Journal of Structural and Construction Engineering (Transactions of AIJ)
Online ISSN : 1881-8153
Print ISSN : 1340-4202
ISSN-L : 1340-4202
Volume 85 , Issue 771
Showing 1-12 articles out of 12 articles from the selected issue
  • Yoshitomo YAMADA, Yoshiki UEHARA, Kohei SAKIHARA, Shinji URANO
    2020 Volume 85 Issue 771 Pages 663-672
    Published: 2020
    Released: May 30, 2020

     Concrete flow analysis is important, because it is possible to visualize how high fluidity concrete flows in the concrete placing form and in the gap between reinforcing bars.

     In this study, MPS (Moving Particle Semi-implicit) method was used as a concrete flow analysis method. First, the flow of Bingham fluid between parallel plates was analyzed, and the validity of the analysis method was verified. Furthermore, the effectiveness of the proposed method was confirmed by comparing the results of slump flow analysis and test results. The research results of Kokado et al.3) were used for the rheological constants (yield value, plastic viscosity) of concrete used for slump flow analysis.

     The results obtained in this study can be summarized as follows.

     The flow of Bingham fluid between parallel plates was reproduced using the proposed MPS analysis. As the result, when the particle size was 20 mm, it was different from the theoretical value, and when the particle size was 10 mm, 5 mm, 2.5 mm, it agreed with the theoretical value.

     The flow of Bingham fluid between parallel plates was reproduced by changing the value of the stress growth index m of the regularized Bingham model with m = 1 and 10,100. As a result, the theoretical value agreed well with the MPS analysis result in the fast range of flow velocity. In the range of slow flow, the MPS analysis results at the stress growth index m = 1 were faster than the theoretical solutions at the center of the parallel plate. Therefore, in order to represent the Bingham fluid, it is necessary to increase the value of the stress growth index m. In this study, the stress growth index m was set to 100 and used for MPS analysis.

     The slump flow test was reproduced by MPS analysis using a regularized Bingham model with a particle size of 5 mm and a stress growth index of m = 100, assuming that the high fluidity concrete was a Bingham fluid. As the result, it was well reproduced in the range of slump flow value of about 500 mm, however, it tended to flow in the experimental result as the slump flow value increased from 600 mm to 700 mm.

     When the slump flow value is less than 600 mm, the spread curve of the flow can be reproduced within the range of the rheological constant within the 95% upper and lower limit prediction interval. However, at the slump flow value of 700 mm, the experimental value flows more than the analytical result using the yield value of 95% lower limit. According to the results of the finite difference analysis in the literature (4), at a slump flow value of over 600 mm, the measured value tends to flow more than the analytical result. Therefore, if the slump flow value exceeds 600 mm, the yield value by the lifting sphere viscometer test may have been evaluated higher. Furthermore, high fluidity concrete with a slump flow value exceeding 600 mm may exhibit non-Bingham properties in the low shear rate region.

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  • Masaro KOJIMA, Tomoyuki KOYAMA
    2020 Volume 85 Issue 771 Pages 673-682
    Published: 2020
    Released: May 30, 2020

     High strength concrete columns are largely used for high-rise reinforced concrete buildings. Such concrete with low water to cement ratio has a dense structure and may experience explosive cover spalling when subjected to fire. The decrease of column cross-section could cause the steel reinforcements exposed to a high temperature at an early stage, and consequently, reduce the load-carrying capacity of the column. For 80 – 150MPa high strength concrete, we developed a method of suppressing the spalling behavior by adding a small number of synthetic fibers in 2000. Moreover, in 2009, we applied hybrid fibers of synthetic fibers and steel fibers to 120 – 200MPa high strength concrete. It is now common knowledge that adding both synthetic and steel fibers is indispensable for improving the fire resistance of the concrete with compressive strength more than 150MPa. However, there is a lack of study concerning the effects of fiber content and fiber geometry. This study carried out several heating tests on 200 – 300MPa high strength concrete using the ISO834 standard fire curve, in which various fiber types and fiber contents were considered. The effects of fiber type, fiber content, and geometry of both synthetic fiber and steel fiber on spalling depth were discussed. The following conclusions can be derived from this study.

     (1) Adding 2.0%.vol synthetic fibers and 0.25 – 0.5%.vol steel fibers can greatly suppress the fire-induced cover spalling for 200 – 300MPa high strength concrete.

