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Diaphragm Analysis

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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.

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Method & scope

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Scope: simple flexible diaphragms without torsion

The Diaphragm Analysis calculator models a simple flexible diaphragm without torsion, essentially a 1D load path between braced wall lines. You define the diaphragm length and the braced walls that support it; the calculator distributes lateral load to those walls. It is not a general 2D diaphragm model. Systems that need torsion, irregular plan geometry, or open/cantilevered layouts should be analyzed with another method outside this calculator.

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Calculation method

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)

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How to use it

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Linking wind loads: MWFRS Envelope Procedure is required

Only the MWFRS (Envelope Procedure) wind method exports pressures that can be linked into the Diaphragm Analysis calculator. Components and Cladding (the default wind method on a new Wind Loads sheet) and MWFRS (Directional Procedure) do not support linking. If the link icon does not offer your Wind Loads sheet, or the linked pressures are missing, this is almost always the reason.
To link wind pressures into the Diaphragm Analysis calculator:
  1. Open your Wind Loads calculation.
  2. Set Calculation Type to MWFRS (Envelope Procedure). Do this before linking; switching after the fact will not backfill the linked values.
  3. Return to the Diaphragm Analysis calculator and link the wind load from the Envelope Procedure Wind Loads sheet.
If you need Components and Cladding pressures for cladding design, keep that as a separate Wind Loads sheet; one project can have both.

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Common questions

A diaphragm is a horizontal structural element (floor or roof) that transfers lateral loads (wind or seismic) to the vertical resisting system (shear walls or braced frames). It acts like a horizontal beam spanning between braced wall lines.
Enter the total lateral force or a uniform wind pressure along the diaphragm span. For ASCE 7 wind loads, compute the design wind pressure and enter the resulting tributary load per unit length.
Both are vertical resisting elements. The calculator treats them identically, they are simply lateral force resisting elements at specified positions. The distinction matters for capacity design but not for the force distribution this calculator performs.
The calculator uses an 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.
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.
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.
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).
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.
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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Next steps

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