⎜⎜ ⎟⎟ η + − η ψ ⎝ ⋅ α⎠ 3 , 1.28 3 0.5 with 0 57 (1 ) eff bp bp bp t b b t t m k ψ . d (11) Sj/3 Sj Mj,exp M φ experimental curve
Figure 8. Evaluation of the experimental flexural resistance
Connections in Steel Structures VII / Timisoara, Romania / May 30 - June 2, 2012 61 62 Connections in Steel Structures VII / Timisoara, Romania / May 30 - June 2, 2012
where η is the bolt preloading level expressed as a fraction of that leading to a bolt tensile stress equal to 80% of the yield stress, db is the bolt diameter and α=m/db.
The results of the comparison between the values of the stiffness predicted by means of the proposed approach and those provided by the experimental tests are given in Table 5. It can be observed that the introduction of a more appropriate evaluation of effective width both in compression and in tension allows to improve the predictions leading to an average value of the ratio predicted/experimental value of the base plate stiffness equal to 1.09, while the standard deviation is almost unchanged.
Table 5. Comparison between experimental stiffness and the proposed approach
Specimen [kNm] SEC3 [kNm] Sexp exp , 3 , j EC j S S
Specimen [kNm] SEC3 [kNm] Sexp exp , 3 , j EC j S S
Specimen [kNm] SEC3 [kNm] Sexp ,exp 3 , j EC j S S W7-4.20-110 20533 24093 0,85 Pic.Beau. 1F 1326 1733 0,77 Pic.Beau. 13F 18441 10523 1,75 W8-4.20-0 14010 11500 1,22 Pic.Beau. 2F 1326 1733 0,77 Pic.Beau. 14CF 28529 26092 1,09 W9-4.20-0 14010 11500 1,22 Pic.Beau. 3F 1326 1733 0,77 Pic.Beau. 15CF 28458 26092 1,09 S220-010 2317 5061 0,46 Pic.Beau. 5CF 3379 8626 0,39 HE 240 B-15- 585 55505 59193 0,94 S220-040 9660 19991 0,48 Pic.Beau. 8F 18461 12484 1,48 HE 160 A-15- 34 10131 7494 1,35 S220-150 63017 70404 0,90 Pic.Beau. 9F 18461 12120 1,52 HE 160 A- 15233 12945 10437 1,24 S220-190 60369 53000 1,14 Pic.Beau. 10CF 37634 26092 1,44 HE 240 B-25-585 41452 43851 0,95 S140-010 1543 1131 1,36 Pic.Beau. 11CF 37634 26092 1,44 S140-100 17861 18132 0,99 Pic.Beau. 12F 18441 10523 1,75
Legend: W (Wald et al., 1995); S (Vandegans, 1997); Mean value 1.09
Pic.Beau. (Picard and Beaulieu, 1984); Standard deviation 0.38 HE (Latour et al. 2009)
CONCLUSIONS
The results of the first part of a research program aimed at the theoretical and experimental prediction of the cyclic behaviour of base plate joints have been presented. In particular, the analysis of the experimental tests carried out at the University of Salerno and of those collected from the technical literature has pointed out the degree of accuracy of Eurocode 3 component method for predicting the stiffness and resistance of column-base connections. An improvement of the approach for the stiffness predictions has been also obtained by means of a better definition of the effective width in elastic range, both in compression and in tension, according to models already proposed in the technical literature.
ACKNOWLEDGMENTS
This work has been partially supported with research grant DPC-RELUIS 2010-2013.
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