A sawn timber floor beam designed to China's GB 50005 uses a design-value approach built around an adjustment coefficient system comparable in spirit to the NDS adjustment factors covered elsewhere in this series, though with its own specific coefficient set and base design values tied to Chinese timber grading standards.
Every check above runs automatically inside the Structyze Timber Beam module — this particular module is part of the full desktop catalogue rather than the free browser trial, since the trial only carries a curated flagship set. You can still try the free trial modules right now to see the same calculation style in action on RCC Beam, Column, Slab and a handful of others.
The Example We'll Use
- Beam: TC15 grade sawn timber, 100 mm × 250 mm, span 3.6 m, simply supported
- Design bending strength f_m: 13 MPa (TC15 grade, per GB 50005 Table 4.3.1-2)
- Design shear strength f_v: 1.4 MPa
- Duration of load adjustment coefficient and moisture adjustment coefficient both taken as 1.0 for this normal, dry-service application
Working Through It
Maximum factored bending moment: 2.8 kN·m. Section modulus: (100 × 250²)/6 = 1,041,667 mm³.
Actual bending stress = 2.8 × 10⁶ / 1,041,667 = 2.69 MPa. Utilization for bending: 2.69 / 13 = 0.21.
Maximum factored shear force: 3.1 kN. Actual shear stress (rectangular section) = 1.5 × V / A = 1.5 × 3100 / (100 × 250) = 0.186 MPa. Utilization for shear: 0.186 / 1.4 = 0.13.
Deflection under service live load, checked against GB 50005's span/250 limit for this beam type, gives a utilization of 0.44 — the closest of the three checks, though still with a comfortable margin.
What Governs, and Why
Deflection governs at utilization 0.44, well ahead of both bending (0.21) and shear (0.13) — a pattern consistent with the general tendency for moderate-span timber floor beams to be deflection-controlled rather than strength-controlled, directly echoing the span-to-depth and deflection discussions elsewhere in this series' timber content, here confirmed under a specifically Chinese design-value framework.
Where Structyze Takes Over
Every check above runs automatically inside the Structyze Timber Beam module — this particular module is part of the full desktop catalogue rather than the free browser trial, since the trial only carries a curated flagship set. You can still try the free trial modules right now to see the same calculation style in action on RCC Beam, Column, Slab and a handful of others.
For Timber Beam and the rest of the full catalogue — 80+ components across RCC, Steel, Composite, Timber, Masonry and more, across 20+ design codes — buy the full Structyze desktop app or see everything it does on the Software page.
Quick FAQ
Are GB 50005's timber grade designations (like TC15) comparable to grades used in other countries' timber codes?
Not directly — TC grades reflect Chinese timber species grouping and strength grading standards specific to GB 50005, and translating between TC grades and another country's grading system (like NDS's species-and-grade combinations) requires a proper strength-based comparison rather than assuming equivalent grade numbers mean equivalent strength.
Does GB 50005 use the same deflection limit for all beam types?
No — like most timber codes, GB 50005 varies its deflection limit by member type and application (floor beams, roof beams, members supporting brittle finishes), similar to the support-condition-dependent span-to-depth variation discussed in the RCC slab deflection content elsewhere in this series.
Final Thoughts
This beam's comfortable strength margins but noticeably tighter deflection utilization is a familiar pattern across timber design generally — regardless of which country's specific design-value system is applied, timber's relatively low stiffness tends to make deflection, not raw strength, the number actually worth watching closely for ordinary floor spans.