Browsing by Author "Ribeiro, Fernando"
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- Computational simulation of the thermal effects on composite slabs under fire conditionsPublication . Piloto, P.A.G.; Balsa, Carlos; Ribeiro, Fernando; Rigobello, RonaldoA computational model is presented to evaluate the thermal effects on composite slabs with still deck, originated by standard fire exposure. Composite slabs with profiled steel deck are widely used in buildings which require fire resistance. Computational simulations are of great importance in this field and consist of an alternative to experimental fire tests that are expensive, time-consuming and require semi-specialized technical equipment. However, computational simulations must be reliable and realistic. The resulting transient and non-linear thermal problem is solved by the Finite Element Method in ANSYS and Matlab. The finite element models are three-dimensional, full scale, and multi-domain. Additionally, the models also include an air gap between the steel deck and the concrete part of the slab, in order to simulate the thermal effects induced by the debonding between the steel deck and the concrete, verified in previous experimental investigations. The results of the numerical simulations are validated against the results of experimental fire tests. The fire resistance of the composite slabs determined computationally is also compared with simplified calculation methods available in standards
- Critical temperature for the components of composite slabs with steel deck under fire for load-bearing ratingPublication . Piloto, P.A.G.; Balsa, Carlos; Ribeiro, Fernando; Santos, Lucas Manoel Cunha; Rigobello, Ronaldo; Kimura, Érica Fernanda AikoComposite slab made with concrete and steel deck are widely used in building structures. They also include other components, such as steel rebars for positive bending and a steel mesh for negative bending. The fire rating of this type of elements can be determined by standard fire tests, accounting for load (R), Integrity (E) and Insulation (I). This investigation deals with the fire resistance for load (R) and insulation (I), using a numerical model validated with experimental tests. This model considers material and geometric non-linear behaviour, using perfect contact between materials. The 3D finite element mesh uses solids, shells and bars, to model a simple supported composite slab with 3.2m long, 0.65 m wide and total height of 143mm, using a trapezoidal steel deck PRINS PSV73. Different load levels are simulated (live load ranging from 1.0 K/m2 to 21 kN/m2) in addition to the dead load (2.8 kN/m2). The fire resistance is determined according to standards, looking for the maximum displacement or the rate of displacement. The critical temperature of each steel component decreases with the load level. A new proposal is presented for the critical temperature of each steel component.
- Critical temperature for the components of composite slabs with steel deck under fire for load-bearing ratingPublication . Piloto, P.A.G.; Balsa, Carlos; Ribeiro, Fernando; Santos, Lucas Manoel Cunha; Rigobello, Ronaldo; Kimura, Érica Fernanda AikoComposite slabs made with concrete and steel deck are widely used in building structures. They also include other components, such as steel rebars for positive bending and a steel mesh for negative bending, preventing cracks in concrete. The fire rating of this type of elements can be determined by standard fire tests, accounting for load (R), Integrity (E) and Insulation (I). This investigation deals with the fire resistance for load (R) and insulation (I), using a numerical model validated with experimental tests. This model considers material and geometric non-linear behaviour, using perfect contact between materials. The 3D finite element mesh uses solids, shells and bars, to model a simply supported composite slab with 3.2m long, 0.65 m wide and total height of 143mm, using a trapezoidal steel deck PRINS PSV73. Different load levels are simulated (live load ranging from 1.0 kN/m2 to 21 kN/m2) in addition to the dead load (2.8 kN/m2). The fire resistance (R) is determined according to standards, looking for the maximum displacement or the rate of displacement, while the fire resistance (I) looks for the average or for the maximum temperature increase at the unexposed side. The critical temperature of each steel component decreases with the load level. A new proposal is presented for the fire resistance depending on the load level.
- Effects of the ESD protections in the behaviour of a 2.4 GHz RF transceiver: problems and solutionsPublication . Carmo, João Paulo; Mendes, P.M.; Ribeiro, Fernando; Correia, José H.This paper identifies the main problems related to the Electrostatic Discharge (ESD) in submicron CMOS processes. The mitigation of this problem is made with the use of protections, in order to avoid the destruction of the internal and nput/output circuits connected to the bondingpads. In the 2.4 GHz ISM band, the parallel capacitance and the serial resistance of the ESD protections have effects in the behavior of RF transceivers. The major identified effect was the transmission range. It is proposed two strategies to solve the secondary effects, due to the protections. All the measurements and simulations were made for a 2.4 GHz RF CMOS transceiver, designed and fabricated using the UMC 0.18 µm RF CMOS process.
