Designers generally pay more attention to the design of transfer beam, but through calculation, it is found that in some cases, the reinforcement of transfer beam seems to change as much as expected, in some cases, the reinforcement of transfer beam changes little, and even in some cases, the reinforcement is basically unaffected whether the transfer beam is defined or not. Designers will have a series of doubts. How does the software realize the relevant requirements of the code for reinforcement change and internal force adjustment and amplification of transfer beams? Moreover, the internal force adjustment of transfer beam is not completely consistent with the anti-code, high-code and special-shaped column code. According to which code is the software implemented? Why is there little difference between defining transfer beam and steel bar?
Based on the internal force adjustment requirements of transfer beam in various codes, this paper analyzes the similarities and differences between codes, explains the internal force amplification and special reinforcement design of transfer beam by software, and shows the design details of transfer beam for designers.
2. Requirements and procedures for internal force adjustment of transfer beam
(1) The internal force adjustment requirements of the transfer beam violate the specifications.
3.4.3 and 3.4.4 Structures with discontinuous vertical lateral force resisting members are classified as vertical irregularities. For the horizontal transfer members in this structure, the code requires that the earthquake internal force be multiplied by the increase factor 1.25~2.0, as shown in figure 1.
Note that the internal force adjustment coefficient of transfer beam is a range, which cannot be accurately implemented in design. To adjust the internal force of transfer beam, it is necessary to refer to the relevant requirements of other codes.
Looking at the seismic grade of frame-supported transfer structure in the code, as shown in Figure 2, we can see that the seismic grade of frame-supported frame is only Grade I and Grade II, and there are no other seismic grades. Therefore, the seismic internal force amplification of horizontal transfer members is only Grade I and II.
② Adjustment requirements for high gauge of transfer beam.
The seismic internal force amplification factor of transfer beam in frame-supported transfer structure is as follows, as shown in Figure 3:
According to the specification 10.2.4, the specification 65438 gives the specific amplification factor for the internal force amplification of the transfer beam, which can be implemented in the design. Code 65438 only gives the amplification factor of transfer beams with special first, first and second grades, and there is no third and fourth seismic grade.
In the high code, as shown in Figure 4 and Figure 5, it can be seen that the frame-supported shear walls in some frame-supported shear wall structures have only special first, first and second levels, but no third and fourth levels. Therefore, there is no internal force amplification in the third and fourth transfer beams in the high code, and the special transfer members have more internal force amplification requirements than those in the anti-code.
③ Adjustment requirements of special-shaped column regulations for transfer beam.
Article 4 of Code for Special-shaped Columns (3.2.5) requires that the seismic design of irregular special-shaped columns should be consistent with the seismic internal force of horizontal transfer members transmitted by members when the vertical lateral force resisting members are discontinuous, and the amplification factor of 1.25~ 1.5 should be multiplied according to different situations and conditions.
The internal force adjustment coefficient is almost the same as that required by the seismic code, both within a range, but the specific range of values is slightly different from that required by the seismic code of 1.25~2.0.
Appendix A of Code for Special-shaped Columns puts forward special requirements for special-shaped column structures with transfer stories at the bottom. The transfer member of special-shaped column structure with transfer floor at the bottom should be beam; In the seismic design, the seismic grade of the transfer beam and the column under the transfer beam of the special-shaped column structure with transfer floor at the bottom should be raised to the first level according to the relevant provisions of Article 3.3. 1 of this code.
In addition, section 3.3. 1 of the code for special-shaped columns gives the seismic grade of frames and frame-shear walls, as shown in Figure 6 below.
According to the seismic grade of the above-mentioned special-shaped column structure, a transfer beam with three seismic grades will appear if there is column replacement in actual design. In this case, it is necessary to amplify the internal force of the transfer beam according to the amplification factor of code 1.25~ 1.5. However, it should be noted that the amplification factor of the diverted beam needs to be specified by the designer.
3. The software performs the relevant adjustment of the transmitted light beam.
Based on the above three codes, it can be seen that the internal force amplification factor of transfer beam is clearly required in the high code, and the internal force amplification factor of horizontal transfer member in the other two codes is a range. Therefore, in the process of program execution, the internal force of transfer beams with special seismic grade I, I and II will be automatically amplified according to the requirements of high codes.
