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Timber Beam Notch Reduction Factor Calculator

Calculate the shear stress amplification factor for a timber beam notched at its end, per NDS 3.4.3.

Computed in your browser — nothing you enter is uploaded. Figures are presented for United States against IRC 2024, and every formula is cited under regulatory standards below.

Last verified 2026-08-26 · v1.0.0

Market
Imperial · sales tax

Shear stress amplification factor

1.44 ×

High confidence
Net depth after notch
9.84 in

At the values currently entered, the shear stress amplification factor works out to 1.44 ×. Figures are shown for United States, where IRC 2024 is the governing residential reference; switch the market above if you are building elsewhere.

Preliminary estimate, not certified engineering. This tool produces an indicative quantity calculation for planning purposes only — it is not a certified structural analysis, a guaranteed material takeoff, or a substitute for building department approval. Always verify measurements on-site and have a licensed contractor or structural engineer review any load-bearing, code-sensitive, or safety-critical work before purchasing materials or starting construction. Spotted an arithmetic or standards error? Report it to contact@craftquantities.com with your inputs — confirmed fixes become pinned regression tests.

[Schema Verified] Computed in alignment with American Concrete Institute (ACI 318-19) formulas and International Residential Code (IRC 2024) spatial boundaries.

Regulatory standards & verification citations

  • NDS (National Design Specification) 3.4.3: for a beam notched on the tension side at the end of the member, actual shear stress is amplified relative to net depth by the factor (full depth ÷ net depth)²

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Frequently asked questions

Why does a notch increase shear stress so much more than the depth loss alone suggests?
Per NDS 3.4.3, an end notch on the tension side amplifies actual shear stress by the square of (full depth ÷ net depth), not just the simple ratio — this accounts for the stress concentration that develops at the sharp re-entrant corner of the notch, which is more severe than a uniform depth reduction.
Does this factor apply to any notch location on the beam?
No — this specific factor is for a notch on the tension side at the end of the member. Notches elsewhere along the span, or on the compression side, are governed by different NDS provisions and are not covered by this calculator.
What should I do if the amplification factor seems too high?
A large factor means the notch is removing a significant fraction of the beam's depth right where shear is being checked — consider reducing the notch depth, tapering it gradually instead of cutting it square, or reinforcing the notched area, and verify the resulting actual shear stress against the allowable shear design value.