- Improved calculation methods for the temperature of the steel components of composite slabs under fire conditionsPublication . Piloto, P.A.G.; Balsa, Carlos; Ribeiro, Fernando; Rigobello, RonaldoA composite steel-concrete slab consists of a profiled steel deck that acts as permanent formwork to the concrete topping, which is normally reinforced with individual rebars and an anti-crack mesh. The Annex D of the EN 1994-1-2 provides guidelines for the calculation of the temperature of the rebars and the parts of the steel deck of composite slabs subjected to the standard fire from below. However, no revisions were made to these methods during the last two decades. This work proposes an improved method for the estimation of the temperature of the parts of the steel deck and the rebars as well. The proposed expressions are derived from numerical simulations using a 3-D finite element model, considering perfect thermal contact between all the materials.
- Low‐ and moderate‐intensity aerobic exercise acutely reduce blood pressure in adults with high‐normal/grade I hypertensionPublication . Lopes, José Carlos; Fonseca, Márcio Daniel Duarte; Torres‐Costoso, Ana; López‐Muñoz, Purificacion; Alves, Alberto Jorge; Magalhães, Pedro; Ribeiro, FernandoThe aim of the study is to compare the acute effects of low‐ and moderate‐intensity aerobic exercise on post‐exercise blood pressure in active adults with high‐normal/grade I hypertension. Thirteen physically active adults (67.0 ± 8.7 years) randomly completed two aerobic exercise sessions of 30 minutes at low (30% heart rate reserve [HRres]) and moderate (60% HRres) intensity. Blood pressure was assessed pre‐session and every hour until 3 hours after. Systolic blood pressure decreased after both exercise intensities without significant differences between sessions at 1 hour after the session (30%: −10.0 ± 12.6% vs 60%: −11.4 ± 12.7 mm Hg, P > .05). Three hours after the 60% session, the systolic blood pressure remained significantly lower than baseline (139.9 ± 12.9 to 129.3 ± 11.9 mm Hg, P < .05), but without significant differences between sessions. No relevant changes were observed in diastolic and mean blood pressure. In conclusion, a single session of aerobic exercise acutely reduces systolic blood pressure in active adults with high‐normal/grade I hypertension.
- Moderate but not low intensity aerobic exercise promotes postexercise hypotension in older adults with hypertension and regular exercise participationPublication . Ribeiro, Fernando; Fonseca, Márcio Daniel Duarte; Alves, Alberto Jorge; Magalhães, PedroThe reduction in blood pressure as a result of a single session of exercise seems especially relevant for assisting in the control of blood pressure during day-time periods when blood pressure is typically at its highest levels and to allow the performance of activities of daily living at lower levels of blood pressure. Despite evidence related to post-exercise blood pressure reduction as a result of a single session of aerobic exercise in adults with hypertension, there is a lack of studies enrolling adults with hypertension and regular exercise participation. The results could be less predictable in this group of subjects, because the magnitude of the acute effect could be lower due to a better blood pressure control in response to the regular exercise practice. Previous studies showed that exercise intensity could influence the blood pressure response after exercise cessation. To compare the effects of low (30% heart rate reserve, HRres) and moderate (60% HRres) intensity on the blood pressure response immediately following aerobic exercise in older adults with hypertension and regular exercise participation. We aim to determine whether the phenomenon designated as postexercise hypotension, i.e. the transient reduction in blood pressure in the period after exercise to values less than those observed before exercise, is modulated by exercise intensity. 19 older adults (12 women; age: 66.0±1.7 years; weight: 65.8±8.5 kg; BMI: 24.8±2.6 kg/m2) who exercise at least 3 times per week on a regular basis, completed two randomly assigned aerobic exercise bouts at low (30% heart rate reserve, HRres) and moderate (60% HRres) lasting 30 minutes each. The exercise bouts were performed in the morning (9 a.m.), 7 days apart from each other, on a treadmill.Anthropometric variables, medication status and clinical history were collected. Blood pressure was measured at baseline after 10 min of rest, at 10 min after exercise, and at every hour until 6 hours post exercise. Over the course of 6 hours, systolic and diastolic blood pressure did not change significantly after the low intensity exercise bout in comparison to the baseline values (134.9±14.5 mmHg and 80.0±8.0 mmHg, respectively). In the moderate exercise bout, the systolic pressure at 1 hour (125.3±15.3 mmHg, p=0.029) and up to 5 (126.2±17.1 mmHg, p=0.041) and 6 hours (126.3±13.8 mmHg, p=0.040) postexercise was significantly lower in comparison with baseline (135.0±16.0 mmHg). Diastolic pressure did not change. A single session of moderate aerobic exercise acutely reduces blood pressure in hypertensive older adults with regular exercise practice and may be a nonpharmacological coadjuvant tool to control hypertension in this age group. The intensity of exercise seemed to be a key factor to post-exercise blood pressure reduction. Even hypertensive subjects with regular exercise practice benefit from an acute bout of exercise. These results add evidence supporting the importance of aerobic exercise as a non-pharmacological strategy to control hypertension.