     (2) A critical amount of synthetic fibers Vofcr was found for avoiding the appearance of cover spalling. The average spalling depth seems to be inversely proportional to synthetic fiber content (Vof) with a proportional coefficient of α.

     (3) The parameters of Vofcr and α are influenced by several factors including fiber material, geometry, the water content of concrete, and concrete strength. Within the test range, it was found that the polypropylene fiber was the most effective to suppress the fiber-caused spalling.

     (4) As the diameter of synthetic fiber decreases and fiber length increases, Vofcr and α become smaller, which is favorable for fire-resistance. These parameters are proportional to the power of fiber diameter and inversely proportional to the fiber length.

     (5) The steel fiber with a smaller diameter is preferable for improving fire resistance. Even the same fiber content can lead to a larger number of fibers per unit volume that can bridge more cracks during the heating process.

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  • Kiichiro SAWADA
    2020 Volume 85 Issue 771 Pages 683-692
    Published: 2020
    Released: May 30, 2020

     Brace structures have been used for a lot of steel buildings because they can give high stiffness and strength to buildings economically. However, brace members have a possibility that buckling or yielding occur in early stage under major earthquakes. In 2016 Kumamoto earthquake, tensile fractures and buckling residual deflection for braces were observed. In addition, damages of many column bases and foundations were observed. Therefore it was surmised that there might have been gymnasiums which could not be used as emergency evacuation. There are a lot of studies to improve hysteresis property of braces, i.e. NC braces, buckling restrained braces and metallic yielding damper braces. However, seismic upgrading by adding above braces needs attentions on large variation of natural periods and increase of seismic load on foundation structures.

     On the other hand, large deformable elastic devices have been studied by authors, which show only slight variation in natural period and seismic load of foundation for the upgraded structures. Large deformable elastic devices can realize elastic performance under even major earthquakes. In previous studies, the various devices were produced by trial and error using laser cutting machine from steel plates. It was overserved from previous tensile tests that the devices kept elastic performance until around 1.5% of the original length and from previous time history analyses that the devices reduced maximum and residual response of story drift.

     This paper explore effective topologies of the devices which show larger elastic deformation capacity by a topology optimization technique from a rectangle two dimensional plate shown in Fig. 1. The optimization technique uses a formulation with a function similar to Inverse Fourie Transformation and real coded Genetic algorithm.

     Concluding remarks are as follows.

     (1) No checkerboard patterns are found from Fig. 10-Fig. 25 and Fig. 27 by using a function similar to Fourier Inverse transformation for topology optimization.

     (2) It is observed from Fig. 11-Fig. 26 and Fig. 27 that the optimization technique can give regular periodical patterns by setting of large values of γ1.

     (3) It is observed from numerical results in Table 1-Table 3 .that using high strength steel can give very large elastic performance until 2.5% of the original length and using even standard steel can give large elastic performance until 1.4% of the original length.

     (4) Mechanism of the optimization technique of this study is shown by visualization of Function F(xk, yk) in Fig. 2-Fig. 7.

     (5) Small differences between elastic linear analyses and material and geometrical nonlinear analyses are observed until 2% to original length of elastic deformation.

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  • Yoshihito IKEDA, Ryuji ENDO, Masahito KOBAYASHI
    2020 Volume 85 Issue 771 Pages 693-703
    Published: 2020
    Released: May 30, 2020

     Inverse analyses are performed for 5-story frame model to identify lateral stiffness using projection and parametric projection filtering algorithms. The actual measurement 1st vibration mode measured by Experimental Modal Analysis (EMA) is adopted as observation data. Because the mode vector are obtained as ratio of displacement, the natural frequency of 1st vibration mode as well as mode displacements was used to determine the necessary inverse solutions. And also, to compose the observation vector by dimensionless displacements and frequency of 1st mode, not projection filter, but parametric projection filter with regularization parameter to control the iterative calculations of the filter equation were used as inverse analysis procedure effectively. In series of our inverse calculations, applicability and effectiveness of regularization parameter are presented through the comparison with inverse calculations used various kind of positive- definite parameter.

     New results obtained in this study are summarized as follows.

     (1) It is also possible to obtain highly precise solutions using projection filtering algorithm, when initial values are given as the neighborhood of the designed lateral stiffness.

