715 Walls

If tall walls (or shear walls) and combined walls (or core walls) subjected to axial load and bending behave like a column, the design procedures and formulas presented in the previous sections are generally applicable. The reinforcement detailing of wall differs from that of columns. Boundary elements, as shown in Figure 7.26, may be attached to the wall ends or corners to enhance moment capacity. The ratio pn of vertical shear reinforcement to gross area of concrete of horizontal section should not be less than rn = 0.0025 + 0.5 ^2.5 - ^ (ph - 0.0025) > 0.0025 (7.55)

The spacing of vertical wall reinforcement should not exceed (w/3, 3h, or 18 in. To prevent buckling, the vertical bars opposite each other should be tied together with lateral ties if the vertical reinforcement is greater than 0.01 the gross concrete area.

7.15.1 Shear Design of Walls

The general shear design procedure given in Section 7.12 for determining shear reinforcement in columns applies to walls. For walls in compression, the shear strength provided by concrete Vc may be taken as 2yf hd. Alternatively, Vc may be taken from the lesser of

In lieu of a strain compatibility analysis, the depth of walls d may be assumed to be 0.81«. Shear strength provided by the horizontal reinforcement in walls is also calculated by the equation Vs = Avfydls. The shear capacity of walls f Vn = f (Vc + Vs) should not be greater than f10^/f0hd.

The spacing of horizontal wall reinforcement should not exceed W5, 3h, or 18 in. The minimum ratio of horizontal wall reinforcement should be more than 0.0025 (or 0.0020 for bars not larger than No. 5). The vertical and horizontal wall bars should be placed as close to the two faces of the wall as cover allows.

7.16 Torsion Design

Torsion will generally not be a serious design issue for reinforced concrete structures if the structural scheme is regular and symmetrical in layout and uses reasonable member sizes. In building floors, torsion may need to be considered for edge beams and members that sustain large unbalanced loading. Concrete members are relatively tolerant of torsion. The ACI permits torsion design to be neglected if which corresponds to about one quarter of the torsional cracking capacity. For hollow sections the gross area of section Ag should be used in place of Acp. If an axial compressive or tensile force Nu exists, the the factored torsional moment demand Tu is less than torsion design limit becomes

If the torsional moment demands are higher than the above limits, the redistribution of torque after cracking may be taken into account, which occurs if the member is part of an indeterminate structural system. Hence, in torsion design calculations, the torsional moment demand Tu need not be taken greater than f4pffi(!p) (7 ■6o) If axial force is present, the upper bound on the design torque Tu is

7.16.1 Design of Torsional Reinforcement

The torsional moment capacity maybe based on the space truss analogy (see Figure 7.27). The space truss formed by the transverse and longitudinal reinforcement forms a mechanism that resists torsion. To be effective under torsion, the transverse reinforcement must be constructed of closed hoops (or closed ties) perpendicular to the axis of the member. Spiral reinforcement or welded wire fabric may be used.

To prevent failure of the space truss from concrete crushing and to control diagonal crack widths, the cross-section dimensions must be selected to satisfy the following criteria. For solid sections xfß-

V \bw d and for hollow sections

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