Reinforced concrete (RC) exerts its structural performance due to the bond stress between rebar and concrete. According to the past research, rebar corrosion is thought to lead to a decrease in bond performance of RC. In order to improve corrosion resistance of rebar, hot-dip galvanized rebar has been developed and put into practical use as one of RC anticorrosion technologies. Hot-dip galvanized rebar is a rebar coated with a thin hot-dip galvanized layer on the surface. On the other hand, it is not clear whether the galvanized coat formed on the surface of the rebar affects the bond performance between concrete and rebar.
The purpose of this study is to evaluate the bond performance of hot-dip galvanized rebar embedded in reinforced concrete. The bond stresses between rebar and concrete under compressive, tensile, and bending and shearing stress field were investigated by measuring the stress distribution along the rebar embedded in concrete using neutron diffraction method. The neutron diffraction method for measuring stress is one of the complete non-destructive and non-contact measurement technologies that utilize the diffraction phenomenon of neutron rays. By taking advantage of the excellent permeability of neutron rays, it is possible to evaluate the stress state in the deep part of the material quantitatively. The series of experiments in this study was conducted at the engineering diffractometer, TAKUMI, in MLF, J-PARC, Japan.
In Chapter 3, the bond performance of hot-dip galvanized rebar embedded in concrete at a compressive stress field was evaluated. As a result, for both introduced stresses (195MPa and 250MPa), the stress distribution along the hot-dip galvanized rebar was almost the same as that of ordinary rebar, and the difference in average bond stress was within 5%.
In Chapter 4, the bond performance of hot-dip galvanized rebar embedded in concrete at a tensile stress field was evaluated. As a result, although the approximate stress distribution along hot-dip galvanized rebar had a little difference compared with that of ordinary rebar due to the degree of cracking, the difference in average bond stress between these was within 10%.
In Chapter 5, the bond performance of hot-dip galvanized rebar embedded in concrete at a bending and shearing stress field was evaluated. As a result, for both introduced stresses (140MPa and 280MPa), the approximate stress distribution along hot-dip galvanized rebar was almost the same as that along ordinary rebar, and the difference in average adhesive stress was less than 10%.
In conclusion, the present study has demonstrated that the bond performance of hot-dip galvanized rebar was equivalent to that of ordinary rebar. In addition, it was found that the neutron diffraction technique could be a useful technique to evaluate the anchorage performance. Furthermore, further studies are needed in order to elucidate of structural performance of larger members.
Wood used for exteriors, if left uncoated, experiences discoloration in the initial stage, and goes on to develop small checks on its surface. As exterior exposure progresses, the surface of the wood partially deteriorates due to weathering of the earlywood and latewood at the exposed surface and increased checking caused by differential shrinkage, resulting in an uneven wood surface. Hereinafter, this phenomenon is referred to as surface erosion.
Wood that experiences surface erosion may develop various types of degradation in its surface layer. Surface coating of exterior wood will not lastingly protect it from surface erosion unless the wood is recoated in a timely manner as the coating wears down.
On the other hand, although surface erosion is some of the well-known degradation phenomena of wood, few studies have attempted to quantify the degradation process. One of the reasons is that it is relatively difficult to quantify the depth of degradation of uneven wood surfaces. Another factor is that protective coating and recoating of exterior wood is commonly included in building planning in line with the original design concept. For this reason, building maintenance planning rarely envisages the possibility that exterior wood will be left in an unprotected state owing to protective coating degradation, with resulting progression of surface erosion. However, in actual buildings, recoating is often not performed in a timely manner.
Therefore, in this study, we conducted an experimental verification of the process by which surface erosion and chipping progress on the surface layer of wood and the effect of various coatings to suppress such degradation. The following conclusions were reached.
1) Measurement of surface erosion using a laser displacement sensor on a rail confirmed that the surface of the wood, which had surface erosion, exhibited a very complicated pattern of height differences with a large number of depressions too narrow to be measured with a ruler or calipers.
2) The depth of the surface erosion of the uncoated specimens increased linearly with age. Within the scope of this experiment, the degradation process was found to be the same for all types of wood, whether softwood (conifers) or hardwood (broad-leafed trees).
3) When the difference in the depth of surface erosion of the uncoated test specimens was rearranged by the density of each tree species, the depth of surface erosion tended to grow shallower as the density became higher. The surface erosion is suggested to be less likely to occur as the density is higher.
4) We examined how much three different types of wood preservative coating, i.e. penetrating-type, semi-film-forming type, and film-forming type, can prevent surface erosion at the material age equivalent to two years and five months of outdoor exposure. As a result, the effect of suppressing surface erosion becomes smaller in the order of film-forming type, semi-film-forming type, and penetrating type.
