JP6393170B2 - Design method for prestressed concrete beams. - Google Patents

Description

  The present invention relates to a method for designing a prestressed concrete girder.

Prestressed concrete beams (hereinafter referred to as PC beams) give compressive force to the concrete via tension materials such as PC cables and PC steel, so that the tensile force generated in the cross section when a load is applied to the beams is applied. It has a structure to suppress.
In order to apply such a PC beam to a large span frame, there is a case where a tension material is disposed at the lower part of the beam cross section (see, for example, Patent Document 1).
In this case, since it is effective to arrange more tension members at positions away from the center of the beam cross section, it is desirable to arrange the tension members in the lateral direction.
According to this PC beam, a large lifting effect due to the eccentric moment can be expected near the center in the beam axis direction due to the compressive stress of the tendon.

Japanese Patent No. 5325313

Conventionally, post-tension type PC beams are the mainstream for large-span girder beams, and post-tensioned precast prestressed concrete beams (when using precast construction methods during construction) (Hereinafter referred to as “PCaPC beam”) has been applied. In the design of the PCaPC beam, the response characteristics when an earthquake load is applied and the amount of decrease in tension on the beam end side are not necessarily considered.
Therefore, the cross section of the PC beam is designed as a PC beam over the entire length of the beam, and the cross section is designed as a PC beam where a considerable amount of compressive stress acts on the cross section even at the end of the beam. It was.
As a result, in the post-tension type PC beam end, in addition to the state in which compressive stress is always acting, a large earthquake load acts in the event of a large earthquake, and there is a possibility that the PC beam end will be damaged greatly. It was.
Therefore, if the pre-tension type PCaPC beam that does not introduce tension at the beam end face can be used, the above-mentioned problems can be solved, but there is an unclear point about the damage range on the PC beam end side during an earthquake, Moreover, because it is out of the scope of application in the current Building Standard Law, PCaPC beams have not been adopted for large beams, which are main structural members that resist seismic loads.
From such a viewpoint, an object of the present invention is to propose a method for designing a pretension type PCaPC beam as a large beam.

In order to solve such a problem, in the present invention, a pre-tensioned precast prestressed concrete girder is constructed with a reinforced concrete structure at the end of the beam where no effective tension is introduced, and the center of the beam other than the end of the beam. As a design method for prestressed concrete girder with prestressed concrete structure as a prestressed concrete structure, presumed cross-sectional shape of precast prestressed concrete girder, tension assumption and eccentricity distance of tension introduction position, and assumption of arrangement of main reinforcement and tension material A steel material placement step, a beam column structure including the precast prestressed concrete beam, and a frame model stress calculation step for calculating a shear force, a bending moment and an axial force at the time of long-term load action and short-term load action, Tension The first fixing length, which is the fixing length of the tendon at the time, and the second fixing length, which is the fixing length of the tendon after the long-term design load action or the tension length of the tension material after the design seismic load action set, and Bond Splitting test step for Bond Splitting test tendon during tensioning force introduced at the first fixing length, the pre-Symbol beam end is regarded as zero tension the second fixing length and reinforced concrete structures In addition, the other central part of the beam is a prestressed concrete structure, and the amount of deformation is calculated by multiplying the elastic deformation amount of the precast