JP2012057311A - Design method of reinforced concrete beam and reinforced concrete beam - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a design method of a reinforced concrete beam, capable of accurately capturing the behavior of a beam for which the concrete of different strengths is vertically separately placed and performing rational design.SOLUTION: In the design method of a reinforced concrete beam, a composite cross section where high-strength concrete 1 and ordinary concrete 2 of different strengths are vertically separately placed on a cross section orthogonal to the long-length direction of a beam 10 is formed. Then, the equivalent concrete strengthσof the beam is calculated on the basis of an equation (1) and an equation (2) by using variation factors rand r, acting stress acting on the beam is calculated by using the equivalent concrete strengthσ, and it is confirmed that a value based on the acting stress is smaller than an allowable value for allowing generation in the beam.

Description

  The present invention relates to a design method for a reinforced concrete beam and a reinforced concrete beam for rational design of a reinforced concrete beam constructed by placing concrete having different strengths one above the other.

  Conventionally, the lower part of the beam (precast part) is manufactured in advance at a factory, etc., the precast part thus manufactured is transported to the site and mounted at a predetermined position, and the upper part of the beam and the floor (slab) above it are cast into the concrete. Are known as half precast beams (see Patent Documents 1 and 2).

  As this cast-in-place concrete, concrete having the same strength as that of the half precast beam may be cast, but it is often unnecessary to use high-strength concrete as used for the beam for the slab.

  Therefore, Patent Documents 1 and 2 disclose inventions of reinforced concrete beams constructed by placing concrete having different strengths in the vertical direction. However, Patent Documents 1 and 2 do not disclose the structure design method.

  On the other hand, Patent Document 3 refers to a rational shear strength calculation method when concrete having different strengths is divided, and a reinforced concrete beam design method using the method is proposed.

JP 7-305443 A JP 2000-336746 A Japanese Patent No. 4421196

  However, the design method of the reinforced concrete beam in Patent Document 3 focuses only on the shear strength, and the actual stress generated in the beam in which the concrete having different strengths is divided up and down and a series of this kind of structural design. The calculation method of the bending strength required for the flow has not been studied.

  In other words, to design a beam, the acting stress that can be generated in the beam is calculated, and the bending stress, shear strength, and adhesion strength to the reinforcing bar are all acting stresses against the acting stress. You must make sure that Therefore, it can be said that there has not been an appropriate design method for a beam in which concrete having different strengths is divided up and down.

  Therefore, the present invention is a reinforced concrete beam design method capable of accurately grasping the behavior of a beam in which concretes having different strengths are divided up and down and performing rational design, and is designed and constructed by the design method. The purpose is to provide reinforced concrete beams.

In order to achieve the above object, the method for designing a reinforced concrete beam according to the present invention forms a composite cross section in which the first concrete and the second concrete having different strengths in the cross section perpendicular to the longitudinal direction of the beam are vertically divided. The strength of the first concrete is _cb σ _B , the strength of the second concrete is _ct σ _B , and the ratio of the height of the second concrete to the height of the beam is r _a, wherein the ratio of the intensity of the second concrete to the intensity of the first concrete equivalent concrete strength of the beam when the _{(ct σ B / cb σ B} ) was r _{b ce} sigma _B1 equation (1) and (2 ), And the working stress acting on the beam is calculated using the equivalent concrete strength _ce σ _B1, and the value based on the working stress is less than an allowable value that is allowed to occur in the beam. It is characterized by confirming.

Here, f (x) represents an arbitrary function expressing the tensile strength using the compressive strength x of the concrete.
Here, r _c is the ratio of stiffness _ct E ₁ of the second concrete against rigid _cb E ₁ of the first concrete _{(ct E 1 / cb E 1} ), Young's modulus ratio n is the first concrete rigid _cb E the ratio of stiffness _s E rebar for _{1 (s E / cb E 1} ), reinforcing bars as is referred to what is arranged what is disposed on the upper side of the beam upper rebar, the lower the lower reinforcing bars, reinforcement ratio the upper reinforcing bars for p _t Liang (height D, width b) lower reinforcing bar cross-sectional area _s ratio of _{a t (s a t / bD} ), the lower reinforcing bars cross the double muscle ratio γ area _s a _t to the cross-sectional area of the the ratio of the cross-sectional area _{s a c (s a c /} s a t), d is up to the lower reinforcing bar effective height, d _c denotes the effective height to the upper reinforcing bar.

