Specialist design required
Open-web joists are not sized in this tool.
Open-web metal-web joists are proof-tested truss systems. Each
manufacturer designs their product using purpose-built software
against their own ETA-tested capacity data — published span tables
are the authoritative source, not a beam-theory hand calculation.
Why
Capacity is governed by chord axial action through metal-web triangulation, plus nail-plate connection and diagonal buckling — none of which reduce to σ = M/W.
Shear
OWJ shear deflection is a major component of total deflection, often exceeding bending. EC5 first principles understates it significantly.
Use this view for
Form-factor visualisation, services-zone discussion, and floor build-up appreciation only.
For sizing, request a layout from the OWJ supplier with loads, spans
and centres as your input.
Indicative — geometric estimate
The calculator does not yet hold published characteristic capacities for this series. Spans shown are estimated from section geometry using first-principles EC5 and may differ from the manufacturer's tabulated values. Verify against the current manufacturer span tables or design software before specifying.
Advisory — exceeds stock length
The design span exceeds the standard 5.4 m CLS stock length. The section is structurally valid at this span, but timber will need to be spliced or special-ordered. Confirm with your supplier.
Design span verdict
PASS
All checks satisfied
Max indicative span: 4.32 m
Rule
Span ≤ 2.5 m: none · 2.5 – 4.5 m: mid-span · > 4.5 m: third points
Design loads
Permanent (Gk)
0.50 kN/m²
Variable (Qk)
1.50 kN/m²
ULS combination
3.43 kN/m²
UDL on joist (w)
1.37 kN/m
Section properties
Second moment of area (I)
— cm⁴
Section modulus (W)
— cm³
Mean E (E0,mean)
— kN/mm²
Bending strength (fm,k)
— N/mm²
Calculation basis
EC5 (BS EN 1995-1-1) + UK NA. Simply-supported single span, service class 1, kmod medium-term, ksys = 1.1. Deflection limits per EC5: L/333 or 14mm absolute on instantaneous, L/250 on final (incl. creep, kdef = 0.6).
Trademarks
JJI-Joists, Steico, Metsä Finnjoist, Boise BCI / AJS and Masonite Beams MFB are trademarks of their respective owners. Knight Designs Ltd is not affiliated with, endorsed by, or partnered with these manufacturers. Product names are used here solely to identify the joist systems for which the tool calculates indicative spans.