prestressed concrete beam calculated from the cross-sectional shape and the bar arrangement specification of the main reinforcement by the deformation increase rate. A stress / deflection test step for calculating and performing a stress / deflection test is provided . If the amount of elastic deformation of the beam calculated in the stress / deflection test step exceeds the allowable value, the cross-section assumption step and the steel material placement step are performed again, and the design conditions of the precast prestressed concrete beam are changed. The frame model stress calculation step, the bond split test step, and the stress / deflection test step are performed again. The two verifications at the design stage are not stipulated in the current standards and guidelines in the current design of pre-tensioned PCaPC beams approved by the Building Standards Act for small beams. This is a new design method focused on the use of large beams.
In the present invention, the bond splitting test is performed using the first fixing length, which is the fixing length provided at the beam end necessary for the design when the tension force is introduced.
Next, the central part of the beam where the tensile force is effectively introduced is a prestressed concrete structure (hereinafter referred to as “PC structure”), and the end of the beam other than the central part of the beam is referred to as a reinforced concrete structure (hereinafter referred to as “RC structure”). ) Stress / deflection test. In the stress / deflection test, the anchorage length that becomes longer due to cracks at the end of the beam due to seismic load is defined as the second anchorage length. In addition, the first fixing length and the second fixing length on the beam end side of the PCaPC beam can be evaluated based on many member experiments and theoretical studies.
As described above, the present invention uses the first anchoring length to perform the bond splitting test at the time of introduction of tension, and has both ends of the beam serving as the second anchoring length as RC structures, and the other central part. This is a design method of a composite prestressed concrete beam (hereinafter referred to as “composite PC beam”) characterized by performing a stress / deflection test on the beam as a PC structure.
According to this composite PC girder design method, the second fixing length which is the longer fixing length of the tension material fixing length after the long-term design load action and the tension material fixing length after the design seismic load action. The range of the beam end (RC structure) where the tension force is considered to be zero is set, and the beam end and the beam center are divided into sections to perform stress / deflection test of the concrete beam. Design according to the response characteristics after the seismic load action (design that does not cause a deflection failure) becomes possible.
Since the adhesion stress in the anchoring area is greatest when tension is introduced, the bond splitting test is performed for three anchoring lengths (when tension is introduced, after long-term design load action and after design earthquake load action). It is verified by using the fixing length (first fixing length) at the time of introducing the tension force among the fixing length of the tension material). In other words, if the bond strength considering the bond effect between concrete and reinforcing bars exceeds the average bond stress between the tension material in the first anchorage length and the concrete, even during long-term design loads and after design earthquake load effects Bond splitting does not occur in concrete.
Further, in the step of performing the stress / deflection test, for example, for the RC structure section and the PC structure section of the composite PC girder calculated from the cross-sectional shape and the reinforcement specification of the beam main reinforcement, the deformation increase rate is set to the elastic deformation amount, respectively. If the cross section design of the beam central portion and the beam end portion is performed by multiplying, the cross section design of each part of the composite PC large beam can be performed with higher accuracy.