For the third and fourth transfer beams, even if the transfer beams are defined, the program will not enlarge the internal force by default. Although there are no horizontal transfer members with seismic grade 3-4 in anti-code and high-code, the seismic grade of transfer members in this kind of structure may be grade 3 because of the special-shaped column structure with transfer floor at the bottom in the special-shaped column code. At this time, the seismic effect of transfer members should be amplified according to the code of special-shaped columns. Or some frame structures are supported by beams, and designers want to define beams as transfer beams and consider certain amplification. At this time, it is necessary to manually enlarge the beams with seismic grade III and IV.
The PKPM program provides the following options as shown in Figure 7 for designers to fill in the corresponding amplification factors, so as to amplify the internal forces of the three-stage and four-stage conversion members under the above circumstances. The program will automatically amplify the internal force when the horizontal seismic effect amplification coefficient of the conversion components (level 3 and level 4) is filled in.
4. Manually check the internal force adjustment of the transfer beam.
① Automatic amplification of internal force adjustment of transfer beam (special primary and secondary)
The height of a 7.5-degree frame-supported transfer structure is more than 60m, and the structural transfer story is in five floors. The seismic grade of the shear wall in the non-bottom strengthening area is Grade II, and the seismic grade of the frame-supported frame is Grade I ... Figure 8 below shows the 3D drawing of the project and the arrangement of beams, columns and walls of the fifth floor transfer floor.
After the calculation, select the calculation result of one of the transfer beams and output the relevant calculation information of the beam. Fig. 9 shows the position and geometry information of the beam, fig. 10 shows the relevant adjustment coefficient information and attribute information output by the beam, and fig. 1 1 shows the internal force calculation result of the beam under a single working condition. Figure 12 shows the combined internal force and reinforcement results of the beam output.
According to the manual inspection of the frame-supported transfer beam, it can be seen from the results of the frame-supported transfer beam in Figure 12 that the corresponding combination number of the I end of the beam is 49, and the specific composition of this combination is shown in Figure 13 below:
Manually check the bending moment combination at the I end of the transfer beam, as shown below:
m = 1.2 *(856.09)+0.6 * 124.78-0.28 *(237.0 1)- 1.3 *(949.88)= 2403.383 kn·m，
The results of manual check are completely consistent with those calculated by software.
From the combination process, it seems that the internal force of the transfer beam has not been adjusted, but in fact, the internal force of the single working condition output by earthquake in the component information has been adjusted according to the specification, and the internal force of the transfer beam involves shear-weight ratio, weak layer and frame-supported beam and has been enlarged and adjusted. Therefore, it is necessary to check the internal force of earthquake action before and after adjustment in the software and see the adjustment process.
Figure 14 shows the comparison of seismic internal forces of the transfer beam before and after adjustment under the positive eccentric earthquake in Y direction. It can be seen that the bending moment at the I-end of the transfer beam is-391.13kn m before adjustment and-949.99kn m after adjustment, and all adjustments have been considered in the adjusted earthquake internal force output of the beam. The adjustment coefficient X of the weak layer has been taken into account in the internal force of the beam end under earthquake, and the Y direction is 1.25, the shear-weight ratio direction is 1.2 1, and the transfer beam is 1.6.
Under the positive eccentric earthquake in Y direction, the transfer beam has three adjustment coefficients, and the comprehensive adjustment coefficient is:1.25 *1.21*1.6 = 2.42. Therefore, according to this adjustment coefficient, the internal force after adjustment is: 2.42 * 391.48667.68688686886
(Note: As the shear-weight ratio adjustment coefficient here is the result of rounding, the actually calculated shear-weight ratio adjustment coefficient has four decimal places: 0.0237/(6080/311288.73) =1.2135. )
Therefore, the accurate value for calculating the adjusted internal force should be:
1.25 *1.2135 *1.6 * 391.13 = 949.3 kn m, which is completely consistent with the software calculation results.
From the above check, it can be seen that the program will automatically consider the internal force adjustment of earthquake action for the special type I, type I and type II transfer members required by the code, and the amplification factor does not need human intervention.
(2) manually specify and manually check the internal force adjustment of transfer beam (level 3 and level 4)
Also in the above project, the seismic grade of the transfer beam is modified to level 3, and the program outputs the relevant calculation result information of the beam. According to the component information in Figure 15, the seismic internal force amplification factor of the transfer beam is 1.0, that is, the internal force of the transfer beam program has not been adjusted.
It should be noted that as long as the transferred beam is limited, the characteristics of the beam become non-amplitude-modulated. Although the internal force of the transfer beam has not been adjusted, the reinforcement result is different from that of the undefined transfer beam.
