Journal of Structural and Construction Engineering (Transactions of AIJ)
Online ISSN : 1881-8153
Print ISSN : 1340-4202
ISSN-L : 1340-4202
Volume 85 , Issue 769
Showing 1-15 articles out of 15 articles from the selected issue
  • Kento ONOMOTO, Koji TAKASU, Hidehiro KOYAMADA, Hiroki SUYAMA
    2020 Volume 85 Issue 769 Pages 311-319
    Published: 2020
    Released: March 30, 2020
    JOURNALS RESTRICTED ACCESS

     Currently, various studies are being conducted on recycled aggregates. Use cases of recycled aggregates have been increasing in recent years. However, since there are still many problems with recycled aggregate, some problems need to be improved for further dissemination. For example, creep characteristics. There are few previous studies on the creep phenomenon of concrete using recycled aggregates.

     This research has two purposes. The first purpose is to clarify the compressive creep characteristics of concrete when mixed recycled aggregate and fly ash. The second purpose is to investigate the applicability of the traditional formula for predicting compressive creep strain. We also examined the effects of physical properties of the recycled aggregate used on the compression creep strain.

     The compressive creep strain of concrete using recycled aggregate showed a tendency to increase the strain compared to the mix using ordinary aggregate. Moreover, the compression creep strain tended to increase due to the deterioration of the quality of the recycled fine aggregate rather than the quality of the recycled coarse aggregate. It was confirmed that even when fly ash was mixed, it was easily affected by the quality of the recycled fine aggregate. In addition, it was shown that the increase in compressive creep strain could be suppressed by mixing fly ash, compared without fly ash. It was confirmed that the correction factor proposed in the prediction formula is greatly influenced by the water absorption rate. Therefore, when using recycled aggregate, it is considered necessary to propose a prediction formula for compressive creep strain considering water absorption. In addition, it was confirmed that it was difficult to uniformly evaluate the compressive creep strain of concrete using recycled aggregate mixed with fly ash even if a correction coefficient representing the effect of fly ash was used.

    Download PDF (1176K)
  • Palamy PHOMMAHAXAY, Saya IOROI, Shigemitsu HATANAKA
    2020 Volume 85 Issue 769 Pages 321-330
    Published: 2020
    Released: March 30, 2020
    JOURNALS RESTRICTED ACCESS

     Geopolymer (GP) has gained attention as a new and environment-friendly material that can be hardened without using cement which emits large volume of carbon dioxide gas (CO2) in the manufacturing process.

     GP could be produced through chemical reaction between active filler which is mainly composed of amorphous aluminosilicate, and alkaline solution. Fly ash (FA), metakaolin, blast furnace slag fine powder (BFS), sewage sludge incineration ash and rice husk ash are used as active fillers; while sodium hydroxide solution, potassium hydroxide solution, water glass, sodium carbonate solution and potassium carbonate solution are used as alkaline solution.

     According to previous studies, GP can exhibit the same level of compressive strength compared to that of concrete made from ordinary Portland cement. In addition, GP shows in general high adhesion performance to cement concrete and reinforcing bars, and has high resistance to fire and acid attacks. Therefore, GP is expected to be used effectively as repair material for concrete structures.

     In this study, we aim to further develop GP as a crack repairing material by using silica fume (SF) as one of main powders. SF is an ultrafine particle material whose particle size is from 0.1 to 10 µm in diameter, while sizes of common inorganic materials such as cement, FA, BFS are approximately 10µm. It is expected that producing GP with SF as the main powder can provide good injection properties. As an initial step, investigation was carried out on the basic characteristics of GP using SF, before studying on its characteristics as a crack repairing material, i.e. on mixing performance, fluidity and compressive strength. Then, the characteristics of GP with FA as base material and those replaced with SF and BFS were also compared in the discussion.

     As the results, it was observed when substitution of SF increased, mixing became difficult, the fluidity of GP paste decreased, then the workability was lost. This is considered due to the large specific surface area of SF. In addition, the hardened GP paste containing SF showed volume expansion and lower compressive strength as compared to that of specimens without SF. These phenomena are getting remarkable with the increase in the substitution of SF content and the decrease in BFS substitution to 40% or less. The volume expansion is considered mainly due to the production of H2 gas in GP.

