Keywords

1 Introduction

With the rapid development of China’s economy, social productivity and people’s living standards have been greatly improved. Nowadays, many old buildings, due to the technical and economic of construction restrictions, there are already hidden safety hazards or cannot continue to use the old building reinforcement design, functional transformation and upgrading, to enhance the comprehensive seismic capacity of housing and buildings is the current urgent need to solve the problem [1]. Old buildings in the use of function, load, structural form and other changes, the original housing structure of the test and identification is essential to the development of reinforcement design should be collected before the design program, consult, analyze the relevant information on housing, through the testing and identification to determine the weak points of the house and the bearing capacity of the components, through the modeling and calculation of the selection of safe and suitable, economically rational, technologically advanced reinforcement and transformation of the design program. Reinforcement construction is completed again after the completion of testing and identification of base modeling, according to the actual test results of the reinforcement construction quality assessment, verification of the rationality of the reinforcement design [2].

2 Project Overview

An office building houses built in the 1990s, the original as equipment room and office use, now the use of the building functions and room area has been unable to meet the owner’s use of the needs of the owner, and thus the owner intends to remodel all the office building and new housing one floor area to continue to use the house all the original information is missing, the house basic information questionnaire see Table 1 the original house one floor plan and reinforced after the transformation The building plan of the first floor of the original house and the reinforcement and remodeling after the removal of the wall, new columns and beams and plate design drawings are shown in Fig. 1.

Table 1. Basic Housing Questionnaire
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Fig. 1.
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Floor plan of the first floor of the original house and the design of wall removal and new columns and beams after reinforcement and remodeling

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3 Structural Inspection and Appraisal

In November 2021, an office building house was inspected and appraised [3].

3.1 Appearance Quality Inspection

Appearance quality inspection of the appraised house’s foundation foundation, superstructure, enclosure system and other areas with inspection conditions: according to the site field survey, the appraised house site has no bad geology and is suitable for construction. There is no corrosion, crispy alkali, loose and obvious subsidence defects in the part of foundation foundation, the house is slightly disconnected around the connection with the ground, the measured maximum tilt value of the house is 13 mm, the tilt rate is 1.37‰, the lateral displacement in the plane of the structure has not exceeded the requirements of the limits of the “Reliability Appraisal Standard for Civil Buildings” [4](limit: ≤ H/300) and the “Code for the Design of Building Ground Foundations” (limit: ≤ 4‰); the house is partially Water seepage occurs in the roof panel, the stucco layer falls off, the wall surface of the balustrade is seriously skinned by moisture, and the floor tiles are seriously hollowed out.

3.2 Testing of Component Material Properties

With reference to the provisions of Technical Procedures for Testing Concrete Strength by Rebound Method, 4 concrete members were sampled on the spot, and the presumed value of concrete strength of 2 members did not meet the specification requirements [5]; with reference to the provisions of Technical Standards for On-site Inspection of Masonry Works, 8 members were sampled on the spot, and the strength of the sampled masonry bricks were MU10 The strength grade of masonry bricks sampled is MU10, which meets the specification requirements; with reference to the Technical Procedures for Masonry Mortar Detection by Penetration Method, 8 members are sampled on site, and the strength conversion values of masonry mortar of all the sampled members do not meet the specification requirements. Specific test results are shown in Table 2.

Table 2. Material performance test results of components
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3.3 Detection of Reinforcement in Members

Referring to the provisions of Code for Quality Acceptance of Construction of Concrete Structural Engineering (GB 50204-2015), on-site sampling of 8 concrete members (4 ring beams + 4 floor slabs) of the reinforcement: member batch of reinforcement protective layer qualified point rate of 78.8%, does not meet the specification requirements of the batch of the qualified rate of 90%; member batch of reinforcement spacing to meet the specification requirements. With the picking method to check the diameter of 3 bars, the diameter of the measured bars meets the minimum requirements of the specification.