In addition, the confirmation regarding the allowable value of the bending moment using the working stress is that the strength _cb σ _{B of} the first concrete is taken into consideration that the neutral axis moves when concrete having different strengths is divided up and down. And a tolerance calculated respectively based on the strength _ct σ _B of the second concrete.

Furthermore, the confirmation regarding the allowable value of the shear force using the working stress is made by comparing the strength _cb σ _B of the first concrete and the strength _ct σ _B of the second concrete, and the strength of the concrete having the lower strength. This can be done using the tolerance calculated based on

  In addition, confirmation regarding the allowable value of the adhesion stress degree using the acting stress should be performed using an allowable value calculated based on the strength of the concrete in which the lower reinforcing bar or the upper reinforcing bar is embedded. Can do.

  In addition, the reinforced concrete beam of the present invention is characterized by being designed and constructed by the reinforced concrete beam design method described above. Here, high-strength concrete can be used for the first concrete, and concrete having lower strength than the first concrete can be used for the second concrete.

  Furthermore, it can also be constructed by previously forming a preceding part including the lower part of the beam with the first concrete and placing the second concrete on the preceding part.

  The method of designing a reinforced concrete beam according to the present invention configured as described above calculates an equivalent concrete strength indicating the strength of a composite cross section formed by concrete having different strengths, and calculates an acting stress of the beam based on the equivalent concrete strength. To do.

  For this reason, it is possible to accurately grasp the actual behavior of a beam in which concrete having different strengths is divided up and down. Moreover, rational design can be performed based on such equivalent concrete strength.

Furthermore, in confirming the acting stress related to the bending moment, the strength of the first concrete _cb σ _B and the strength of the second concrete are considered in consideration of the fact that the neutral shaft moves when concrete with different strengths is laid up and down. Occurs when the allowable value is calculated using the equivalent concrete strength _ce σ _B1 by using the allowable values (short-term allowable stress level, long-term allowable stress level, etc.) calculated based on _ct σ _B. It can prevent overestimation.

It is explanatory drawing which showed the cross section typically in order to demonstrate the design method of the reinforced concrete beam of embodiment of this invention. It is explanatory drawing which showed typically the reinforced concrete beam which receives a reverse symmetrical bending. It is a skeleton curve for explaining the behavior of a reinforced concrete beam. FIG. 4 is a stress distribution diagram at both ends of a beam in a first gradient section of the skeleton curve of FIG. 3. FIG. 4 is a stress distribution diagram at both ends of a beam in a second gradient section of the skeleton curve of FIG. 3. It is a figure explaining the structure of the reinforced concrete beam demonstrated in Example 1, Comprising: (a) is a cross-sectional view, (b) is a longitudinal cross-sectional view. BRIEF DESCRIPTION OF THE DRAWINGS It is sectional drawing of the reinforced concrete beam demonstrated in Example 1, Comprising: (a) is all the single cross section which cast high strength concrete, (b) is a composite cross section of high strength concrete and normal concrete, (c) is All show a single cross section with ordinary concrete. It is a skeleton curve for demonstrating the behavior of three types of reinforced concrete beams of Example 1. FIG. It is a perspective view explaining the structure of the reinforced concrete beam of Example 2. FIG.

  Hereinafter, a method for designing a reinforced concrete beam according to an embodiment of the present invention will be described with reference to the drawings. 1 and 2 are schematic views for explaining the configuration of the beam 10 as a reinforced concrete beam.

  As shown in FIG. 1, the beam 10 has a composite cross section in which concretes having different strengths are vertically divided in a cross section perpendicular to the longitudinal direction. Here, the first concrete placed below the beam 10 is referred to as high-strength concrete 1, and the second concrete placed above the beam 10 is referred to as ordinary concrete 2. Further, the lower reinforcing bars 31 of the beam 10 are embedded in the high-strength concrete 1, and the upper reinforcing bars 32 of the beam 10 are embedded in the ordinary concrete 2.

  Furthermore, the composite cross section of the high-strength concrete 1 and the ordinary concrete 2 is formed over the entire length in the longitudinal direction of the beam 10 as shown in FIG. Both ends of the beam 10 are fixed to the columns 4 and 4 (fixed end support). Here, the left end in FIG. 2 is the A end, and the right end is the B end.

  FIG. 2 is a diagram schematically showing a state in which an antisymmetric bending is generated in the beam 10 by displacing the A end side and the B end side of the beam 10 in the upside down direction. In the beam 10 subjected to the reverse symmetrical bending, a tensile force is generated on the upper side at the A end that moves relatively upward, and a tensile force is generated on the lower side at the B end that moves relatively downward.

