JP6223225B2 - Reinforced concrete columns - Google Patents

Description

  The present invention relates to a reinforced concrete column.

  In the conventional structural design of reinforced concrete columns, although concrete strength and the amount of reinforcement that can express sufficient strength against external force during an earthquake, etc. are examined (for example, see Patent Document 1), buckling failure of axial force is considered. Consideration was not common.

  Reinforced concrete columns have a diameter-to-length ratio (column height / small diameter of cross section) limited to 15 or less by the Building Standard Law Enforcement Ordinance Article 77 No. 5, so the column height (column length) It had a sufficiently large cross-sectional dimension (thickness) and was considered to have sufficient strength against buckling failure.

  Patent Document 2 discloses a reinforcing bar material having different strengths in a column head and a column base (high strength portion), which are portions where stress during an earthquake increases, and in a central portion (ordinary strength portion) other than that. Although the reinforced concrete column used as is disclosed, the high strength portion and the normal strength portion of the main reinforcement are not set for the purpose of preventing buckling.

However, if it was confirmed that the reinforced concrete columns would not be buckled by the 2011 revision of the Building Standards Law, the diameter-to-length ratio could be increased to 15 or more.
For such a long column, the buckling strength is generally calculated using the Euler buckling formula.

JP 2010-261285 A Utility Model Registration No. 3147699

  In the case of a reinforced concrete column at the time of a fire, the temperature of the member surface layer concrete rises earlier than the center of the section due to heating from the outside. As a result, the compressive strength and Young's modulus of the member surface concrete are reduced (see FIGS. 5 and 6), so that the stress acting on the main bars arranged in the surface layer portion is increased. When the stress acting on the main bar exceeds the yield point and the main bar yields, the Young's modulus of the main bar rapidly decreases, and the Young's modulus of the entire member also decreases significantly. Therefore, even if the reinforced concrete column is designed so as not to buckle in the structural design at room temperature, there is a concern about the occurrence of buckling during a fire.

  The present invention has been made to solve the above-described problems, and an object of the present invention is to propose a reinforced concrete column in which safety is ensured even when heated by a fire or the like.

In order to solve the above-mentioned problem, the reinforced concrete column according to the first aspect of the present invention is a normal reinforcing bar in which a central portion in the longitudinal direction of the main bar is made of a high-strength reinforcing bar and the other part of the main bar is connected to the high-strength reinforcing bar It is characterized by consisting of.
The high-strength reinforcing bar and the normal reinforcing bar are preferably joined by friction welding.

According to such a reinforced concrete column, since the yield point of the main reinforcement is increased by arranging the high-strength reinforcement in the central portion in the longitudinal direction where buckling is likely to occur, the buckling strength can be improved.
In addition, since relatively expensive high-strength reinforcing bars are used only in the central part of the main reinforcing bars, the cost can be reduced.
Further, according to the present invention, the buckling strength can be increased without increasing the cross-sectional area of the column and the amount of reinforcing bars, so that an increase in material costs and labor during construction can be suppressed.

  Further, the reinforced concrete column according to the second invention is characterized in that a reinforcing bar (reinforcing bar) is attached inside the main reinforcing bar in the central portion in the longitudinal direction.

According to such a reinforced concrete column, since the yield point of the main reinforcement is increased by the reinforcing bars, the buckling strength can be improved.
Further, since the reinforcing bars are arranged inside the main bars, the temperature rise is slower than that of the outer main bars, so that the compressive strength and Young's modulus are slow to be delayed and the buckling strength can be lowered. Furthermore, it is not necessary to increase the cross-sectional area of the column in order to secure the cover thickness of the reinforcing bar.

  According to the reinforced concrete column of the present invention, safety can be ensured even when heated by a fire or the like.

It is a perspective view which shows the reinforced concrete pillar which concerns on 1st embodiment. (A) The perspective view which shows the reinforced concrete pillar which concerns on 2nd embodiment, (b) is the same sectional drawing. (A)-(c) is a schematic diagram which shows the main reinforcement of a reinforced concrete pillar, and a calculation model. (A)-(c) is a graph which shows the relationship between the elapsed time and buckling proof stress of the calculation model shown in FIG. It is a graph which shows the relationship between the temperature of a reinforcing bar, and a yield point. (A) is a graph showing the relationship between concrete temperature and compressive strength, (b) is a graph showing the relationship between concrete temperature and Young's modulus.