If the horizontal seismic effect amplification factor of transfer beam with seismic grade III is defined manually under "Adjustment Information" in SATWE (the amplification factor in special-shaped column code is 1.25~ 1.5), as shown in figure 16, check the information output and reinforcement results of the beam.
As shown in figure 17, the information such as the properties, seismic grade and internal force adjustment of the corresponding beam are output, and the combined internal force and reinforcement results of the converted beam are output in figure 18.
The bending moment combination of I-end of transfer beam with three-level seismic grade is still No.49, and the bending moment of I-end of transfer beam before Y-direction positive eccentricity adjustment is 39 1. 13kN.m, shear-weight ratio adjustment of transfer beam 1.2 135, and weak layer adjustment 1.25.
1.25* 1.2 135* 1.4=2. 1236
After adjustment, the earthquake action is:
2.1236 * 391.13 = 830.6 kn.
Manually check the combined internal force at the I end of the beam as follows:
m = 1.2 *(856.09)+0.6 * 124.78-0.28 *(237.0 1)- 1.3 *(830.6)= 2248.32 kn·m，
Obviously, the results of manual inspection are consistent with those calculated by software.
The internal force amplification program of the three-level and four-level transfer members will not be automatically executed. If the internal force adjustment coefficient is defined artificially, the program will consider the internal force amplification of the beam under earthquake.
5. Whether to define the comparison of transfer beams.
For transfer beams with special seismic grade I, I and II, the definition of transfer beams will generally affect the strengthening results due to the different internal force amplification factors. However, for transfer beams with seismic grade Ⅲ and Ⅳ, the program does not amplify the internal force by default, so the definition of transfer beam should not affect the reinforcement calculation. Of course, the structural reinforcement of transfer beam is different from that of ordinary beam. Under the same conditions, check whether a beam is defined as a transfer beam and compare the calculation results.
As shown in figure 19, the basic information about the beam is displayed. The seismic grade of the beam is Grade II, and the internal force combination belongs to the combination of earthquake participation. According to the calculation of ordinary beam and transfer beam respectively, the results are shown in figure 19.
By comparing the reinforcement results of ordinary beam and frame-supported beam, it can be seen that the reinforcement calculation of frame-supported beam is very different from that of ordinary beam, and only a single reinforcement section is considered in the tensile reinforcement calculation of frame-supported beam support. When the seismic grade of ordinary frame beam is Grade I, II and III, it should be designed according to the double reinforcement section considering the compressed reinforcement. Even under the same bending moment, the calculated tensile reinforcement area of frame-supported beam support is larger than that of ordinary beam. At the same time, more importantly, if the transfer beam is defined, it is the non-amplitude modulation beam by default, and the moment amplitude modulation is not done, which leads to the difference of combined internal forces even if the internal forces of the transfer beam are consistent under a single working condition, and whether the transfer beam is defined or not will also lead to a slight difference in reinforcement.
6. Summary of internal force adjustment and reinforcement design of transfer beam
The design of internal force adjustment and graded reinforcement of frame-supported transfer structure is summarized as follows:
(1) The program will automatically adjust the seismic action of the transfer beam according to the requirements of the high code under the condition of meeting the requirements of anti-code and high code, because there are only three situations in the code: super-class, first-class and second-class.
(2) By default, the program does not automatically consider the internal force adjustment under earthquake action for transfer beams with seismic grade Ⅲ and Ⅳ; If the internal force amplification factors of the third and fourth transfer members are defined according to the special-shaped column specification or artificially, the program will also consider the amplification of the third and fourth transfer members.
(3) In the case of beam supporting column, in some cases, the designer defines the transfer beam. Although the third and fourth grades do not adjust the earthquake internal force by default, because the transfer beam is a non-amplitude modulation beam, this will lead to a slight change in the bearing and mid-span reinforcement of the beam compared with the undefined one.
④ Whether transfer beam is defined or not, the structural reinforcement of transfer beam is different from that of ordinary beam. Generally speaking, the structural reinforcement of transfer beam is greater than that of ordinary beam.
⑤ The reinforcement of transfer beam is quite different from that of ordinary beam. The transfer beam is only designed according to the single reinforcement section, the ordinary frame beam is designed according to the specification requirements, and the first, second and third stages are designed according to the double reinforcement section. This results in that under the same bending moment, the tensile reinforcement area of the bearing designed by transfer beam is generally larger than that designed by ordinary frame beam.
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