    Download PDF (1321K)
  • Judgment criterion of snow removal on roofs based on risk analysis for fatalities and structural damage
    Takahiro CHIBA, Takuya TSUTSUMI, Toru TAKAHASHI
    2020 Volume 85 Issue 769 Pages 331-341
    Published: 2020
    Released: March 30, 2020
    JOURNALS RESTRICTED ACCESS

     In Hokkaido, accidents involving people due to snow have been occurring in every winter. The fatalities caused by snow in Hokkaido on 2006 heavy snowfall or after is largest number in Japan. 60 to 70% of all accidents were caused by snow removal of roofs such as falling from roof and falling from ladder in contents of the damage factors. According to Building Standard Act of Japan, it is possible to reduce snow load on roofs conducted by snow removal only in the regions that to rise on roofs is traditional. However, the reduction of snow load isn’t recognized by prefectural ordinance. Nevertheless, those accidents caused by the snow removal have been occurring frequently. To reduce the accidents caused by the snow removal, the authors analyzed damage risk using statistics data of 2006 or after, and then analyzed performance against snow load subjected the wood-frame houses build in Hokkaido. The authors proposed also methods of the snow removal based on those results obtained in the risk analyses.

     The fatality ratio affected by snow conditions and aging ratio in each subprefectural bureau was focused in the risk analysis using statistics data for accidents involving people. As the results, the fatality ratio increased with normal value of annual maximum snow depth increased. Also, the aging ratio had a larger impact on the fatality ratio. In relationship between the fatality ratio and snow depth ratio, which is annual maximum snow depth divided normal maximum snow depth, the fatality ratio of 2014 to 2019 winter was larger than the ratio of 2006 to 2013 even if the snow depth ratio was similar both. This means that the damage risk is raising by recent social situation such as increasing aging ratio. It is also important to focus that the fatality ratio was large even if the snow depth ratio was smaller than 1.0. And then the authors analyzed relationship between the damage factors and accident period. The peak of accidents caused by falling from roofs appeared in February when snow load increased. The fact that snow removal on roofs was caused by the snow load was clarified through the analysis.

     In analysis of performance against snow load for wood-frame houses build in Hokkaido, the authors calculated damage probability related roof snow depth of girder and rafter in each constructed period. The snow depth when damaging in girder with the probability 5% was 1.0 m, and was unrelated the constructed period. This means that snow load on roofs was set considering reduction by the snow removal. On the other hand, the snow depth when damaging in rafter was deeper as new house.

     Finally, to deter unnecessary snow removal on roofs, the authors proposed judgment criterion of snow removal on roofs considering capability to withstand snow load the eaves based on the above results of the analyses. The authors consider that the criterion is effective to reduce the accidents caused by situation remained to unclear motive of the snow removal.

    Download PDF (1011K)
  • Yoshimasa HANYUDA, Daiki FUJIKAWA, Hideki IDOTA, Tetsuro ONO
    2020 Volume 85 Issue 769 Pages 343-353
    Published: 2020
    Released: March 30, 2020
    JOURNALS RESTRICTED ACCESS

     The authors’ previous studies on the structural characteristics of Nuki-to-column joints using wedges for Japanese traditional wooden frames showed that the problem with such joints is the extraction of wedges due to cyclic loading. This paper proposes a joint with a spring device to solve this problem. The proposed joint maintains the embedment characteristics of joints at all times and retains the advantages of conventional Nuki-to-column joints. The proposed joint is a Nuki-to-column joint with a spring device attached to the wedge of the traditional joint.

     We show the superiority of the proposed joint through full-scale and element tests, and demonstrate the possibility of practical application.

     Chapter 1 describes the significance of this study.

     Chapter 2 outlines the problem with the traditional joint and describes the proposed joint.

     Chapter 3 shows the structural characteristics of the proposed joint through full-scale tests and demonstrates the superiority of the joint.

     Chapter 4 shows the results of element tests that were conducted to examine the proposed spring device in detail. Furthermore, a method for determining the hysteretic characteristics of the joint based on element tests is demonstrated.

     Chapter 5 shows the superiority of the proposed joint using evaluations of energy absorption and time history response displacement.

     The following conclusions were obtained in this study:

     1) Through full-scale and element tests, this study showed that the parameters for the proposed joint are wedge type, spring constant, tree species, and wedge angle.

