Experimental study on the flexural behaviour of demountable steel-concrete composite beams at room temperature and under fire exposure

Shear connectors are required to build composite (concrete and steel) beams. They are placed at the interface of concrete and steel to transfer shear and normal forces between two structural components. Such composite beams are sensitive to provide structural integrity when exposed to fire as they loss strength, stiffness, and ductility at elevated temperature. The present study is designed to investigate the shear resistance and the failure modes of the headed stud shear connectors at fire exposure and post-fire exposure. The study includes ordinary concrete and concrete with carbon nanotubes (CNTs) to build composite (concrete-steel) beams with structural steel. Experimental push tests were conducted on composite beams at ambient and elevated temperatures, such as 200, 400 & 600℃. Moreover, push tests were performed on the composite beams after being exposed to 200, 400 & 600℃. Push test results illustrated the reduction of ultimate shear capacity and stiffness of headed stud shear connectors as the temperature increased. Although similar values of ultimate shear were obtained for the headed stud connectors in both ordinary and CNT concrete, the CNT modified concrete reduced the concrete spalling and cracking compared to ordinary concrete and was observed to be effective at temperatures greater than 400°C. All specimens showed a lower shear resistance at fire exposures compared to the corresponding post-fire exposures. Moreover, numerical simulation by Finite Element (FE) analyses were carried out at ambient temperature and at fire conditions. The FE analysis results show a good agreement with the experimental results. In the experimental studies, failure of all specimens occurred due to shear failure of headed stud, which was later validated by FE analyses using ABAQUS.

This study experimentally and theoretically investigated the behaviour of concrete beams reinforced with different glass fibre-reinforced polymer (GFRP)–steel combinations without/with fire exposure. Experiments were performed on 12 concrete beams reinforced with 4 GFRP-steel combinations, which are represented by the ratios (ρf/fs ) of GFRP area to total reinforcement area. The test results indicated that the increase in ρf/fs and fire duration shifted the flexural failure of the tested beams to shear failure. At room temperature, the ultimate load of concrete beams reinforced with both GFRP and steel bars increased by approximately 20%, while that of concrete beams reinforced with only GFRP increased by 11.8% compared with beams reinforced with only steel. Exposed to 30-min and 60-min fires, the ultimate loads of concrete beams with ρf/fs =0.0–1.0 decreased by 13.3%–77.3% and 39.3%–96.0% compared with those of their counterpart, respectively. The elastic stiffness was significantly affected by both ρf/fs ratio and fire duration. Concrete beams reinforced with only GFRP had 67.4% lower stiffness than those reinforced with only steel. 30-min and 60-min fires significantly decreased the elastic stiffness by 44.6%–54.3% and 71.8%–76.0% compared with their counterpart, respectively. Finally, a method for estimating the load-carrying capacity of postfire GFRP-steel reinforced concrete (RC) beams was proposed.

