One of the most important additions in LAVA v26.0.6 is the ability to automatically design and analyze continuous tie-rod systems for wood shear walls. Along with the new design workflow, we also took a careful look at how drift is calculated for these systems.
During development, we compared the published equations in SPDWS with the actual behavior of a continuous tie-rod system. That review led us to modify one portion of the flexural drift equation to better represent the structural mechanics of a tie-rod building.
- Flexural drift
- Shear drift
- Anchor (assembly) deformation
As shown below, the total drift is expressed as:
The flexural component published in SPDWS 4.3-1 is:
where:
- V = shear force
- h = wall height
- E = modulus of elasticity
- A = chord area
- b = shear wall length (distance between chords)
The Assumption Behind the Equation
The published equation is based on a conventional wood shear wall.
In a traditional shear wall:
- One chord carries compression.
- The opposite chord carries tension.
Because both chords participate in resisting the overturning moment, the flexural term includes the deformation contributed by both members.
This assumption is appropriate for conventional hold-down systems.
What Changes with Continuous Tie-Rod Systems?
A continuous tie-rod system behaves differently.
Instead of the wood tension chord resisting overturning forces, the continuous steel tie rod carries the tensile force through the height of the building.
The wood chord is no longer acting as a tension member.
Instead:
- One wood chord carries compression.
- The continuous steel rod carries tension.
- The wood tension chord is effectively removed from the flexural mechanism.
This changes how deformation is distributed within the system.
Where the Tension Deformation Is Already Accounted For
SPDWS already includes a separate deformation term for the tie-down assembly:
where the assembly deformation includes:
- Rod elongation
- Take-up device (TUD) deformation
- Upper TUD deformation
- Bearing plate deformation
- Crushing deformation
In other words, the tension-side deformation is already explicitly modeled in the third term of the equation.
Avoiding Double Counting
If the original flexural coefficient of 8 is used for a continuous tie-rod system, the calculation effectively counts tension deformation twice:
- Once within the published flexural equation, which assumes two active wood chords.
- Again in the assembly deformation term, where rod elongation and tie-down deformation are already included.
Since the steel tie rod replaces the wood tension chord, only the compression-side wood chord contributes to flexural shortening.
To reflect this behavior, LAVA modifies the flexural term to:
Reducing the coefficient from 8 to 4 removes the contribution of the nonexistent wood tension chord while preserving the compression-side deformation.
Why LAVA Uses the Modified Equation
This adjustment is based on the mechanics of continuous tie-rod systems rather than simply reproducing the published equation.
The resulting drift calculation:
- Represents the actual load path in a continuous tie-rod system.
- Avoids counting tension deformation twice.
- Separates wood compression deformation from steel tie-rod deformation.
- Produces a drift prediction that more closely matches the physical behavior of the structure.
Summary
The SPDWS Equation 4.3-1 was developed assuming a traditional shear wall with two wood chords participating in flexure. In a continuous tie-rod system, that assumption no longer applies because the steel tie rod replaces the wood tension chord.
LAVA therefore uses:
This modification ensures that compression-side wood shortening is captured in the flexural term, while tension-side deformation is accounted for separately through the tie-rod assembly deformation term. The result is a drift calculation that better reflects the behavior of continuous tie-rod systems without introducing duplicate deformation into the analysis.
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