     2) The bending moment, stiffness, and bilinear hysteresis for the proposed joint were compared with those for a conventional joint. A method for determining the hysteretic characteristics of the joint based on element tests was demonstrated.

     3) A hysteretic model for the proposed joint was proposed for time history response analysis based on a bilinear-slip model.

     4) The proposed joint showed higher energy absorption performance than that for a conventional joint. The time history response analysis showed that the maximum response for the proposed joint was less than half that for the conventional joint.

    Download PDF (3061K)
  • Masatoshi SHINOHARA, Hiroshi ISODA
    2020 Volume 85 Issue 769 Pages 355-365
    Published: 2020
    Released: March 30, 2020
    JOURNALS RESTRICTED ACCESS

     This research is about equivalent linearization method based on calculation of response and limit strength for SDOF model of timber structure with oil damper installed on the slip type main frame. By integrating many literatures, the evaluation of oil damper effects in vibration control design by extended limit strength calculation method is shown. By comparing with numerical analysis and full scale shaking table experiments the accuracy has been verified. The results are summarized below.

     ・Complying to “Manual for Design and Construction of Passively-Controlled Buildings” etc., refer to what current calculation of response and limit strength can do to evaluate the effect of oil damper on ”Seismic Isolated Building”, by replacing Maxwell model with equivalent Voigt Model, the extended method is considered and shown.(Extended Limit Strength Calculation Method)

     ・By assigning each element specification value as a parameter, time history response analysis of SDOF model of timber structure with inelastic-bilinear type of oil damper installed in the slip type main structure was performed. By comparing the analysis result with the result of extended limit strength calculation method, the overall response trend can almost be predicted, but in case the additional damping of damper is large, the decrease of accuracy, and the variation of predicted value due to the difference of the rigidity of the support material was confirmed.

     ・As a result of improving the accuracy of extended limit strength calculation method based on the previous research results by Kasai et al., the improvement of overall result accuracy and the reduction of variation was confirmed. (Kasai Extended Limit Strength Calculation Method)

     ・By comparing shaking table experiment of 1 story timber frame structure result with response prediction value from Extended Limit Strength Calculation Kasai Extended Limit Strength Calculation Method, the correspondence of experimental values and the predicted values of various input levels (level 1 to level 3) was confirmed.

    Download PDF (2446K)
  • Susumu TAKAHASHI
    2020 Volume 85 Issue 769 Pages 367-372
    Published: 2020
    Released: March 30, 2020
    JOURNALS RESTRICTED ACCESS

     Wing walls and spandrel walls had been separated from columns or beams before the revision of AIJ Standard in 2010. After the revision of AIJ Standard in 2010, wing walls and spandrel walls are recommended to be cast monolithically so as to resist seismic load. However, elastic shear stiffness, which is necessary for constructing a model of structural members, of columns with wing walls and beams with spandrel walls had been investigated only limited conditions such as a column with same length wing walls on both sides. In this research, elastic shear stiffness of columns with wing walls and beams with spandrel walls is theoretically investigated based on the equilibrium of external work and internal work.

     As a result, the coefficient for shear deformation, which is 1.2 for a rectangular section as widely known, is obtained. In this research, the coefficient is defined to total cross sectional area of a structural member. The coefficient become more than 2 in the case of columns with thin wing walls and beams with thin spandrel walls. In the case of wall thickness larger than 0.4 times of column width or beam width, the coefficient is between 1.2 and 1.4.

     In this paper, a simple approximation method to estimate the coefficient is also proposed and it is similar to the method used in AIJ Standard for shear walls with boundary elements on both sides of wall panel. In this method, effective cross section is defined as the product of wall thickness and total depth of the column with wing walls or the beam with spandrel walls. The estimated value by this method is appropriate if the total depth of a member is 5 times larger than the depth of the column or the beam. It means that elastic shear stiffness of columns with short wing walls and beams with short spandrel walls cannot be obtained by using this method.

     Theoretical value of elastic shear stiffness is verified by using three series of experimental results. The specimens used in the experiments are the columns with thin wing walls on both sides of the column, the beams with different length of spandrel walls on the sides and the columns with a long wing wall on one side. The elastic stiffness obtained from these experiments are from 64 % to 137 % of the calculated value, and the validity of the calculation in this paper was shown.