  According to the design method of the PC girder of the present invention, it is possible to design based on the response characteristics of the pretension type PCaPC girder at the time of introduction of tension, long-term load action, and design earthquake load action. As a result, the PC girder can be rationally designed by designing the cross section of the PC girder separately into the PC structure section and the RC structure section. It should be noted that there is no problem even if a value longer than the second fixing length is adopted in the stress / deflection test.

(A) is an elevation view schematically showing a composite PC girder according to an embodiment of the present invention, and (b) is a sectional view of the same. (A) is a schematic diagram which shows the fixing length of the tension material at the time of the prestress introduction of a precast beam, (b) is the same sectional drawing. (A) is a schematic diagram which shows the fixation length of the tension material at the time of the long-term load effect | action of a composite PC girder and a design earthquake load effect | action, (b) is the same sectional drawing. (A) is an enlarged cross-sectional view schematically showing the relationship between the adhesion stress level and the adhesion strength within the precast beam anchoring length at the time of introduction of tension, and (b) is the result of long-term load action on the frame structure including columns. Bending moment diagram, (c) is a curvature distribution diagram when a long-term load is applied to the frame structure.

As shown in FIG. 1, the composite PC girder 1 according to the embodiment of the present invention is a girder laid across between the left and right columns 3 and 3 (inside the columns 3 and 3). At the joint portion between the composite PC large beam 1 and the columns 3 and 3 (upper portions of the columns 3 and 3), the cast-in-place concrete is placed so that the main beam bars 4 and 5 protruding from both ends of the composite PC large beam 1 are wound. The composite PC girder 1 may be placed on the left and right columns 3 and 3 as a full precast member.
In the design method of the composite PC girder of this embodiment, the beam center portion 11 of the beam 1 is designed as a PC structure section, and the other beam end portions 12 and 12 are designed as an RC structure section. In addition, the PC structure section of the beam central portion 11 is a portion where the tension force by the pretension method is effectively applied.
As shown in FIG. 1 (b), the composite PC girder 1 is rectangular in cross-sectional view, and is composed of a so-called half precast member that includes a precast portion 13 and a spot casting portion 14. The composite PC girder 1 may be a full precast member.
The precast portion 13 is a concrete member having a rectangular cross section, and a plurality of lower main bars 4, 4,... Arranged at the bottom of the cross section of the composite PC large beam 1 along the axial direction, and at predetermined intervals. Are provided with a stirrup 6 wound around the lower main muscles 4, 4, 4, and a plurality of tendon members 2, 2,.
The cast-in-place portion 14 is formed so as to cover the precast portion 13 and has a height equivalent to the thickness of the slab 7. In addition, the thickness of the spot striking part 14 is not limited.
The cast-in-place portion 14 is formed by placing concrete on the upper surface of the precast portion 13, and upper main bars 5, 5... Are arranged along the axial direction of the composite PC large beam 1. It is formed with the stirrups 6 and 6 protruding from the upper surface of the portion 13 being wound. In the present embodiment, the concrete of the spot casting portion 14 is placed simultaneously with the slab 7. The spot casting portion 14 may be placed separately from the concrete of the slab 7.

The composite PC girder design method of this embodiment includes a cross-section assumption step, a steel material placement step, a frame model stress calculation step, an adhesion split verification step using the first fixing length, and a stress / deflection test using the second fixing length. Has steps.
In the cross-section assumption step, the cross-sectional shape of the composite PC girder 1, the tension force, and the eccentric distance of the tension force introduction position (tension material) are assumed.
The cross-sectional dimensions, the introduction tension and the eccentric distance of the composite PC girder 1 are set so that the stress at the upper and lower edges of the concrete section when the tension is introduced and when a long-term load is applied does not exceed the allowable stress.
The cross-sectional design is performed so that the cross-section has sufficient strength against the bending moment when a long-term load is applied. At this time, the allowable stress level of the concrete is calculated in accordance with, for example, PC standards ("Prestressed Concrete Design and Construction Standards / Explanation (1998)", Architectural Institute of Japan). The allowable stress levels of the main bars 4 and 5 and the stirrup 6 are calculated in accordance with, for example, the RC standard (“Reinforced Concrete Structure Calculation Standards / Description (2010)”, Architectural Institute of Japan).
The prestressing force used for the design is calculated using the prestress effective rate η = 0.80 in the pretension method shown in the PC standard.
It should be noted that, as necessary, the decrease in tension generated in the production stage and the decrease in tension over time are taken into consideration. Moreover, you may calculate the effective tension | tensile_strength which considered the decline amount by the method of using the effective rate shown by the said PC standard.

In the steel material placement step, it is assumed that the main bars 4 and 5 and the tension material 2 are placed.
In this embodiment, an RC structure is designed at the beam end for the seismic load. Four lower main bars 4, 4,... Are arranged (arranged) so as to be parallel to the bottom surface of the composite PC large beam 1. In addition, the number and arrangement | positioning of the lower main muscle 4 are not limited. For example, two-stage bar arrangement may be used. Moreover, although a deformed reinforcing bar is used in this embodiment, the lower main reinforcing bar 4 is not limited to a deformed reinforcing bar, and may be a screw reinforcing bar or a steel bar, for example.
The tendon 2 is disposed at a position where the eccentric distance assumed in the cross-section assumption step is secured. In the present embodiment, the seven tendon members 2, 2,... Are juxtaposed so as to be parallel to the bottom surface of the composite PC large beam 1. The number of tendons 2 is not limited. Although the material which comprises the tendon 2 is not limited, PC steel strand is used in this embodiment. A fixing member may be formed at the end of the tendon 2 as necessary.
The space between the tendons 2 is 1.5 times the nominal diameter or more than the maximum dimension of the coarse aggregate. Further, the clearance between the tendon 2 and the lower main muscle 4 is 1.5 times or more the nominal diameter of the lower main muscle 4.

  In the frame model stress calculation step, the beam column frame with the composite PC girder 1 is converted into a frame model, and each stress (shearing force, bending moment, axial force) at the time of long-term (vertical) load action and short-term load (earthquake force) action Is calculated (see FIG. 4B).

