## 1 Introduction  
Steel-concrete composite members show high load-carrying capacity, high local stability, and high stiffness due to the composite action of steel and concrete [1, 2]. To achieve the desired composite action or to transfer longitudinal shear between steel and concrete, steel anchors or shear connectors should be provided at the interface between the steel and concrete [1, 3]. For this purpose, steel-headed stud anchors (hereafter, stud anchors) have been most widely used, but recently steel shape anchors such as C-, L-, T- and Z-channels (hereafter, channel anchors collectively) have also gained popularity since conventional welding equipment can be used and the required performance can be obtained with a smaller number of connectors [3, 4]. In general, channel anchors are welded to the base steel sections along the full length of the anchors. However, in some cases of composite members using U-shaped or built-up sections [4, 5, 6], only both ends of the channel anchors are welded to base steel sections. In those cases, the partially welded channel anchors can function as (1) steel anchors as intended, (2) battes to form a closed or built-up section, and (3) walk-plates or steps during construction. Figure 1 shows an example that utilizes partially welded Z-channel anchors to a composite beam: an H-beam (or wide-flange beam) with a concrete-filled U-shaped flange (hereafter, UH-beam). As shown in the figure, the partially welded channel anchors can be used as an alternative to the combined conventional stud anchors and batten For the performance and design of channel anchors, numerous studies have been conducted. [PERSON] et al. [7] carried out tests for push-out and beam specimens (consisting of steel beams and concrete slabs) using C-channel anchors and found that the strength of the concrete and the web thickness, flange thickness, and length of the channel anchor affected the load-carrying capacity of the test specimens, but the height and orientation of the channel anchor had negligible effects. They provided a theoretical analysis model, assuming that the channel anchor acts as a dowel. [PERSON] and [PERSON] [8] also carried out push-out and beam tests, and proposed a design equation for the strength of C-channel anchors. The design equation of [PERSON] and [PERSON] [8] is adopted in ANSI/AASC 360-16 [9] and KDS 41 31 00 [10] with a modification, and CSA S16-09 [11] also specifies a similar form of design equation. It is noteworthy that slip between the steel beam and concrete slab is inevitable [7], but the slip is not an important factor of the ultimate load of test specimens if a sufficient number of steel anchors are provided [8]. [PERSON] and [PERSON] [12] carried out push-out tests using L-, C-, and T-channel anchors and found that the test specimens failed by the tensile fracture of steel anchors, crushing of concrete, or shear failure of concrete, and the failure modes were affected by the shape and direction of the steel anchors and the strength of concrete. [PERSON] and [PERSON] [13] performed experimental and analytical studies for C-channel anchors embedded in unconfined (without stirrups) normal or polypropylene concrete slabs and found that the specimens without stirrups failed in a more brittle way and show much lower strength than a specimen with stirrups. [PERSON] and [PERSON] [14] carried out push-out tests with test parameters of the concrete strength and the length and web thickness of C-channel anchors, and suggested a design equation, which was simplified from their previous study. [PERSON] [15] developed a type of sandwich beam test method and proposed equations for the design of L-channel anchors as a function of strut angle and also for the shear force-relative displacement relationship. [PERSON] and [PERSON] [3] formulated a design equation for C-channel anchors based on the typical failure mechanism in push-out tests and plastic analysis principles. [PERSON] et al.