<First embodiment>
In this embodiment, a reinforced concrete column 1 having a diameter / length ratio exceeding 15 will be described.
As shown in FIG. 1, the reinforced concrete column 1 of the present embodiment is composed of a hardened concrete body 2 having a square cross section and main bars 3, 3,... Arranged at each corner of the hardened concrete body.

The main reinforcing bar 3 is composed of a high-strength reinforcing bar 3a at the longitudinal center of the main reinforcing bar 3 and a normal reinforcing bar 3b at the other part.
The high-strength reinforcing bar 3a and the normal reinforcing bars 3b, 3b constituting the main reinforcing bar 3 are joined by friction welding in a state where the end faces are in contact with each other.

In this embodiment, USD685 having a high yield point is used as the high-strength reinforcing bar 3a, and SD490 having a yield point lower than that of USD685 is used as the normal reinforcing bar 3b (see FIG. 5).
In addition, the material which comprises the high intensity | strength reinforcing bar 3a and the normal reinforcing bar 3b is not limited to the above-mentioned thing.

The high strength reinforcing bar 3 a has a length of about 40% with respect to the entire length of the main reinforcing bar 3. Note that the length of the high-strength reinforcing bar 3a is not limited.
The high-strength reinforcing bar 3a and the normal reinforcing bar 3b have the same reinforcing bar diameter.

  In the reinforced concrete column 1, the temperature of the member surface concrete at the time of fire rises earlier compared to the center of the cross section, and as a result, the compressive strength and Young's modulus of the member surface layer concrete decrease (see FIGS. 5 and 6). The force applied to the main muscles arranged on the surface layer increases. When the stress on the main bar exceeds the yield point and the main bar yields, the Young's modulus of the main bar suddenly decreases and the Young's modulus of the entire member decreases, so the bending stiffness (E (t) · I) of the entire member increases. descend.

Therefore, by using the high-strength reinforcing bar 3a as the main reinforcement 3, the yield point of the main reinforcement 3 is raised so that it does not yield in the event of a fire.
Note that the length of the high-strength reinforcing bar 3a is not limited.

According to the reinforced concrete column 1 of this embodiment, since the high strength reinforcing bar 3a is arranged in the central part and the yield point of the central main part 3 is increased, the main part is difficult to yield in the central part, and part of the main part is Even if it yields, a significant reduction in the buckling strength of the entire member can be suppressed (see FIG. 4B). By increasing the strength of the main reinforcement only in the vicinity of the location where the deformation increases during buckling, and without increasing the strength of the main reinforcement over the entire length of the reinforced concrete pillar 1, the initial reinforcement of the reinforced concrete pillar 1 and uneven heating The influence of secondary bending on the reduction of buckling strength can be reduced.
Further, the high-strength reinforcing bar 3a constituting the main reinforcing bar 3 and the normal reinforcing bars 3b and 3b are joined by friction welding in the factory with the end faces butted together, so that the gas pressure welding joint, mechanical joint, weld joint, etc. In comparison, when heated from the outside with less bulge, it is possible to reduce the portion close to the concrete surface that is susceptible to heat, which is preferable.

Moreover, since the high-strength reinforcing bar 3a is adopted only in the central part of the main reinforcing bar 3, the material cost can be reduced as compared with the case where all the main reinforcing bars 3 are made of high-strength reinforcing bars.
In addition, the reinforced concrete column 1 increases buckling strength without increasing the cross-sectional area and the amount of reinforcing bars compared to the case where all the main bars are made of ordinary reinforcing bars, thereby suppressing the increase in material costs and labor during construction. Can do.
Therefore, an open space can be provided, and the filling property of the concrete is not impaired.

<Second Embodiment>
As shown in FIG. 2, the reinforced concrete column 1 of the second embodiment is a concrete hardened body 2 having a square cross section, main bars 3, 3... Arranged at each corner of the hardened concrete body 2, and a reinforced concrete column. It consists of reinforcing bars 4, 4,... Arranged at the center of 1 in the longitudinal direction.

The reinforcing bar 4 of the present embodiment is composed of a high-strength reinforcing bar. In this embodiment, USD685 is used as the reinforcing bar 4.
On the other hand, the main bars 3 are made of ordinary reinforcing bars. In this embodiment, SD490 having a yield point lower than that of USD685 is used as a normal reinforcing bar (see FIG. 5).
In addition, the kind of reinforcing bar which comprises the main reinforcement 3 and the reinforcement reinforcement 4 is not limited.

