Elsevier

Engineering Structures

Volume 28, Issue 11, September 2006, Pages 1494-1502
Engineering Structures

Effects of plastic hinge properties in nonlinear analysis of reinforced concrete buildings

https://doi.org/10.1016/j.engstruct.2006.01.017Get rights and content

Abstract

Due to its simplicity, the structural engineering profession has been using the nonlinear static procedure (NSP) or pushover analysis. Modeling for such analysis requires the determination of the nonlinear properties of each component in the structure, quantified by strength and deformation capacities, which depend on the modeling assumptions. Pushover analysis is carried out for either user-defined nonlinear hinge properties or default-hinge properties, available in some programs based on the FEMA-356 and ATC-40 guidelines. While such documents provide the hinge properties for several ranges of detailing, programs may implement averaged values. The user needs to be careful; the misuse of default-hinge properties may lead to unreasonable displacement capacities for existing structures. This paper studies the possible differences in the results of pushover analysis due to default and user-defined nonlinear component properties. Four- and seven-story buildings are considered to represent low- and medium- rise buildings for this study. Plastic hinge length and transverse reinforcement spacing are assumed to be effective parameters in the user-defined hinge properties. Observations show that plastic hinge length and transverse reinforcement spacing have no influence on the base shear capacity, while these parameters have considerable effects on the displacement capacity of the frames. Comparisons point out that an increase in the amount of transverse reinforcement improves the displacement capacity. Although the capacity curve for the default-hinge model is reasonable for modern code compliant buildings, it may not be suitable for others. Considering that most existing buildings in Turkey and in some other countries do not conform to requirements of modern code detailing, the use of default hinges needs special care. The observations clearly show that the user-defined hinge model is better than the default-hinge model in reflecting nonlinear behavior compatible with the element properties. However, if the default-hinge model is preferred due to simplicity, the user should be aware of what is provided in the program and should avoid the misuse of default-hinge properties.

Introduction

Since inelastic behavior is intended in most structures subjected to infrequent earthquake loading, the use of nonlinear analyses is essential to capture behavior of structures under seismic effects. Due to its simplicity, the structural engineering profession has been using the nonlinear static procedure (NSP) or pushover analysis, described in FEMA-356 [1] and ATC-40 [2]. It is widely accepted that, when pushover analysis is used carefully, it provides useful information that cannot be obtained by linear static or dynamic analysis procedures.

In the implementation of pushover analysis, modeling is one of the important steps. The model must consider nonlinear behavior of structure/elements. Such a model requires the determination of the nonlinear properties of each component in the structure that are quantified by strength and deformation capacities. Lumped plasticity idealization of a cantilever (Fig. 1) is a commonly used approach in models for deformation capacity estimates. The ultimate deformation capacity of a component depends on the ultimate curvature and plastic hinge length. The use of different criteria for the ultimate curvature and different plastic hinge length may result in different deformation capacities. Several plastic hinge lengths have been proposed in the literature [3], [4], [5].

In practical use, most often the default properties provided in the FEMA-356 [1] and ATC-40 [2] documents are preferred, due to convenience and simplicity. These default properties can be implemented in well-known linear and nonlinear static and dynamic analysis programs such as DRAIN-2DX, DRAIN-3DX, PERFORM-2D, and SAP2000 [6], [7], [8], [9]. Some programs (i.e. SAP2000) have already implemented these default nonlinear properties. The use of this implementation is very common among the structural engineering profession and researchers.

Although there may not be significant differences in the modeling of steel structures, the use of guidelines requires special care for reinforced concrete (RC) structures. As mentioned above, the deformation capacity of reinforced concrete components depends on the modeling assumptions. FEMA-356 and ATC-40 guidelines are prepared on the basis of some assumptions related to typical reinforced concrete construction in the United States. While the documents provide the hinge properties for several ranges of detailing, the programs (i.e. SAP2000) may implement averaged values. Also, there may be some differences in construction techniques and detailing in other countries. If the user knows the capability of the program and the underlying assumptions, then he/she can take advantage of the feature provided to avoid an extensive amount of work. In some cases, the default-hinge properties are used without any considerations due to simplicity.

This paper aims to study the possible differences in the results of pushover analysis due to the default and user-defined nonlinear component properties. Two structures are used with different hinge properties. Since there is no torsional effect in the selected structures, two-dimensional (2-D) modeling is employed. The SAP2000 [9] program is used for pushover analysis.

Section snippets

Description of structures

Two structures are considered to represent low- and medium-rise RC buildings for study. These consist of two typical beam–column RC frame buildings with no shear walls, located in a high-seismicity region of Turkey. Since the majority of buildings were constructed according to the 1975 Earthquake Code [10], 4- and 7-story buildings are designed according to this code, considering both gravity and seismic loads (a design ground acceleration of 0.4g and soil class Z3, which is similar to class C

Modeling approach

Analyses have been performed using SAP2000 [9], which is a general-purpose structural analysis program for static and dynamic analyses of structures. In this study, SAP2000 Nonlinear Version 8 has been used. A description of the modeling details is provided in the following.

A two-dimensional model of each structure is created in SAP2000 to carry out nonlinear static analysis. Beam and column elements are modeled as nonlinear frame elements with lumped plasticity by defining plastic hinges at

Pushover analysis

Five cases are considered in the pushover analyses of each frame, as shown in Table 3: the default-hinge properties of SAP2000 termed “Case A”, and four user-defined hinge properties, including the variation of plastic hinge length and transverse reinforcement spacing. These four cases are: (i) Case B2: Lp with Eq. (2) and transverse reinforcement spacing, s=100 mm; (ii) Case B3: Lp with Eq. (3) and s=100 mm; (iii) Case C3: s=150 mm and Lp with Eq. (3); and (iv) Case D3: s=200 mm and Lp with

Discussion of results

The interior frames of 4- and 7-story buildings were considered in pushover analyses to represent low- and medium-rise reinforced concrete (RC) buildings for study. Beam and column elements are modeled as nonlinear frame elements with lumped plasticity by defining plastic hinges at both ends of the beams and columns. The frames were modeled with default and user-defined hinge properties to study possible differences in the results of pushover analyses. The following findings were observed:

  • 1.

    The

Acknowledgements

The authors acknowledge support provided by Scientific and Technical Research Council of Turkey (TUBITAK) under Project No: 105M024.

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