  The behavior of the beam 10 subjected to such antisymmetric bending can be defined as a curve having two singular points. FIG. 3 is a diagram schematically showing the behavior of the beam 10 with a trilinear skeleton curve with the material end rotation angle R on the horizontal axis and the material end bending moment M on the vertical axis.

  Here, the range from the bending moment Mc to the bending moment My where the bending yield occurs until the bending moment Mc at which the bending crack occurs at the end is defined as a first gradient interval.

Then, an equivalent concrete strength (compressive strength) of the beam 10 is calculated in order to simulate the beam 10 having the composite cross section of the high-strength concrete 1 and the ordinary concrete 2 by the skeleton curve of FIG. Here, the equivalent concrete strength of the beam 10 in the first gradient section is _ce σ _B1, and the equivalent concrete strength of the beam 10 in the second gradient section is _ce σ _B2 .

  On the other hand, FIG. 4 is a diagram showing the distribution of the stress degree σ of the cross section at the A end and the B end of the beam 10 in the first gradient section. This stress distribution diagram is drawn on the assumption that “plane retention is established even in a cross section in which concrete having different strengths is divided”. Here, the plane holding means an assumption that a cross section perpendicular to the axis when the beam 10 is bent is a plane perpendicular to the deformed axis even after deformation and does not deform in a direction perpendicular to the axis.

  When it is assumed that plane holding is established, the strain of the cross section (indicated by a function such as ε (x) in FIG. 4) can be represented by a linear distribution, and the stress degree drawn based on the strain distribution The distribution is a distribution diagram in which steps are generated at boundary positions having different intensities. This level difference is caused by the difference in material rigidity between the two types of concrete. At this time, the boundary where the stress σ that occurs in the cross section changes from tension to compression, that is, the neutral axis is accompanied by the separation of strength. There is a tendency to move to a higher strength concrete side than ordinary beams.

Next, symbols used in the calculation formulas of the present embodiment will be described. First, the compressive strength of high-strength concrete 1 is _cb σ _B , the compressive strength of ordinary concrete 2 is _ct σ _B (= r _{b · cb} σ _B ), and the height _t D of ordinary concrete 2 occupying the height D of the beam the ratio r _a, the ratio of the intensity of the ordinary concrete 2 with respect to the intensity of the high-strength concrete _{1 (ct σ B / cb σ} B) and r _b.

Further, with respect to r _c is the ratio of stiffness _ct E ₁ plain concrete 2 for rigid _cb E ₁ of high strength concrete _{1 (ct E 1 / cb E} 1), Young's modulus ratio n rigidity _cb E ₁ of high strength concrete 1 The ratio ( _s E / _cb E ₁ ) of the rebar stiffness _s E is shown.

Furthermore, reinforcement ratio p _t Liang (height D, width b) the ratio _(s a _t / bD) of the lower reinforcing bar cross-sectional area _s a _t to the cross-sectional area of, multiple muscle ratio γ is lower reinforcing bar cross-sectional area _s a It indicates the ratio of the upper reinforcing bar cross-sectional area _{s a c (s a c /} s a t) for _t.

D indicates the effective height up to the lower rebar 31 and d _c indicates the effective height up to the upper rebar 32. Here, the effective height refers to the distance from the top of the beam to the centroid of the cross section of the reinforcing bar.

  On the other hand, as a result of the experiment, it has been confirmed that bending cracks occur at the A end and the B end of the beam 10 simultaneously. Therefore, a point in time when the stress burden of the concrete at the tensile edge of each cross section at the A end and the B end reaches the tensile strength of the concrete placed in that portion is defined as a stress distribution at the time of occurrence of bending cracks.

  Furthermore, it was confirmed by experiments and analyzes that when the beam 10 with different strengths was loaded in an anti-symmetric bending, the position of the inflection point generated at the center of the beam having a single cross section moved.

  The movement distance of the inflection point is obtained by determining the stress distribution due to the bending moment so that bending cracks occur at both ends (A end, B end) of the beam 10 simultaneously.

Therefore, when the equivalent concrete strength _ce σ _B1 of the first gradient section is derived under these conditions, the following equation is obtained.
Here, f ( _cb σ _B ) and f (r _{b · cb} σ _B ) indicate arbitrary functions that express the tensile strength using the compressive strengths _cb σ _B and r _{b · cb} σ _B of the concrete.

