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Steel Girder Lateral-Torsional Buckling Limit (Lp) Calculator

Calculate the maximum unbraced length (Lp) within which a steel girder retains full plastic moment capacity.

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

Limiting unbraced length (Lp)

5.56 ft

Check your inputs

This checks only whether your unbraced length is within Lp (full plastic capacity retained). It is NOT a full moment capacity calculator — once the unbraced length exceeds Lp, the actual reduced capacity requires Lr, Cb, Fcr, and Sx per AISC 360 Eqs. F2-2 through F2-6, which a structural engineer should evaluate.

With the figures above, the limiting unbraced length (lp) comes to 1.70. The method behind this is well established, though site conditions and material batches will move it somewhat. This is calculated for United States and cites IRC 2024. Building in another market? Change the selector above so the units and code reference follow.

Estimated cost — your price

This site holds no price list for this material — local prices vary too much to publish honestly. Enter your supplier's price and the result is costed with it.

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

  • AISC 360 Chapter F, Eq. F2-5: Lp = 1.76×ry×√(E/Fy), the limiting laterally unbraced length below which lateral-torsional buckling does not reduce a doubly-symmetric compact I-shaped member's nominal moment capacity below Mp

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Frequently asked questions

What does it mean if my unbraced length is within Lp?
If the actual laterally unbraced length of the compression flange is at or below the calculated Lp, AISC 360 Chapter F treats lateral-torsional buckling as not governing — the member's nominal moment capacity can reach its full plastic moment Mp, for a doubly-symmetric compact I-shaped member.
What happens if my unbraced length exceeds Lp?
This calculator does not evaluate that case. Once the unbraced length exceeds Lp, the nominal moment capacity is reduced and must be calculated using the additional Lr, Cb, Fcr, and Sx terms in AISC 360 Eqs. F2-2 through F2-6 — a structural engineer should perform that check.
Does this apply to any steel beam shape?
The Lp formula used here (AISC 360 Eq. F2-5) applies specifically to doubly-symmetric compact I-shaped members and channels bent about their major axis — other shapes (e.g. singly-symmetric or non-compact sections) are governed by different AISC provisions.