     (2) Inverse analysis used parametric projection filtering algorithm are able to obtain a lot of collect inverse solutions by using appropriate value of regularization parameter for many initial values.

     (3) Physical quantity which are each element of sensitivity matrix, filter gain and determinant of sensitivity matrix change gently to regularization parameter of large value, oppositely, these quantities change drastically to parameter of small number. Namely, iterative process of filtering algorithm is similar to that of Kalman filter to large values of regularization parameter, on the other hand, iterative process of that is similar to projection filter to small values of regularization parameter.

     (4) Even if, filtering process is similar to projection filter, the tendency which goes suddenly up and down has not formed in the calculation process of parametric projection filtering step.

     (5) In practical use of this procedure, when regularization parameter which does not fit in with structure is used, a lot of same inverse solutions can be not obtained on each initial value. However, when regularization parameter which can fit in with structure is used, because straight line is formed on each initial value, the appropriate parameter which corresponds to the structure can be inspected.

     (6) The parametric projection filtering algorithm which used regularization parameter appropriately can develop the effective and practical computer programming easily.

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  • Ryutaro KADOTA, Hiroshi KAWASE, Masami GOTOU
    2020 Volume 85 Issue 771 Pages 705-713
    Published: 2020
    Released: May 30, 2020

     Many retaining walls for residential houses were damaged by the 2016 Kumamoto earthquake. There is always a risk that collapsed wall blocks would create damage to structures as well as people and block the road for evacuation or restoration work. Therefore, we need to clarify the collapse mechanism and then construct a method how to design more ductile retaining walls.

     In this study, we first carried out the survey on the collapsed retaining walls in Mashiki Town, Kumamoto prefecture, Japan. From the results of the damage survey, we found that concrete-block walls were most severely damaged, and it seems that the degree of the damage were affected by the height and the direction of them.

     Second, in order to understand the dynamic behavior and find out the collapse mechanism of retaining walls, we conducted the shaking table tests with small-scale retaining walls and tried to reproduce the dynamic behavior through their simulations. As a result of the first experiment using the shaking table in DPRI, Kyoto University, the model wall was not collapsed because of the stronger model wall than the design based on the regulations. One reason is that the strength of the concrete behind the model wall did not follow the scaling low. The other reason is that the model wall was supported by the steel container due to the frictional force. Therefore, we conducted the second experiment with a re-designed model wall. In the second experiment, even though the friction problem was solved, the model wall was not collapsed either, because of the smaller maximum input acceleration due to the capacity of the shaking table in KIT. However, since the frictional force was eliminated, we could simulate the results of the second experiment with the dynamic DDA (Discontinuous Deformation Analysis) nicely.

     Finally, we analyzed the nonlinear behavior of the retaining walls to find key factors leading to the collapses by the dynamic DDA of the real-scale retaining walls. In our parametric study, we made two types of models which have different heights, and different strengths of the concrete behind the blocks. As a result, we found that the degree of the damage is affected by the factors such as height, direction of walls, and the strength of concrete behind the blocks. Furthermore, it is inferred that the overburden pressure from structures too close to the wall, which are not allowed in the current standards, affects the dynamic behavior of the walls.

     Since the actual blocks were found to be completely separated whenever the walls were collapsed or severely damaged, it is suggested that whether blocks of walls were connected with concrete or not is much more important than the strength of concrete in order to prevent severe damage of block walls. Therefore, we strongly recommend that the construction practice for and the immediate inspection of “the quality of the concrete behind the blocks” must be implemented as soon as possible, which will ensure the inside integrity of the retaining walls and prevent their severe damage.

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  • Atsuo TAKINO, Kyosuke MUKAIBO, Yasuhiro ARAKI, Yuji MIYAMOTO, Hiroshi ...
    2020 Volume 85 Issue 771 Pages 715-723
    Published: 2020
    Released: May 30, 2020

     In this study, we conducted the experiments of the restoring force characteristics of column rocking found in the temple buildings using large-diameter columns on the basis of the Japanese traditional construction methods, and especially experimental verification was conducted on the effects of varying axial force and the horizontal cross members, because experiments with these parameters had not been examined so far. The configuration of the specimens is shown in Fig. 1, and the parameter of specimens is given in Table1. There were two types of test specimens: a test specimen with only a column and one with horizontal cross members at the top and bottom of the column. The obtained results are summarized below.