3.4 Appraisal Unit Safety Appraisal Ratings

Based on the actual inspection of the house, PKPM software JDJG module is used for modeling and calculation. Referring to the “Reliability Appraisal Standard for Civil Buildings”, the structural safety rating is made. Structural safety rating is divided into components, sub-units (foundation, upper load-bearing structure, load-bearing part of the enclosure system), and appraisal unit at three s layer by layer: (1) safety appraisal of masonry structural components, according to the four inspection items of load-bearing capacity, structure, displacement and cracks or other damages which are not suitable to continue to carry: empirically calculated, the walls of the house are under the action of normal load, and the ratio of resistance to effects of the limbs of the wall of the first floor and second floor is less than 1, and the compressive load-bearing capacity is not suitable to continue to carry. The ratio of resistance to effect is less than 1, and the compressive bearing capacity does not meet the specification requirements. The statistical results are shown in Table 3, and the results of the first-floor wall limb compressive bearing capacity checking are shown in left of Fig. 2; the structural items of the structure’s main load-bearing member construction are comprehensively evaluated as cu; the items of the deformation that is not suitable for continued load-bearing are comprehensively evaluated as bu; and the items of the cracks and other items are comprehensively evaluated as bu; (2) The subunit safety rating of the foundation is comprehensively evaluated as Bu, the safety appraisal rating of the upper load-bearing structure subunit is assessed as Du according to its structural bearing function, structural integrity and structural lateral displacement, and the load-bearing part of the enclosure system is assessed as Du; (3) In conclusion, the safety rating of the housing appraisal unit is assessed as Dsu, and the safety of the housing seriously fails to comply with the standard regulations for Asu, which severely affects the overall bearing, and the safety of the housing structure does not meet the subsequent normal use under the action of static loading. Under static load, the structural safety of the house does not meet the subsequent normal use requirements [6].

Table 3. Statistical table for wall limbs with insufficient compressive bearing capacity
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Fig. 2.
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Results of inspection for compressive bearing capacity and Seismic Calculation of first floor wall limb

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3.5 Seismic Identification

With reference to the relevant provisions of the Building Seismic Identification Standard, the house is located in Yunnan Province, built in the 1990s, the seismic intensity of 7º, the design of the basic seismic acceleration value of 0.10 g, the design of the seismic grouping of the third group, according to the seismic appraisal of class B masonry houses: seismic measures identification of structural columns set up, the house’s local dimensions, the strength of the mortar, the wall tension reinforcement does not meet the specification requirements. In the seismic load capacity calculation, the ratio of the resistance of the wall limb on the first floor of the house under seismic load (the percentage of the seismic load capacity of the wall is 8.4%) to the load effect is less than 1.0, and the seismic load capacity does not meet the requirements, and the results of the seismic calculation of the wall limb on the first floor are shown in the right of Fig. 2. In conclusion, the comprehensive seismic capacity of the house structure does not meet the standard requirements.

4 Reinforcement and Renovation Program Design

4.1 Engineering Characteristics

This project has a large remodeling surface and complex remodeling content, which is a typical comprehensive remodeling project. In addition to a variety of different reinforcing technology reinforcing trades, but also involves demolition, civil engineering, decoration, water, electricity and other specialties. Various types of work procedures are complex, more cross operations, how to coordinate between different floors, different components, different technologies, different processes, is the project’s major difficulties. Another tight schedule, spring and summer remodeling construction efficiency is low, the construction area adjacent to the residential construction noise, to a certain extent, increased the difficulties of strengthening and remodeling.

4.2 Reinforcement and Retrofit Program Design

Parameter value. The design adopts the standard value of uniform live load: 2.0 KN/m2 for functional rooms, 2.5 KN/m2 for corridors and stairwells, 2.5 KN/m2 for bathrooms, 0.5 KN/m2 for uninhabited roofs, and 0.3 KN/m2 for basic wind pressure. Floor constant load = plate weight + 1.5 KN/m2 decoration load, roof constant load = plate weight + 3.0 KN/m2 decoration load. Strength of materials: strength of brick and mortar is taken according to actual test value, strength of new concrete column is taken as design value C40, strength of high ductility concrete surface is taken as design value C30. Structural calculation parameters are taken as follows: seismic intensity 7º, design basic seismic acceleration 0.10 g, design seismic grouping group III, characteristic period 0.45 s, building site soil category II, ground roughness class B, class B ground floor. Frame structure building (subsequent service life 40 years), seismic defense category C, structural safety class II, structural importance coefficient 1.0.

Reinforcement program design. According to the owner’s demand for use and the results of the inspection and appraisal report, the structural form of the house is transformed from a brick-concrete structure to a ground-floor frame brick house structure, with the first floor using columns, beams, and walls for joint load-bearing. The original structure of the beam bottom reinforcement insufficient position using increased cross-section and sticky carbon fiber cloth way to reinforcement. The wall limbs with insufficient seismic load bearing capacity are reinforced by adding high ductility reinforced concrete slab walls. The demolition position of the wall is reinforced with additional reinforced concrete columns and new conversion beams. The position of wall end without structural column is reinforced by adding four sides of reinforced concrete surface layer to form structural column [7]. This project is the existing building reinforcement design, mainly based on the “Building Seismic Reinforcement Technical Regulations”, according to the above design ideas modeling calculation analysis, the results meet the expected assumptions. The sketch of the reinforcement design model is shown in Fig. 3.

