C-H5H5 Composite special shear walls (C-SSW)

H5.5a Ductile Elements

Coupling beams must be able to undergo substantial inelastic deformation reversals; therefore, coupling beams are designated as protected zones. Well-established guidelines for shear links in eccentrically braced frames need to be followed.

H5.5b Boundary Members

Concerns have been raised that walls with encased steel boundary members may have a tendency to split along planes 1 and 2 shown in Figure C-H5.3. Transverse reinforcement within a distance $2 h$, where $h$ is the width of the wall, will resist splitting along plane 1 while the wall horizontal reinforcement will be adequate to prevent failure along plane 2. The detailing of web cross-ties is governed by the requirements ACI 318, Chapter 18.

H5.5c Steel Coupling Beams

A coupling beam rotation equal to 0.08 rad reflects the upper limit of link rotation angle in EBF. It should be noted that 0.08 rad may be conservative for coupled walls, in which case using this rotation will result in extra stiffeners in the coupling beam.

Figure description:

Location of coupling beam over building height vs. Coupling beam shear force

Distribution of Coupling Beam Shear Force and Strength over Building Height

Legend

• $V_u$ — color: black; symbol: Solid line
• $\phi V_n$ — color: black; symbol: Dashed line

Annotations

• $V_{u, max}$ (text_label - position: at the peak of the $V_u$ curve (height ≈ 0.25))
• $\phi V_n > 0.8V_{u, max}$ (text_label - position: distance from y-axis to dashed line at mid-height (height ≈ 0.5))

Fig. C-H5.2. Vertical distribution of coupling beam shear.

A smaller value of link rotation may be used if established by rational analysis to determine the inelastic deformation demands expected at the design earthquake displacement.

In addition to the potential use of stiffeners along the span between the reinforced concrete walls, face bearing plates must be provided at the face of the wall. Face bearing plates are full-width stiffeners located on both sides of the web that, in effect, close the opening in the concrete form required to install the beam. Face bearing plates provide confinement and assist in transfer of loads to the concrete through direct bearing. If it is convenient for formwork, face bearing plates may extend beyond the flanges of the coupling beam, although the plate must be installed on the inside of the form and is thereby flush with the face of the wall. The face bearing plates are detailed as a stiffener at the end of a link beam as required in Section F3.5b.4. Near the end of the embedded region, additional stiffeners similar to the face bearing plates need to be provided. In cases where vertical transfer bars are used, these stiffeners are to be aligned with the vertical transfer bars near the end of the embedded region.

In addition to boundary element reinforcing, two regions of vertical “transfer bars” may be provided to assist in the transfer of vertical forces and thus improve the embedment capacity (Shahrooz et al., 1993; Gong and Shahrooz, 2001a, 2001b; Fortney, 2005). Evaluation of experimental data in which transfer bars had been used (Gong and Shahrooz, 2001a, 2001b; Fortney, 2005) indicates that the minimum required area of vertical transfer reinforcement is as follows (see Figure C-H5.4):

(C-H5-2)

where

$\begin{aligned} F_{y s r} & =\text { specified minimum yield stress of transfer reinforcement, ksi (MPa) } \\ L_{e} & =\text { embedment length of coupling beam, in. (mm) } \\ b_{f} & =\text { width of flange, in. (mm) } \\ f_{c}^{\prime} & =\text { specified compressive strength of concrete, ksi (MPa) }\end{aligned}$

The transfer bars need to be placed close to the face of the wall and near the end of embedment length in order to develop an internal force couple that can alleviate the bearing stresses around the flanges and improve the energy dissipation characteristics of coupling beam-to-wall connections (Gong and Shahrooz, 2001a, 2001b).

Figure description:

Key Information:

• Subject: Reinforcement detail for a coupling beam-to-wall connection to prevent splitting failures.
• Key Entities:
  • Steel Section: An I-shaped structural steel member embedded in concrete.
  • Reinforcement Bars: Longitudinal and transverse bars surrounding the steel section.
  • Transfer Bars (indicated by 1 and 2: Placed near the face of the wall and the end of the embedment.
• Dimensions & Specifications:
  • h: Total depth of the concrete section.
  • Development Length: Labeled as "Adequate development length" extending across the reinforcement zone.
  • Spacing: Two distinct regions of length h are shown for placement of reinforcement around the embedded steel.

