1. Introduction
Reinforced concrete buildings constructed before the implementation of modern seismic design standards present a major global risk for earthquake safety. These older structures are highly vulnerable to severe damage or even collapse during strong earthquakes, which has historically resulted in considerable loss of life. Many fatalities in past earthquakes are directly linked to the collapse of such buildings. Since the introduction of the capacity design concept in seismic codes in the 1980s, the safety disparity between earthquake-resistant buildings and those built before 1980 has widened, heightening concerns worldwide. Earthquakes such as those in Athens (1999), Turkey (1999), L’Aquila (2009)—which the author personally witnessed while residing there—and the 2023 Turkey-Syria earthquakes underscore the critical need for improved assessment and retrofitting of older reinforced concrete structures. Over the past 20 years, extensive research and code advancements have targeted this issue, as the detailing in these older buildings often falls significantly short of current standards for earthquake-resistant design.
Reinforced concrete (RC) columns are crucial to a building’s overall performance, as their failure can lead to extensive, disproportionate damage throughout the structure. The behavior of RC columns under the combined effects of axial load, shear, and flexure has been widely studied. For columns primarily exhibiting flexural behavior, sectional analysis or a fiber model in a one-dimensional stress field can reasonably estimate both ultimate strength and yielding deformation. However, when a column’s behavior is driven by shear or shear and flexure, sectional analysis alone falls short, as shear forces generate stress fields that extend through the member to its supports [
1,
2].
Recently, researchers have shown increased interest in the lateral load behavior of columns, particularly regarding axial failure that can lead to building collapse [
3,
4]. Before specific design requirements were introduced in the 1970s, reinforced concrete building frames in high-seismicity areas were built with detailing and proportions similar to those designed mainly for gravity loads. In these structures, columns were not typically designed to be stronger than beams, so column failure mechanisms are common in buildings from that period, especially in areas without infill walls, such as soft-story structures like the Imperial County Hospital or buildings with window-framing columns, as seen in the Van Nuys Holiday Inn [
5,
6]. Columns often featured widely spaced transverse reinforcement, which contributed to failure modes involving shear or combined flexure-shear failure. As shear failure advances, the degradation of the concrete core can reduce the column's capacity to carry axial loads. When this capacity declines, gravity loads must be redistributed to adjacent structural elements. A sudden loss of axial capacity can trigger a rapid, dynamic redistribution of internal forces within the frame, potentially leading to progressive collapse. This type of structural response has been observed in numerous strong earthquakes worldwide, including the Perachora Earthquake in Greece (1982), the L’Aquila Earthquake in Italy (2009), and others [
7].
One example of a member-based approach to modeling shear effects is the strut-and-tie mechanism, which is used in the D-regions of beams and columns. Here, a 45° diagonal strut extends through the concrete member, covering a distance at least equal to the member's depth. Despite this, many design codes treat shear strength as a cross-sectional property [
8], though alternative approaches like strut-and-tie models [
8,
9,
10,
11] are available, albeit less commonly used and often unfamiliar to many practitioners.
More advanced approaches, such as variable-angle strut-and-tie models, adjust the angle of the strut based on the level of transverse reinforcement. For instance, Eurocode 2 (2004) [
13] permits a strut angle between 22.5° and 45°, with the specific angle varying according to the required transverse reinforcement. A more detailed approach, as outlined in AASHTO 2013 [
10] and Model Code 2010 [
9], is based on the Modified Compression Field Theory (MCFT), developed by Vecchio & Collins (1986) [
14], which is widely regarded as the most comprehensive framework for understanding the shear behavior of reinforced concrete members.
The most advanced seismic design and assessment techniques available today still rely on some form of nonlinear analysis, whether static or dynamic. These analyses are typically carried out using frame elements with differing degrees of approximation. The two primary approaches used are lumped-plasticity models and distributed-inelasticity models.
Distributed-inelasticity elements allow for the direct integration of section response [
15,
16]. In this approach, fiber beam elements are especially effective for studying the behavior of RC structures under reversed cyclic loading, as they accurately capture moment-axial force (M-N) coupling and the interaction between concrete and steel within the section. While many fiber beam-column elements have been developed to reliably represent axial force and flexural effects, the interaction between normal and shear forces is more complex, and only a limited number of modeling strategies have been fully implemented to address this [
17].
