Keywords: flat-slab, connections, high-strength, concrete, slabs, punching shear, fiber, shear reinforcement, finite element method.

This paper shows that we can use a finite element model to represent the punching behavior of high strength steel fibers slab column connections. Because of reinforced concrete slab-column connections are usually subjected to large amount of bending moments and shearing forces at their connections with columns. Their combined effect can cause punching failure. The accurate prediction of the stresses in this case of study is complex and difficult. Several fundamental obstacles remain in reaching a practical analysis of such problems using finite element methods. The non-linear finite element analysis for reinforced concrete structure is largely dependant on the stress-strain relationships, failure criteria used, simulation of steel reinforcement and interaction between steel and concrete. A model for predicting punching shear failure at interior slab-column connections was developed based on experimental results obtained from the experimental program carried out by the authors. This model has been incorporated into a new reinforced concrete slab element for the non-linear analysis program, ANSYS.

In this paper, a general description of the finite element method, theoretical modeling of concrete and reinforcement are presented. In order to verify the analytical model used in this research using test results of the experimental data of the experimental branch, the finite element analysis were performed then a parametric study of concrete compressive strength, volumetric ratio of steel fibers and slab thickness were investigated. To be able to propose a design formula to predict the ultimate punching shear force for slab-column connections made with high strength steel fiber concrete subjected to concentric and eccentric punching loads, wide range of numerical analysis is conducted.

Before using the non-linear finite element program in the analysis of the parametric study of this research, a verification between the experimental results (carried out by authors, [1]) and that obtained from the finite element analysis for these specimens were performed. The numerical values of maximum deflections at central point of the slab for pure and eccentric punching were plotted against the applied load in comparison with experimental values of the slab column connections.

It can be seen that the numerical maximum loads were in good agreement with those observed from test results for all specimens. The load-deflection relations obtained from numerical analysis have a linear relation up to crack load and almost coincide with the experimental values. The ratio between the experimental and numerical central deflections at maximum load was varied from 0.60% to 0.99%, while the ratio between the experimental and numerical ultimate moment was varied from 0.97% to 1.03%. These results indicate that the proposed finite element models are valid and can be applied in the analysis of punching shear of high strength steel fibers slabs.

Due to the validity of the finite element analysis used, a parametric study of 24 specimens slab column connections subjected to pure punching were conducted numerically. The aim of this parametric study is to get more information and results to be able to propose a design formula, which can be used to predict the ultimate punching shear force of high strength steel fiber concrete slab subjected to punching loads taking into consideration the effect of fiber content.

Three values of different concrete compressive strengths were used. Moreover, two values of slab thickness and four values of fiber content were analyzed. The obtained results of numerical analysis for the studied parameters are shown that concrete compressive strength was significantly increase the ultimate punching load. Increasing the compressive strength of slabs from 700 to 900 kg/cm caused an increase of ultimate punching loads by 35.4%, 12%, 11.5% and 15.1% for of 0.0%, 0.50%, 1.00% and 1.50%, respectively for 15 cm slab thickness. Increasing slab thickness from 15 cm to 18 cm caused an increase in punching ultimate loads by 13.6%, 24.4%, 20.3% and 20% for of 0.0%, 0.50%, 1.00% and 1.50% respectively at 900 kg/cm while, the ratios of increasing were 31.6%, 19.7%, 16.6% and 20.5% for =800 kg/cm and were 34.6%, 20.9%, 19.3% and 26.3% for =700 kg/cm.

A good agreement between the numerical results of punching shear loads and the obtained values from the proposed formula is found. The ratio between the punching shear force calculated by the proposed formula and those obtained from numerical analysis is varied from 0.86 to 1.07 while the ratio between the shear force predicted by the proposed formula and these obtained experimentally is varied from 0.85 to 1.05.

1

Hassan, A.A., "Punching Shear for High-Strength Concrete Flat Slab Reinforced with Fibers," Ph.D Thesis, Cairo University, 2004.

2

ANSYS 1998. Ver. 5.4 finite element program manual.

3

Gardner, N.J., "Relationship of the Punching Shear Capacity of Reinforced Concrete Slabs with Concrete Strength," ACI Structural Journal, V. 87, No. 1, pp. 66-71, 1990.

4

Imre K., Franz-Josef U. and Gyorgy L.B. "Modeling of Plastic Matrix-Fiber Interaction in Fiber Reinforced Concrete", 2nd Int. PhD Symposium in Civil Engineering 1998 Budapest Technical University of Budapest, Budapest, Hungary

5

Ahmed A.H., "Behavior of High-Strength Fiber Reinforced Concrete Columns Subjected to Concentric and Eccentric Loads", Master Thesis, Civil Engineering Department, Cairo University, 1999.

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