Further, the coefficient Ω ₁ in the equation (1) is a coefficient determined so that the relative position between the neutral axis position and the main axis (1 / 2D) of the beam 10 with different strengths is as shown in FIG. Can be shown.
This coefficient Ω ₁ is a variable factor r _a (= _t D / D (ratio of the height of ordinary concrete 2 to the height of the beam 10)), and a variation factor. using _{r b (= ct σ B /} cb σ B ( ratio of the intensity of the ordinary concrete 2 for high-strength concrete strength 1)) is a coefficient for consideration.

On the other hand, FIG. 5 is a diagram showing the distribution of the stress degree σ of the cross section at the A end and the B end of the beam 10 in the second gradient section. The equivalent concrete strength _ce σ _{B2 of} the second gradient section is also developed in accordance with the development starting from the stress intensity distribution of the cross sections at both ends of the beam in FIG. It can be expressed as:
Here, _e α ₂ is a coefficient indicating the influence of strength striking on the member rigidity of the beam 10, and is obtained by the following equation.
Further, _B α ₂ in the above equation indicates a numerical value that expresses the reduction in the cross-sectional performance on the B end side by the strength classification, and is obtained by the following equation.
Also, _A Omega ₂ and _B Omega ₂ in the above equation, intensity positional relationship between the neutral axis and the spindle in the second slope section like the Omega ₁ each cross section in FIG. 5 (A end, B end) This coefficient is determined by reflecting the influence of the classification, and can be expressed by the following equation.
Next, the design method of the reinforced concrete beam of this Embodiment is demonstrated.

As shown in FIG. 1, a high-strength concrete 1 having _a thickness (D-r _a D) is placed on the beam 10 to which this design method is applied, and on the high-strength concrete 1. An ordinary concrete 2 having _a thickness _t D (= r _a D) is placed.

  Further, lower rebars 31 are arranged below the beam 10, and upper rebars 32 are arranged above the beam 10. Further, the lower rebars 31 are embedded in the high-strength concrete 1, and the upper rebars 32 are embedded in the ordinary concrete 2.

  Such a beam 10 is subjected to a load such as its own weight, the weight of the floor 5, an upper load acting on the floor 5, and an earthquake load. In addition, which load is applied to the beam 10 depends on the design to be implemented, such as a short-term design or a long-term design. And it is confirmed by this design method whether the beam 10 can endure even if these loads (short-term load, long-term load) act.

First, the equivalent concrete strength _ce σ _B1 of the beam 10 in which the high-strength concrete 1 and the ordinary concrete 2 are divided is calculated using the equations (1) and (2). Subsequently, the elastic stiffness E is calculated on the assumption that the beam 10 has a single cross section with the equivalent concrete strength _ce σ _B1 , and the acting stress when the above-described load is applied to the beam 10 is calculated. Then, it is confirmed whether or not the applied stress is less than an allowable value that is an allowable stress in the beam 10.

  The confirmation using the acting stress is performed on, for example, a shearing force, a bending moment, and an adhesion stress level (rebar pulling force). Further, as the allowable value, for example, an allowable stress calculated based on “Reinforced Concrete Structure Calculation Standards / Description” (Architectural Institute of Japan, 2010) is used.

For example, as an allowable value for confirming the shear force, to calculate the short-term allowable shear force Q _A for damage control of the beam 10. The short-term allowable shear force Q _A of the beam 10 can be calculated by the following equation.
Here, j = 7 / 8d, α = 1, f s are usually short-term allowable shear stress of the concrete 2, short-term allowable tensile stress of the _w f _t is shear reinforcement, p _w is shows the shear reinforcement ratio Yes. That is, the beam 10 has a composite cross section of the high-strength concrete 1 and the ordinary concrete 2, but the short-term allowable shear force Q is assumed on the assumption that the ordinary concrete 2 having a low strength is placed on the entire cross section of the beam 10. _A is calculated.

Then, shear force Q of the beam 10, which is calculated using the equivalent concrete strength _ce sigma _B1 confirms that it is less than the short-term permissible shear force Q _A.

Subsequently, as an allowable value for confirming the bending moment, it calculates a short-term permissible bending moment M _A of the beam 10. The calculation of the short-term permissible bending moment M _A is, (i) assuming a planar holding is established, (ii) attachment of the concrete and the reinforcing bars (main reinforcement) is complete, do not bear a (iii) concrete tensile stress, (Iv) It is performed under the condition that the compression part and the reinforcing bar of concrete are regarded as elastic bodies.