In the bond splitting test step using the first fixing length, the first fixing length l _d1 that is the fixing length of the tendon material 2 when the tension force is introduced is set (see FIG. 2).
In the present embodiment, the first fixing length l _d1 is set based on the result (see Table 1) obtained by the tension introduction experiment by the full-scale cross section test. When the design conditions are different, the set value of the first fixing length l _d1 is not limited to this.
Tensile force introduction test by full-scale cross section test is, for example, “Structural experiment of pretension type PCaPC large beam using large diameter strand” (The Architectural Institute of Japan, Vol. 77, No. 672, pages 265-272, (February 2012).
In the bond splitting test, for example, as shown in Equation 1, it is confirmed whether or not the ratio of the _bond strength τ _bu and the average bond stress τ _b within the first fixing length can be 1.0 or more. That is, if the adhesion strength τ _bu is greater than the adhesion stress τ _b , the adhesion splitting failure will not occur. In this embodiment, since it is assumed that a calculation formula (for example, an RC strength bond strength formula) including a safety factor is used for the _bond strength τ _bu , in this example, it is 1.0 or more. However, the present invention is not limited to this. For example, a large safety factor of 1.2 or more may be expected.
By doing so, adhesion splitting can be prevented by the concrete tensile resistance and the stirrup tensile resistance (see FIG. 4A).
Moreover, it can confirm that the necessary perforation dimension is ensured about the tension material arrange | positioned in a line.
In the above bond splitting test, when the ratio of the _bond strength τ _bu and the bond stress degree τ _b cannot be ensured to be 1.0 or more and bond split fracture is assumed, the design condition is changed again, A cross-section assumption step, a steel material placement step, and a frame stress calculation step are sequentially calculated, and a design is made so as not to cause bond splitting fracture.
If the gap between the tension members is more than three times the nominal diameter of the tension member, which is accepted for small beams in the current standards and guidelines, and the concrete design standard strength is 30 N / mm ² or more, adhesion It is not necessary to perform the split test.

In the stress / deflection test step using the second anchorage length, the anchorage length of the tendon after the design seismic load action (the anchorage length l _d3 after the seismic load action) is set as the second anchorage length (see FIG. 3). ).
In the present embodiment, the fixing length l _d3 after the seismic load action is set based on the results of experiments performed on the full-scale test specimen (see Table 2). When the design conditions are different, the fixing length l _d3 after the seismic load is not limited to this set value. The fixing lengths shown in Table 2 are values that allow for a safety factor in the fixing lengths obtained by alternating positive and negative alternating experiments for earthquakes with full-scale sections.
In addition, the positive / negative alternating repetition experiment by a full-scale cross section test is, for example, “Structural experiment of pretension type PCaPC large beam using large-diameter strand” (The Architectural Institute of Japan, Vol. 77, No. 672, 265-272). Page, February 2012).

In the stress / deflection verification step, the stress and deflection of the composite PC girder 1 are verified.
In the stress test, the beam center portion 11 into which the tension force is effectively introduced is used as the PC structure section, and the beam end portions 12 other than the beam center section 11 are used as the RC structure section.
Note that the beam end 12 is in a range having a length equal to or longer than the second fixing length (Table 2).
In the present embodiment, the anchoring length l _d3 after the seismic load action is in the range of the beam end 12, but the tensioning anchor anchoring length (long-term anchoring length l _d2 ) at the long-term load action is the beam end 12. It is good also as the range. In many cases, the anchoring length l _d3 after the seismic load action is greater than the anchoring length l _d2 during long-term loading. However, in buildings equipped with advanced seismic isolation devices, the anchoring length during long-term loading l _d2 > seismic loading In some cases, the fixing length l _d3 after the action is used, and in this case, the fixing length l _d2 under long-term loading is set as the second fixing length (range of the beam end portion 12) in the stress / deflection test step.
In the stress / deflection test step, for example, the elastic deformation amount M / EI (see FIG. 4B) of the RC structure section of the beam end portion 12 calculated from the cross-sectional shape and the reinforcement arrangement of the beam main reinforcement is used for the RC structure. deformation amount by multiplying the deformation increase rate phi _RC of calculated (shown in FIG. 4 (c) refer) to conduct a cross-sectional design.
Similarly, the PC structure section of the beam center part 11 calculates the deformation amount by multiplying the deformation increase rate phi _PC for PC structure elastic deformation M / EI (shown in FIG. 4 (c) refer) and the cross-sectional design Do.
Here, the deformation increase rates φ _PC and φ _RC are coefficients for adjusting an increase in deformation caused by a long-time load.
If the calculated deformation amount of the beam exceeds the allowable value (impeding the use of the structure), design the cross section assumption step and the steel material placement step again, and change the design conditions of the composite PC girder, Again, a frame model stress calculation step, an adhesion split verification step, and a stress / deflection verification step are performed.