    Download PDF (834K)
  • Investigation of increase of axial yield displacement and buckling restraint effect
    Masaya HADA, Kenichi TAKEUCHI, Keiji KITAJIMA, Mitsukazu NAKANISHI
    2020 Volume 85 Issue 769 Pages 373-381
    Published: 2020
    Released: March 30, 2020
    JOURNALS RESTRICTED ACCESS

     In steel framed buildings, braces are widely used as a main earthquake- resistant element. Braced structures are economically superior to pure moment resisting frame structures. In conventional brace structures, however, stiffness is much greater in braces than in the frame and therefore most of the seismic forces concentrate in the brace. Well-balanced arrangement of braces in the building is therefore required. Conventional braces yield due to small deformation at a story drift angle of approximately 1/500 rad. No braces can therefore be arranged in small numbers because of the restrictions of the primary design that allow no yielding of members. In cases where only a few braces can be arranged or no well-balanced arrangement of braces is possible because the appearance or functions of the building are given priority, therefore, pure moment resisting frame structures are adopted abandoning the use of brace structures. To solve the problem, expanding the range of elasticity to prevent seismic forces from concentrating in the brace is effective.

     Therefore, the authors developed a folded brace that would not yield at a story drift angle of less than 1/200 rad. Specifically, the actual length of the member was made approximately 2.5 times larger than the apparent length by folding three steel bars unicursally. Axial yield displacement increase 2.5 times in proportion to the length of the member. Elastic limit deformation can be controlled arbitrarily by varying the length or frequency of folding even in cases where the material strength is at the level in ordinary steel. Thus, braces that would not yield at a story drift angle of less than 1/200 rad are realized. Also, the folded brace has a buckling restraining effect. Thus, it shows a stable historical performance up to large deformation of 1/50 rad. Using folded brace enables the application of fewer braces and the eccentric arrangement of braces.

     In this paper, we examined the increase in axial yield displacement and the buckling restraint effect, which are the structural characteristics of folded braces.

     Chapter 3 describes a full-scale experiment of folded braces. As a result of the experiment, the folded brace showed elastic behavior up to the deformation level of the story drift angle 1/200 rad. Furthermore, even after axial yielding, it showed stable hysteresis performance up to the deformation level of the story drift angle of 1/50 rad.

     Chapter 4 organized the buckling restraint mechanism unique to folded braces. In a folded brace, the medium steel tube (tensile material) restrains the entire buckling of the core steel (compressed material). As a result of organized, the limit value (critical axial force NC) of the axial force not buckling was derived.

     Chapter 5 describes an element experiment that reproduces the relationship between the core steel(compressed material) and the medium steel tube (tensile material). As a result of the experiment, the validity of the buckling restraint mechanism and the critical axial force NC shown in Chapter 4 was confirmed.

    Download PDF (3286K)
  • Takayuki NANBA, Keiji KITAJIMA, Mitsukazu NAKANISHI
    2020 Volume 85 Issue 769 Pages 383-393
    Published: 2020
    Released: March 30, 2020
    JOURNALS RESTRICTED ACCESS

     Concrete filled steel tube (CFT) is a member consisting of a steel tube filled with concrete. It is well known that the strength and ductility of CFT columns increase due to the effect of mutual confinement between the concrete and the steel tube. However, the prescriptive design rules of ultra-high strength CFT is less established due to lack of the laboratory database.

     In this study, experiments on circular CFT stub columns consisting of 780N/mm2 or 550N/mm2 class steel with 23 to 45 diameter thickness ratio and 100N/mm2 class concrete (ultra-high strength CFT) were carried out. Steel tube columns and plain concrete columns were also individually tested to characterize the behavior of CFT stub columns.

     The biaxial strain is measured to calculate the axial and circumferential stress in the steel tubes by the incremental method. Then, the relationship between axial and confinement stress in infilled concrete is quantitatively evaluated to understand the details of mutual confinement effect in ultra-high strength CFT stub columns.

     Results are summarized as follows:

     1) The strength ratio of the CFT column calculated by concrete cylinder strength was almost proportional to a steel tube strength ratio.

     2) Clear relations were not observed between the strength ratio calculated by concrete material strength and the diameters of CFT specimens.

     3) The load of CFT stub column with 550N/mm2 class steel and 100N/mm2 class concrete becomes larger than the simple cumulative load of steel tube and plain concrete when strain increased by 0.03 to 0.06% after the fracture strain of plain concrete. During this strain level, the circumferential strain of steel tube in CFT stub column becomes larger than that of individual steel tube.