Fig. 3.
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Sketch of house structural reinforcement design model

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(3) Reinforcement and remodeling content and method.

Remove part of the wall of 1–5/A-F axis on the first floor of the house, remove the wall and add new concrete columns with dimensions of 400 mm × 400 mm, 600 mm × 600 mm and other four models, and add new conversion beams with dimensions of 400 mm × 500 mm, 400 mm × 550 mm and other three models. The new columns and beams in the area of 1–5/A-F axes on the first floor, the location and reinforcement diagram are shown in Fig. 1. The new columns are added by adding new independent foundations, and the new columns are added by removing the walls and using the method of enlarging the original foundation cross section. The retained wall of the first floor is reinforced with additional high ductility reinforced concrete slab wall, with the thickness of 50 mm on one side; the wall of the second and third floors is reinforced with high ductility concrete slab wall, with the thickness of 20 mm on one side; the bottom of the beams and plates of the second and third floors, as well as all the staircases, are reinforced with sticky carbon fiber cloth. The contents and methods of house reinforcement and renovation are shown in Table 4, and the construction of the reinforcement site is shown in Fig. 4.

Table 4. House reinforcement and remodeling content and methods
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Fig. 4.
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Construction plan of the reinforcement site

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5 Results of House Inspection and Appraisal After Reinforcement and Remodeling

In April 2023, after the completion of strengthening and remodeling, the house was re-inspected and appraised, and no obvious quality defects were found in the exterior quality. The strength presumption value of the new concrete columns meets the design requirements, and the strength presumption value of the high ductility concrete surface layer meets the design requirements, and the test results are shown in Table 5. Modeling calculations under static load, checking the reinforcement of the first floor of the concrete members of the house, the stress ratio of the steel members, and the stress sketch of the lower flange stability check, the frame columns and beams did not show any over-reinforcement; checking the calculation of the axial compression ratio of the first floor of the concrete columns, the axial compression ratio was between 0.05 and 0.17, which was lower than the limit value of 0.05. 0.05–0.17, which is lower than the limit value of 0.9 and meets the specification requirements; the compressive bearing capacity of the wall limb and the height-thickness ratio of the second and third floors meet the specification limits.

Table 5. Concrete strength test result table
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Comprehensive testing indicators and results, with reference to the “Civil Building Reliability Appraisal Standards” to make the structural safety rating: normal use of housing appraisal unit safety rating for Bsu level, that is, the safety of the house is slightly lower than the standard provisions of the Asu level, is not yet a significant impact on the overall load bearing.

Referring to the relevant provisions of “Building Seismic Identification Standard”, the relevant parameters of reinforcement design and measured data are taken for modeling and calculation, and the seismic identification is carried out according to Class B ground floor frame brick house: the maximum height of the house, the number of floors, the maximum spacing of seismic transverse wall, the strength of the material, and the overall connecting construction of the house satisfy the requirements of verification of seismic measures of the specification. Seismic load capacity calculation, the house of the first floor of concrete structural members reinforcement to meet the requirements of the concrete column axial compression ratio is less than the normative limit, the second floor, the third-floor wall limb seismic calculation results are shown in Table 6. In summary, the house structure of the comprehensive seismic load capacity to meet the standard requirements.

Table 6. Seismic calculations of wall limbs at the second and third floors
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6 Conclusion

This paper comprehensively considers the old brick-concrete structure of the house itself problems, reinforcement goals, subsequent use of the life and the owner’s needs and other factors, an office building as an example, choose a reasonable reinforcement design program targeted to the house seismic structural measures do not meet the standard requirements, the comprehensive seismic bearing capacity is insufficient to enhance the use of functionality and other issues to deal with the house reinforced before the assessment of the safety level of the Dsu level, after the reinforced transformation to increase the safety level to Bsu level, effectively guaranteeing the normal safe use of the house to enhance the comprehensive seismic capacity and the use of functionality. The safety grade of the house was Dsu grade before reinforcement, and the safety grade was upgraded to Bsu grade after reinforcement and remodeling, which effectively guarantees the normal safe use of the house and improves the comprehensive seismic capacity of the house and the use function [8]. This project has a lot of transformation contents, complex reinforcement process, and many kinds of cross work, through the comparison of testing and appraisal data before and after reinforcement and transformation, it verifies that the reinforcement design scheme is reasonable and feasible, and it will provide reference for other similar projects in the future.