5) The suppression effect of the surface erosion of the semi-film-forming type and the film-forming type differs depending on the density of the wood. The smaller the density of the tree species is, the easier the suppression effect can be obtained.
In recent years, the demand for maintaining building functions after large earthquakes has been increasing especially for high-rise buildings. For this reason, the importance of vibration control systems is increasing which efficiently absorb the vibration energy of buildings during an earthquake. Among several vibration control dampers, a buckling restraint brace (BRB) is widely used in practice design of high rise building structures because of its ease of installation and maintenance. In this paper, a new design method is proposed to determine optimal placement and performance of BRBs which minimizes seismic response of building frame structure with the same amount of BRB. Different from most of other previous researches related to optimization of BRB placement, this paper uses nonlinear time-history analysis directly considering nonlinear hysteresis characteristics of BRB. Furthermore, proposed method can pursue the optimal placement continuously by increasing the total amount of BRB.
The validity and the versatility of the of the proposed method is examined through numerical examples of 15 storied building structure. As for the difference of the structural modelling, the results of shear model and frame structure model are compared. As for the difference of input ground motion, the results using three types of seismic waves, i.e. El Centro 1940 NS, Hachinohe 1968 NS and TAFT 1952 EW, are compared. Furthermore, as for the difference of BRB grouping in the frame structure model, the results using two types of grouping, i.e. "story grouping" and "subdivided grouping", are compared. The "story grouping" gives the same cross section to BRBs in the same story. The "subdivided grouping" gives the same cross section to BRBs in symmetrical place of the same story.
As a result, the findings are summarized as follows.
1) An optimal algorithm proposed for the shear model is extended a method for the frame structural model. This algorithm makes candidate solutions with different installation locations of minute amounts of BRB and selects the candidate solution with the smallest maximum response estimated by nonlinear time history response analysis. The effectiveness and validity of the proposed method is demonstrated through comparison of responses of structural models with random placement and optimal placement of BRB.
2) In some cases of numerical examples using three types of input seismic waves, the seismic responses increase in spite of additional damping effect due to BRB. It is caused by the combination of the shortening of the natural period of the building and the periodic characteristics of the response spectrum of the seismic waves.
3) The optimal placements of BRBs and seismic responses of the frame structural model and shear model generally correspond when the total amount of BRBs is small. However, the difference between two models increases as the total amount of BRBs increases. The seismic response reduction is overestimated when the shear model is used because there are BRBs that do not work effectively depending on the location in frame structural model.
4) It is demonstrated that the optimal BRB placement and amount of BRB can be selected in more detail by using subdivided grouping. However, in the examples in this paper, the difference between the two types of grouping is small and further examination is necessary for the models in which the overall bending deformation is remarkable.
When a large pulse-like ground motion strikes in urban areas, significant structural damages may occur to super-high-rise residential buildings along with human injuries or fatalities. Herein, to evaluate human injury, human responses were investigated in super-high-rise residential buildings using the Ricker wavelet as the pulse-like ground motion. Using nonlinear seismic response analysis models of super-high-rise RC buildings and those of the human body, the variation in the maximum displacement of the center of pressure (CoP) and head velocity was examined based on the predominant periods and amplitude level of the input seismic waves. The following conclusions were drawn.
1) The maximum CoP displacement and head velocity of the human body in super-high-rise buildings excited using the Ricker wavelet tended to increase from the lower to the upper floors.
2) Diagrams were proposed for evaluating the maximum responses of the human body in super-high-rise buildings. The Diagrams evaluated human responses using the maximum value of the pseudo-displacement response spectrum (pSdmax), maximum value of the pseudo-velocity response spectrum (pSvmax), and ratio of the predominant period of the input motion to the primary natural period of the building (Tp/T0).
3) If the predominant period of the input motion matched the natural period of the building, humans can suffer a minor injury when pSvmax=80 (cm/s) and a fatal injury when pSvmax = 130 (cm/s) at the building top floor.
4) As the maximum CoP displacement calculated using the human model was larger, the action difficulty became greater based on the questionnaire survey for the residents of super-high-rise residential buildings during an earthquake.
5) The maximum CoP displacement and head velocity of the human body obtained using the human model were compared with those obtained using the human body response evaluation diagram. The results obtained using the human model roughly corresponded to the evaluated values, thus confirming the validity of the human body response evaluation diagram.
