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For composite walls, the 2nd and 3rd terms of the 4-term equation, shear and nail slippage, and the 2nd-term of the 3-term equation, combined shear and slippage, are calculated separately for each side of the shear wall, which may have different materials. It is necessary to determine single deflection from the two-sided wall, or equivalently, a combined stiffness.
3-Term Deflection Equation
SDPWS 4.3.5.4 provides ways to calculate a combined effective shear stiffness Ga(c) for use in the 3-term equation depending on the material configuration on either side of the wall. Ga(c) is then used in the second term of the deflection equation to determine wall deflection.
From 4.3.5.4.1, for walls sheathed with the same type of materials on both sides of the wall, and for which both sides are blocked or both are unblocked, the stiffnesses on either side can simply be added. For walls with identical material specifications on either side, this reduces to taking twice the Ga from one side.
From 4.3.5.4.2, for walls with one type of material on one side and a different material on the other, or if one side is blocked and the other is not, the combined Ga is calculated as the greater of twice the Ga from the side with the lower shear capacity, compared with the Ga from the side with the lower shear capacity.
More details are found in Combination of Sheathing Capacities and Stiffness.
4-Term Deflection Equation
When using the 4-term equation, Ga is not calculated and the rules given in 4.3.5.5.4 do not apply. A rigorous approach based on engineering mechanics is used.
Shearwalls apportions shear to each side of the wall by determining the shear force v on each side that causes the deflection due to shear plus nail slippage to be the same on both sides of the wall. For the non-linear 4-term equation, a Newton-Raphson solution to the 2-equation system is used.
Distribution to Segments in Line
The equalization of deflections to each side of the shear wall is done regardless of whether the program is equalizing deflections on all wall segments on the line, that is, it is done for both capacity-based distribution and stiffness-based distribution. These methods are also described in the Shear Wall Rigidity Design settings.
Design Forces
The forces on each side of the wall are used for deflection calculations only; for shear wall design, the combined force is used and sheathing capacities on either side are combined according to procedures in SDPWS 4.3.5.4.
Zero Shear on One Side
Slippage for non-wood-panel materials is a constant when using the 4-term equation, which in many cases creates a larger slippage deflection than is possible for shear plus slippage even when all load is placed on the wood panel. In these cases, all the force is placed on the wood panel side. The deflection for that segment is the nail slippage plus shear from the wood panel side, not the constant non-wood-panel slippage.
Note that in this case, despite the fact that all of the load is assigned to the wood side for purposes of deflection analysis and story drift, the program still uses the sheathing on both sides of the shear wall for shear wall capacity calculations according to the procedures for combining shear wall capacity.