1.0.1 This code is formulated with a view to strengthening the concrete structure, ensuring technical reliability, safety and applicability, economic rationality and guaranteeing the quality.
1.0.2 This code is applicable to the design of reinforced concrete structures for buildings and general structures.
1.0.3 Before the strengthening, the concrete structure shall, based on types of buildings, be subjected to testing or appraisal in accordance with the current national standard “Standard for Appraisal of Reliability of Industrial Buildings and Structures” (GB 50144) or “Standard for Appraiser of Reliability of Civil Buildings” (GB 50292). When combined with seismic strengthening, the concrete structure shall also be subjected to seismic appraisal in accordance with the current national standard “Standard for Seismic Appraisal of Buildings” (GB 50023) or “Criteria for the Appraisal of Antiseismic Performance of Industrial Construction”(GBJ 117).
1.0.4 Strengthening design of the concrete structure shall not only meet the requirements of this code, but also comply with relevant current national standards.
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2 Terms and Symbols
2.1 Terms
2.1.1 Strengthening of structure
Measures such as reinforcement, partial replacement or adjustment of internal forces taken for load-bearing structures, structure members and related parts that are less reliable or requested by the Owner to improve reliability, so that they will meet the safety, durability and applicability specified by the design codes and requested by the Owner.
2.1.2 Existing structure member
The existing member before the strengthening of the structure.
2.1.3 Important structure
A load-bearing structure in a building with safety grade I.
2.1.4 General structure
A load-bearing structure in a building with safety grade II.
2.1.5 Important structure member
A load-bearing structure whose failure will affect or jeopardize the overall work of the load-bearing structural system.
2.1.6 General structure member
A load-bearing structure whose failure, an isolated incident, will not affect the overall work of the load-bearing structural system.
2.1.7 Structure member strengthening with increasing section area
A direct strengthening method of increasing the cross-sectional area of the existing structure member and adding steel bars to increase its bearing capacity and stiffness, or to change its natural vibration frequency.
2.1.8 Structure member strengthening with externally wrapped shaped steel
A strengthening method that enables to apply a common force to the framework welded of reinforced concrete beam, externally wrapped shaped steel and steel batten plate and constrain the existing structure member.
2.1.9 structure member strengthening with externally bonded reinforced material
A direct strengthening method of bonding the strengthening material onto the concrete surface of the existing structure member by applying a structural adhesive or a high-strength polymer modified cement mortar (“PMCM”) to form a composite cross section, so as to improve its bearing capacity and ductility Based on different strengthening materials, it can be divided into structure member strengthening with externally bonded shaped steel, structure member strengthening with externally bonded steel plate, structure member strengthening with externally bonded fibre reinforced polymer, structure member strengthening with wire rope mesh and polymer modified cement mortar layer, etc.
2.1.10 Structure member strengthening with wire wrapped
A direct strengthening method that enables to constrain the concrete of the strengthened compression member by winding the annealed steel wire, so as to improve its ultimate bearing capacity and ductility.
2.1.11 Structure member strengthening with externally applied prestressing
An indirect strengthening method that improves or adjusts the force of the existing structure and its member by applying external prestressing.
2.1.12 Embedded steel bar
One of the post-installed fastening methods used to embed the ribbed steel bars or full-thread screws into the substrate concrete with a special structural adhesive.
2.1.13 Structural adhesive
The adhesive that is used for bonding the load-bearing structural members and can withstand long-term design stress and environmental effects.
2.1.14 Fibre reinforced polymer (FRP)
A composite material, with a fibre-reinforced effect, formed by arranging high-strength continuous fibre in accordance with a certain rule, impregnating with an adhesive and then bonding and curing.
2.1.15 Polymer modified cement mortar
A cement mortar prepared by using a high molecular polymer as a modified material for enhancing adhesion properties The polymer modified cement mortar for load-bearing structure shall not only be capable of improving its own physical and mechanical properties, but also significantly enhance its capabilities to anchor steel bars and bond the concrete.