In contrast, lumped-plasticity elements require parameter calibration based on the response of an actual or ideal frame element under simplified loading conditions. This calibration is crucial because the behavior of concentrated plasticity elements depends on the moment–rotation relationship of their components. For an actual frame element, the end moment–rotation relationship is determined by integrating the section response, similar to the process used in a fiber beam element [
19].
To model the behavior of prismatic members, where normal stresses and strains vary across a cross-section depth in response to flexural moment demands (maintaining plane sections), Vecchio and Collins (1988) [
18] introduced the MCFT within a layered analysis framework, commonly known as a fiber model [
19]. In this method, kinematic assumptions for flexure and shear (represented by sectional curvature and shear strain) drive the algorithm, while principal stress and strain orientations are calculated at multiple layers across the member’s depth. Nonlinear constitutive material laws, defining uniaxial stress-strain behavior in the principal directions, are used to determine the stress state and ensure equilibrium of the stress resultants. Here, concrete fibers are treated as biaxially stressed elements within the cross-section, with their in-plane stresses analyzed through MCFT. This approach was later refined to enhance the accuracy of shear stress distribution across the section. These advanced formulations were implemented in Response 2000 [
20], a nonlinear analysis program for structural members.
When applying the MCFT to seismic assessments, several modifications are required to address the unique demands of cyclic loading. One challenge is that most experimental data supporting the MCFT is based on tests with monotonic loading, offering limited understanding of the model's behavior under cyclic displacement reversals and related degradation mechanisms. Additionally, the method assumes uniformly distributed reinforcement, which does not adequately represent older structures with sparse reinforcement. Another limitation is the absence of explicit modeling for bond-slip degradation effects on the shear behavior of RC members.
This limitation, along with the distinctive behavior of lightly reinforced concrete columns where shear and flexure interact, was recently investigated by developing a fiber beam model grounded in the MCFT [
21]. This theory was applied using an exact Timoshenko fiber element that also accounts for the substantial effect of tensile reinforcement pullout due to anchorage or short lap splices on the column’s overall lateral drift. These capabilities were incorporated into a standalone Windows program named "Phaethon" [
21], with a user interface developed in C++. The program aids engineers in analyzing substandard reinforced concrete columns with both rectangular and circular cross-sections.
Utilizing the moment-rotation envelope results from a cantilever shear-critical column analyzed using Phaethon software, one can model an inelastic frame structure subjected to shear, axial, or pull-out failures by placing a rigid plastic spring at the expected shear failure location. This approach also accounts for the impact of anchorage or lap-splice pullout slip on total drift and incorporates a negative degradation slope effect. The slope of the degradation links the moment-rotation envelope point where shear failure occurs to the axial failure point, beyond which the column cannot sustain its gravity loads. The section of the member between the two rigid plastic springs remains perfectly elastic. Giberson [
22,
23] generalized the original one-component model. A significant benefit of this method is that inelastic deformation at the ends of the member is determined solely by the moment applied there, allowing for any moment-rotation hysteretic model to be assigned to the spring. While this straightforward model has received some reasonable criticism, it is anticipated to perform effectively for relatively low-rise frame structures, particularly where the inflection point of a reinforced concrete column is situated near mid-height.
The main objective of Performance-Based Earthquake Engineering is to determine an "acceptable" probability of collapse. Collapse should be assessed as accurately as possible using nonlinear dynamic analysis. A thorough set of guidelines will provide a framework for tackling the complexities associated with nonlinear softening responses during significant displacements and deformations, thereby facilitating the acceptance of nonlinear response analyses in professional practice [
24,
25,
26,
27,
28,
29,
30,
31]. The introduction of straightforward yet effective column models, such as those presented in this study, which incorporate localized effects like shear and anchorage or lap splice slip within a coherent element formulation, will help mitigate issues of non-convergence and reduce computational time.
This paper contributes to the field of seismic assessment of older RC frames through nonlinear dynamic analyses in the following ways as also
Figure 1 depicts:
The formulation of path-dependent one-component element response with strength degradation due to shear and axial failures is described in detail.