The burden stress of the concrete compression edges, bending moment that causes the short-term tolerance compressive stress of f _c of concrete obtained by the following equation is the short-term permissible bending moment M _A.
Here, Fc represents the concrete strength on the compression side. For example, in the case of antisymmetric bending loading as described above, Fc becomes the compressive strength _cb σ _B of the high-strength concrete 1 on the A end side, and Fc becomes the compressive strength _ct σ _B of the ordinary concrete 2 on the B end side. .

As described above, since the neutral axis is moved by beat classification of strength, calculation results to be of short-term permissible bending moment M _A using short allowable compressive stress of f _c of the equation (9), of course The effect will be reflected in. That is, to calculate the short-term permissible bending moment _{M A} using coefficients _B Omega ₂ coefficients _A Omega ₂ of the formula (6) (7).

The bending moment of the beam 10, which is calculated using the equivalent concrete strength _ce sigma _B1 confirms that it is less than the moment M _A bending the short acceptable.

Further, as an allowable value for performing confirmation of the bond stress degree, to calculate the short-term tolerance bond stress of _s f _a for damage control.
Here, Fc indicates the concrete strength at the position where the reinforcing bar is embedded. It should be noted that the short-term allowable adhesion stress degree of the upper rebar 32 is set to be smaller than that of the lower rebar 31 in the above-mentioned “Reinforced Concrete Structure Calculation Standards / Description”, when the concrete is launched from below. This is because the adhesion stress level of the reinforcing bars arranged is lower than the adhesion stress level of the reinforcing bars arranged on the lower side.

On the other hand, in the reinforcing bars (main bars) arranged in the beam 10 on which the shearing force Q acts, an adhesion stress τ shown in the following equation is generated.
Here, j = 7 / 8d, ψ indicates the circumference of the main muscle.

Therefore, to make sure that the bond stress of calculating the tau, which is less than the short-term tolerance bond stress of _s f _a by equation (11) from the shear force Q of the beam 10, which is calculated using the equivalent concrete strength _ce sigma _B1 .

  Next, the effect | action of the design method of the reinforced concrete beam of this Embodiment is demonstrated.

Design method of reinforced concrete beams thus configured present embodiment calculates an equivalent concrete strength _ce sigma _B1 indicating the strength of the composite cross-section intensity is formed by different concrete, of the equivalent concrete strength _ce sigma _B1 The acting stress is calculated as the beam 10 having a single cross section.

  For this reason, it is possible to accurately grasp the actual behavior of the beam 10 in which concrete having different strengths is divided up and down.

  Here, in the design method of the composite cross section beam 10 in which the concrete having different strengths is divided, the design is performed on the assumption that the concrete having the lower strength is placed on the entire cross section. It seems to have an advantage in ensuring safety.

  However, in the actual composite cross-sectional beam 10, it has been found that the position of the neutral axis moves to the higher material rigidity side when subjected to bending. For this reason, when subjected to bending that compresses the low-strength concrete, the conventional design method that does not move the neutral shaft is more advantageous than the actual stress due to bending, that is, on the dangerous side. There is a risk of estimating.

In contrast, in the reinforced concrete design method of the present embodiment, the equivalent concrete strength _ce σ _B1 is calculated by equation (1) using a coefficient Ω ₁ (equation (2)) that takes into account the moving distance of the neutral axis position. in order to, for calculating a short-term permissible bending moment M _a using the coefficients _{a Ω 2, B Ω 2,} it can be said that accurately reflect the behavior of the beam 10 which the actual strength was divided out.

Moreover, by designing based on such equivalent concrete strength _ce σ _B1 , it is not necessary to perform an excessive design on the safe side, and a rational design can be achieved.

  Hereinafter, the validity of the design method of the reinforced concrete beam described in the embodiment will be described with reference to FIGS.

  FIG. 6 is a diagram showing various cross-sectional quantities of a beam 10A as a reinforced concrete beam described in the first embodiment. 6A shows a cross section obtained by cutting the beam 10A in the direction perpendicular to the axis, and FIG. 6B shows a vertical cross section obtained by cutting the vicinity of the side surface of the beam 10A in the axial direction. In this beam 10A, a plurality of shear reinforcement bars 33,... Are arranged at intervals in the longitudinal direction.

