Y

FIGURE 20.24 Column tie details [37].

20.8.1.5.3 Transverse Reinforcement in Plastic Hinge Zones

Figures 20.23 to 20.25 illustrate the typical transverse reinforcement. For confinement in plastic hinge zones, the ratio of transverse reinforcement, ps, shall not be less than f0

D0 s

Ash Ash r

—- H--for rectangular sections

for circular sections

Acc = area of column core concrete, measured to the centerline of the perimeter hoop or spiral Ash = total area of transverse reinforcement in one layer into the direction of the applied shear h h

Spiral reinforcement

FIGURE 20.25 Column interlocking spiral details [37].

Spiral reinforcement

Interlocking bars

FIGURE 20.25 Column interlocking spiral details [37].

Ash = total area of transverse reinforcement in one layer perpendicular to the direction of the applied shear

B" = core dimension of tied column in the direction of the applied shear D" = core dimension of tied column perpendicular to the direction of the applied shear D' = center-to-center diameter of perimeter hoop for spiral fy = minimum specified yield strength of rebars s = vertical spacing of transverse reinforcement, not exceeding 100 mm Usf = strain energy capacity of transverse reinforcement (= 110 MPa) ashape = 12 and 15 for circular sections and rectangular section, respectively

To restrain buckling of longitudinal reinforcement in plastic hinges, the transverse reinforcement shall satisfy the following requirements:

For circular section where D is the diameter of the circular column, db is the diameter of longitudinal reinforcement bars being restrained by hoop or spiral, fy is the minimum specified yield strength of longitudinal reinforcement bars, fyh is the minimum specified yield strength of transverse reinforcement bars, Ab is the area of longitudinal reinforcement bars being restrained by hoop or spiral, Abh is the area of hoops or spiral or ties restraining the longitudinal steel, and s is the vertical spacing of transverse reinforcement restraining the longitudinal steel.

The transverse reinforcement shall be provided in the plastic hinge zones defined in Section 20.6.1.5. The spacing the transverse reinforcement shall meet

20.8.1.5.4 Joint Reinforcement

• Moment-resisting integral connections shall be designed to resist the maximum plastic moment.

• The principal tension stress pt and compression stress pc shall be calculated by

For rectangular section

where fh is the average axial stress in the horizontal direction within the plans of the connection under consideration (positive is compressive stress) (MPa), fh is the average axial stress in the vertical direction within the plans of the connection under consideration (positive is compressive stress) (MPa), and vhv is the average shear stress within the plans of the connection (MPa).

• The principal compression stress pc shall not exceed 0 . 25f'.

• When the principal tension stress pt < 0 . 29yf for circular columns or columns with intersecting spirals, the volumetric ratio of transverse reinforcement in the form of spirals or hoops to be continued into the cap or footing ps shall not be less than 0 . 29 yf /fh where fyh is yield stress of horizontal hoop/tie reinforcement in the joint.

• When the principal tension stress pt > 0 . 29 yf the additional reinforcement shown in Figure 20.26 shall be provided as follows:

• On each side column, vertical stirrups: Ajv = 0.16Ast.

• Inside joint: the required vertical tie: Ajt = 0.08Ast.

• Longitudinal reinforcement: Ajt = 0.08Ast.

• Column hoop or spiral reinforcement into the cap: ps > 0 . 4Ast/l2c.

where Ast is the total area of longitudinal steel anchored in the joint and lac is the length of column reinforcement embedded into the joint.

20.8.1.5.5 Plastic Rotation Capacities

In SDRs 3 to 6, the plastic rotational capacity shall be based on the appropriate performance level and may be determined from tests and a rational analysis. The maximum plastic rotational capacity 0p should be conservatively limited to 0.035 (0.05), 0.01, and 0.02 radians for life safety (for liquifiable pile foundation), operational performance, and in ground hinges, respectively. For the life safety performance, the plastic hinge of a column shall be calculated by

where

Nf = number of cycles of loading expected at maximum displacement amplitude for liquefiable soil, take Nf = 2 Lp = effective plastic hinge length

0 0

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