A self-developed MATLAB [
32] code is created in order to run a nonlinear dynamic analysis on one-story, two-bay reinforced concrete frame experiencing both shear and axial failures and was simulated with the above formulated beam element.
The proposed analytical model can also address the stress state of a column under full cyclic load reversals, accounting for both flexure- and shear-dominated response conditions in RC columns, while also considering the contribution of anchorage or lap-splice pullout slip to the total drift.
A reduced computational model for prediction of dynamic response of old reinforced concrete structures under seismic loads is developed based on the moment-rotation envelope results from cantilever shear-critical columns analyzed by Phaethon Windows software.
Inelastic frame structure experiencing shear, axial or pull-out failures are modeled in this study by placing a rigid plastic spring at the location where shear failure is predicted considering the contribution of anchorage and pullout slip in the total drift and applying a degradation slope. The negative slope connects the point on the moment-rotation envelope where shear failure occurs to the point of axial failure.
The proposed approach advantage is the inelastic deformation at the member ends depends solely on the moment applied at the end, allowing any moment-rotation hysteretic model to be assigned to the spring hence simplifying the analytical and numerical modeling.
This study is organized as follows: Following the introduction, which outlines the objectives of this research paper,
Section 2 details the formulation of a path-dependent, one-component element response with strength degradation.
Section 3 provides a comprehensive comparison of the proposed analytical model with experimental results found in the literature. Lastly,
Section 4 discusses the output results, while
Section 5 presents the conclusions and suggestions for future work.
4. Discussion
During an earthquake, columns can experience a wide range of loading histories, which may include a single large pulse or several smaller-amplitude cycles. These cycles can sometimes result in shear failure or even collapse, where the column loses its ability to support gravity loads. Previous research [
1,
2] has shown that such collapse cannot be explained by a simple combination of shear force and axial load. Instead, it is governed by an interaction envelope that depends on both the loading history and the peak deformation exerted on the column (maximum drift demand).
To understand how the loading history affects a column's response, it is important to note that structural members undergoing lateral displacement reversals tend to lengthen due to the accumulation of permanent tensile strains in the longitudinal reinforcement crossing diagonal shear cracks. As displacement cycles increase in amplitude, the cracks widen. This is depicted in the axial stress-strain diagram of the reinforcement after yielding, where permanent strains are biased in tension due to the neutral axis shifting towards the compression side of the member's cross-section after cracking. Axial load plays a key role in this process, as it helps keep the cracks partially closed, thereby delaying the elongation and ratcheting of the column.
Additionally, research [
1,
2] shows that increasing the number of cycles beyond the yield displacement can reduce a column’s drift capacity at shear failure. One of the goals of this research is to better understand these effects and develop simplified tools to identify the failure characteristics at the loss of axial load-bearing capacity, as well as the impact of drift demand on the column’s deformation capacity.
As already mentioned, collapse should be quantified as accurately as possible through nonlinear dynamic analysis. A comprehensive set of guidelines will serve as a foundation for addressing the complexities of nonlinear softening responses under large displacements and deformations, helping to promote the acceptance of nonlinear response analyses in professional practice. The introduction of simple but effective column models, like those presented in this study, which account for localized effects such as shear and anchorage or lap splice slip within a consistent element formulation, will reduce non-convergence issues and computational time. The correlation of the proposed model with the experimental results produces acceptable results especially in terms of drifts and permanent damage and the model succeeds in reducing the computational effort.
Finally, it should be noted that the goal of this study is to simplify the assessment of collapse of RC frame structures so the presented methodology could be used in large-area-scale seismic assessment. It is not intended to substitute the introduced in the literature review advanced methods and, in a way, as for example with MCFT it is based also on its estimates through Phaethon Windows software. The intended improvement lies upon its simplifications without losing reasonability in its results.
Figure 1.
Graphical research framework of this study (Δshear cantilever lateral displacement due to shear mechanism, Δslip cantilever lateral displacement due to pull-out slip of anchorage or lap-splice, Δflex cantilever lateral displacement due to flexure, Δtot total lateral displacement, lr yield penetration length in the anchorage, fby local bond strength of the anchorage, lp plastic hinge length, γe elastic shear strain, γp plastic shear strain. θ cantilever lateral rotation, θslip cantilever lateral rotation due to pull-out slip, VR shear strength, Ls shear span, d column section effective depth, V seismic shear force, Δ lateral displacement, Δs lateral displacement at shear failure, Δa lateral displacement at axial failure) .
