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The end reaction load the I-joist transfers into its bearing support.
The reaction at ONE end, not the total load on the joist — for a simple span that is roughly half the load, and for a joist continuous over a support the middle reaction is larger than either end. Take it from the joist layout or the supplier's design output rather than deriving it, because an I-joist's reactions are what the manufacturer's tables are written against and the web stiffener requirement follows from them.
The allowable compression stress perpendicular to the flange, from the manufacturer's design values.
Commonly 2.5-3.5 MPa (360-500 psi) for I-joist flange bearing.
The width of the I-joist's bottom flange, which is the bearing contact surface.
The flange width, which is the only part of an I-joist that bears — the web carries nothing into the support. Flange widths are product-specific and are not a series you can guess from the depth: two joists of the same depth from different makers can differ enough to change whether a stiffener is needed. Read it from the product's own data sheet.
Required bearing length
1.77 in
A simplified proxy check — always follow the specific I-joist manufacturer's installation guide for actual web stiffener requirements at bearings and concentrated loads.
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I-Joist Web Stiffener Bearing Length Calculator: 1.77 in — shown in imperial, US market. The link sets both, so the result they see is the one on your screen.
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How this was calculated
Formula source(s)
- Required bearing length = reaction load ÷ (allowable bearing stress × flange width); if the provided/actual bearing length is less than this, a web stiffener is typically required per the I-joist manufacturer's installation guide
Inputs used
- Reaction Load
- 1124.04 lbf
- Allowable Bearing Stress
- 362.59 psi
- Flange Width
- 1.75 in
Confidence note: A simplified proxy check — always follow the specific I-joist manufacturer's installation guide for actual web stiffener requirements at bearings and concentrated loads.
What this calculation does not cover
- Arithmetic can return a bearing shorter than any manufacturer permits. I-joist installation guides set an absolute minimum regardless of how small the reaction is — commonly 45 mm (1¾ in) at an end bearing and 89 mm (3½ in) where a joist runs over an interior support — so the 44 mm (1.7 in) these defaults produce is not a permitted detail. Round up to the published minimum before anything else is decided.
- The flange is only one half of the bearing, and usually the stronger half. The plate, beam or hanger seat underneath has its own compression capacity across the grain, and SPF at about 2.9 MPa (425 psi) crushes well before an LVL flange rated 3.5 MPa does — so on a softwood wall plate it is the plate that governs the bearing length, not the stress entered above. Check both and design to the lower.
- Bearing length says nothing about holding the joist upright. An I-joist end has to be restrained from rolling, by a rim board, blocking panels or a hanger with sides tall enough to grip the top flange — and where a post or a stud pack lands over the bearing from above, that load goes down through squash blocks fitted tight alongside the web, not through the joist. A web stiffener does neither of those jobs.
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This result is a specification — 1.77 in — not a quantity. Put the thing it sizes into your project: how many, what you call it, and your supplier’s price.
Computed in your browser — nothing you enter is uploaded. Presented in US customary units and US trade terminology. Where a formula follows a published standard, that standard and its edition are cited beside it on this page; where none governs, the page says so. Local amendments override model codes — verify against the code in force where you build.
Sources checked 2026-09-06 · in the site-wide review of 2026-09-06 · v1.0.1
Regulatory standards & verification citations1
- Required bearing length = reaction load ÷ (allowable bearing stress × flange width); if the provided/actual bearing length is less than this, a web stiffener is typically required per the I-joist manufacturer's installation guide
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