Keywords

1 Engineering Situation

The building is located in Fucheng District, Mianyang City, Sichuan Province. It was built in 1996. The building has 7 floors and the height of the house is 24.90 m. The height of the first and seventh floors is 4.2 m, and the height of the rest floors is 3.3 m. The building area is about 3500 m2.

2 On Site Inspection and Testing Analysis

In order to have a comprehensive understanding of the structural safety performance of the inspected structure, the original data of the inspected structure is first verified before testing, and then on-site testing is carried out on the structure based on the actual situation [1].

2.1 An Analysis of Engineering Structure Documents

Before on-site testing, investigated the usage history of engineering structures and relevant design and construction history data of the inspected structures [2]. The seismic fortification intensity of the building during design was 6°; the foundation is pile foundation, with pebble layer as the holding layer, and the characteristic value of bearing capacity of roadbed should not be less than 700 kPa; the main structure adopts C20 grade concrete. The longitudinal reinforcement of beams and columns adopts HRB335 grade reinforcement. The stirrup adopts HPB300 grade rebar. Prefabricated hollow slabs are used for floor and roof panels.

2.2 Check the Building Foundation

The construction site is flat, no adverse geological phenomena such as landslides, collapses, subsidence, or ground fissures were observed on site. On site inspection did not find any obvious settlement or adverse phenomena such as foundation cracks or local damage caused by settlement in the main structure foundation, there is no lateral displacement or crack caused by uneven settlement in its upper structure.

2.3 Detection of Structural Component

Component Section Dimension Inspection

Part of the concrete column components and concrete beam components were selected on site for Component section dimension inspection [3]. The detection values are shown in Tables 1 and 2.

Table 1. Inspection Data of Cross Section Dimensions of Concrete Column Components
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Table 2. Test Data for Cross Section Dimensions of Concrete Beam Members
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Component Reinforcement, Concrete Strength and Carbonation Depth Testing

On site, a steel bar detector was used to detect the quantity of steel bars in concrete components, and the test results showed that the steel bar configuration met the requirements for construction quality acceptance.

Since the building commissioner was in use and the site did not have the conditions for core drilling and sampling, the compressive strength of the concrete members of the main structure of the first floor of the project was tested by the rebound method [4].

In accordance with the “Technical standard for in-site inspection of concrete structure”(GB/T50784-2013), When the standard deviation of the inspection lot is unknown, the upper and lower limits of the presumptive interval of the eigenvalue (0.05 quantile) xk of the metrologically sampled inspection lot with 95% guarantee can be calculated according to the following formula:

$$ {\text{x}}_{{\text{k}}1} = {\text{m}} - {\text{k}}_{1} {\text{s}} $$
$$ {\text{x}}_{{\text{k2}}} = {\text{m}} - {\text{k}}_2 {\text{s}} $$

where xk1 is the upper limit of the eigenvalue (0.05 quantile), xk2 is the lower limit of the eigenvalue (0.05 quantile), m is the arithmetic mean of the sample, s is the standard deviation of the sample, k1 and k2 are the coefficients of the upper and lower limits of the presumptive interval, the values taken are shown in the values corresponding to the sample capacity in the 0.05 quartile value column in GB/T50784-2013 “Technical standard for in-site inspection of concrete structure” [5]. The evaluation results are shown in Table 3.

Table 3. Evaluation results of compressive strength of concrete elements of the first floor main structure (Mpa)
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Measurements of the carbonation depth of the reinforced concrete members were taken on site and the measurements showed that: The carbonation depth values ranged from 13.52 to 46.35 mm, and the combination of the appearance quality of the members and the detection of the protective layer thickness of the steel reinforcement shows that the carbonation depth of some reinforced concrete members was close to or exceeded the protective layer thickness of the steel reinforcement. [6].

