Abstract
ABSTRACT: Provided that the anchorage capacity is guaranteed at the supports, the bearing capacity of hollow core slabs depends on the shear capacity of the pretensioned concrete webs, wherein the critical section is in a region between h and 2h from the support. For line loads acting within 2h to 6h from the supports, especially for shallow slabs 150 to 200 mm deep, it is likely to have flexure-shear cracks within the transfer region, wherein the bearing capacity is highly affected by the actual prestressing forces at the critical section. Therefore, one of the major questions pondered by structural engineers is to determine the effective amount of prestressing force that affects the shear resistance mechanism near to the support. According to ABNT NBR 14861:2011, the shear capacity is based on the flexure-shear mechanism, wherein the shear strength is a sum of the tensile concrete strength in the slab webs plus the contribution of the prestressing forces at the critical section, wherein a coefficient of 0.15 is considered. However, in both codes NBR 14861 and NBR 6118 it is required that this coefficient 0.15 should be further multiplied by an additional reduction factor in order to take into account the effect of the transmission length near to the support. Considering the current revision of the NBR 14861, this paper presents a theoretical-experimental comparison from standard shear tests of hollow core slabs with nominal depths from 150 to 200 mm carried out in different research at NETPre-UFSCar. Based on the analytical study of each term of the equation for the flexure-shear capacity, it has been observed that the coefficient 0.15 provides a conservative limit for the contribution of the actual prestressing force. Therefore, there is no need to apply any additional reduction factor in order to guarantee a safe design limit for the shear capacity.
Highlights
The use of precast systems provides an effective improvement for the site construction, with a higher rationalization of the production processes and better quality control of the final building systems
Considering the current revision of the NBR 14861, this paper presents an experimental-theoretical comparison from results of standard shear tests of hollow core slabs with depths varying from 150 to 200 mm, which were carried out at NETPre-UFSCar, analyzing the contribution of each parcel of the expression for the flexure-shear capacity in order to verify if the coefficient 0.15 is enough to consider the contribution of the actual prestressing force or if there is still the need to apply any additional reduction factor in order to take in account the effect of the transmission length for the flexure-shear capacity
It is noteworthy that the importance of the transfer length is related to the fact that in cases where the failure of slab occurs due to flexure-shear model, the maximum tensile stress will occur in a place with the greatest interaction between the bending moment and shear stress, that is between 2.5∙d and 4∙d
Summary
The use of precast systems provides an effective improvement for the site construction, with a higher rationalization of the production processes and better quality control of the final building systems. Hollow core slabs are the main product within the precast concrete industry. This trend is due mainly to a combination of high structural performances with standardized production methods, with choice of depth and capacity, speed of construction, environmental friendliness without the need for molding and propping on site. For the flexure-shear mechanism, wherein there is a possibility of occurrence of bending cracks within 2h from 6h from the slab support, it is essential that the anchorage capacity is secured in order to avoid any failure by bond-slip at the supports. In the presence of imposed concentrated loads, according to Lindström [1], there is a real possibility of occurrence of interaction between flexure-shear and bending mechanisms, wherein they cannot be utilized at their respective maximum capacities
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