Engineering Structures and Technologies (May 2009)
Analysis of methods for calculating the width of normal cracks in reinforced concrete structures
Abstract
The width of normal cracks at the level of tensile reinforcement was calculated according to various methods using the data obtained from experimental tests on reinforced concrete beams (without reinforcement pre-stress), pre-cast reinforced concrete slabs and ribbed roof slabs. Th e numerical results were compared to actual crack widths measured during the experimental tests. Also, the crack widths of pre-stressed reinforced concrete beams were calculated according to various methods and compared with each other. Th e following conclusions were reached based on the analysis of numerical and experimental results: 1) Design stresses in tensile reinforcement calculated according to [STR] and [EC] design codes are very similar, although the calculation of such stresses is more logical and simple according to [EC]. Design stresses calculated according to [RU] are greater due to the estimation of the plastic deformations of concrete in the compressive zone. Th e method proposed by Rozenbliumas (Розенблюмас 1966) estimates tensile concrete above the crack peak, and thus allows a more accurate calculation of stresses in tensile reinforcement (Fig 3). Therefore, the latter stresses in pre-stressed RC beams may be decreased by 10–12 %, when height hct ≠ 0 (Fig 1, c) and ratio M/MRd varies between 0,65 and 0,75; 2) The widths of normal cracks in conventional RC beams (subjected to load that corresponds approx. 70 % of their carrying capacity) calculated according to [STR] and [EC] design codes are almost equal to the experimentally obtained crack widths. When beams and slabs are loaded by approximately 52 % of their carrying capacity, design crack widths wk [EC] are approximately 12 % less than wk [STR], although the design crack width wk [RU] is signifi cantly greater. Here, ratio β in the beams and slabs is equal to 2 and 3.3 respectively. Th erefore, the design code [RU] ensures higher probability that the crack width will not reach the limit value (for environmental class XO and XC1) equal in all design codes mentioned in this article; 3) In case of loaded prestressed reinforced concrete beams, the calculated increases of crack widths wk [EC], wk [RU] and w [5] are greater if compared to wk [STR] (Fig 6). Th e increased reinforcement ratio ρ has more signifi cant infl uence on the increases of crack widths calculated according to other design codes if compared to wk [STR]. Tensile concrete above the crack peak has signifi cant infl uence on the design crack width when pre-stressed RC beams are lightly reinforced (ρ ≤ 0,008); 4) During the evaluation of the state of fl exural RC members, expression (5) could be used for calculating the crack width or a position of the neutral axis when the heights of the crack and the tensile zone above the crack are known (calculated or measured experimentally). Design crack widths w (5) are very similar to the experimentally obtained results. Gelžbetoninių konstrukcijų normalinių plyšių pločio skaičiavimo metodų analizė Santrauka Remiantis eksperimentinių gelžbetoninių sijų (be išankstinio armatures įtempimo) ir surenkamųjų gelžbetoninių perdangų bei stogo briaunotųjų plokščių tyrimų duomenimis, pagal skirtingas metodikas apskaičiuotas atsivėrusių normalinių plyšių plotis tempiamosios armatūros lygyje. Skaičiavimo rezultatai palyginti su tyrimų metu išmatuotais plyšių pločiais. Taip pat palyginti pagal įvairias metodikas apskaičiuoti įtemptojo gelžbetonio sijų plyšių pločiai. Atlikta analizė parodė, kad pagal Lietuvoje galiojantį statybos techninį reglamentą ir euronormas beveik vienodai įvertinamas normalinių plyšių atsivėrimas. Pagal Rusijos projektavimo normų metodiką apskaičiuotos paprastojo gelžbetonio sijų plyšių pločio vertės yra gerokai didesnės. Pateikta plyšio parametrų tarpusavio priklausomybė gali būti panaudota apskaičiuojant plyšių atsivėrimą ar nustatant neutraliosios ašies padėtį elemento skerspjūvyje, kai atliekami konstrukcijų būklės tyrimai ir vertinimas. First Published Online: 16 May 2013 Reikšminiai žodžiai: gelžbetoninės sijos, plokštės, armatūros įtempiai, plyšio plotis
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