Moreover, in Example 1, the beam 10B which made the whole cross section high strength concrete 1 as shown to Fig.7 (a), and high strength concrete 1 and ordinary concrete 2 as shown in FIG.7 (b) are divided. The three types of the composite cross-sectional beam 10A and the beam 10C in which the entire cross-section is ordinary concrete 2 as shown in FIG. 7C were examined. Here, in Example 1, the strength of the high-strength concrete 1 was 45 N / mm ² and the strength of the ordinary concrete 2 was 36 N / mm ² .

  The three beams 10A-10C were each subjected to a shearing force Q having a magnitude that causes a forced shear displacement of 30 mm between both ends (see the A end and B end in FIG. 2).

As can be seen from the value of the shear displacement of 30 mm in FIG. 8, the shear force Q is Q ^B = 360 kN for the beam 10B of the high-strength concrete 1 in the entire cross section, Q ^A = 349 kN for the beam 10A in which the strength is divided, and all Q ^C = 334 kN in the beam 10C of the ordinary concrete 2 cross section. As described above, it can be seen that the performance of the beam 10A with different strengths shows intermediate performance between the beam 10B of the high-strength concrete 1 and the beam 10C of the ordinary concrete 2 in the entire cross section.

Further, the bending moment M acting on both ends of each beam 10A-10C is equal at both ends in the single-section beams 10B and 10C in which the antisymmetric bending is established, and in the beam 10A in which the anti-curvature point moves, the A end and B The size is different at the ends. Here, both ends of the bending moment ^{M B} of the beam 10B is 937KNm, bending moment ^{M C} across the beam 10C becomes 868KNm. Furthermore, the bending moment _A M ^A of the A end of the beam 10A 917KNm, bending moment _B M ^A and B ends of the beam 10A becomes 898KNm.

Furthermore, the degree of adhesion stress τ acting on the main reinforcement of each beam 10A-10C is calculated by the equation (11), and τ ^B = 1.131 N / mm ^{2 for} the beam 10B and τ ^A = 1.095 N / mm for the beam 10A. ² and in the beam 10C, τ ^C = 1.049 N / mm ² .

Then, the short-term allowable shear force Q _A for damage control of each beam 10A-10C is calculated using equation (8), Q _A ^B = 888 kN for beam 10B, Q _A ^A = 855 kN for beam 10A, beam In the 10C was the _Q ^{a C} = _855kN.

Moreover, short-term permissible bending moment M _A for damage control of each beam 10A-10C, when calculated using Equation (9) taking into account the influence of the movement of the neutral axis by dividing out the strength, both ends of the beam 10B The bending moment M _A ^B is 1580 kNm, and the bending moment M _A ^C at both ends of the beam 10 ^C is 1343 kNm. Furthermore, the bending moment _A M _A ^A of the A end of the beam 10A 1580KNm, bending moment _B M _A ^A B-end of the beam 10A becomes 1345KNm.

Moreover, short-term tolerance bond stress of _s f _a for damage control acting on the main reinforcement of the beams 10A-10C, when calculated from the equation (10-1) and (10-2), the upper the beam 10B rebar 32 ^{_{s f a B = 3.150N / mm}} 2 and the lower reinforcing bar 31 ^{_{s f a B = 4.725N / mm}} 2, the upper reinforcing bars 32 in the beam 10A is ^{_{s f a A = 2.790N / mm}} 2 and the lower rebar 31 ^{_{s f a a = 4.725N / mm}} 2, the upper reinforcing bars 32 in the beam 10C is ^{_{s f a C = 2.790N / mm}} 2 and the lower reinforcing bar 31 becomes ^{_{s f a C = 4.185N / mm}} 2 .

  Using the relationship between the acting stress and the allowable value of each beam 10A-10C calculated as described above, the present invention is compared with a design method (hereinafter referred to as a “comparative method”) that seems to be on the safe side at first glance. Explain that the reinforced concrete beam design method (hereinafter sometimes referred to as “this method”) is superior.

  Here, the comparison method is a method of designing assuming that the entire cross section is constituted by the concrete having the lower strength, even if the beams have different strengths. For this reason, the result of the beam 10 </ b> C in which the entire cross section described above is constructed with the ordinary concrete 2 is the result of the comparison method.

  And the concept of margin is used as an evaluation standard for comparing these two methods. The margin is a value obtained by dividing the applied stress by an allowable value. If the margin is larger than 1, it indicates that the structural design is not established. In addition, when the margin is 1 or less, it is established as a structural design. However, if the numerical value is too small, there is a possibility of overdesign.