The formula for the full plastic nail arrangement modulus Zpxy which is used in the calculation of the ultimate shear capacity of the sheathed shear walls with any nail arrangement was theoretically derived by using the upper bound theorem.
The unknown values are the rotation ratio (θx/θy) and the neutral axis positions (xo and yo) in the calculation.
The difference between the calculation with the elastic values and the plastic values as unknown values is within 1.5%.
Even if the houses are built based on the same seismic resistance standards, some are damaged or not, and some may or may not collapse during an earthquake. This difference is considered to be due to the difference in local seismic motion or the target design level, the certainty of construction, the degree of deterioration, and the like. Besides, in wooden constructions, the strength and rigidity of the materials used, the joints, and the shear walls are not constant, so-called variations, which causes a difference in the load-deformation relationship of the layers. It is thought that the above factors cause differences in the response deformation during earthquakes. In this study, we discuss variations in response deformation caused by variations in materials, with the same input seismic motion, building design level, and load capacity.
Five or three full-scale box-shaped test specimens manufactured under the same conditions were simultaneously vibrated on the same shake table, and the variation in response displacement was quantitatively clarified. Also, the differences in the metal joints at the ends of the braces and the effects of finishing materials and orthogonal walls on the yield strength were examined. The experiments performed for three different purposes are the A-, B- and C-series shown below (Table 1). The A-series is to examine the variation of the response by simultaneously vibrating 5 test pieces with the uniform wall magnification and construction method on the same shaking table (Fig.1-2, Table2-3). The B-series examines the effect of hardware on the displacement response, using the hardware at the end of the brace with the same wall magnification as a parameter (Fig.8, Table9-10). Finally, the C-series examines the effect of differences in the plane type of hanging walls and frames on the displacement response, as well as the wall magnification (Fig.16-17, Table16-17).
As for the shear walls specimens in the A-series, the variation in response displacement up to the maximum load was small, about 7%, but where the maximum load is exceeded or not exceeded, the variation in response displacement was about 28%. For the brace specimens in the B series, the variation in response displacement increased when buckling in the brace and damage at its ends occurred. It is considered that this is influenced by the material strength and the performance of the brace end hardware. These tendencies were suggested in the C-series to be similar with or without lintel or orthogonal walls.
After an earthquake, it is important to evaluate the residual seismic capacity of damaged buildings in order to determine the necessity of repair and retrofit and to make an efficient recovery plan. In this regard, a Japanese guideline developed by the Japan Building Disaster Prevention Association (JBPDA) “Standard for Post-earthquake Damage Level Classification of Buildings 1) ” is currently in use in Japan to estimate the residual seismic capacity of structures based on an index named R. The R-index represents the ratio of seismic capacity before and after the earthquake. The R-index calculation is intended to be a simple seismic evaluation method that does not require complicated analysis. However, this simplified method is based on the assumption that the ultimate deformation capacity of all members is the same; thus, it is not practical for assessing dual systems, such as reinforced concrete (RC) buildings containing both moment resisting frames and walls. The main objective of this study is to propose a new evaluation method to determine the R-index for buildings of mixed structure types. The proposed method uses two factors: (i) explicit consideration of different deformation capacities of structural members, θu, and (ii) a seismic capacity reduction factor, ηW 7) which considers the hysteretic energy absorption reduction of each structural member based on the observed level of damage. The available JBDPA method and the proposed method are assessed using results obtained from a shake-table test of an RC building.
Firstly, the proposed simplified calculation method is explained in detail. Next, the results from a 1/4 scale 4-story RC frame-wall shaking table test are summarized (conducted jointly by Tohoku University and Obayashi Corporation). The 4-story specimen was designed with different frame and wall strength contributions in the longitudinal and transverse directions to investigate the impact that this has on the evaluation of residual seismic capacity ratio. The RC walls were designed to fail first, followed by the formation of a ‘strong column-weak beam’ frame-sway mechanism. Finally, each simplified calculation method (the proposed method and the existing Japanese guideline method) are applied to the building. The accuracy of the resulting R-index is verified by comparing with the results of the experiment.
In general, the results showed that both methods identified the correct tendency of residual seismic capacity observed from the experimental values. In the longitudinal X-direction, though the proposed method was closer the experimental residual capacity results, both methods were considerably lower than the experimental data as the structure did not shown strength degradation despite extensive wall damage. In the transverse Y-direction the proposed method resulted in a higher residual seismic capacity ratio compared with experimental results. The reason for this being that a flexural failure mode was assumed for the wall in the calculation, whereas a less ductile shear failure was observed in the experiment.
