Wood I-joist

A prefabricated wood I-joist to NDS 2024 Chapter 7 with ASCE 7-22 combinations, in ASD or LRFD. An I-joist is a proprietary product: the NDS gives no values (7.2), so the calculation works from the design resistances of the product, not from stresses. It holds the APA Performance Rated I-joists (PRI) of APA Z725H-2022 Table 9, and takes values entered from a manufacturer's report.

Inputs of the I-joist #

• Joist: the APA depth and series, or the moment Mr, shear Vr, intermediate and end reactions, EI, K, the vertical load capacity, depth, flange width and flange Fc⊥ from the manufacturer. Values are at normal duration of load. K is as APA defines it: a simple span under a uniform load deflects 5wℓ⁴/384EI + wℓ²/K. A manufacturer who writes the shear term as wℓ²/8GA has K = 8 GA. The APA values come from a copy of the 2022 revision (APA's current one is March 2024): confirm them, and the product you specify.
• Spacing and loads: area loads per square foot over the spacing and the full length, plus line and point loads with their load types. Own weight is entered (about 2.5 to 5 plf).
• Partial loading: on by default. L, Lr and S also act on part of the spans (ASCE 7-22 4.3.3, 7.5), as on the sawn lumber beam. The APA span tables were not calculated this way: read their note on it.
• Bearing: the lengths at the ends (at least 1-3/4 in.) and at intermediate supports (at least 3-1/2 in.; the support next to a cantilever counts as intermediate), bearing stiffeners at the ends, an end in a hanger whose sides do not reach the top flange (it needs a stiffener), load stiffeners at point loads, and the Fc⊥ of the plate. Table 9 note i limits a reaction to the bearing on the flange or the plate, whichever is smaller.
• Lateral stability: whether the ends are held against rotation at the bearings (required, NDS 7.3.5.2), and the unbraced lengths of the top and bottom flanges. 0 = held throughout, CL = 1.0. An unbraced flange is designed as a column by itself, buckling sideways (7.3.5.3), which needs the flange's Fc and Emin from the manufacturer: the prefilled values are sawn No. 2 values and matter only where a length is entered.
• Deflection and service: limits as span / n (APA's tables use 480 for live load; the IBC floor value is 360), creep (Kcr = 1.5, NDS 3.5.2), temperature, and the APA cantilever limits, which you can clear where you engineer the cantilever.
• LRFD format conversion: NDS 7.3.8 takes KF from the manufacturer. 2.54 and 2.88 are prefilled, the Table 2.3.5 values for bending and shear: check them against the product report. A reaction takes the shear value, as Table 7.3.1 has φ = 0.75 for it.

Auto-design varies the depth and series from the APA table (shallowest then lowest series, or lowest stiffness that passes); the spacing when unlocked. Entered values stay as entered.

Checks performed #

Limitations #

• Web holes (7.4.3), notches (7.4.4), wet service (7.1.4: dry use only), pressure treatment and floor vibration are not checked.
• Composite action with glued sheathing is not used: Table 9 is the bare joist, so spans come out shorter than the APA span tables, which assume a glued-nailed floor.
• On a continuous joist the moments and reactions are those of a bending analysis; shear deformation is added to the deflection but does not redistribute them.
• A wall or roof load at the end of a cantilever usually needs reinforcement: APA Tables 5 and 6 are not applied; the module warns.
• Web and bearing values with stiffeners at intermediate supports are the manufacturer's: only the Table 9 intermediate value is used.
• NDS 7.3.5.3 calls the flange column method "one acceptable method"; the manufacturer may have another.

Verification examples #

Verification status: 22 values compared in 9 examples (5 published, 4 hand calculations from the cited equations): 22 within tolerance, 0 recorded deviations. Code basis: the provisions it uses were read in the target edition.

No free worked example of an I-joist calculation was found. The NDS has none; the APA guide gives design values and span tables only. So the module is checked three ways, and the reader should weigh them as such:

The module's table of APA values is checked by test against the printed rows. Open: the current APA revision and a product evaluation report with its KF values.