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Diaphragm Analysis

Structural engineers who need to distribute lateral wind and seismic loads across flexible diaphragms to braced wall lines. Unlike rigid diaphragm approaches, this calculator uses FEA-based tributary area analysis - so you get accurate per-wall-line demands without working through the tributary areas by hand.

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What it calculates

Perform FEA-based analysis of simple flexible diaphragms with unlimited loads and braced wall lines. Results include governing shear demand and per-wall-line load distribution for wind and seismic. Available on the free plan.

How it calculates

The Diaphragm Analysis calculator models a flexible diaphragm as a one-dimensional beam spanning between braced wall lines, using an FEA beam solver to distribute lateral wind and seismic demands.

Flexible diaphragm assumption

The calculator assumes a fully flexible diaphragm. Under this assumption, lateral loads are distributed to each vertical resisting element (braced wall line) by tributary area only - each span between adjacent wall lines is treated as an independent simple beam. This is the approach required by ASCE 7 and IBC for wood-sheathed diaphragms that meet the flexible diaphragm classification criteria.

To model this correctly, the calculator inserts a hinge immediately adjacent to each interior braced wall line. The exterior wall lines are pinned supports. The result is that each span resolves independently with no moment transfer between spans.

Load input and load cases

Line loads are entered as distributed lateral forces with a start position and end position along the diaphragm. Each load entry specifies separate magnitudes for two load types:

  • (W) Wind: lateral wind load on the diaphragm
  • (E) Seismic: lateral seismic load on the diaphragm

Partial and variable distributed loads are supported (start magnitude can differ from end magnitude).

FEA solver and shear distribution

The FEA beam solver processes all load combinations simultaneously and returns the following for each load case:

  • Reaction at each braced wall line (R_w for wind, R_E for seismic)
  • Shear diagram along the diaphragm length
  • Governing shear demand V - the maximum in-plane shear at any cross-section

The governing load case is identified by comparing the maximum shear under wind versus seismic and reporting whichever is larger.

Demands table

The demands table collects the per-wall-line reactions from the FEA results and presents wind load and seismic load for each named braced wall line. These values feed directly into shear wall and diaphragm nailing design.

V_governing = max(V_wind, V_seismic)

Related calculators

Diaphragm Analysis (Load Linking)→Beam Analysis→Beam Analysis→Truss Analysis Wizard→

Frequently asked questions

What analysis method does this calculator use?
The calculator uses a FEA beam solver to model the diaphragm as a simple flexible beam spanning between braced wall line supports. For flexible diaphragms, loads are distributed to vertical resisting elements by tributary area only - each span between adjacent wall lines is treated as an independent simple beam. This is consistent with the flexible diaphragm assumption in IBC and ASCE 7 for wood-sheathed diaphragms.
What are the key inputs?
Key inputs are the diaphragm length (perpendicular to the loading direction), the positions of each braced wall line measured from the left end, and the lateral loads applied as distributed line loads. Each load entry requires a start location, end location, and magnitude for wind (W) and seismic (E) load types independently.
What does the calculator output?
The calculator reports the governing shear demand in the diaphragm (maximum in-plane shear), the governing load type (wind or seismic), and a demands table showing the total wind and seismic load carried by each braced wall line. Diagrams show the shear distribution, free body diagram, and reaction summary for the selected load case.
Can I model a diaphragm with more than two wall lines?
Yes. Any number of intermediate braced wall lines can be added. Each interior wall line gets a hinge inserted adjacent to it so that each span is treated as a simple beam, consistent with flexible diaphragm tributary area assumptions. The minimum configuration is two wall lines (at each end).
Does this calculator handle torsion or rigid diaphragms?
No. Only simple flexible diaphragms without torsion are supported. Loads must act perpendicular to the diaphragm span direction. For buildings where the diaphragm cannot be classified as flexible, a rigid or semi-rigid analysis is required and is outside the scope of this calculator.
Is this the free version? What is different from the full Diaphragm Analysis calculator?
Yes, this is the free-plan version. It provides full FEA-based flexible diaphragm analysis with unlimited wall lines and loads. The free version is available without a paid subscription, making it a practical tool for quick checks or for engineers who are evaluating the platform.

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