Collapse test of reinforsced concreate columns with increased axial load

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Collapse test of reinforsced concreate columns with increased axial load

Sidorov М., Shimokawa S., Takeda T.,

Saito S., Miyajima K., Nakamura T.

NU, Niigata, Japan

Abstract

The study is devoted to the study of the behavior of RC columns under load in seismically dangerous areas under various loading methods, the appearance of which is possible with seismic loads perceived by the building. To achieve the goal in the laboratory of the University of Niigata in Japan, samples were tested with the creation of conditions close to real loads.

Keywords. Column, seismic, loading, destruction, shear, axial load, horizontal load, reinforcement, collapse, collapse drift.

Абстракт

Тест на разрушение колонн с повышенной осевой нагрузкой

Сидоров М. Е., Симокава С., Такэда Т., Сайто С., Миядзима К., Накамура Т. УН, Ниигата, Япония

Исследование посвящено изучению поведения ЖБ колонн под нагрузкой в сейсмоопасных районах при различных способах нагружения, появление которых возможно при сейсмических нагрузках, воспринимаемых зданием. Для достижения цели в лаборатории университета г. Ниигата Япония были испытаны образцы с созданием условий нагружений близких к реальным.

Ключевые слова: Колонна, сейсмика, нагружение, разрушение, сдвиг, осевая нагрузка, горизонтальная нагрузка, армирование.

Introduction

Many reinforced concrete (RC) buildings with brittle columns are in danger of collapse in the event of gravity load collapse of columns following shear failures from future earthquakes. In structural frames, once the brittle columns are severely damaged, some of the axial load sustained by them is transferred to neighboring columns through girders (Fig. 1).

In other words, the axial load of the first damaged columns decreases, while the axial load of the neighboring columns increases to more than the initial value. In the past, tests with increased axial load are yet to be conducted. As a result of the increase in the axial load, the structural integrity of the columns will deteriorate such that they are in danger of collapse. However, the process of such collapse is still unclear. This study aims to investigate the effects of axial load increase on column collapse behavior. Half-scale column specimens were fabricated and loaded horizontally until the loss of axial-load carrying capacity under increased axial load and constant axial load. The findings are useful in order to accurately evaluate the seismic performance variation of RC buildings with axial load redistribution.

1. Outline of test

Two half-scale column specimens were fabricated and designed to ensure shear failure after flexural yielding. Table 1 lists the structural properties of the specimens.

The two specimens have the same reinforcement specifications and loading history, but different loading methods for the axial load. The reinforcement details and the column sections of the specimens are shown in Fig. 2.

The column height was 1200 mm, the column section was 300 mm x 300 mm, and the height-to-depth ratio was 4.0. The longitudinal bar ratio (pg), defined as the total main reinforcement area divided by the column section, was 1.69%. The transverse bar ratio (pw) was 0.21%. The concrete strength was 25.6 N/mm². The yield strength of steels for longitudinal reinforcement and transverse reinforcement were 364 N/mm² and 423 N/mm², respectively.

The specimens with increased and constant axial load were compared. The test variables were the axial stress ratio. Note that the axial stress ratio is defined as the axial load multiplied by the concrete strength multiplied by the column section. For the specimens with increased axial load (see Table 1, FS2-3D), the initial axial stress ratio was 0.2. The axial stress ratio increased to 0.3 during loading. The drift at axial load increase was 0.75%.

The axial load was increased before the maximum load when the drift was small. For the specimens with constant axial load, the axial stress ratios were 0.2 and 0.3 for specimens FS2 and FS3, respectively (see Table 1). Here, the axial stress ratios = 0.2 and 0.3 are the same as the initial and increased axial stress ratio of specimens with increased axial load, respectively. Table 1 also lists the shear and flexure strengths computed for each specimen using the conventional method in Japan.

Fig. 3 illustrates the test apparatus that realizes double-curvature deformation. The specimens were laterally loaded under increased or constant axial load. The tests were terminated when the specimens could not sustain the prescribed axial load. With respect to the loading history, we used cyclic loadings. The specimens were finally loaded to the positive direction for as long as the axial load could be maintained.

2. Test results

All specimens failed in shear after flexural yielding, and finally lost their axial load-carrying capacity. In this study, “collapse” and “collapse drift” are defined as the column's loss of axial load-carrying capacity and the maximum lateral drift experienced prior to collapse, respectively. The collapse behavior is presented below in order to compare specimens FS-2-3D and FS2.

Figs. 4 and 5 show the lateral load and the lateral drift relations for specimens

FS2-3D and FS2, respectively. Fig. 6 shows the lateral drift and the axial deformation relations for the two specimens. The drifts and axial deformations were divided by the column height. Photo 1 shows the damage stages observed at the collapse for specimens FS2-3D and FS2.

FS2-3D. The axial stress ratio increased to 0.3 from 0.2 when the drift reached at 0. 75%. The strength reduction after the axial load increase was great. The collapse drift was 1.79%. The axial deformation at collapse was 0.15%. The collapse occurred accompanied shear failure at the column end (see photo 1). FS2. The axial stress ratio was 0.2 (constant axial load). The collapse drift was 5.83%. The axial deformation at collapse was 0.25%. The collapse occurred accompanied shear failure over the entire length of the column (see photo 1).

The collapse drift was smaller in columns with increased axial load (FS2-3D: 1.79%) than in columns with constant axial load (FS2: 5.83%). The former was 0.31 times smaller than the latter. The axial deformation at collapse was smaller in columns with increased axial load (FS2-3D: 0.15%) than in columns with constant axial load (FS2: 0.25%). The former was 0.6 times smaller than the latter. seismic japan column reinforced concrete

In summary, columns for which the initial axial load increased exhibited smaller collapse drift and axial deformation at collapse than columns for which the initial axial load was kept constant. These results indicate that the near-collapse axial load (increased axial load) significantly affects the collapse behavior.

Conclusions

The effects of increased axial load on the collapse behavior of shear failing RC columns due to axial load redistribution in structural frames were investigated. The major findings of this study are summarized below.

(1) Columns for which the initial axial load increased exhibited smaller collapse drift and axial deformation at collapse than columns for which the initial axial load was kept constant.

(2) The near-collapse axial load (increased axial load) significantly affects the collapse behavior.

Библиографические ссылки на источники

1. Японская ассоциация по предотвращению стихийных бедствий. Стандартная оценка сейсмической уязвимости. Оценка существующих железобетонных зданий (на японском языке), 300 с., 1977, пересмотренная в 1990 и 2001 годах.

2. Японская ассоциация по строительству. Профилактика стихийных бедствий. Руководство по проектированию сейсмической реконструкции для существующих железобетонных зданий (на японском языке), 377 с. 1977, пересмотрено в 1990 и 2001 годах.

3. Японская ассоциация по предотвращению стихийных бедствий. Стандарт для инспекции после землетрясения и рекомендации по ремонту и технологии усиления (на японском языке), 122 стр., 1991 год

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