Investigation of headed bar joints between precast concrete panels

Abstract

The paper addresses the design and behaviour of narrow cast in - situ joints between precast concrete elements in which continuity of reinforcement is achieved through overlapping headed bars. Using headed bars minimises the lap length required with in the cast - in - situ joint region. Confining reinforcement in the form of transverse bars and vertical shear studs is also installed in the joint. Th e paper describes a series of tensile tests which were carried out to simulate the tensile zone of a joint loaded in pure flexure . The headed bars used in the tests were 25 mm in diameter with 70 mm square heads and yield strength of 5 30 MPa. The tests studied the influences of concrete strength, headed bar spacing, splice length, transverse reinforcement and conf ining shear studs on joint strength . A lap length of 100 mm in concrete with 28 MPa cylinder strength was found to be sufficient to develop the full strength of the headed bars . A strut - and - tie model (STM) is presented for determining joint strength. Analys is shows that the STM gives safe results even though it does not fully capture the observed joint behaviour. An upper bound plasticity model is found to give relatively good predictions of joint strength in most cases , although it also does not always capt ure the correct failure mechanism . The tests provide insights into joint behaviour which, in conjunction with numerical modelling, will facilitate the development of an improved design method. W idespread use of this system would lead to improvements in bui ldability, sustainability and health and safety in the construction of concrete structures

Headed Bar Connections Between Precast Concrete Elements: Design Recommendations and Practical Applications

“…Joergensen and Hoang [15,16] developed an upper bound plasticity model for U-bar splices which is also pertinent to headed bars. The geometry of a typical tensile test specimen is shown in Figure 2 while Table 1 gives details of the 32 tested tensile specimens of which 27 have been reported previously [17]. The headed bars used in the tensile specimens were 400 mm in length, measured between the inside faces of the heads.…”

“…Load was applied under displacement control at a rate of 0.2 mm per minute up to failure. More details can be found in references [1,17]. The test IDs in Table 1 fully Table 2 gives the material properties of the reinforcement bars used in all tensile and flexural specimens.…”

Numerical modelling of headed bar joints subjected to tension

“…However, it should be noted that the measured and calculated stress in the shear studs at joint failure was at most around 50% of yield. This has implications for the calculation of concrete strength in the diagonal struts of the STM in Figure 3 (Vella et al, 2017a) since shear studs are assumed to provide out of plane confinement. Currently, the confining stress is calculated in the STM assuming shear studs yield at failure (Vella et al, 2017a) which is unrealistic.…”

“…The strength of the inclined struts in the STM is assumed to be increased by confinement when shear studs are present. Analysis of splice joints tested by the authors shows that the STM gives overly conservative estimates of joint resistance, when concrete failure governs, without always capturing the observed failure mechanism (Vella et al, 2017a). Headed bar tension splices have been tested by several researchers (Chun, 2015, Li et al, 2010a, Li et al, 2010b, Thompson et al, 2006, Thompson et al, 2003) but there appears to be a complete lack of published literature on the nonlinear finite element analysis (NLFEA) of such connections.…”

“…This has implications for the calculation of concrete strength in the diagonal struts of the STM in Figure 3 (Vella et al, 2017a) since shear studs are assumed to provide out of plane confinement. Currently, the confining stress is calculated in the STM assuming shear studs yield at failure (Vella et al, 2017a) which is unrealistic. However, it should be noted that even with the assumption of shear stud yield the STM provides a lower bound estimate of joint strength in the authors' tension and flexural tests (Vella et al, 2017a, b) with shear stud diameters of 10 mm and 12 mm respectively.…”