  First, when comparing the shearing force, it was 0.391 in the comparative method, whereas it was 0.408 in the present method. Here, since the denominators are the same in both methods, this method means that the acting shear force is greatly evaluated. In other words, designing the beam 10A with different strengths using the comparative method may underestimate the acting shear force from the actual level, and considering the actual behavior of the beam 10A, It can be said that efficient design is performed under appropriate working stress.

  Subsequently, when the bending moments were compared, the comparison method had a single cross section, so that the A end and the B end were 0.647, whereas the present method was 0.580 at the A end and 0.667 at the B end. That is, in this method, since the action of strength classification is taken into consideration, the margin is different between the A end and the B end. At the A end, since this method appropriately evaluates the influence of the high-strength concrete 1, the margin is smaller than that of the comparative method that ignores it. However, at the B end, the margin of the comparison method is conversely large, and the result of the B end of the comparison method that does not take into account the strength classification may underestimate the acting bending moment. It can be said.

  As for the degree of adhesion stress, since the surrounding concrete strength in which the reinforcing bar is embedded is used in the comparative method, the upper reinforcing bar 32 has a comparison method of 0.376, the present method becomes 0.393, and the lower reinforcing bar 31 has Compared to 0.222, this method is 0.232. Here, since the denominator is the same between this method and the comparative method, the difference in the margin can be said to be a difference in the evaluation of the applied stress. Therefore, there is a possibility that the comparison technique with a small margin underestimates the applied stress.

Further, when the present method of calculating the short-term permissible bending moment M _A is an allowable value, without using an equivalent concrete strength _ce sigma _B1. That is, in confirming the allowable value regarding the bending moment of both ends caused by the acting stress, the short-term allowable stress degree calculated based on the strength _cb σ _B of the high-strength concrete 1 and the strength _ct σ _B of the ordinary concrete 2 respectively. (or long-term allowable stress, etc.) to reflect the influence of the movement of the neutral axis using a short-term permissible bending moment M _a is obtained.

The reason for this is that if the allowable bending moment is calculated using the equivalent concrete strength _ce σ _B1 , the influence of the movement of the neutral axis cannot be reflected in the calculation process, so the allowable bending moment is overestimated. This is because there is a possibility that (the evaluation in which the portion where the low-strength concrete is cast is estimated to be higher than the actual strength). Then, specializing the equivalent concrete strength _ce σ _B1 only for the calculation of the applied stress as in this method leads to prevention of setting an excessive allowable value.

  Other configurations and functions and effects are substantially the same as those in the above-described embodiment, and thus description thereof is omitted.

  Hereinafter, a beam 60 that can be designed using the method for designing a reinforced concrete beam described in the embodiment or Example 1 will be described with reference to FIG. The description of the same or equivalent parts as those described in the above embodiment or Example 1 will be given the same reference numerals.

  The beam 60 according to the second embodiment is a composite in which a PCa portion 61 as a leading portion formed by the first concrete and a spot placing portion 62 formed on the PCa portion 61 by the second concrete are divided. It has a cross section.

  And the high strength concrete (1st concrete) with high intensity | strength is used for the PCa part 61, and the normal concrete (2nd concrete) whose intensity | strength is lower than the PCa part 61 is used for the site placement part 62. . Further, the on-site placing portion 62 of the beam 60 and the floor 5 are integrally formed of the same ordinary concrete.

  The beam 60 having such a structure is an economical structure in which high-strength concrete can be used specifically for the lower part of the beam 60 where a large bending moment acts during an earthquake or the like.

  On the other hand, high-strength concrete is not a problem when it is used in a stable environment such as a factory, but when it is placed on site, cracks are more likely to occur than normal concrete. Management during curing is particularly difficult. Moreover, high-strength concrete causes a decrease in workability due to its high viscosity. On the other hand, by using ordinary concrete for the upper side of the beam 60 and the floor 5, the occurrence of cracks can be suppressed and the workability can be improved. Moreover, since the material cost of ordinary concrete is lower than that of high-strength concrete, the material cost can be reduced.

  However, the conventional method cannot appropriately design the beams 10 and 60 with different strengths. In particular, there was no appropriate design method for the bending moment. For this reason, the beam 60 as described above has not been constructed under an appropriate structural design method in spite of having structural and construction advantages.