  The reinforcing bars 4 are arranged (attached) along the inner side of the main bars 3 as shown in FIG. That is, the reinforcing bars 4 are arranged in an area surrounded by the four main bars 3, 3,... (A rectangular area having the four main bars 3, 3,. In the present embodiment, as with the main reinforcing bar 3, four reinforcing bars 4, 4,.

  The reinforcing bar 4 of this embodiment has a length of about 40% of the length of the main bar 3. The length of the reinforcing bar 4 is not limited, and may be the same length as the main bar 3, for example.

The reinforcing bars 4 are arranged so that the height position of the axial middle point coincides with the height position of the axial middle point of the reinforced concrete column 1.
Further, in the present embodiment, the separation distance (opening) between the main reinforcement 3 and the reinforcing reinforcement 4 can be ensured to be 25 mm. In addition, the magnitude | size of the separation distance (opening) of the main reinforcement 3 and the reinforcement reinforcement 4 is not limited, If it is an inner side, you may touch the main reinforcement 3 and a planar view column center may be sufficient as it.

The high-strength reinforcing bar 3a is arranged at a location where deformation due to buckling increases during a fire.
In the reinforced concrete column 1, the temperature of the member surface concrete at the time of fire rises earlier than the center of the cross section, and as a result, the compressive strength and Young's modulus of the member surface concrete decreases (see FIGS. 5 and 6). The force applied to the main muscles arranged in the section increases. When the stress applied to the main bar surpasses the yield point and the main bar yields, the Young's modulus of the main bar suddenly decreases and the Young's modulus of the entire member decreases, so that the bending rigidity of the entire member significantly decreases.

  Therefore, the reinforcing bar 4 is attached to the main bar 3 to raise the yield point of the main bar 3 so that it does not yield in the event of a fire.

  According to the reinforced concrete column 1 of the present embodiment, the reinforcing bars 4 are arranged in the central part, and the stress applied to the central main part 3 is reduced. Therefore, it is difficult for the main bars to yield in the central part, and some of the main bars yield. Even so, a significant reduction in the buckling strength of the entire member can be suppressed (see FIG. 4C). By placing reinforcing bars in the main reinforcement only at and near the location where deformation is large when buckling, initial reinforcement and uneven heating of the reinforced concrete long column is possible without reinforcing reinforcement throughout the length of the column. It is possible to reduce the influence of the secondary bending due to the reduction of the buckling strength.

Moreover, since the reinforcing bar 4 (high-strength reinforcing bar) is adopted only in the central part of the reinforced concrete column 1, the cost can be reduced. Since the reinforcing bar 4 is arranged inside the main bar 3, the temperature rise is slower than that of the outer main bar, so that the compressive strength and Young's modulus are slow and the buckling strength can be lowered. It is not necessary to increase the cross-sectional area of the reinforced concrete column 1 to ensure the length.
Therefore, an open space can be provided, and the filling property of the concrete is not impaired.

When the main reinforcing bar 3 is formed by connecting the high-strength reinforcing bar 3a and the normal reinforcing bars 3b and 3b (see FIG. 3B), the reinforcing bar 4 is located at the center in the longitudinal direction of the reinforced concrete column 1 on the main reinforcing bar 3 made of the normal reinforcing bar. In the case of being attached to the part (see (c) of FIG. 3), the buckling strength at the time of fire was calculated.
In addition, as a comparative example, when the main bar 3 is composed of high-strength reinforcing bars (USD685) over the entire length (see FIG. 3A), reinforced concrete in which only main bars made of ordinary reinforcing bars (SD490) are arranged. For the column 1 as well, the buckling strength at the time of fire was calculated.

In this calculation, as shown to (a)-(c) of FIG. 3, it performed about the reinforced concrete pillar of the cross section square whose one side is 250 mm.
The specific calculation method is as follows.

First, the cross section of the reinforced concrete column 1 is modeled as an assembly of 196 elements.
Next, the thermal stress analysis of the reinforced concrete column 1 to which the assumed axial force is given is performed, and the instantaneous Young's modulus of each element at time t after the start of heating is calculated.
Strain relationship and axial stress in the axial force concrete column 1 given as the external force (axial force strain epsilon _sigma) is different for each temperature.

Further, in the reinforced concrete column 1, a thermal expansion strain ε _th accompanying thermal expansion and a transient strain ε _tr which is a contraction strain per unit stress are generated.
For example, when the transient strain ε _tr is 400 ° C., the shrinkage strain per unit stress is about 1.5 × 10 ² μ / (N / mm ² ), so that the axial stress is 20 N / mm ² . For example, 1.5 × 10 ² × 20 = 3.0 × 10 ³ μ.

