Abstract

     Anchorage of shear reinforcement, such as links or stirrups, can be performed by providing hooks, bends or heads at their ends, by welding transverse reinforcement or by closing open stirrups with pins. Hooks and bends, also used to enhance the anchorage of flexural reinforcement at the end regions of beams and slabs, have often been preferred because of their simple and cost-effective production. Such anchorages present nevertheless several peculiarities that shall be accounted for. They are relatively sensitive to their detailing (mandrel diameter, length of the tail segment between bar end and bent region) as well as to the cracking state of the surrounding concrete. Also, brittle failures can occur due to spalling of the concrete cover in case of bars near to a free surface.

Introduction

     Since the beginning of reinforced concrete construction, mechanical anchorage of the reinforcement in tension by means of bends and hooks has been extensively used in a large number of applications, such as beams, walls and slabs (Fig. 1a-f). Arrangement of hooks was first required for the anchorage of plain bars due to their poor bond performance. Such solution was observed to be efficient and was also adopted for the anchorage of various types of shear and punching reinforcement.

     The first research works on hooks were performed for the anchorage of flexural reinforcement at the extremity of beams. On the basis of beam tests, Hyatt in 1877 [1] observed that flat reinforcing bars bent at 90° at their ends gave higher performances. As a result of this work, 90° bends became popular in the following years in the USA [2]. In Europe, in 1908 Mörsh [3] encouraged the use of hooks to enhance the performance of anchorages. He also proposed to use higher bend angles than 90°, in contrast to the practice in the USA. The reason for this was that 90°-angle bends caused failure issues in the case of too small concrete cover (as demonstrated by the tests of Wayss and Freytag [4], [5]). It was Considère in 1907 [6] who formally proposed a 180° hook bent with a mandrel diameter equal to 4 times the bar diameter to avoid failures due to concrete crushing. In addition to the use of hooks at beam ends, the hook anchorage of shear reinforcement (Fig. 1a-e) was also used in the early developments of reinforced concrete.

Conclusions

     This paper presents the results of an experimental programme and a mechanical model allowing to better understand the behaviour and to predict the resistance of bends and hooks in tension. The case of the anchorage of shear reinforcement near to a free surface is investigated. In addition, the influence of bending cracks on the performance of shear reinforcement is also investigated. The main conclusions are listed below:

۱٫ Three failure modes potentially govern the strength of a bend anchorage: (i) spalling of the concrete cover, (ii) pull-out (bond) failure and (iii) reinforcement yielding. The governing failure mode depends upon the size of the concrete cover, the transverse crack opening and the detailing of the region.

۲٫ The mechanical response of bend anchorages is complex, with an interaction between normal, bond stresses and longitudinal steel stresses due to axial forces and bending. Such interaction is confirmed by means of an experimental programme performed with detailed fibre-optic measurements. Also, the experimental results presented in this paper show a significant influence of the state of cracking, consistently with previous researches on bond.

۳٫ Spalling of the concrete cover is originated by a combination of the tensile stresses associated to bond and the uplift forces in the tail region of bends (lever effect). Such uplift forces allow, by equilibrium conditions, to develop an additional tensile force in the reinforcement at the end of the bend (enhancement of the anchorage capacity in case of pull-out failures).