Influence of reinforcement arrangement in flexural fire behavior of hollow core slabs
Abstract This paper describes standard scale fire tests conducted on four precast prestressed hollow core slabs configurations under flexure. The influence of the reinforcement arrangement was investigated using two test parameters: the average axis distance (from 30.1 to 46.7 mm), and the type of reinforcement (5 mm diameter wires and 9.5 mm diameter strands). An ISO 834 standard fire was applied maintaining the load level in a four-point test arrangement constant. The results presented different failure mechanisms, types of cracks, spalling patterns, and fire resistance ratings and it was shown that wires displayed optimal behavior as opposed to strands. A comparison was carried out with the current verification methods provided in Eurocode 2 part 1–2, and showed that verification methods tend to overestimate the calculation of fire resistance when both wire and strands are moved away from the exposed surface.
A 3D finite element model for predicting the fire behavior of hollow-core slabs
Abstract A nonlinear finite element three-dimensional model is presented and validated in this work against standard fire tests. The aim is to develop a numerical tool in order to study the behavior of prestressed Hollow Core (HC) slabs in the event of fire. A sequentially coupled approach is considered in order to perform the thermo-mechanical analysis at an affordable computational cost. The model includes the effect of prestress and a realistic constitutive inelastic behavior of concrete, for both tension and compression. It takes into account the temperature dependency of the material properties, and thus the crack patterns due to bending, shear, thermal cracking and longitudinal cracking are reproduced. The model is validated by comparing the simulation results with 11 real fire resistance tests on single elements, obtaining satisfactory results in terms of both failure mechanism and failure time. Finally, the suitability of using the simple calculation models existing in the European codes (Eurocode 2, EN 1168) for the verification of the HC resistance in fire situation is discussed. In particular, the numerical results are compared to the simple calculation model given in the EN 1168.
Effect of Web reinforcement on high-strength concrete deep beams
Results of an experimental investigation on the behavior and ultimate shear strength of 18 high strength concrete deep beams are summarized. The concrete cylinder compressive strength f' c ranges from 55 to 86 MPa (approx. 8000 to 12500 psi). The test specimens are divided into three series based on the shear-span-to-overall-height ratio a/h. Each series consists of six beams with different arrangements of horizontal and vertical web reinforcements, i.e., the main variables are the horizontal and the vertical web steel ratios. Observations are made on mid-span deflections, crack widths, failure modes and ultimate strengths. The test results show that for deep flexural members with a/h exceeding 1.00 (or shear-span-to-effective-depth ratio a/d ≥ 1.13), the vertical web reinforcement is more effective than the horizontal web reinforcement. It is also shown that orthogonal web reinforcement comprising both vertical and horizontal reinforcements is the most efficient in increasing the beam stiffness, restricting the diagonal crack width development and enhancing the ultimate shear strength. The test results are then compared with the ultimate strength predictions obtained using the current ACI Code, the Canadian Code, and the UK CIRIA Guide. The deep-beam provisions in the ACI Code overestimate the contribution of the horizontal web steel to shear strength. Based on the test results, a revision to ACI Eq. (11-31) for web steel contribution is suggested. The Canadian Code shows the most consistent and yet conservative predictions of the test beams with different web reinforcements, while the UK CIRIA Guide is unconservative for beams with horizontal web reinforcement.