Fig. C-H5.3. Reinforcement to prevent splitting failures.

Although the required embedment length of the coupling beam may be reduced if the contribution of these bars is taken into account (Qin, 1993), to avoid excessive inelastic damage in the connection region, it is recommended by Harries et al. (1997) and Shahrooz et al. (1993) that the contribution of the transfer bars be neglected in the determination of the required embedment length. The vertical transfer bars may be attached directly to the top and bottom flanges or be passed through holes in the flanges and mechanically anchored by bolting or welding. The use of mechanical half couplers that are welded to the flanges has been successfully tested (Gong and Shahrooz, 2001a, 2001b; Fortney, 2005). U-bar hairpin reinforcement anchored by the embedded coupling beam may also be used, as shown in Figure C-H5.5. These hairpins will be alternated to engage the top and bottom flanges. The transfer bars have to be fully developed in tension either by providing an adequate tension development length or through the use of headed bars. In order to prevent congestion, the sum of the areas of transfer bars and wall longitudinal bars over the embedment length ( $A_{s}$ shown in Figure C-H5.4 or the area of U-bar hairpins in Figure C-H5.5) is limited to $8 \%$ of the wall cross section taken as the wall width times the embedment length.

The vertical transfer bars shown in Figure C-H5.4 are a suggested detail for beams located at a floor level where the wall piers extend far enough above the floor/roof level to accommodate the vertical transfer bars. For coupling beams located at the roof level where the wall piers do not extend far enough above the floor/roof level, alternate details will need to be considered. Such alternate details are presented and discussed in El-Tawil et al. (2009).

Equations H5-1 and H5-1M are derived using the same method as described for Equations H4-4 and H4-4M (see Commentary Section H4.5b).

Figure description:

Structural Diagram: Transfer Bars (Fig. C-H5.4

• Key Entities:

  • Embedded Steel Beam: Defined by depth $d$ and embedment length $L_e$.
  • Wall Face: Vertical boundary of the concrete structure.
  • Transfer Bars ($A_{tb}$: Transverse reinforcement bars required at specific locations.
  • Vertical Bars ($A_s$: Longitudinal reinforcement through the connection area.

• Governing Formula for Transfer Bar Area ($A_{tb}$:

  • $A_{tb} \geq 0.03 f'_c L_e b_f / F_{ysr}$
  • Where $f'_c$ is concrete strength, $L_e$ is embedment length, $b_f$ is flange width, and $F_{ysr}$ is reinforcement yield strength.

• Spatial Constraints:

  • The first set of transfer bars must be placed at a distance of at least $d/2$ from the embedded end of the beam.
  • Reinforcement is distributed within the embedment zone $L_e$ relative to the wall face.

Fig. C-H5.4. Transfer bars.

H5.5d Composite Coupling Beams

The required embedment length needs to be calculated to ensure that the capacity of the composite coupling beam is developed. Based on analytical studies, which were verified against experimental data, Gong and Shahrooz (2001a) proposed an equation in which a single material overstrength factor had been specified for computing the contribution of concrete and transverse reinforcement towards the shear strength of composite coupling beams. In that equation, the specified concrete compressive strength, $f_{c}^{\prime}$, and specified minimum yield stress of transverse reinforcement, $F_{y s r}$, were to be used. Equations H5-5 and H5-5M are revised versions of the original equation in order to more transparently differentiate between the material overstrength factors for concrete and reinforcing steel. The coefficients in this equation were calibrated in order to obtain the same values as those from the original form of the equation published by Gong and Shahrooz (2001a, 2001b).

H5.5e Protected Zones

Coupling beams are expected to undergo significant inelastic deformations. With the exception of face bearing plate and web stiffener attachments, the entire clear span is designated as a protected zone.