According to the design method of the composite PC girder of this embodiment, the response characteristics of the composite PC girder 1 at the time of pre-stress introduction, long-term load and seismic load design are evaluated as a design method based on experimental research and the like. It becomes possible, and it is possible to realize a column beam structure combining a composite PC girder and a column.
Moreover, in order to set the RC structure section of the beam end 12 based on the anchoring length of the tension material at the time of long-term load or after the design seismic load action, and to perform the stress test of the concrete beam with the beam end 12 as the RC structure, It is possible to design according to the response characteristics at the time of long-term load and after design seismic load action.
In addition, in order to perform a deflection test by multiplying the elastic deformation amount of the composite PC large beam calculated from the cross-sectional shape and the bar reinforcement specification by the deformation increase rate of each section of the beam central portion 11 and the beam end portion 12, The cross-section design for each section of the composite PC girder can be performed with higher accuracy.
By dividing the composite PC large beam body into a PC structure section and an RC structure section and designing a cross section, the composite PC large beam body can be reduced in cross section.

Although the embodiment of the present invention has been described above, the present invention is not limited to the above-described embodiment, and the above-described components can be appropriately changed without departing from the spirit of the present invention.
The present invention relates to pre-tensioned PCaPC beams by performing “adhesion split test” and “stress / deflection test” which are not defined at the time of design by current standards and guidelines, etc. It is a design method that avoids adhesion failure and can maintain the function of the building even when the adhesion performance of the tendon material has deteriorated over the long term or after experiencing a large earthquake, and enables the application of large beams of pre-tensioned PCaPC beams. .
Moreover, although this specification demonstrated as a PCaPC member which uses a strand from PC steel as a tension material, a tension material is not limited to this, PC structure member which uses a PC steel rod and a high-strength reinforcing bar as a tension material In this case, the member and the frame structure to which the member is attached can be designed by two design tests performed using the same design concept, that is, the first fixing length and the second fixing length.

DESCRIPTION OF SYMBOLS 1 Composite prestressed concrete beam 11 Beam center part 12 Beam end part 13 Precast part 14 Site cast part 2 Tensile material 3 Column 4,5 Main reinforcement 6 Star wrap 7 Slab

Claims (1)

A pretension type precast prestressed concrete beam design method,
A cross-section assumption step that assumes the cross-sectional shape of the precast prestressed concrete beam, the tension force, and the eccentric distance of the tension force introduction position;
A steel placement step that assumes the placement of main bars and tendons;
A frame model stress calculation step for calculating a shear force, a bending moment and an axial force at the time of a long-term load action and a short-term load action by modeling the beam column frame including the precast prestressed concrete girder,
The first fixing length, which is the fixing length of the tendon when the tension force is introduced, and the second fixing length, which is the fixing length of the tendon after long-term loading or after the action of the seismic load An adhesion split test step for performing an adhesion split test of a tendon material at the time of introduction of the tension force in the first fixing length ;
Wherein with the second fixing length tension was zero and all the beam end and reinforced concrete structures, as prestressed concrete structures the beam center portion of the otherwise be calculated from the reinforcement specification of the said cross-sectional shape main reinforcement And a stress / deflection test step of calculating a deformation amount by multiplying the elastic deformation amount of the precast prestressed concrete girder by a deformation increase rate and performing a stress / deflection test,
If the elastic deformation amount of the beam calculated in the stress / deflection test step exceeds an allowable value, the cross-section assumption step and the steel material placement step are performed again, and the design conditions of the precast prestressed concrete girder are changed to re-frame the frame. A method for designing a prestressed concrete girder, comprising performing a model stress calculation step, an adhesion split verification step, and a stress / deflection verification step .

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