We often construct reinforced concrete beams with openings for plumbing, and the longitudinal bars are spliced or cut off within the beams. As designing a building structure of reinforced concrete, we verify bond around cut off bars and, lap splices, and shear capacities around openings, each by each. Although these parts are weak points in the beam, we don’t pay much attention to the combined influence among them. If an opening is close to a discontinuous point generated by cutting off the bar or lap splicing, they may influence each other. On the other hand, in order to verify serviceability and repairability, allowable shear capacities for a RC beam with opening have been added to AIJ Standard for Structural Calculation of Reinforced Concrete Structure (2010). Because examination on the allowable shear capacities with the test data are limited, more data of RC beams with opening are required to make clear a relationship between cracks on the concrete surface and structural performance of the beams. In this study, we investigated the influences of longitudinal bars cut off and lap splice in a beam with opening. We prepared three RC beam specimens, and applied anti-symmetric bending in both positive and negative directions repeatedly and statically. The specimens were scaled down by about a half. Test parameters are the presence of openings and the presence of discontinuous points generated by cutting off bars or lap splicing. One specimen had two openings and the discontinuous points of bars, another had two openings but no discontinuous points of bars, the other had the discontinuous points of bars but no openings. The following conclusions were obtained from the experimental results.
(1) The specimen without opening, after flexural yielding, failed in bond between longitudinal bars and concrete. The specimens with opening, after flexural yielding, failed in shear at the beam end region, not at the opening. That is because the inclined reinforcement transferred bond stress around the longitudinal bars.
(2) The load-deformation curves of the beams with opening were better than that of the beam without opening because the embedded part of the inclined reinforcement contributed to the resistance to shear load.
(3) It is considered that the margin of bond capacity on inner cut-off bar is on the safe side considerably because the beam failed in shear though we designed the beams to fail in bond around inner cut-off bars.
(4) The beams failed in shear had almost the same ratio of calculated shear capacity to required value at the opening as at the region without opening. However, the inclined reinforcement around opening didn’t yield. It is considered that the margin of the shear capacity in the region with opening is higher than the region without opening.
(5) When the inner bar of the beam with opening was cut off, shear forces resisted by concrete and inclined reinforcement decreased, and shear force resisted by transverse reinforcement increased.
(6) The load when an initial shear crack occurred around opening was lower than the load when an initial shear crack occurred on the beam without opening. However, the width of shear cracks around openings was less than approximately 0.1mm after unloading from the load of allowable shear capacity, which the width was narrower than that in the region without opening.
In this paper, the results of load-bearing fire test are introduced that targets a composite flooring system composed of a flat RC slab and an unprotected beam. When the unprotected steel beam applied as a secondary beam is exposed to fire, it loses strength at an early stage of fire. On the other hand, for flat RC slabs supported in 2 ways, when deflection increases, membrane stress occurs within the cross section of the slab, and as a result the load bearing capacity of the slab is improved due to this membrane action. In RC slabs in which two-tier reinforcements are arranged, when subjected to fire heating, the temperature at the upper-side reinforcement becomes lower than that at the lower-side reinforcement, and thus the tensile strength at the upper-side reinforcement is kept to a high level for a long time. As a result, it is forecast that the retention of strength by the slab due to the membrane action during fire will become more conspicuous. In order to understand these behaviors, a load-bearing fire test was carried out by means of standard fire heating in accordance with ISO 834, targeting a composite flooring system composed of a two-tier reinforcement-arranged flat RC slab and an unprotected steel beam. The following will summarize the results of the tests.
• The RC slab to which the bending collapse load based on the yield-line theory was given sustained load-bearing capacity even after being subjected to 3.5-hours of standard fire heating while involving deflection surpassing 1/20 that of the short-side span.
• In the RC slab in which two-tier reinforcements were arranged, even when the temperature of the lower-side reinforcement reached 700 °C, the temperature of the upper-side reinforcement was about 400 °C, thereby allowing for the slab to retain its load-bearing capacity. The reason for this successful retention was considered to be that the tension resistance of the upper-side reinforcement worked effectively due to the membrane action of the RC slab.
• In the composite flooring system used in this test, the effect of the application of an unprotected steel beam on fire resistance was small.
• The effect of the headed studs arranged in the four peripheral sections of the RC slab on the deflection behavior of the RC slab during fire was small. Due to the small effect, even when the rotational and horizontal displacement at the end of the slab was not restricted, the membrane action of the RC slab worked effectively, and it was thus shown that the load-bearing capacity of the composite flooring system was retained during fire.
• It was shown from these test results that the application of an unprotected secondary beam in the composite flooring system is made possible by means of the membrane action of the RC slab.