- A new calculation method for the temperature of the components of composite slabs under firePublication . Piloto, Paulo A.G.; Balsa, Carlos; Ribeiro, Fernando; Rigobello, RonaldoComposite steel-concrete slabs are structural elements composed of a profiled steel deck which acts as a permanent formwork to the concrete topping. This layer is commonly reinforced with individual rebars and an anti-crack mesh. The Annex D of the EN 1994-1-2 provides guidelines for the calculation of the temperature of the steel components of composite slabs subjected to the standard fire. However, no revisions were made to these calculation rules during the last years. This paper proposes a new method for the estimation of the temperature of the parts of the steel deck and the rebars as well. The proposed methodology is derived from numerical analyses using a 3-D finite element model, considering perfect thermal contact between the materials.
- Numerical simulation of composite slabs with steel deck under fire conditionsPublication . Balsa, Carlos; Ribeiro, Fernando; Piloto, P.A.G.; Rigobello, RonaldoA composite slab consists of a concrete topping cast on the top of a steel deck. The concrete is usually reinforced with a steel mesh on the top and may also be reinforced using individual rebars. The steel deck also acts as reinforcement and may be directly exposed to accidental fire conditions. This composite solution is widely used in every type of buildings which require fire resistance, in accordance to regulations and standards. The scope of this investigation concerns the fire rating for insulation (I). Numerical simulation was developed, using Matlab PDE toolbox and ANSYS, to find out the thermal effects of standard fire exposure. The results of the numerical simulation are compared with experimental results available in the literature in order to validate our approach. The results are also compared with the simplified method proposed by Eurocode 4-part 1.2.
- Numerical simulation of composite slabs with steel deck under fire conditionsPublication . Ribeiro, Fernando; Piloto, P.A.G.; Balsa, Carlos; Rigobello, RonaldoA composite steel-concrete slab consists of a concrete topping cast on the top of a profiled steel deck. Normally, the concrete is reinforced with an anti-crack mesh positioned on the upper part and individual reinforcing bars placed within the ribs. The steel deck acts as a permanent formwork and the composite action between the steel and concrete is generally achieved by indentations or embossments in this component. Composite slabs play an important role in the overall stability of buildings during fire exposure, and should be designed in accordance with regulations and standards. The fire rating of this structural element is defined with respect to fire exposure from below and is normally determined through standard fire tests. Three different criteria should be taken into consideration: load bearing (R), integrity (E) and thermal insulation (I). The Annex D of the EN 1994 – 1-2 and the Annex C of the NBR 14323 provide guidelines for the calculation of the fire resistance (I), as well as the temperature of the rebars and the parts of the steel deck of unprotected fire exposed composite slabs. However, no revisions were made to these methods during the last two decades. The underlying work presents the development of numerical models for 3-D thermal analysis in the software ANSYS Mechanical APDL 18.2, and MATLAB R2018a through the PDE Toolbox. A total of 208 numerical simulations should be performed considering perfect thermal contact between all the materials, with the aim to investigate the influence of different parameters on the fire resistance (I) and the temperature of the steel components. During experimental fire tests, the steel deck separates from concrete, which increases the thermal resistance in this interface. In order to simulate debonding effects, another thermal model is established for more 40 numerical simulations, including an air gap with a constant thickness between the steel deck and concrete topping. The thermal models shall be validated against the results of different experimental fire tests. It is concluded that the calculation rules given in the European and Brazilian standard are generally on the unsafe side and do not consider important parameters. This work proposes improved equations for the estimation of the fire resistance (I) and the temperature of the parts of the steel deck and the rebars as well.