Ante Mihanović

The comparative body model in material and geometric nonlinear analysis of space R/C frames

Purpose – This paper aims to present a new numerical model for the stability and load‐bearing capacity computation of space reinforced‐concrete (R/C) frame structures. Both material and geometric nonlinearities are taken into account. The R/C cross‐sections are assumed to undergo limited distortion under torsional action.Design/methodology/approach – A simple, global discretization using beam‐column finite elements is preferred to a full, global discretization using 3D elements. This is more acceptable from a practical point of view. The composite cross‐section is discretized using 2D elements to apply the fiber decomposition procedure to solve the material and geometrical nonlinear behavior of the cross‐section under biaxial moments and axial forces. A local discretization of each beam element based on the comparative body model (i.e. a prismatic body discretized using brick elements, element by element, during the incremental‐iterative procedure) allows determining the torsional constant of the cross‐se...

Non‐linear finite element analysis of post‐tensioned concrete structures

Presents a non‐linear numerical model for the computations of post‐tensioned plane structures. Generally curved prestressing tendons and reinforcing bars are embedded into the concrete and they are modelled independently of the concrete mesh using one‐dimensional curvilinear elements. Among the losses which influence the decrease in the prestress force, it is possible to compute the losses caused by friction between tendons and the concrete, the losses which result from the concrete deformation and the losses in the anchorage zone. The computation for post‐tensioned structures is organized in phases: the phase preceding prestressing (Phase I), the prestressing phase (Phase II) and the phase following prestressing (Phase III). The load is applied incrementally until failure. The model is tested on a number of examples.

Finite element solution improved by full clamping element functions

Presents a procedure for obtaining an improved finite element solution of boundary problems by estimating the principle of exact displacement method in the finite element technique. The displacement field is approximated by two types of functions: the shape functions satisfying the homogeneous differential equilibrium equation and the full clamping element functions as a particular solution of the differential equation between the nodes. The full clamping functions represent the solution of the full clamping state on finite elements. An improved numerical solution of displacements, strains, stresses and internal forces, not only at nodes but over the whole finite element, is obtained without an increase of the global basis, because the shape functions are orthogonal with the full clamping functions. This principle is generally applicable to different finite elements. The contribution of introducing two types of functions based on the principle of the exact displacement method is demonstrated in the solution procedure of frame structures and thin plates.

Target acceleration method for analysis of RC structures

Purpose – The purpose of this paper is to present a new modification of the multimodal pushover method, named the target acceleration method. The target acceleration is the minimum acceleration of the base that leads to the ultimate limit state of the structure, i.e., the lowest seismic resistance. Design/methodology/approach – A nonlinear numerical model is used to determine the target acceleration, which is achieved using the iterative procedure according to the envelope principle. Validation of the target acceleration method was conducted on the basis of the results obtained by incremental dynamic analysis. Findings – The influence of higher modes is highly significant. The general failure vector corresponding to the target acceleration differs from the first load vector and the form of the load with uniform acceleration according to the height of structure, as contained in the European Standard EN 1998-1. Comparison between the target acceleration, including the equivalent structural damping, and the ...

Multimodal Pushover Target Acceleration Method Versus Dynamic Response of R/C Frames

A new multimodal pushover target acceleration method for the nonlinear analysis of reinforced concrete (R/C) frames subjected to seismic action is presented in chapter. The aim of research shown in this chapter was to find the influence of multimodal combinations in assessing the bearing capacity of R/C frames based on linear (L) combination of modes, and to compare the target ground acceleration a gr,t (a gr,u ) of the multimodal pushover target acceleration method with the failure peak ground acceleration a gr,d obtained by a dynamic transient response of R/C frames. The target acceleration presents the lowest seismic resistance and it is the minimum acceleration of the base that leads to the ultimate limit state of structure, and it is reached by an iterative procedure. In accordance with the Eurocode 8 rules, application of the pushover method favors access utilizing the first mode. Examples of presented 5-storey spatial R/C frame show the significant influence of higher modes. A formulation for determining the equivalent structural system damping by equalizing the dissipated energy during one cycle of vibration of the nonlinear system and the equivalent linear system is presented in this chapter. Results of the dynamic response of R/C frames are also presented in this chapter. As seismic excitation eight real earthquake accelerograms are taken. On the basis of the results obtained by nonlinear dynamic time-history analysis validation of the procedure of searching the target ground acceleration was made. Taking into account the elastic spectrum with calculated equivalent structural damping and usability of the capacity curve up to 3/3 of a displacement, the comparison of the target acceleration a gr,u of the multimodal pushover method with the failure peak ground acceleration a gr,d , obtained by a dynamic response of the structure, shows a very good agreement between the target acceleration and the failure peak ground acceleration.

Extreme Modal Combinations for Pushover Analysis of RC Buildings

The pushover method is a practical procedure for comprehensive nonlinear analysis of structures subjected to seismic action. Application of this method, in accordance with the Eurocode 8 rules and due to engineering simplicity, favours application utilizing the first mode. The aim of the presented research in this paper was to find the influence of multi modal combinations in assessing the bearing capacity of reinforced concrete (RC) frames and walls. This paper presents a procedure in which the most extreme state is defined by the lowest ground acceleration caused by a predetermined shape of an elastic spectrum. The extreme bearing value is obtained by the envelope principle. Mode shapes and period sizes are determined on a linear elastic model while the limit state of the load bearing system is evaluated in a nonlinear state of structures. Results of the analysis show that influences of higher modes are significantly higher and that the safety/reliability, indicated by the criteria for the target displacement, in accordance with Eurocode 8 (Annex B), is not achieved. Inclusion of higher modes, in some presented examples, decreases the peak ground acceleration by more than two times, which is significantly less favourable than the target displacement criteria.

Nonlinear analysis of space r/c frames with non-uniform torsion

This paper presents a numerical model of stability and load-bearing capacity of space reinforced concrete (R/C) frame structures taking into account the material and geometric nonlinearity. The developed model describes the behavior of space frames with composite cross sections under a monotonically increasing load, from zero up to the ultimate load, i.e. collapse of the structure. The collapse of the structure occurs due to exceeding the limit load and/or loss of stability of space beams or whole structure.