     In the case of the specimens with horizontal cross members, the hysteresis loop at the initial deformation due to the nonlinearity was expanded (Fig. 5), and the embedment of wood by transverse compression was observed in the horizontal members (Photo1). Especially when the axial force was changed, the horizontal members are remarkably damaged.

     The theory-based formula is valid for the specimens with only a column in comparison with the test results (Fig. 8), but the determination method of bearing stiffness which is an important constant is a future issue. On the other hand, in the case of the specimens with horizontal cross members, the restoring force calculated by the theory-based formula is much lower than the experimental results because the assumed failure mode is different between the theory and experimental result.

     Comparing the relationship between bending moment and rotational angle of the end of the column where the P-Δ effect has been removed (Fig. 9), in the case of the specimens with only a column, the bending moment remains constant after yielding near 1/100 radian. However, in the case of the specimens with varying axial force, the rigidity increased after yielding in response to the increased axial force. Also, in the case of the specimens with horizontal cross members, the rigidity gradually increased after yielding because the embedment of wood by transverse compression of the horizontal cross members is a dominant failure mode. As a result of the comparison of non-dimensional data (Fig. 10), there is a significant difference in the specimens with horizontal cross members in comparison with other test conditions, and the maximum yield strength is about 50% in comparison with the specimens with only a column.

     Based on the conclusions in this paper, we will establish an evaluation formula for the case with horizontal cross members and useful application methods for actual design and analysis in the future.

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  • Tomoaki SUGIYAMA, Hideo ONO, Yutaka ISOZUMI, Hideyoshi WATANABE
    2020 Volume 85 Issue 771 Pages 725-735
    Published: 2020
    Released: May 30, 2020

     In the case of repair and reinforcement of an R/C structural frame, and installation of new members for extension and reconstruction, it is necessary to anchor such as main bars and wall reinforcement bars of new members to the existing frame. In this case, the anchor is required to reliably transfer the tension of the rebar to the existing frame. The purpose of this research is to propose a post-installed headed rebar anchor method with high reliability both in structural performance and workability, and to evaluate the anchorage performance of the method. In this paper, experiments in a short fixing length were carried out to clarify the bond behavior of the surface of the rebar and the surface of the hole wall and the bearing effect of the small headed plate.

     In the first part of this paper, pull-out experiments are conducted to understand the effects of various factors on the basic bond performance and bearing performance. The following results were obtained.

     1) The bond strength on rebar surface of proposed method is equal to or higher than that of the cast-in-place deformed bar and is affected by the concrete strength.

     2) The bond strength difference on hole surface of proposed method is small between the case of installation by advanced rock-drilling system and advanced core drilling system. So, the performance difference due to the difference of the roughening degree is small within the scope of those methods and this experiment. Moreover, the bond strength is approximately equal to that of the cast-in-place deformed bar, and since the hole diameter is large, good bond performance can be obtained by the proposed method.

     3) The headed plate improves the anchor performance by bearing pressure resistance, and the effect is increased especially after the bond resistance on rebar surface is lost. Due to the effects of bond and bearing pressure, good anchor performance can be obtained by the proposed method.

     4) It can be said that the bond strength on rebar surface of this method is estimated to the allowable bond stress of AIJ reinforced concrete calculation standard and bond strength of AIJ design recommendation for composite construction.

     5) The local bond stress-slip relationship can be estimated by the same evaluation model used for cast-in-place deformed bars.

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  • Ryo KUWABARA, Fuhito KITAMURA, Taku OBARA, Susumu KONO, Tomohisa MUKAI ...
    2020 Volume 85 Issue 771 Pages 737-747
    Published: 2020
    Released: May 30, 2020

     According to the Guidelines for Performance Evaluation of Earthquake Resistance Reinforced Concrete Buildings published from the Architectural Institute of Japan, damage state of RC members is defined by concrete/steel stress level, residual deformation/ crack width. However, these indices are not enough to evaluate repair strategy. In this study, crack simulation procedure is introduced for building performance design method based on damage evaluation. Crack width/ length/ spacing in the tests on two full-scale five story RC buildings were extracted and quantified. Crack width/ length observed in the tests were numerically simulated with finite element analysis.

