Flat Slab Structures Research Articles

The forces in backprops during construction of flat slab structures

The forces in backprops during construction of flat slab structures

Criteria for the loading of slabs during construction

Two inequalities are derived to establish the loads that may be imposed on slabs at an early age. These loads may arise from the slab self-weight immediately after striking the formwork or, slightly later in the construction cycle, when the slab is supporting construction loads from the slabs above. In deriving the inequalities, it was concluded that the criteria defining acceptable loading were related not to safety but to the possible impairment of serviceability performance. Results obtained during the construction of a seven-storey flat slab structure at BRE Cardington as part of the European Concrete Building Project were used to verify the proposals.

Criteria for the loading of slabs during construction

Two inequalities are derived to establish the loads that may be imposed on slabs at an early age. These loads may arise from the slab self-weight immediately after striking the formwork or, slightly later in the construction cycle, when the slab is supporting construction loads from the slabs above. In deriving the inequalities, it was concluded that the criteria defining acceptable loading were related not to safety but to the possible impairment of serviceability performance. Results obtained during the construction of a seven-storey flat slab structure at BRE Cardington as part of the European Concrete Building Project were used to verify the proposals.

Influence of Viscoelastic Dampers on the Seismic Response of a Lightly Reinforced Concrete Flat Slab Structure

A 1/3-scale model of a section of a three-story lightly reinforced concrete flat slab structure was constructed. Researchers installed a viscoelastic damper (VED) seismic rehabilitation system on the model and subjected it to seismic simulations on a shaking table. Successive simulations were conducted with increasing base accelerations until the horizontal displacement limit of the shaking table was reached. The dampers were then removed, and the simulations were repeated until structural failure occurred. This paper summarizes the responses of the model. Brief comparisons of the responses of the model with and without VEDs installed are made. The VEDs improved energy dissipation characteristics and serviceability of the structure by reducing interstory displacements. Earthquake simulation responses indicated that the damper configuration that was employed in the experimental investigation permitted rotations of the VEDs, which lowered their energy dissipation efficiency. An alternate arrangement for the VEDs is proposed to alleviate this problem.

Effect of spandrel beams on strength and behaviour of column-slab connections

Spandrel beams are commonly used to strengthen the connections between slabs and edge columns in flat slab structures. The present study uses experimental and analytical means to improve understanding of the behaviour of these connections, especially when spandrel beams are used. The parameters considered are the spandrel beam size and reinforcement ratio, the width of the inside column slab interface, and the type of straining actions to be transferred through the connection. The efficiency of using a slab extension (beyond column outer face) instead of a spandrel beam is also studied. Based on the failure mechanism observed during the tests, a new analytical method to predict the strength of edge column-slab connections is suggested. A second analytical method is also developed while being based on a simplified failure surface. The accuracy and reliability of both methods are thoroughly assessed in this study.

Analysis of Flat Slab Structures Subjected to Combined Lateral and Gravity Loads

The effective width and equivalent frame analysis methods are applied to a flat plate test specimen. The theoretical moment distribution and lateral drift show poor agreement with the test specimen results. A modified two-beam analytical model is proposed. This model allows for more appropriate modeling of the slab cracking in negative and positive bending regions. The modified model is able to reproduce both the slab moment distribution and lateral drift observed in the test specimen. Effective width factors, a, and slab cracking factors, β, are suggested for use in modeling of flat plates for lateral and gravity load analysis.

Computer-Aided Dimensioning of Reinforced Concrete Wall and Flat Slab Structures

A new computer-aided procedure for the ultimate load dimensioning of reinforced concrete wall and flat slab structures is presented. The stress distributions for all required load cases are first determined by means of standard finite element linear elastic analysis. Based on these results, the minimum amount of steel for any number of user-defined reinforcement fields is found by applying well-known linearised dimensioning conditions. In order to “rationalise” the distribution of the reinforcement bars and to reduce the amount of steel, self-equilibrating stress states due to arbitrary plastic strains are then introduced. In contrast to standard nonlinear elasto-plastic analysis, these plastic strains are not determined by a step-by-step incremental procedure, but chosen at will, i.e. either manually by the program user or by means of linear programming optimisation techniques. This approach is justified by the classical theory of plasticity which states that self-equilibrating stress distribution does not alter the ultimate load-carrying capacity of a sufficiently ductile, slightly deformed elasto-plastic structure.