The growing awareness of environmental impacts and sustainability of the construction industry has increased interest in the use of lightweight aggregate concrete (LWAC) due to its low density and good insulation properties. However, the inferior mechanical properties of LWAC compared to its normal-weight equivalent have led to reluctance in its adoption in practice. Both metallic and polymer fibres have been used to enhance mechanical behaviour of fibre-reinforced lightweight aggregate concrete (FRLWAC) to overcome this weakness, and positive results at material level has been reported in the literature. However, limited work has been conducted in assessing the feasibility of applying FRLWAC at structural level, particularly on slabs and beams where FRLWAC applications are more relevant. Moreover, while it is traditionally believed that LWAC behaviour under fire exposure is superior to normal-weight concrete, it is largely unknown if the addition of fibres affects fire resistance of FRLWAC, particularly at structural level. Therefore, this research work was carried out to fill the knowledge gaps on the behaviour of FRLWAC at structural level at both ambient and elevated temperatures. To this end, experimental investigations were carried out flexural and shear behaviour of FRLWAC slabs and beams at both ambient and fire conditions. Additionally, the bond-slip behaviour of reinforcement in FRLWAC was also studied as it holds an importance on the flexural behaviour of FRLWAC. These investigations were divided into four series of tests. In addition to the experimental studies, analytical models were proposed to equip engineers with straightforward tools to predict behaviour of FRLWAC beams and slabs. The test series involving bond-slip behaviour of FRLWAC consisted of 64 bond-slip specimens with different concrete covers, steel fibre contents and temperature levels. This test series highlighted the change in bond failure mode due to fire exposure, and the benefit of steel fibre in preventing brittle bond failure. From the experimental observations, an integrated bond-slip model that caters for different bond failure modes and the shift of failure modes due to exposure to elevated temperatures was developed. The experimental investigation on flexural behaviour of FRLWAC was conducted through a series of tests on six one-way slabs: two at ambient conditions and another four at elevated temperatures. These slabs were designed based on the prescriptive 2-hour fire resistance requirement of EN 1992-1-2 with different continuity conditions and degrees of moment redistribution, which affected their reinforcement configurations. The tests revealed that the continuous specimens survived much longer than the prescribed fire resistance rating, highlighting the benefit of structural continuity on fire resistance of FRLWAC slabs. It was also discovered that designing slabs with design moment redistribution towards the hogging region improved fire resistance of continuous slabs. An analytical model based on the rigid-body rotation analogy that adopted the novel FRLWAC bond-slip model was also proposed and verified at both ambient and fire conditions. The investigation on shear behaviour of FRLWAC was split into two series of tests: one series consisted of seven beams tested at ambient condition, while another series consisted of six beams tested at elevated temperatures. The ambient series focused on the effects of continuity, shear-span-to-depth ratio (a/d), steel fibre content and amount of shear reinforcement on shear behaviour of FRLWAC beams. It was discovered that while beam continuity did not increase shear resistance, it prevented a sudden collapse of FRLWAC beams upon attaining shear failure. Two shear-resisting mechanisms were also noted: direct strut action and truss action. The second series studied the effects of continuity, steel fibre contents and a/d on shear behaviour of FRLWAC beams under fire conditions. Unlike the ambient tests, the fire tests highlighted a notable benefit of continuity on fire resistance of FRLWAC beams failing in shear. It also highlighted the sensitivity of steel fibre contribution on shear resistance to fire exposure. A strut-and-tie model that predicts shear resistance of FRLWAC continuous beams was also proposed. This model considers contributions of steel fibre and high-temperature exposure on shear resistance of FRLWAC. When assessing the proposed model against different test results from this study and the literature, it outperformed design equations from different codes of practice. This model could also be extended to predict shear failure duration of FRLWAC beams under fire.

Experimental and numerical study on post-fire flexural capacity of corroded reinforced concrete beams under various cooling methods

Engineered Cementitious Composite (ECC) is a new kind of random short fiber reinforced cementitious composite with ultra high ductility. Unlike common FRC, ECC is developed by optimizing the microstructure of the composite which exhibits tensile strain-hardening behavior with strain capacity more than 3%, yet the fiber content is typically 2% by volume or less. The flexural behavior of concrete beams reinforced with CFRP grid (named NEFMAC) and ECC is studied in this paper. The behavior of concrete beams reinforced with ECC-NEFMAC is compared with the behavior of a beam reinforced with steel and carbon fiber sheet. The research investigates their flexural behavior including pre-cracking state, cracking pattern and width, deflections, ultimate capacities and strains, and the mode of failure. The information obtained throughout this investigation is valuable for future field application and development of design guidelines for ECC and FRP grids.

Steel-concrete composite box beams have been widely used in high rise buildings and long-span bridge structures. But so far, almost all researches have been aimed at the static behavior of the composite beams and dynamic behavior of steel and concrete composite beams have been rarely studied. In this paper, by using general finite element program ANSYS to analyze the dynamic performance of the composite box beam under different geometric parameters. Research is focused on the slip stiffness、width-to-thickness ratio、depth-span ratio and the height ratio of cross section to the vibration characteristics of composite box beam. The results indicate that these factors affect the seismic dynamic response of steel-concrete composite box beams most and they should be controlled according to different situations in seismic design stage.