     4) The axial stress of infilled concrete almost agrees with the stress of plain concrete until the fracture strain of plain concrete is reached. The circumferential stress in the steel tube of CFT stub column was generated between the fracture strain of plain concrete and that of cylinder concrete. After fracture strain of cylinder concrete, the axial stress in infilled concrete has climb gradient.

     5) If the fracture strain of concrete is lower than the yield strain of steel tube in CFT stub column, the gradient of confining stress – axial stress curve changes smoothly. On the other hand, if the yield strain of steel tube in CFT stub column is lower than the fracture strain of concrete, only the confining stress was observed and the axial stress did not grow for a moment.

     6) The confinement factor of ultra-high strength CFT stub column becomes lower than the cases under constraint by a hoop reinforcement.

    Download PDF (10255K)
  • Toshiaki FUJIMOTO, Ryoma ARAMAKI
    2020 Volume 85 Issue 769 Pages 395-402
    Published: 2020
    Released: March 30, 2020
    JOURNALS RESTRICTED ACCESS

     1. INTRODUCTION

     Steel reinforced concrete structures are typical composite structural systems comprising steel and reinforced concrete (RC), which exhibit excellent resistance against earthquake through high capacities and deformability. The design process and construction work for such structures are technically more complicated than those for RC and steel structures. To overcome these limitations, concrete encased steel (CES) structural system comprising fiber reinforced concrete (FRC) and encased steels have been proposed herein as a new composite structural system. CES has been a subject of continuous and comprehensive studies to make its use practical. Herein, the seismic behaviors of CES members are experimentally investigated to evaluate the ultimate shear strength. A summary of the corresponding test results are laid and a discussion is presented for an approach of evaluating the required characteristics, namely cracking, yield, and ultimate strength and deformation of CES members, for structural design practices.

     2. TEST PROGRAM

     Six specimens were tested under monotonic loading to investigate the effects of the section shape of encased steels and the sectional width of the FRC cover on the shear strength of CES members.

     3. EXPERIMENTAL RESULTS AND DISCUSSIONS

     All specimens showed considerably ductile and stable behaviors. Brittle failure was not significant during testing, although flexural and shear cracks propagated at small deformation. The steel shape affected the observed cracking patterns of the CES members. As revealed by a comparison of the deformation behavior and damage situations of these specimens, the FRC improved the structural performance and reduced the damage in composite members.

     4. SHEAR STRENGTH EVALUATION

     An existing study 3), 4) was referred to for the numerical evaluation of the shear strength results. All specimens displayed larger maximum strengths than those obtained through the formulas; therefore, their strength was evaluated within the safety limits. Moreover, the sectional width of the FRC cover was found to display an influence on the ultimate strength of the CES members; thereby, a design formula was proposed taking into account this influence. Consequently, the formula provided numerical results that showed good agreement with those obtained from the experiment.

     5. CONCLUSIONS

     Specimens with different built-in steel shape and member widths were subjected to beam-column tests, after which the structural performance of CES members with shear failure was examined. The main findings of this study can be generalized as follows:

     1) Shear failure occurred in each specimen. The basic data of shear behavior of CES members were obtained.

     2) Shear cracking occurred in each specimen. After bending, cracks were noticeable in the FRC cover, although there was no indication of a significant decrease in strength or major falling of the cover.

     3) Regardless of the built-in steel shape, such as H- or cross-H-shaped steel, the maximum shear strength increases with an increase in the sectional width.

     4) The ultimate shear strength of CES members was proposed by taking the effective width of the FRC into account. This improves the evaluation accuracy of the ultimate shear strength of the CES members.

    Download PDF (1756K)
  • Kazushi SADASUE, Hideki OGOMORI
    2020 Volume 85 Issue 769 Pages 403-413
    Published: 2020
    Released: March 30, 2020
    JOURNALS RESTRICTED ACCESS

     Headed studs are widely used as shear connecters between steel and concrete. Usually, headed studs are vertically welded in a steel surface. However, if we expect the shear resistance in headed studs, the resistance mechanism is changed such that the headed stud is subject to a force (either tensile or compressive) along the axial direction of the stud, which is predicted to produce considerable resistance. Therefore, it is hypothesized that composite beams using inclined headed studs would improve shear stiffness and strength.

