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-concreto, denominado ensaio APULOT, para estimativa da resistência à . In NBR , the bond stress (τu) is given by the follo-. A complete example using the expressions of the Brazilian code for the design of reinforced concrete structures, NBR, is also presented. PDF | The Brazilian code, NBR , recommends the usage of truss (θ) e do tirante transversal (α) para o dimensionamento de vigas de concreto.
As the building presents obstacles to extraction of concrete cores, since the structure is made of concrete-filled steel columns, a study of concrete compressive strength evolution was made, in order to helps in the safety assessment of structure.
It is noteworthy that this analysis is complementary and must be performed together with other verifications, as the design review with the obtained concrete compressive strength and realization of non-destructive tests. Concrete mix proportions are presented in table Table 2. Results are presented in Table 3. In this case, NBR states that the acceptance criterion is when all of the individual samples meet the fck specified by designer.
As can be seen in Table 3 , the potential strength, at 28 days, does not show strength above or equal than the 40 MPa specified.
Another pointed aspect is strength evolution after 28 days. In a study conducted by concrete supplier, in a year, the concrete presented a strength gain of Values are presented in Table 5.
Results are presented in Table 6.
Both analyzed standards, ACI and the NBR , consider the structural safety verifications through the LRFD Load Resistance Factor Design , following a semiprobabilistic approach, which can be represented by this symbolic requirement: Uncertainties are inherently present in both sides of the inequality, i.
In this semi-probabilistic approach, these uncertainties are accounted for by the use of the load and resistance factors defined in the design standards.
These factors are applied for reducing the nominal values of the structural strengths as well as for increasing the nominal values of the actions, accounting for possible under-strength or over-loading of the structural members.
The nominal values of resistances and actions are defined as values with a given probability of being attained or exceeded, respectively, in the service life of the structure.
It is nowadays recognized that a rational basis for evaluating the actual risks of failure in a structure can only be achieved by a full probabilistic approach. However, it should be recognized that this approach can give only a nominal evaluation for the probability of the actual failure, as long as several relevant variables are not included in the reliability models, such as human errors, deterioration of the structure, etc.
In this nominal reliability evaluation of the safety in a given concrete structure, the several variables involved, such as the concrete and reinforced steel strengths, geometric dimensions and acting loads shall be treated as random variables.
It has been considered herein that for the comparison between ACI and NBR , a mere analysis of the load and resistance factors defined by the standards, or even the direct comparison between examples of structures designed according their requirements, would be useless in a quantitative point of view, as long as their design philosophies are quite different.
It is to be pointed out that the Brazilian Standard is strongly influenced by the European technical tradition and also by the characteristics of low seismicity of Brazil.
The proposed comparison is then done herein from the results of reliability analyses for structural members designed according the two standards. It is considered that the obtained reliability indexes express a quantitative measure of the reliability of the structures with respect to actual risks of failure.
For obtaining a representative set of numerical values that would permit the comparisons, several reliability analyses have been done, for reinforced rectangular sections with different values of cross section sizes and reinforcement ratios, and considering all the possibilities regarding the relationship between applied dead and live loads.
In this way, a consistent comparison between the standards is possible, based on actual quantitative results.
The considered reliability models use typical values found in the literature for the average values, standard deviations and bias factors for both resistance and loads variables. Reliability based studies for defining load factors for the ASCE have been performed by Ellingwood et al. Reliability analyses of reinforced concrete structures designed according to ACI have been performed for combinations of dead, live, wind and snow loads for instance, by Nowak and Collins5; Nowak and Szerszen6; Szerszen and Nowak7; and Szerszen et al.
The present study is focused in structural members, such as beams and slabs, subjected only to bending and shear, typical of building structures, designed considering the code provisions of the ACI and of the NBR The reliability indexes for the possible loading combinations of dead and live loads are then evaluated and compared. Research Significance The evaluation of the safety levels implied in the adoption of a given technical standard can be a significant parameter in the establishment of the confidence of the standard for their users.
The use of the reliability analysis provides a rational tool for this evaluation. The results presented herein, of reliability indexes obtained with structural elements designed according to ACI and NBR , for bending and shear, provide some quantitative elements for the discussions related to the comparisons and use of these two standards.
Analyzed Examples Without loss of generality regarding any other cases of reinforced concrete structural members subjected to bending or shear, a simply supported element beam or slab is considered, as shown in Figure 1.