Reinforcement In Beams Parts

along the entire length of the bea (and not just in the critical regions). In this expression, fctm is the mean value of concrete tensile strength as defined in Table 3.1 of EC2 and fyk is the characteristic yield strength of the reinforcement.

To ensure that yielding of the flexural reinforcement occurs prior to crushing of the compression block, the maximum amount of tension steel provided, pmax, is limited to:

0.0018 fcd

Here, tsyd is the design value of reinforcement strain at yield, p' is the compression steel ratio in the beam and fcd and fyd are the design compressive strength of the concrete and design yield strength of the reinforcement respectively.

The development of the required local curvature ductility, p , is also promoted by specifying that the area of steel in the compression zone should be no less than half of the steel provided in the tension zone in addition to any design compression steel.

Since bond bettveen concrete and reinforcement becomes less reliable under conditions of repeated inelastic load cycles, no splicing of bars should take place in critical regions according to Clause 8.7.2(2) of EC2. All splices ust be confined by specially designed transverse steel as defined in Equations 5.51 and 5.52 of EC8.

nother area here particular attention needs to be paid to bond stresses is in beam/column joints of primary seismic frames, due to the high rate of change of reinforcement stress, generally varying from negative to positive yield on either side of the joint. To cater for this, the diameter of bars passing through the beam/column joint region is limited according to Equations 5.50a and 5.50b of EC8 Part 1. For DCM structures, these become:

dbL < 7.5.fctm / fyd)(1 + 0.8vd) / (1 + 0.5p / pmax) for interior columns (5.12)

hc dbL < 7.5.fctm / fyd)(1 + 0.8vd) for exterior columns. (5.13)

where dbL is the longitudinal bar diameter and hc is the depth of the column in the direction of interest.

HOOP (TRANSVERSE) STEEL

any of the detailing provisions in 8 revolve around the inclusion of transverse reinforcement to provide a degree of triaxial confinement to the concrete core of compression zones and restraint against buckling of longitudinal reinforcement. As confinement increases the available compressive capacity, in terms of both strength and more pertinently strain, it has enormous benefits in assuring the availability of local curvature ductility in plastic hinge regions. EC2 gives relationships for increased compressive strength and available strain associated with triaxial confinement, illustrated in Figure 5.6. These indicate that for the minimum areas of confinement reinforcement required at column bases and in beam column joints, the ultimate strain available would be between about ttvo and four times that of the unconfined situation, dependent on the effectiveness of the confinement arrangement, as defined later.

Column Beam Joint
Figure 5.6 Stress-strain relationships for confined concrete
Parts Beam Reinforcements
Figure 5.7 Transverse reinforcement in beams, from Eurocode 8 Part 1

The requirements set out in EC8 to achieve this through detailing of critical regions are briefly summarised below.

• Hoops of at least 6 mm diameter dbw must be provided.

• The spacing, s, of hoops should be less than the minimum of: hw/4;

• The first hoop should be placed not more than 50 mm from the beam end section as shown in Figure 5.7.

Hoops must have 10 bar diameters anchorage length into the core of the beam.

126 A. Campbell and M. Lopes 5.6.4.2 Columns

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