2.1.16 Effective cross-sectional area
The cross section after removing the weakened and failed parts such as hole, defect, rust layer, weathering layer, etc.
2.1.17 Design working life for strengthening of existing structure or its member
The intended working time without the need of re-testing or re-appraisal after the strengthening of structure or its member specified in the strengthening design.
2.2 Symbols
2.2.1 Properties of materials
Es0 - the elastic modulus of the steel bar of the existing structure member;
Es - the elastic modulus of newly added steel bar;
Ea - the elastic modulus of newly added shaped steel;
Esp - the elastic modulus of newly added steel plate;
Ef - the elastic modulus of newly added fibre reinforced polymer;
fc0 - the design axial compressive strength of the existing structure member concrete;
fy0, f′y0 - the design tensile and compressive strength of the existing structure member;
fy, f′y - the design tensile and compressive strength of newly added steel bar;
fa, f′a - the design tensile and compressive strength of newly added shaped steel;
fsp, f′sp - the design tensile and compressive strength of newly added steel plate;
ff - the design tensile strength of newly added fibre reinforced polymer;
ff,v - the design bond strength of the fibre reinforced polymer and the concrete;
fbd - the design bond strength of the structural adhesive;
fud - the design tensile strength of the anchor bolt;
εf - the design tensile strain of the fibre reinforced polymer;
εfe - the effective design tensile strain of the FRP circumferential hoop.
2.2.2 Action effect and bearing capacity
M - the design bending moment after the strengthening of the structure member ;
M0k - the standard initial bending moment under the original action on the checking cross section of the flexural member before strengthening;
N - the design axial force after the strengthening of the structure member;
V - the design shear after the strengthening of the structure member;
σs - the tensile stress of newly added longitudinal steel bar;
σs0 - the stress of the longitudinal tensile bar for the existing structure member or for the steel bar on the side under smaller compression;
σa - the stress of the tension leg for newly added shaped steel or of the leg under smaller compression;
εf0 - the lagged strain of the fibre reinforced polymer;
w - the deflection or prestressed camber of the structure member.
2.2.3 Geometric parameters
As0, A′s0 - the cross-sectional area of the steel bar in the tension zone and the compression zone of the existing structure member;
As, A's - the cross-sectional area of the steel bar in the tension zone and the compression zone of the newly added structure member;
Afe - the effective cross-sectional area of the fibre reinforced polymer;
Acor - the cross-sectional area of the concrete in the circumferential hoop;
Asp, A′sp - the cross-sectional area of the newly added tension steel plate and compression steel plate;
Aa, A′a - the cross-sectional area of the newly added tension leg and compression leg;
D - the drilling diameter;
h0, h01 - the effective height of the cross section before and after the strengthening of the structure member;
hw - the height of the web plate of the cross section of the structure member;
hn - the displacement depth of the concrete in the compression zone;
hsp - the vertical height of the bonded steel stirrup plate on the beam side;
hf - the vertical height of the bonded FRP stirrup plate on the beam side;
hef - the effective anchorage depth of the anchor bolt;
ls - the basic anchorage depth of the embedded steel bar;
ld - the design anchorage depth of the embedded steel bar;
ll - the tension lap length of the embedded steel bar.
2.2.4 Calculation coefficients
α1 - the ratio of the stress value in the rectangular stress diagram of the concrete in the compression zone to the design axial compressive strength of the concrete;
αc - the strength utilization coefficient of newly added concrete;
αs - the strength utilization coefficient of the newly added steel bar;
αa - the strength utilization coefficient of the newly added shaped steel;
αsp - the calculation coefficient introduced for preventing the concrete from splitting;
βc - the concrete strength influence coefficient;
β1 - the ratio of the height of the compression zone in the rectangular stress diagram to the height of the neutral axis;
Ψ - the reduction coefficient, correction coefficient or influence coefficient;
η - the amplification coefficient or improvement coefficient.