     We proposed an appropriate arrangement and direction of inclined headed studs for beams subjected to unidirectional loading and for girders subjected to a positive–negative cyclic load. In addition, the parallel-inclined type was considered desirable for the beam and the cross-inclined type for the girder. Moreover, we proposed an evaluation method of the ultimate shear strength for the headed studs with positive and negative inclination. The ultimate shear strength was determined on the basis of the assumption that the shear force in a composite beam connection using positive-inclined headed studs is brought into equilibrium with the bearing force of the concrete and the tensile force of the headed stud. In addition, it was assumed that the bearing force of concrete carries the shear force of composite beams using negative-inclined headed studs. The bearing strength of concrete is obtained by multiplying modified coefficients κp and κn for inclined headed studs in the J. W. Fisher design formula. The tensile strength of headed studs is thus the smallest allowable stress as limited by the yielding of the headed stud or concrete cone failure.

     In this research, first, we confirmed the mechanical behavior of parallel-inclined and cross-inclined headed stud steel and concrete connectors by conducting a structural test under a cyclic shear force. The direction, arrangement, diameter, and length of the headed stud as well as concrete strength were used as experimental parameters. A total of 58 specimens were tested. For the shear tests, the cone failure of the concrete from the stud head, the splitting crack of the concrete from the stud base, the bearing failure of the concrete at the stud base, the shaft fracture at the stud base, and the stud welding part fracture were observed. From the test results, the modified coefficient κp used for determining the ultimate shear strength of the headed studs with positive inclination was identified. Second, to identify the modified coefficient κn used for determining the ultimate shear strength of the headed studs with negative inclination, we tested four specimens in additional experiments for the parallel-inclined headed stud.

     Finally, we confirmed that the results of the proposed evaluation method for the shear strength are consistent with the test results.

    Download PDF (2343K)
  • Required conditions for attainment of full plastic moment and framework and issues of strength design
    Masae KIDO, Keigo TSUDA, Masayuki HARAGUCHI
    2020 Volume 85 Issue 769 Pages 415-425
    Published: 2020
    Released: March 30, 2020
    JOURNALS RESTRICTED ACCESS

    1. Introduction

     The objective of this study is to propose the axial load ratio ny-slenderness ratio sλ1 relations which assure that the CFT beam-columns attain the full plastic moment Mpc. While the CFT beam-columns can be designed by using the strength formulas specified by the “Recommendations for Design and Construction of Concrete Filled Steel Tubular Structures”, the required conditions for attainment of full plastic moment are not apparent. In this study, the strengths of concrete filled steel square tubular beam-columns are obtained by an analytical work, and the required conditions are proposed. Moreover, the framework of strength design are presented.

    2. Analytical work

     Analytical model is shown in Figure 1, and concrete filled steel square tubular (CFT) beam-columns are subjected to constant vertical load N and monotonic increasing moment M1 and κM1. The stress-strain relations of steel tube and concrete are assumed as shown in Figure 2. As the analytical parameter, the moment gradient ratio κ, the value of α defined as Eq. (8) and strengths of materials are selected. The load-deflection relations and the maximum strengths of CFT beam-columns are obtained by the shooting method.

    3. Results and Discussion

     After showing the moment M1-slope θ1 relations in Figure 5, effects of end moment ratio (Figs. 6 and 7), effects of the strength of steel tubes (Figs. 8 and 9) and effects of the strength of concrete (Figs. 10 and 11) on the maximum strength M1max are discussed. It has been shown that the maximum strengths increase when the end moment ratio and/or the strength of steel increases and the strength of concrete decreases.

    4. Required conditions for attainment of full plastic moment

     Axial load ratio ny-slenderness ratio sλ1 relations are shown in Figures 12 and 13. In these figures, white circle ○ marks indicate the condition where the maximum strengths M1max are greater than 0.95Mpc. In the case of the α is smaller than 0.05, beam-columns can attain the full plastic moment when the axial load ratio is lower than 0.75. Required conditions for attainment of full plastic moment Mpc are examined and consequently summarized as Eqs. (12)-(15). Moreover, an example to calculate the maximum axial load ratio limitny which assures Mpc is presented.

