# Shear Behavior of Superposed Perfobond Connectors Considering Lateral Constraints
## 3 Experimental Results
### Load-Slip Curve  
Figure 13 shows the load-slip curves of the specimens. The perfobond connectors exhibited considerable shear stiffness in the elastic stage. When the curve reached the evident inflection point, the slippage of specimens was less than 2 mm, which conforms to the design of the rigid connectors. Nevertheless, the load-slip curves show clear distinction at the post-yield stage. For the single-hole connectors (Specimen P-4), the load-slip curve varied markedly: as shown in Figure 13d, the load-slip curve was similar to that of an inserted perfobond connector with strong constraints. When the specimen was loaded to the yield state, the bearing capacity continued to increase slowly due to the reinforcing effect of perforating rebar. Finally, the perforating rebar was cut off; meanwhile, the capacity fell suddenly at the moment the displacement reached 32 mm. As shown in Figure 7a-cj-l, the bearing capacity of the connectors without perforating rebar gradually fell to 80% after the peak load (e.g., Specimens P-3 and CP-3). On the contrary, the bearing capacity of perfobond connectors with perforating rebar fell slightly after the peak load. Subsequently, the curve rose again thanks to perforating rebar, showing the phenomenon of secondary reinforcement. Until the termination of loading, the curve stayed stable without a significant decrease (e.g., Specimens P-1, P-2, CP-1 and CP-2). Based on Figure 13a,h,i, even if each specimen has perforating rebar in the hole, Specim NP-1 had a lower bearing capacity because of no transverse reinforcement in the slab, the capacity attenuation rate was the fastest after the peak load, and soon fell to 80% of the peak value, the specimen did not exert the reinforcing effect of the perforating rebar. However, for the Specimens P-1 and SP-1 with ordinary stirrups and segmental stirrups, respectively, the bearing capacity gradually decreased after the peak load, and then the curve rose again slowly. The load-slip curve shows a secondary strengthening effect. As illustrated in Figure 13a,e-g, Specimens P-1, P-5, LP-1 and HP-1 (featuring different-sized holes) featured a perforating rebar and transverse reinforcement. Hence, the load-slip curves exhibited little difference, and the loads did not attenuate significantly after the peak value, meaning that a good load-holding capacity was maintained. However, there was no secondary strengthening section in the curve of specimens P-5, LP-1 and HP-1 due to the larger hole areas. According to the characteristics of the load-slip curve, the static performance indexes of each specimen were calculated, as shown in Table 4. Considering the mechanical differences between specimens, the values of the performance indexes were stipulated as follows: the ultimate bearing capacity \(P_{\text{u}}\) was the maximum load during the test; for Specim P-4, which did not exhibit a descending section in its curve, the ultimate bearing capacity \(P_{\text{u}}\) was the corresponding load when the slip reached 20 mm [33]; according to EC4 [28], the characteristic value \(P_{\text{RK}}\) was defined as \(0.9P_{\text{u}}\); the design value \(P_{\text{Rd}}\) was computed according to  
\[P_{\text{Rd}}=\frac{f_{\text{u}}}{f_{\text{ut}}}\frac{P_{\text{RK}}}{\gamma_{ \text{v}}}\leq P_{\text{RK}}/\gamma_{\text{v}} \tag{1}\]  
where \(f_{\text{u}}\) is the design ultimate strength of the material, \(f_{\text{ut}}\) is the actual ultimate strength of the material, and \(\gamma_{\text{v}}\) is the security coefficient, which was set to 1.25 [28]; the shear stiffness \(K\) adopted the secant stiffness with a relative slip of 0.2 mm [34] on the curve; the ultimate slip \(\delta_{\text{u}}\) was the corresponding slip value when the load reached \(P_{\text{RK}}\)[28], which was 20 mm [33] for specimens whose loads did not fall to \(P_{\text{RK}}\); and the ductility coefficient \(D\) was taken as the ratio of the ultimate slip \(\delta_{\text{u}}\) to the design slip \(\delta_{\text{d}}\), corresponding to the design value \(P_{\text{Rd}}\) of the bearing capacity.