  On the other hand, since the design of the beam 60 can be appropriately performed by applying the design method of the reinforced concrete beam described in the present embodiment or Example 1, the beam 60 excellent in structure and construction. Will be able to build.

  Other configurations and functions and effects are substantially the same as those of the above-described embodiment or Example 1, and thus description thereof is omitted.

  The embodiment of the present invention has been described in detail above with reference to the drawings. However, the specific configuration is not limited to this embodiment or example, and the design changes are within the scope of the present invention. Are included in the present invention.

  For example, in the said embodiment and Example, although the case where the intensity | strength of 1st concrete was higher than the intensity | strength of 2nd concrete was demonstrated, it is not limited to this, The direction of 2nd concrete is 1st. Even when the strength is higher than that of concrete, the expressions (1) and (2) are established.

  Moreover, in the said embodiment and Example 1, although the allowance was calculated based on "Reinforced concrete structure calculation standard and description" (The Architectural Institute of Japan), it is not limited to this, Other standards and experiment The allowable value may be calculated based on the value. Furthermore, although the short load design has been illustrated, the present invention is not limited to this, and the present invention can be applied to a long load design.

1 High-strength concrete (1st concrete)
2 Normal concrete (2nd concrete)
10,10A Beam (Reinforced concrete beam)
60 Beam (Reinforced concrete beam)
61 PCa part (leading part)
62 Site placement section

Claims (7)

A method of designing a reinforced concrete beam in which a composite cross section is formed in which a first concrete and a second concrete having different strengths are vertically divided in a cross section perpendicular to the longitudinal direction of the beam,
The first concrete strength _cb sigma _B of the second concrete strength _ct sigma _B of the height ratio of the second concrete occupying the height of the beam r _a, against the intensity of the first concrete first 2 concrete strength ratio of the _{(ct σ B / cb σ B} ) a r _b and equivalently concrete strength of the beam when the _ce sigma _B1 is calculated based on the formula (1) and (2),
Calculate the acting stress acting on the beam using the equivalent concrete strength _ce σ _B1 ,
A method for designing a reinforced concrete beam, wherein a value based on the acting stress is confirmed to be less than an allowable value that is allowed to be generated in the beam.
Here, f (x) represents an arbitrary function expressing the tensile strength using the compressive strength x of the concrete.
Here, r _c is the ratio of stiffness _ct E ₁ of the second concrete against rigid _cb E ₁ of the first concrete _{(ct E 1 / cb E 1} ), Young's modulus ratio n is the first concrete rigid _cb E the ratio of stiffness _s E rebar for _{1 (s E / cb E 1} ), reinforcing bars as is referred to what is arranged what is disposed on the upper side of the beam upper rebar, the lower the lower reinforcing bars, reinforcement ratio the upper reinforcing bars for p _t Liang (height D, width b) lower reinforcing bar cross-sectional area _s ratio of _{a t (s a t / bD} ), the lower reinforcing bars cross the double muscle ratio γ area _s a _t to the cross-sectional area of the the ratio of the cross-sectional area _{s a c (s a c /} s a t), d is up to the lower reinforcing bar effective height, d _c denotes the effective height to the upper reinforcing bar. The confirmation regarding the allowable value of the bending moment using the acting stress is based on the fact that the neutral axis moves when concrete having different strengths is laid up and down, and the strength _cb σ _B of the first concrete and the The method for designing a reinforced concrete beam according to claim 1, wherein the method is performed using tolerance values calculated based on the strength _ct σ _B of the second concrete. The confirmation regarding the allowable value of the shear force using the acting stress is based on the strength of the concrete having the lower strength by comparing the strength _cb σ _B of the first concrete with the strength _ct σ _B of the second concrete. The method for designing a reinforced concrete beam according to claim 1 or 2, wherein an allowable value calculated in the above is used.   The confirmation regarding the allowable value of the degree of adhesion stress using the acting stress is performed using an allowable value calculated based on the strength of the concrete in which the lower rebar or the upper rebar is embedded. A method for designing a reinforced concrete beam according to any one of claims 1 to 3.   A reinforced concrete beam designed and constructed by the design method for a reinforced concrete beam according to any one of claims 1 to 4.   The reinforced concrete beam according to claim 5, wherein the first concrete is high-strength concrete, and the second concrete is concrete having lower strength than the first concrete.   The reinforced concrete according to claim 5 or 6, wherein the first concrete includes a preceding portion including a lower portion of a beam, and the second concrete is placed on the preceding portion. Beams.