Influence of Concrete Material Ductility on Shear Response of Stud Connections. Paper by Shunzhi Qian and Victor C. Li/AUTHORS' CLOSURE

(ProQuest Information and Learning: ... denotes formulae omitted.)

Discussion by Shiming Chen

Professor, School of Civil Engineering, Tongji University, Shanghai, China.

The discusser appreciates the authors' comprehensive work to investigate the potential application of engineered cementitious composite (ECC) in shear stud connections for steel-concrete composite beams with a desired ductile slip capacity. Some test findings that are interesting to the discusser, however, were not well clarified when the test results were compared with the predictions based on the design method (AASHTO LRFD). Discussions are drawn as follows:

Compressive strength of concrete

Accordingly, it is understood that f'^sub c^, the compressive strength of concrete in Table 2, should be the cylinder compressive strength. For a comparison, the measured and predicted strength per stud based on AASHTO equations were drawn in Fig. A.

It is found that the predicted strengths based on AASHTO LRFD method were all greater than the measured strengths for concrete and RC stud connections so that the method would be unsafe, which is argued by the discusser. As being noted that a steel reinforcement ratio of 0.86% had been used for transverse reinforcement in RC specimens that could prevent earlier longitudinal splitting shear failure in the connections, is f'^sub c^ adopted in the paper a cube compressive strength rather than the cylinder compressive strength?

Strength and failure modes

In design practice, the shear stud connections are normally classified as ductile as far as ratio h/d > 4, where h and d are the overall height and diameter of a stud, respectively. Ductile behaviors were observed in RC, SFRC, and ECC specimens, except for concrete specimens.

It is likely that an ECC connection with a higher strength and less stiffer modulus in ECC properties will possess higher shear connection strength and greater slip capacity. This, however, does not occur for concrete specimens, and the main reason may be owing to the longitudinal splitting shear failure. It is still not clear in the authors' investigation that the longitudinal splitting shear failure can be prevented by ECC material itself other than transverse steel reinforcement.

It is agreed that the direct adoption of the AASHTO equation for ECC material will be very conservative, as the actual failure mechanism of ECC specimens is fracturing of the stud shank near the welds as observed in the tests. However, this should not be as simple as to use A^sub sc^F^sub u^ to predict the load capacity, because for Specimen ECC1, the measured load capacity per stud is greater than A^sub sc^F^sub u^ and for Specimen ECC2, the measured load capacity of each stud is much smaller than A^sub sc^F^sub u^, as shown in Fig. A.

A Comparison with EC4 method

In Eurocode 4 (ECCS Technical Committee 11 1993), the shear resistance of a headed stud in a solid slab is determined by

... (1)

... (2)

whichever is smaller. Where ?v is the partial safety factor taken as 1.25, f^sub u^ is the ultimate tensile strength of the stud, f^sub ck^ and E^sub c^ are the characteristic cylinder compressive strength and modulus of elasticity of concrete, respectively; d is the diameter of stud; and a is coefficient a =1.0 for h/d > 4.

Figure B test results and design formulas

To assess the load capacity of test specimens, let γ^sub v^ equal to 1.0. Design curves based on EC4 (solid line) and AASHTO (dashed line) equations are drawn in Fig. B. The measured load capacity of each test specimen is drawn against [the square root of]f^sub c^E^sub c^, a parameter scaling strength and modulus of concrete. Similar test results of push-out tests on studs (d = 19 mm) in high-strength and normal-strength concrete carried out by Li and Krister (1996) are also drawn in Fig. B.

It is demonstrated that the predicted load capacity by AASHTO is greater than that predicted by EC4. Concrete specimens (Concrete 1 and Concrete 2) fail on the margin of EC4 curve due to longitudinal splitting in concrete. The longitudinal splitting failure should be obstructed with transverse steel reinforcement in RC specimens, whereas the test results lie above the EC4 curve, as in Specimen RC1, steel reinforcement ratios are 0.56 and 0.86% in longitudinal and transverse direction, respectively; and in specimens tested by Li and Krister, the reinforcement ratios are 0.67 and 0.69% in longitudinal and transverse direction, respectively. There appears to be good agreement in tendency between the test results and the EC4 prediction for RC specimens for both normal- and high-strength concrete, whereas the hatched triangles plotted above the right side of the curve refer to the test results of high-strength concrete RC connection, and those on the lower and left positions refer to the normal strength concrete RC connection results.

In Fig. B, the measured results for ECC connections are close to that of SFRC connection as the strength of shear connections is concerned. AASHTO (dashed line) likely gives unsafe predictions of load capacity for RC shear stud connections and should also not be appropriate for ECC connections, though the measured values of the load capacity are greater than the AASHTO prediction.