     As to flexural crack, on the premise that concrete between cracks does not deform, flexural crack width measured in the tests and tensile elongation on tensile fiber are compared taking cyclic loading effect into consideration and accumulated flexural crack width has good agreement with accumulated flexural elongation. This result shows that the assumption which concrete between cracks does not deform under tensile stress is reasonable enough. Total shear crack width (shear deformation) and maximum shear crack width are analyzed with test result. Shear deformation of members is compared to shear deformation which comes from shear crack width via geometric model and it has been discovered that shear deformation which comes from shear crack width was less than shear deformation of members and that means shear deformation of members does not consist of only shear crack width. Therefore, other component can contribute to shear deformation of members. Relationship between maximum shear crack width and total shear crack width is examined with an equation in CEB-FIP model code 1987 and the equation can well simulate total and maximum shear crack relationship. However, shear portion of flexure shear crack is not well simulated because it is not pure shear crack and opens by flexural deformation. Therefore, the issue is how to deal flexure shear cracks.

     According to those examination, relationship between flexural crack width and elongation on tensile fiber has been found except relationship between shear deformation and shear crack width. Therefore, flexural crack is simulated with FEM analysis.

     Flexural crack width measured in the tests is compared with computed flexural crack width and well simulated. However, some cracks were not measured due to concrete spalling at 1.0% roof drift. Thus, flexural crack width was over estimated. On the other hand, in this study, elongation on tensile fiber corresponds to flexural crack width. Therefore, it is compared to computed flexural crack width. Accumulated elongation is well simulated and total elongation is simulated within ±30% error. It shows this method well predict flexural crack width except effect of concrete spalling. Maximum flexural crack width is simulated approximately within ±30% error as well. Flexural crack length is also simulated with the method based on Bernoulli-Euler theory and compared on crack length ratio. Cracks progress horizontally such as pure flexural crack is well simulated. However, cracks progress obliquely such as flexure shear crack is not well simulated due to the deference of propagation manner between test and numerical simulation.

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  • Koji HIROISHI, Takanori ISHIDA, Ryota MASEKI, Hiroyuki NARIHARA, Satos ...
    2020 Volume 85 Issue 771 Pages 749-758
    Published: 2020
    Released: May 30, 2020

     Low and middle-rise steel moment frames are generally verified for seismic safety by horizontal load capacity calculation with push-over analysis assuming one-directional force. However, during an actual earthquake, the dynamic behavior and the horizontal bi-directional input may cause the building behavior different from the design assumption. Therefore, to evaluate the seismic performance of a steel moment frame, it is important to grasp the collapse mechanism and response displacement under bi-directional dynamic input.

     Under bi-directional input, in a moment frame, bi-axial bending moment and additional axial force by overturning moment act on the column, which lower the column strength and causes to form the weak column mechanism as compared with one-directional input. While overall sway mechanism in which the beams or panels yield prior leads to a stable plastic deformation behavior where member plasticization is dispersed throughout the building, the week column mechanism leads to damage concentration of the specific story and brittle collapse. Previous studies have shown that not only bi-directional input, but also the deterioration of column's restoring force due to local buckling and composite effect of beams by concrete slab are factors in the weak column mechanism.

     In this paper, as a starting point of research to grasp response behavior of low and middle-rise steel moment frame under horizontal bi-directional input, seismic response analyzed under various conditions for 4-story 2 x 1 span moment frame. For the analysis, to evaluate the collapse mechanism and the maximum story deformation precisely, a three-dimensional frame model was used that can evaluate composite effect of beams and panels by concrete slab and the deterioration of column's restoring force due to local buckling.

     From the analytical results, the following knowledge was obtained.

     1) The response story drift angle of 1st story is in negative proportion to the column strength ratio γ, and the story in which γ is 1.0 can be deformed by more than 1/40rad under horizontal bi-directional input. The smaller γ of the story, the more bi-directional input affects, and the story in which γ is 1.5 or more is hardly affected because plasticization of the beam and panel precedes.

     2) Even in a frame designed to form overall sway mechanism, there is a high possibility that the story collapse will proceed beyond the design assumption due to horizontal bi-directional input and composite effect by concrete slab. In many analysis cases, it can be confirmed that all the ends of the columns yield, and in the story where γ is 1.0, both ends of the column are deteriorated. Thus, even a Lv. 2 ground motion input can cause serious column damage.