Simplified model for estimating the punching resistance of reinforced corete slabs

A method for predicting the punching strength of symmetric slab—column connections in flat slab structures is presented. The rational simplified model used was derived from a more complete one previously proposed by the author. This model leads to a single equation which is all that is required for design, agrees with the observed slab behaviour and correlates well with test results.

Rational Method for Designing R.C. Slabs to Resist Punching

A method for predicting the punching strength of symmetric slab-column connections in flat slab structures is presented. The rational simplified model used was derived from a more complete one previously proposed by the author. This model leads to a single equation, which is mostly required for design, agrees with the observed slab behavior, and correlates well with test results.

フラットスラブのせん断力に関する研究

This paper discusses an analysis of the shearing forces around the drop panel of a flat slab structure. In this study, several shapes of the drop panel are investigated in case of an uniformly distributed load. The analytical solutions of the shearing forces are derived from the analogy between the equation of a slope of the membrane and the equation of a shearing forces of a plate. The results of the shearing forces among the several shapes are obtained by the experiments in membrane.

フラットスラブの柱配置について

Flat slabs are usually supported by rows of columns distributed at the vertices of a square. In this study several manners of column arrangements in the flat slab structure are compared. Comparison is made by means of the total bending moment M, which is obtained by Eq.1. For this purpose the slab is divided into two areas : the portion of the plate around the column (A in Fig.2) and the middle portion surrounded by the boundaries of the others. Considering the former as the circular plate leads to the the solution M_1 (Eq.3). For the latter M_2 (Eq.4) is obtained. In Figs. 4, 6, 8 the lines indicating M_2=0 are found in fairly good agreement with the boundaries of the portion A. The total bending moment M, provided by M_1+M_2, along the diagonal line of the slab is shown in Fig.5, in which the results given by the membrane experiment (Phot.1) and the finite element method are also presented. For various column arrangements at the vertices of regular triangle and hexagon etc., the total bending moments, M_1+M_2, are obtained in the same fashion. From the above discussion it is advised that the flat slab structures should have column arrangements with small middle portions.

Flexural Strength of Flat Slabs at Exterior Columns

The flexural strength of exterior panels of flat slab structures is investigated by use of Yield Line Theory. Six controlling, geometrically admissible, collapse mechanisms are identified, three of which involve local flexural failures around the columns. The loading is uniformly distributed, the columns are rectangular, and the reinforcement is arranged orthotropically. The coefficient of orthotropy and the ratio of negative to positive yield moment may vary arbitrarily. The governing equations for the six mechanisms are derived and presented in dimensionless form. Based on numerical solutions of four of the equations, a set of graphs is provided which covers practical ranges of the important physical and geometrical parameters. These graphs are useful for the analysis or design of the exterior slab panels. In addition, an equation is derived which gives an upper bound on the bending moment which the slab can transfer into the exterior column at collapse load through flexural action.

Linear constraint equations for continuous support conditions in finite element analysis

Discretised structural models such as by finite elements imply discretised support conditions. In some cases such as plates on elastic foundation or slabs on large interacting columns an improved formulation of the continuous support conditions is desirable. This can be achieved by means of linear constraint equations. The numerical treatment of linear constraints is discussed for the method of elimination of variables as well as for the method of Lagrange multipliers. Then specific constraint equations for different accuracy requirements are derived, which can be used to constrain rectangular flat shell elements of arbitrary shape functions. These constraints introduce six generalized displacements according to the rigid body motions of the element and transmit the corresponding generalized reactions on the nodal degrees of freedom in a way consistent with distributed reactions. The effect on the strain energy of a square shell element is shown for the different constraint equations. As an application, the linear constraints are used to represent the continuous interaction of columns with the plate in a flat slab structure. Comparison of the finite element solutions with analytical results shows that the derived constraint equations allow a considerably improved formulation of continuous support conditions.

Punching Failure of Slabs—Transfer of Moment and Shear

A theoretical method of analysis is presented for the calculation of the punching load of an internal column of a flat slab structure subjected to moment and shear transfer. The analytical procedure utilizes elastic thin plate theory and takes into consideration the complex state of stress which exists in the compression zone of the slab around the column periphery. As this prediction is based on lower bound limit analysis a safer estimate than that based on yield line analysis results. A detailed description of the steps involved in the analysis is given in a flow diagram which indicates that four different modes of failure are possible. The validity of the assumptions made is checked by comparing the predicted punching loads with those obtained from tests that are reported in the literature. This method will be of value in supplementing existing empirical procedures.

14) 水平力を受ける無梁版構造

14) 水平力を受ける無梁版構造