    5. Conclusions

     The conclusions derived from the analytical work are as follows:

     1) Required conditions for the attainment of the full plastic moment Mpc as the strength of CFT beam-columns are proposed as Eqs. (12)-(15). These equations are related to axial load ratio ny, end moment ratio κ and slenderness ratio sλ1 of steel tubular column.

     2) In the range α≦0.05, the full plastic moment Mpc can be expected under the conditions of end moment ratio κ≧-0.5 and axial load ratio ny≦0.6.

     3) It becomes easy to exert the full plastic moment Mpc, as the end moment ratio κ and the strength of steel tube sσy becomes large, and the strength of concrete cσB and axial load ratio ny becomes small.

     4) The framework and issues to solve of the strength design are presented.

    Download PDF (4208K)
  • Examination based on Inverted Pseudo Point Source Model
    Kentaro KASAMATSU, Kenichi KATO
    2020 Volume 85 Issue 769 Pages 427-437
    Published: 2020
    Released: March 30, 2020
    JOURNALS RESTRICTED ACCESS

     We simulated strong ground motions of the 2011 off the Pacific coast of Tohoku Earthquake at periods from 2 to 10 s based on the 3D finite difference method. For the source model, we used the space and time distribution of pseudo point sources by Nozu (2012), and the seismic moment, rake angle, and rise time of 9 point sources were estimated by inverting observation records. The theoretical Green's functions were evaluated from the finite difference method using a 3D deep subsurface model by the Headquarters for Earthquake Research Promotion (2012).

     In order to validate the constructed model, the reproducibility of observation records of rock sites not used in the inversion was examined. It was confirmed that Fourier amplitudes and predominant periods of 2 to 10 s of observed records were generally reproduced. Furthermore, we investigated the reproducibility of 289 observation records from Tohoku to Kanto regions including the area where the sedimentary layers are thick. Although the amplitudes at periods from 5 to 10 s at northern Iwate prefecture are underestimated by 0.5 to 1 times, it was found that the observation records at others were well reproduced. In particular, the maximum Fourier amplitudes over 80 cm/s*s observed in the Kanto plain were satisfactorily simulated. It is thought that evaluating the theoretical Green's function in consideration of the complicated 3D deep subsurface structure led the result of well reproducibility.

     Since the pseudo point source model cannot consider a rupture directivity effect, we performed the numerical experiment on the effect by comparing the simulated ground motions between pseudo point source model and SMGA source model. It was confirmed that the rupture directivity effects are limited at the observation located in the forward direction of the rupture. Because the amplitude at periods from 5 to 10 s in the northern area of Iwate Prefecture is underestimated, it might be necessary to add the point source off the coast of the area. This is a future issue for upgrading the source model. Using the current pseudo point source model, observation records over the wide area from Tohoku to Kanto regions are well reproduced. In particular, in the Kanto plain where thick sedimentary layers are distributed, we were able to simulate the characteristics such as large-amplitude ground motions propagated for a long time after S wave arrival.

     In this mega thrust earthquake, large amplitude with long-period ground motions were observed in Osaka bay area seven hundred kilometers away from the hypocenter (e.g. Sato et al., 2012). In the next step, we are going to apply our pseudo point source model to the wider area from Tohoku to Kansai and try to reproduce long-period ground motions.

    Download PDF (6163K)
  • Masakazu HARUHATA, Makoto ARAI, Kenichi IKEDA
    2020 Volume 85 Issue 769 Pages 439-447
    Published: 2020
    Released: March 30, 2020
    JOURNALS RESTRICTED ACCESS

     If a fire occurs, the building will be damaged. And most of them remain in a state where they have been damaged to some extent by structural members.

     In apartment buildings, it is required that the main structural part is a fireproof structure, and the type of structure is often reinforced concrete or steel-frame reinforced concrete.

     In order to reuse a damaged building after a fire, it is necessary to grasp the damage range and depth of the damaged part concrete and secure the same performance as before the damage for the damaged area, the property of the building It is important in terms of value.

     In this study, the core is drilled from the heating surface of concrete after single-sided heating using a gas furnace, in order to grasp the strength reduction and damage depth of the damaged part concrete in the ceiling slab of RC apartment house, The relationship between the maximum temperature of the concrete heating surface and the compressive strength was determined.