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  • Kikuo IKARASHI, Yuichi UTSUKI, Kazuya MITSUI
    2020 Volume 85 Issue 771 Pages 759-769
    Published: 2020
    Released: May 30, 2020

     In factories and warehouses, I-shaped cross-section members are often employed for column and beam members, and an L-shaped joint panel (panel) is formed at a column-to-beam joint. Ordinary, the joint panels are restrained on their four sides by column and beam members; however, because a column and a beam member are connected only to adjacent sides, the behavior of the panels is assumed to be different.

     The current design procedure that allows the joint panels to yield under a destructive earthquake has been established in Japan because the joint panels have been proven to have large deformation capacity. On the other hand, the structural performance of the panels is not entirely explained. In particular, it is not clear that the influencing factors on the buckling strength. Therefore, it is important to clarify the structural performance of the panels in the current design procedure that allows the plasticization of the joint panels.

     The purpose of this study is to establish a buckling strength evaluation formula for the panels employed in steel structures such as factories and warehouses. It is essential to clarify the complex stress state and the buckling displacement function to establish the buckling strength evaluation formula. In this paper, only the panel web is extracted, the stresses acting on the panel web is expressed by Airy stress function, and the shear buckling coefficient is calculated based on the derived stress function by applying the energy method. Also, since a column and a beam are connected to the panels in two directions, the stress distribution changes depending on the load direction. As a result, the buckling strength of the panels changes depending on the load direction. Furthermore, the solution, evaluating the shear buckling strength considering the difference in the load direction, is proposed. Finally, reflecting these findings, a formula for evaluating the shear buckling coefficient is established. The validity of the evaluation formula is verified by comparing it with FE analysis results.

     The following are found.

     1) In the panel web, the boundary conditions of four sides can be regarded as fixed support conditions within the range of realistic width-to-thickness ratio, and the buckling at panel web is dominant.

     2) In the examination only for the panel web, the ratios of shear to bending αb, αc (index of the stress state of beam member, and column member) do not have an impact on the buckling strength. It is shown that the elastic buckling strength of the panel web is determined by the parameters related to the panel shape (section modulus ratio zbw, zcw and panel aspect ratio λp).

     3) It is shown that the distributed axial force acts on the panel web from the column and beam member, and this distributed axial force is evaluated with parameters related to the beam shape and the column shape.

     4) It is confirmed that the elastic buckling strength, which is calculated based on the energy method, is evaluated on the safe side by using the correlation curve between the distributed axial force, which is transmitted from a column and a beam member, and the shear buckling strength. The proposed evaluation formula can evaluate the elastic buckling strength accurately. Finally, the validity is confirmed by comparing eigenvalues obtained from FE analysis.

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  • Masae KIDO, Keigo TSUDA
    2020 Volume 85 Issue 771 Pages 771-778
    Published: 2020
    Released: May 30, 2020

    1. Introduction

     The objective of this study is to examine the amplification factor of moment and rotation angle due to the PΔ effect for symmetrical braced frames. A sub-assemblage frame is analyzed by using the buckling slope deflection method, and amplification factors are obtained. An approximate formula for estimating the amplification factors is presented.

    2. Analytical work

     The sub-assemblage frame shown in Fig. 2 is analysed, taking the geometric nonlinear effect into consideration. It is assumed that the slope θA= θA1 = θA2 and θB= θB1 = θB2 and the beam moments of node A and B are distributed to upper and lower column according to the column stiffness. Equation (6) is obtained by the fundamental formula of buckling slope deflection method. Moment equation at point A and B becomes Eqs. (10) and (13), respectively, where GA and GB are G factors defined as Eqs. (12) and (14). From the story equation (Eq. (15)) together with moment equations, the rotation angle R of the member AB and the moment MAB and MBA of node A and B are obtained as Eqs. (16), (17) and (18). In absence of axial force P, the rotation angle R and the moment MAB and MBA become Eqs. (20), (21) and (22), and then the amplification factors are obtained by Eqs. (23), (24) and (25).