3 The Security Analysis of the Struct

3.1 The Security Analysis of the Main Structural Components

Based on on-site measurement data and calculation results, some main components R/ γ0S ≥ 1.00, the concrete structural components of this part are evaluated as au level based on their bearing capacity level; Part of the main components R/ γ0S ≥ 0.95, the concrete structural components of this part are evaluated as bu level based on their bearing capacity level; Part of the main components R/ γ0S < 0.90, the concrete structural components of this part are evaluated as du level based on their bearing capacity level; on site inspection found that the construction of the concrete structural components of the building is reasonable, the connection method is correct, and there are no defects on the surface. Based on the construction situation, the safety level of the upper concrete structural components is evaluated as bu level; There is no obvious lateral bending or horizontal displacement observed in the concrete components, and the safety of the concrete components is evaluated as bu level based on the displacement or deformation that is not suitable for bearing; Some of the concrete members had force cracks of up to 0.5 mm, and the safety of the concrete structural components is evaluated as cu level based on the cracks.

3.2 Safety Assessment of Buildings

Based on the rating results of its foundation, upper load-bearing structure, and load-bearing sub units of the enclosure system, as well as other safety issues related to the entire building, the safety level of the building is rated as Csu level [7].

4 Anti-Overturning Checking

Based on the analysis of the safety testing results of the project structure, the anti overturning calculation is carried out [8]. During the verification calculation, referring to the original design drawings and based on the actual structural layout and dimensions, the SATWE2021 (V1.3.1 version) from China Academy of Building Sciences Beijing Gouli Technology Co., Ltd. is used for modeling and calculation.

Building width B = 49.8 m; Building height H = 27.9 m; Building length L = 20 m; If the aspect ratio of the building is H/B = 2.325 ＜ 4, the stress zone on the foundation bottom of this project should not exceed 15% of the area of the foundation bottom [8]. The foundation burial depth of this project is set at 2 m.

According to the calculation results of SATWE2021 (V1.3.1 version), the total mass generated by dead load:g = 4580.099KN; The total mass generated by live load:q = 325.352KN.

Bottom shear force and bending moment under wind load: Vx1 = 222.5KN; Vy1 = 824.0KN;Mx1 = 3721.5KN·m; My1 = 13275.6KN·m. The shear force diagram is shown in Fig. 1 and the bending moment diagram is shown in Fig. 2.

Fig. 1.
figure 1

The shear force diagram

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Fig. 2.
figure 2

The bending moment diagram

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Bottom shear force and bending moment under earthquake action: Vx1 = 840.8KN; Vy1 = 714.5KN；Mx1 = 13546.27KN·m; My1 = 11432.31KN·m.

Taking the burial depth of the building: C = 2 m, The overturning moment of the building Mov = VC + M, The calculation point of anti overturning moment is assumed to be the outer edge point of the foundation, and the calculation force of anti overturning moment is the representative value of the total gravity load, Then the anti overturning moment:Mr = GB/2,G = g + 0.5q = 4742.775KN. The calculation results are shown in Table 4.

Table 4. Anti overturning calculation
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From the above table, it can be seen that Mr. is greater than 3Mov, which meets the specification requirements, so there is no stress zone in the base [9].

5 Conclusions and Recommendations

  1. (1)

    Although the compressive strength of reinforced concrete members of the building meets the requirements, the carbonization depth of the members is large, and the protective layer of reinforcement of some members is too thin, which has a greater impact on the safety and durability of the structure, and it is necessary to take corresponding repair, reinforcement and maintenance measures to ensure that the subsequent use of the building requirements [10].

  2. (2)

    Under the existing structural system and existing loading conditions, the overall structural safety of the building is rated at Csu level, and there are major problems with the structural safety performance, which significantly affects the overall load bearing, and treatment measures should be taken to enable it to meet the due safety requirements.

  3. (3)

    Where the shear bearing capacity of the beam is insufficient, it is recommended to take the beam side paste steel plate or paste carbon fiber reinforcement. Where the bending capacity of beam is insufficient, it is recommended to take the bottom of the beam to paste steel plate or increase the cross-section reinforcement. For the column bending bearing capacity is insufficient, it is recommended to take the column outside the package angle or increase the cross-section reinforcement.