     In addition, the small size core was used to capture the change in strength between the surface side and the inner side of the heated and cooled concrete. Furthermore, with the sliced specimen cut and shaped in the depth direction from the heating surface of the concrete after heating and cooling, the flexural strength for each depth was confirmed from the heating surface side inward, and the damage depth was estimated. The findings obtained in this study are summarized below.

     (1) Compressive strength by core

     The compressive strength residual ratio of the core collected from the surface heated by concrete decreased as the maximum temperature reached by the heated surface increased (concrete fire duration time lengthened). In addition, from the relationship between the color change of the concrete and the presence or absence of cracks and the compressive strength residual ratio of the core, during the fire damage investigation, by observing the presence or absence of color change or cracks of the surface of the affected area concrete, It is possible to estimate the compressive strength residual ratio.

     (2) Compressive strength by small size core

     When the compressive strength in the depth direction of the damaged concrete was compared using the small diameter core, a difference occurred between the heated surface side and the inner side. In addition, a correlation was found between the maximum temperature and the compressive strength of the small diameter core collected.

     From these things, it is possible to estimate the degree of damage if the heat receiving temperature of the affected area concrete and the fire duration time are known in the fire damage investigation.

     (3) Flexural strength by core slice test piece

     The damage depth was grasped by finding the flexural strength continuously from the heating surface to the inside with the core collected from the damaged concrete. From this result, at the time of fire damage investigation, it is possible to obtain the damage depth of the damaged concrete and indicate the depth at which it is judged that the repair is necessary.

     By this study, although it is limited, the tendency of the compressive strength from the core or small diameter size core to observe the color change of the heated surface of the damaged concrete in the fire damage investigation of RC apartment buildings and the presence or absence of cracks etc. We have shown the possibility of clarifying the damage area and depth of damaged concrete structural members by capturing It is thought that it is possible to estimate the repair range and depth of a concrete structural member by this.

    Download PDF (1803K)
  • (Masaya OTSUKI, Toru TAKAHASHI, Tsukasa TOMABECHI, Takahiro CHIBA, Takuya TSUTSUMI, Isao KAMIISHI, Hitomitsu KIKITSU, Yoshihiro IWATA, Tadashi ISHIHARA and Yasuo OKUDA, J. Struct. Constr. Eng., AIJ, Vol. 82, No. 739, 1329-1338, Sep., 2017)
    Masaya OTSUKI, Toru TAKAHASHI, Tsukasa TOMABECHI, Takahiro CHIBA, Taku ...
    2020 Volume 85 Issue 769 Pages 449
    Published: 2020
    Released: March 30, 2020
    JOURNALS RESTRICTED ACCESS

     To propose a calculation method for snow loads that takes rainfall after snowfall into account, a rainfall experiment was conducted. Surcharge load increases continuously once rain starts falling, but the rate slows down once the eaves start discharging rainwater. The load reaches its peak once the rainfall intensity and flow rate at the eaves reaches equilibrium. This study tested a regression formula for surcharge load based on roof shape and snow depth. Also, this study evaluated a method for estimating the rain-on-snow surcharge load depending on regional weather conditions, and proposed a modified formula for surcharge load.

    Download PDF (258K)
  • -Young's modulus and bending strength of Cryptomeria Japonica and Zelkova serrate under various water-containing states at elevated temperature- (Chihiro KAKU, Yuji HASEMI, Daisuke KAMIKAWA, Tatsuro SUZUKI, Noboru YASUI, Mikio KOSHIHARA and Hirofumi NAGAO, J. Struct. Constr. Eng., AIJ, Vol. 82, No. 732, 299-308, Feb., 2017)
    Chihiro KAKU, Yuji HASEMI, Daisuke KAMIKAWA, Tatsuro SUZUKI, Noboru YA ...
    2020 Volume 85 Issue 769 Pages 451-453
    Published: 2020
    Released: March 30, 2020
    JOURNALS RESTRICTED ACCESS

     In this paper, a table, figures and considerations with those diagrams about the influence of water content on the mechanical propertirs of wood under heating are corrected in chapter 4 and 5. We showed in the published paper that the residual rates of bending strength under heating on Japanese Cider specimens, under 15% water contents, were comparable with dried specimens; however, we correct the fact that the residual rates of bending strength over 80℃ on Japanese Cider specimens with water content were smaller than that of dried specimens.

    Download PDF (834K)
feedback
Top