Adopting hybrid cellular I-sections can favor optimizing steel consumption, as the higher steel grade is localized only in the compression tee. In addition, continuous steel-concrete composite cellular beams allow the construction of longer spans due to the combination of higher bending stiffness of the cellular I-section and its connection with the concrete slab, further the continuous of the beam, which provides a better distribution of the bending moment diagram. However, due to the hogging moment on the support regions, the steel-concrete composite beam can present lateral instability of the compression flange accompanied by web distortion, named Lateral-DistortionalBuckling (LDB). No investigations have evaluated the behavior of steel-concrete composite beams with hybrid cellular I-section under hogging moment. This way, the present study aims to assess the behavior of the structure in question using a nonlinear finite element model via ABAQUS software. The investigation consists of verifying the beams' failure mode, considering plastic behavior or LDB, and the effectsof different combinations of steel grades for the hybrid I-section, and further comparing the use of hybrid and homogeneous steel profiles on the composite beams.

Behaviour of composite beams with beam-to-girder end-plate connection under hogging moments

Purpose This paper aims to investigate the predicted temperature behaviour of the protected cellular steel beam (CSB) with circular web openings at elevated temperature through finite element simulation. Design/methodology/approach Temperature development along the CSB were analysed and used for parametric investigation. In addition, this research paper investigates the novelty application of various intumescent coating thicknesses covering the whole CSB to cut down the temperature development along the beam section. Findings From the simulation outcomes, it shows that intumescent coating has a significant effect in reducing the temperature development along the CSB section. Thicker intumescent coating contributes to a higher temperature drop at the bottom tee section than the upper tee section. Originality/value The use of structural CSB has gained popularity among engineers and architects. This type of beam allows serviceability ducts and pipes to pass through the main steel web section under the flooring system, thus providing larger headroom for designers. Nevertheless, in any structural steel building, it is highly risky for CSB to be exposed to fire hazard if it were triggered accidentally. To mitigate and reduce fire exposure risk which might compromise the strength and stiffness of CSB, a passive fire protection is proposed to minimise the risk. One of the common passive fire protection materials used for steel beam section is intumescent coating. Intumescent coating is by far the cheapest solution to protect CSB as compared to other passive fire protection system. Intumescent coating can absorb some portion of heat exposure which subsequently translates a lower temperature development along the CSB section.

The experimental behavior of FRP-strengthened RC beams subjected to design fire exposure

Behaviours of novel prefabricated RAC-HSC composite beams subjected to fire: An experimental and numerical study

Behaviour of lightweight concrete slabs acting compositely with steel I-sections

Flexural behavior of shallow cellular composite floor beams with innovative shear connections

Recycled aggregate concrete (RAC) is an environmentally friendly building material. This paper investigates the mechanical behavior of recycled aggregate concrete filled steel tube (RACFST) columns exposed to fire. Two groups of 12 columns were designed and tested, under axial compression, before and after fire, to evaluate the degradation of bearing capacity due to fire exposure. Six specimens were subjected to axial compression tests at room temperature and the other six specimens were subjected to axial compression tests after a fire exposure. The main parameters of the specimens include the wall thickness of the steel tube (steel content) and the type of concrete materials. Several parameters as obtained from the experimental results were compared and analyzed, including the load-bearing capacity, deformation capacity, and failure characteristics of the specimens. Meanwhile, rate of loss of bearing capacity of specimens exposed to fire were calculated based on the standards EC4 and CECS28:90. The results show that concrete material has a large influence on the rate of loss of bearing capacity in the case of a relatively lower steel ratio. While steel content has little effect on the rate of loss of bearing capacity of concrete-filled steel tube (CFST) columns after fire, it has a relatively large influence on the loss rate of bearing capacity of the RACFST columns. The loss of bearing capacity of the specimens from the experiment is more serious than that from the calculation. As the calculated values are less conservative, particular attention should be given to the application of recycled aggregate concrete in actual structures.

Experimental study on uniaxial and nonproportionally multiaxial ratcheting of SS304 stainless steel at room and high temperatures