The axial force strain ε _σ obtained from the relationship between axial force (stress) and strain, thermal expansion strain ε _th generated in the element due to thermal expansion during fire, and transient strain ε generated in the element due to compressive force and temperature during fire _The strain with _tr added is the total strain ε _tot (Equation 1).

When the thermal expansion strain ε _th is added to the axial force strain ε _σ , the overall strain increases, and when the transient strain ε _tr is added, the overall strain decreases, so the total strain ε _tot of each element is the strain It becomes a shape that translates along the axis.

Subsequently, assuming that the total strain ε _{tot of} each element becomes equal (plane holding), the thermal stress analysis at time t is performed to calculate the axial force strain ε _σ of each element, and the axial force strain ε _σ The instantaneous Young's modulus E (t) corresponding to is calculated. When performing the thermal stress analysis, the data obtained in the temperature grasping operation S1 is used.
The instantaneous Young's modulus E (t) was obtained from a tangential gradient with a curve obtained from the relationship between stress and strain.

  Next, the total bending stiffness E (t) · I, which is the bending stiffness of the entire cross section, is calculated by integrating the bending stiffness of each element over the entire cross section.

  The bending stiffness is calculated by multiplying the instantaneous Young's modulus E (t) of each element by the sectional secondary moment I with respect to the centroid axis of the entire section.

  After calculating the bending rigidity E (t) · I of each element, the entire bending rigidity at time t is calculated by integrating the entire cross section using Equation 2.

Then, the total bending stiffness E (t) · I is substituted into the Euler buckling equation shown in Equation 3 to calculate the buckling proof strength P _E (t) at time t.
The buckling strength P _E (t) is calculated using the total bending stiffness E (t) · I and the buckling length l _k assumed in the design.

4A to 4C show the relationship between the elapsed time and the buckling strength.
As shown in FIG. 4 (a), when all the main bars were made of high-strength reinforcing bars, they were broken in 55 minutes, whereas when the main bars 3 were normal reinforcing bars over the entire length, they were broken in 41 minutes.
Therefore, by adopting a high-strength reinforcing bar as the main reinforcing bar 3, early yielding of the reinforcing bar can be suppressed.

  As shown in FIG. 4 (b), when all the main bars are ordinary reinforcing bars, they were destroyed in 41 minutes, whereas when the middle part of the main bars was replaced with high-strength reinforcing bars, it was destroyed in 48 minutes. It was. Therefore, even when only a part of the main reinforcing bar is replaced with a high-strength reinforcing bar, the early yielding of the reinforcing bar can be suppressed, and safety during a fire can be ensured.

  Furthermore, as shown in FIG. 4 (c), when all the main bars are normal reinforcing bars, they were destroyed in 41 minutes, whereas high strength reinforcing bars (reinforcing bars 4) were added inside the main reinforcing bars 3 made of normal reinforcing bars. When installed, it resulted in destruction in 52 minutes. Therefore, by arranging the reinforcing bars 4, it is possible to suppress early yielding of the reinforcing bars and to ensure safety in the event of a fire.

  Thus, according to the reinforced concrete column of each of the embodiments described above, it is possible to reduce the influence of secondary bending due to initial irregularity or uneven heating on the fire resistance time.

As mentioned above, although embodiment of this invention was described, this invention is not restricted to the said embodiment, In the range which does not deviate from the meaning of this invention, it can change suitably.
For example, the concrete constituting the hardened concrete body may be high-strength concrete or ordinary concrete.

1 Reinforced concrete pillar 2 Concrete hardened body 3 Main reinforcement 3a High-strength reinforcement 3b Normal reinforcement 4 Reinforcement reinforcement (high-strength reinforcement)

Claims (3)

A reinforced concrete column with main bars
The central portion of the main bar in the longitudinal direction is made of a high-strength reinforcing bar,
The other part of the main reinforcing bars is composed of ordinary reinforcing bars connected to the high-strength reinforcing bars.   The reinforced concrete column according to claim 1, wherein the high-strength reinforcing bar and the normal reinforcing bar are joined by friction welding. A reinforced concrete column with main bars
A reinforced concrete column, wherein a reinforcing bar is attached to the inside of the main reinforcing bar at a central portion in a longitudinal direction.

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