Figure 1.
Graphical research framework of this study (Δshear cantilever lateral displacement due to shear mechanism, Δslip cantilever lateral displacement due to pull-out slip of anchorage or lap-splice, Δflex cantilever lateral displacement due to flexure, Δtot total lateral displacement, lr yield penetration length in the anchorage, fby local bond strength of the anchorage, lp plastic hinge length, γe elastic shear strain, γp plastic shear strain. θ cantilever lateral rotation, θslip cantilever lateral rotation due to pull-out slip, VR shear strength, Ls shear span, d column section effective depth, V seismic shear force, Δ lateral displacement, Δs lateral displacement at shear failure, Δa lateral displacement at axial failure) .
Figure 2.
Beam (a) displacements and (b) forces in global, local and basic reference systems and c) one-component beam model.
Figure 2.
Beam (a) displacements and (b) forces in global, local and basic reference systems and c) one-component beam model.
Figure 3.
(
a) Specimen 2 of shake table test [
37,
38] (
b) Simplified numerical model implemented in MATLAB 2024b.
Figure 3.
(
a) Specimen 2 of shake table test [
37,
38] (
b) Simplified numerical model implemented in MATLAB 2024b.
Figure 4.
Capacity curve of center shear-critical column of Specimen 2 and lateral displacement contributions for each step of the pushover analysis (16 total pushover steps of 5 kN)[This is a screenshot from Phaethon Windows software user’s interface].
Figure 4.
Capacity curve of center shear-critical column of Specimen 2 and lateral displacement contributions for each step of the pushover analysis (16 total pushover steps of 5 kN)[This is a screenshot from Phaethon Windows software user’s interface].
Figure 5.
Strain, Slip and Bond distributions along the straight anchorage length of center shear-critical column of Specimen 2 for pushover step 15 of Phaethon Windows software. See also
Figure 1 and 4.
Figure 5.
Strain, Slip and Bond distributions along the straight anchorage length of center shear-critical column of Specimen 2 for pushover step 15 of Phaethon Windows software. See also
Figure 1 and 4.
Figure 6.
Time-history responses in terms of drift, base shear, and center column shear of Specimen 2.
Figure 6.
Time-history responses in terms of drift, base shear, and center column shear of Specimen 2.
Figure 7.
Absolute Error Time-history responses in terms of drift, base shear, and center column shear of Specimen 2.
Figure 7.
Absolute Error Time-history responses in terms of drift, base shear, and center column shear of Specimen 2.
Figure 8.
Shear hysteretic response of Specimen 2.
Figure 8.
Shear hysteretic response of Specimen 2.
Figure 9.
Below beam moment-rotation hysteretic response of center column of Specimen 2.
Figure 9.
Below beam moment-rotation hysteretic response of center column of Specimen 2.
Figure 10.
Below beam moment-rotation hysteretic response of outside column of Specimen 2.
Figure 10.
Below beam moment-rotation hysteretic response of outside column of Specimen 2.
Table 1.
Details of central shear-critical RC columns of Specimen 2 (units: mm, MPa, kN).
Table 1.
Details of central shear-critical RC columns of Specimen 2 (units: mm, MPa, kN).
Case |
Axial Load (kN) |
Width (mm) – Depth (mm) |
Shear Span (mm) – StraightAnchorage Length (mm) |
Clear Cover (mm) |
Concrete Strength (MPa) |
Number - Diameter (mm) – Reinforcing ratio of Longitudinal Bars |
Yielding Strength of Long. Bars (MPa) |
Ultimate Strength (MPa) – Spacing (mm) – Diameter (mm) –Ratio of Transv. Reinf. |
Elwood and Moehle [37,38] – (Spec. 2 – Center Column) |
308.132 |
230 230 |
814 298 |
25.4 |
24.27 |
4 and 4 12.7 and 15.875 0.0245 |
479.18
|
717 152 4.9 0.00236 |