The reinforcement in a reinforced concrete structure can be stressed mainly by the bonded anchorage between the reinforcement and the concrete, so the anchorage of the reinforcement is the basis for the stress of the concrete structure. If the anchorage fails, the structure will lose its load-bearing capacity and consequently lead to structural damage.
The anchorage length of reinforcement is generally defined as the total length of the reinforcement of a member such as a beam, slab or column extending into the support or foundation, and can be anchored in a straight line or in a bend. Bending anchorage lengths include straight and bending sections.
Calculation of the anchorage length of reinforcement According to GB50010-2010 8.3.1 of the Code for the Design of Concrete Structures.
When the tensile strength of the reinforcement is fully utilised in the calculation, the basic anchorage length of the tensile reinforcement (common reinforcement) shall be calculated in accordance with the following notation.
Lab = α x (fy/ft) x d where: Lab is the basic anchorage length of the tensile reinforcement; fy is the design value of the tensile strength of the anchorage reinforcement; ft is the design value of the axial tensile strength of the concrete; a is the profile factor of the anchorage reinforcement, 0.16 for light round reinforcement and 0.14 for ribbed reinforcement; d is the diameter of the anchorage reinforcement.
The anchorage length of the tensile reinforcement shall be calculated according to the anchorage conditions in the following formula and shall not be less than 200 mm: La = ξaXLab where: La is the anchorage length of the tensile reinforcement; ξa is the anchorage length correction factor.
When considering seismic resistance the seismic anchorage length of longitudinal tensile reinforcement shall be calculated according to the following formula: LaE = ξaEXLa where: LaE is the seismic anchorage length of longitudinal tensile reinforcement; La is the anchorage length of tensile reinforcement.
ξaE is the correction factor for the seismic anchorage length of longitudinal tensile reinforcement, which is 1.15 for seismic class I and II, 1.05 for class III and 1.00 for class IV. In addition, when the diameter of the reinforcement is greater than 25mm for HRB335 and HRB400, the anchorage length should be multiplied by a correction factor of 1.1.
In seismic zones, a further factor should be multiplied according to the seismic level: 1.15 for seismic level 1 and 2; 1.05 for level 3; and 1.0 for level 4. The anchorage length of the concrete reinforcement in compression is 0.7 times the anchorage length of the reinforcement in tension.
The above is the calculation method for the anchorage length of reinforcement. In the design notes section of the construction drawings, there are generally requirements for the anchorage length of reinforcement, which can be checked according to the requirements in the drawings.
Anchorage length specification for reinforcement: In the basic theory of concrete structures, the ultimate strain value of concrete limits the strength of the reinforcement to too high a level to play its full role (this is the theoretical basis for the concrete design and construction codes not to have Ⅳ reinforcement). Therefore, even for Ⅳ reinforcement, its strength design value can only be taken to 360N/mm2 (the same as Ⅲ reinforcement), and when used in axial tension and small eccentric tensile members can only be taken by 300N/mm2. Therefore, the anchorage length of reinforcement above Grade III can be taken as per Grade III reinforcement.
The plasticity and weldability of Grade Ⅳ reinforcement is worse than the new Grade Ⅲ reinforcement and is not suitable or economical for use in ordinary concrete structures.