When designing a wood building for seismic forces, calculating and distributing forces within the diaphragm is only part of the analysis.
What happens to those forces next?
The forces calculated in the diaphragm aren’t simply passed directly to the shear walls. They have to be transformed into the forces acting along each shear line based on where the lateral load is generated, where it is resisted, and how it moves through the building.
This becomes especially important when shear walls don’t stack from floor to floor, when resisting elements occupy only a portion of a shear line, or when diaphragm design forces exceed the forces used to design the shear walls.
LAVA follows that load path.
From Building Forces to Collector Forces
For each level, LAVA determines the seismic forces required for different parts of the lateral system, including:
- Shear wall design forces
- Diaphragm design forces
- Collector design forces
These forces are not always the same.
For example, the seismic force used to design a shear wall may be based on the vertical distribution of building seismic forces, while the diaphragm is subject to the requirements of ASCE 7-22 §12.10.
When the collector design force is greater than the corresponding shear wall force, LAVA determines an amplification factor:
Af = Collector Seismic Force / Shear Wall Seismic Force
That factor is then used when evaluating collector demand along the shear line.
But that’s only the beginning.
Following the Force Along the Shear Line
Consider a simple shear line with a 12-foot shear wall along a longer diaphragm edge.
The diaphragm is delivering seismic force continuously along the shear line. The shear wall, however, provides resistance only over its 12-foot length.
That means the collector force isn’t constant.
As LAVA moves along the shear line, diaphragm force accumulates:
Diaphragm Force = Tributary Length × Diaphragm Shear
When the analysis reaches the shear wall, LAVA also begins accounting for the force being resisted by that wall:
Collector Force = Accumulated Diaphragm Force − Resisting Shear Wall Force
The result is a collector force that changes along the length of the shear line.
This is why the collector diagram in LAVA has a shape rather than simply reporting one force for the entire line. It represents how force is accumulated, transferred, and resisted along the actual load path.
What Happens at the Floor Below?
This gets even more interesting in a multi-story building.
An upper-story shear wall produces a reaction that has to continue through the structure. How LAVA handles that reaction depends on the load path.
When Shear Walls Align
If an upper-level shear wall aligns with the supporting shear line below, its reaction can transfer directly into the lower-level resisting system.
LAVA recognizes this condition and does not unnecessarily redistribute that reaction through the lower-level collector.
When Shear Walls Don’t Align
Now move that upper shear wall away from the resisting line below.
The physics change.
The upper-level reaction can no longer transfer directly into the lower shear wall. That force must travel through the diaphragm and collector system until it reaches the resisting element.
LAVA accounts for that transfer force as part of the collector analysis.
The total collector demand therefore includes both the current-level collector force and the upper-level reaction that must be transferred because of the offset.
This distinction matters because the two buildings may have identical seismic weights and identical shear walls, but they do not have the same load path.
Why This Matters
Structural software can make it easy to focus on whether an individual member passes or fails.
But lateral design isn’t just a collection of individual checks. The building still needs a continuous load path.
A collector force should represent more than a number pulled from the diaphragm calculation. It should reflect where the force originates, where it needs to go, and what structural elements it passes through along the way.
That’s the approach LAVA takes.
From seismic force distribution, to diaphragm and collector requirements, to shear wall resistance and transfer forces between levels, LAVA follows the forces through the building so engineers can see not only what the demand is, but where it comes from.
That’s what integrated wood building design should look like.
Check out a full example with the calculations here: Diaphragm and Collector Forces in LAVA
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Questions? Contact us at info@cbs2llc.com