    3. Results and discussions

     As the analytical parameters, G factors GA(=GB), slenderness ratios γ axial load ratios ny and non-dimensional horizontal stiffness k* are selected, and they vary as follows: G factors GA(=GB) : 0 (rigid beams), 0.5, 1, 2.5 and 5, slenderness ratio γ: 20, 40 and 80, axial load ratio ny: 0.1, 0.3, 0.5, 0.7, non-dimensional horizontal stiffness k*: 0, 1, 5. Figure 5 shows R/0R-Z relations and M/0M-Z relations. The dotted lines and solid lines show the R/0R-Z relations and M/0M-Z relations, respectively. A similar tendency is observed between R/0R and M/0M. Figure 6 and 7 show the effect of axial load ratio ny and G factors. From Fig. 6, it is shown that the ratios R/0R and M/0M become large as the axial load ratio increases. Approximate effective length factor Kdsn has been proposed by Eq. (27), and using this, the amplification factors am can be obtained by Eq. (31). In Fig. 5, 6 and 7, white circles indicate the results obtained using Eq. (31). The values of am agree well with the R/0R and M/0M, and the relative error is shown in Fig. 8. While the error increases as the value Z becomes large, the am fits with an error of 2% or less when Z is smaller than 1. Figure 9 shows the limit Zα where the amplification factors is guaranteed below the value of α=1.05.

    4. Conclusions

     The conclusions derived from this study are as follows:

     1) The amplification factors of the moments and rotation angle are presented by Eqs. (23) - (25). The governing parameters are G factors, Z and k* defined in Eqs. (12), (14), (9) and (5).

     2) The amplification factors can be approximately calculated by using Eq. (31).

     3) The limit value of Z to assure the restriction of the amplification factors is proposed by Eq. (35).

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  • Shintaro MORISHIMA, Makoto YAMAGUCHI, Soshiro SHIBUYA, Atsuhisa OGAWA, ...
    2020 Volume 85 Issue 771 Pages 779-789
    Published: 2020
    Released: May 30, 2020

     When designing important structures such as industrial plants and public facilities, it is necessary to ensure their safety against accidental or intentional explosions, which happen rarely but can cause severe damage. In particular, the fracture modes of reinforced concrete (RC) slabs subjected to contact detonation are characterized by spalling, which is caused by tensile stress waves reflected from the back side of the slab. To protect the lives of humans inside a structure under such conditions, it is necessary to prevent the launch of concrete fragments that accompanies spalling. Therefore, reducing spall damage is an important problem faced by the designers of blast-resistant RC structures.

     In a previous study, the authors verified the good blast-resistant performance of fiber reinforced cement composite (FRCC) slabs under contact detonation. However, a designing method for obtaining the required blast-resistant performance of the FRCC members has not been developed yet; one of the reasons for this is that it is difficult to obtain dynamic mechanical properties of FRCCs corresponding to this problem where the dominant strain rate is of the order of 103–104/s. Hence, it may be convenient to consider the spall-suppressing performance of FRCC member as a material property of the FRCC. It can be obtained directly based on material factors such as fiber shape, water-binder ratio (W/B) of the matrix, and fiber volume fraction (Vf).

     This study was conducted to evaluate the influence of various material factors on the local failure of FRCC slabs under contact detonation; therefore, contact detonation tests were carried out on polyvinyl alcohol fiber reinforced mortar (PVAFRM) slabs with four different shapes of fibers (Type I: φ0.1 × 12 mm, Type II: φ0.2 × 12 mm, Type III: φ0.2 × 18 mm, and Type IV: φ0.2 × 24 mm), four different values of W/B of the mortar matrix (50, 40, 33, and 25%), and three different values of Vf (2.0, 3.0, and 4.0%). After the tests, the fracture appearances of each specimen were observed in detail, and then the sizes of the local failure created in each specimen were measured and compared each other. The main results obtained are as follows:

     (1) It is more effective to adapt longer fibers to suppress spall if the fiber diameter is constant.

     (2) Regardless of whether the length or the aspect ratio of the fibers is constant, it is more effective to adapt finer fibers to suppress spall.

     (3) The spall-suppressing performance is reduced when the W/B value is too high or too low, and there is an appropriate value of W/B to suppress spall.

     (4) It is more effective to increase Vf value under the appropriate combination of fiber shape and W/B value to improve the spall-suppressing performance of the PVAFRM slabs.

     (5) When combining the fibers of type I and the matrix of W/B = 33%, the scaled concrete thickness of the spall limit in a normal RC slab can be reduced by almost 49% by setting the Vf value to 3.0% or more. The same effect can be obtained by setting Vf value to 4.0% under a combination of the type IV fibers and the matrix of 33%.

     (6) When finer fibers are used in combination with the appropriate W/B value, mentioned in (3) above, the spall depth tends to be reduced even if the spall occurs.

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