International Concrete Abstracts Portal

The assessment of fire safety in a rather slender pretensioned simply-supported beam with a V section is presented in this paper. Such architecturally-valuable thin-walled beams have been used in Italy for the last forty years in the roofs of large industrial or office buildings, where a series of V girders (= secondary beams, with interposed concrete or glass panels) are supported by other girders resting on columns (= primary beams). After several decades since their construction, many of these members exhibit severe symptoms of distress, often in the form of longitudinal cracks, that can significantly reduce the bearing capacity in shear. (These span-wise cracks are mostly due to transverse bending and start propagating close to the supports, along the extrados of the secondary beams, where rain may accumulate because of insufficient or damaged water proofing). The beam in question, with thin and inclined webs, is checked both at the ultimate limit state (ULS) and in fire, with reference to bending, shear (where the traditional truss model is used), and shear-slip close to the supports, where the aforementioned longitudinal cracking may cause a sort of delamination resisted by the partially-corroded stirrups. Reference is mainly made to the provisions of Eurocode 2, but in the check concerning shear transfer along the longitudinal cracks ACI 318-08 comes into play as well. Even if it is rather peculiar for its unusual cross-section, the beam in question offers the opportunity to focus the attention on some general aspects concerning the behavior of prestressed concrete members in fire conditions, and on some weaknesses of past design provisions.

This paper presents an experimental study on the fire behavior of seven restrained RC beams strengthened with carbon fiber sheet (CFS) and provided with fire insulation. The influence of some parameters (i.e., axial and rotational restraint stiffness, thickness of the fire insulation, and load ratio) on the deformations and internal forces of the beams is analyzed. The test results indicate that: (a) for a restrained beam in fire, the maximum axial elongation decreases with increasing stiffness of the axial restraint, and increasing thickness of the fire insulation, while the maximum additional axial force increases with increasing stiffness of the axial restraint, and decreasing thickness of fire insulation. The maximum additional bending moment at the beam ends increases with decreasing thickness of the fire insulation, but on the whole the influence of the thickness of the fire insulation within the investigated range (10~20 mm [0.394~0.787 in]) is rather limited; (b) the additional axial force in a restrained beam recovers slightly during the cooling phase, while the additional bending moment at beam end of a restrained beam recovers significantly in the cooling phase; (c) the peak value of the bending moment of a strengthened and insulated beam occurs much later than in the case of an ordinary RC beam (i.e., without strengthening and fire insulation), while the maximum additional bending moment at beam end is lower; and (d) the influence of the rotational restraint stiffness on the maximum additional bending moments at beam ends is rather limited.

Concrete structures fabricated with high strength concrete (HSC) experience degradation of strength and spalling when exposed to extreme fire conditions. To mitigate fire induced spalling in HSC; different types of fibers are often added to concrete. Presence of fibers influence the properties of HSC and knowledge of high temperature properties is essential for evaluating the fire response of structures made of fiber reinforced HSC. In this paper, thermal and mechanical properties of four types of HSC are evaluated. The four types of concrete comprise of plain HSC, and HSC with 3 types of fibers namely steel, polypropylene and hybrid (steel + polypropylene) fibers. For thermal properties specific heat, thermal conductivity, and thermal expansion are measured, whereas for mechanical properties compressive and tensile strength are measured in the temperature range of 20-800°C (68-1472°F). Results from mechanical property tests show that addition of steel fibers enhances tensile strength of HSC which is beneficial against fire induced spalling. Results from thermal property tests show that presence of fibers increase the specific heat and thermal expansion of fiber reinforced concrete that will affect the development of fire induced thermal gradients and thermal stresses in HSC cross-section. Data generated from these tests was utilized to develop simplified relations for expressing thermal and mechanical properties of fiber reinforced HSC (FRHSC) as a function of temperature. The proposed thermal and mechanical property relationships can be used as input data in computer models for evaluating fire response of structures made of FRHSC.

An earlier paper demonstrated that the cover requirements given in ACI 216.1 will not consistently lead to adequate fire resistance of simply supported, unrestrained, slabs, with current load factors and reinforcement materials. This study leads to a suggested replacement for Table 2.3 of ACI 216.1. Greater cover thicknesses would be required for all fire exposure times beyond one hour, with significant increases for the uncommon design case of 4 hours required fire resistance.

Fire safety is a critical criterion for designing reinforced concrete (RC) structures. As new design codes are moving towards performance-based design, analytical tools are needed to help engineers satisfy code criteria. These tools are also needed to assess the fire performance of critical structures. As full scale experiments and finite element simulations are usually expensive and time consuming options for designers to achieve specific fire performance, a simplified sectional analysis methodology that tracks the axial and flexural behavior of RC square sections subjected to elevated temperatures from their four sides was previously developed and validated by the authors. In the first part of this paper, the proposed methodology is extended to cover rectangular beams subjected to standard ASTM-E119 fire from three sides. An extensive parametric study is then conducted to study the distribution of the concrete compressive stresses at different ASTM-E119 fire durations. Based on the parametric study, simple equations expressing the equivalent stress-block parameters at elevated temperatures are presented. These equations can be utilized by designers to accurately estimate the flexure capacity of simply supported and continuous beams exposed to fire temperatures.