Does a joist lapped over a middle support span farther than a simple-span joist?

research/house-continuous

Does a joist lapped over a middle support span farther than a simple span? FE beam with pattern loading and NDS checks: two-span peak moment equals a simple span's, so bending-governed floors and snow rafters gain nothing; deflection-governed ceiling joists gain up to 18.5%; 1.5 in bearing on the middle support cuts a 2x10 floor joist to 11 ft 4 in. Not validated against published tables. Follow-up to house-spans.

11 tests

What it found

A pure-Python finite-element beam with pattern loading and the NDS bending, shear, bearing and deflection checks of house-spans, for two- and three-span continuous sawn-lumber joists (SPF No. 2). Under full uniform load a two-span joist's peak moment, wL²/8 at the middle support, equals a simple span's, so members whose simple span is governed by bending (most floor joists, all snow rafters) gain nothing from two spans (0.0% in 16 floor cells; three spans add about 5%), while deflection-governed ceiling joists gain 0.6% to 18.5%. With only 1.5 in of bearing on the middle support, the 1.25wL reaction limits a 2x10 floor joist at 16 in to 11 ft 4 in, against 15 ft 3 in with 3.5 in. Stylised: equal spans, no vibration, not compared with published span tables. Not a structural design.

A figure from its own output

Figure produced by research/house-continuous: ceiling_gain — house-continuous/experiments/ceiling_gain.svg, written by the project's own experiment code.

Paper, code and results

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Produced with AI assistance (Claude). Results are from the code in this repository and are simulated or stylised unless the abstract says otherwise; nothing here implies affiliation with or endorsement by any lab, researcher or company cited. MIT licensed.