Design CLT to NDS
Design CLT floor panels in the USA to NDS and ANSI/APA PRG 320, with real product data supporting standard reference values or supplier-specific properties, so your bending, shear, deflection and vibration results reflect what's actually available to specify, including Southern Pine layups. Fire, connection and shear wall modules are in active development. Try the calculator to specify your next CLT floor with confidence.
Start Designing for Free See Integrated Suppliers- Standard: AWC NDS 2018, ANSI/APA PRG 320, ASCE 7 loading
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About CLT Design
The Design of CLT in America
The IBC 2021 updates have ushered in a new era of „Tall Mass Timber“ (Type IV-A, IV-B, and IV-C construction). However, designing Cross-Laminated Timber (CLT) requires navigating complex composite theory often missing from standard software packages.
The SPEC Toolbox CLT platform is built specifically for North American structural engineers. It automates the requirements of the National Design Specification (NDS), allowing you to specify ANSI/APA PRG 320 certified panels with confidence. From simple roof diaphragms to high-rise floor plates, this tool ensures your designs are safe, serviceable, and code-compliant.
The engineer's guide
CLT Design, NDS
What This Calculator Does
Performs a complete rational analysis of CLT panels under gravity loads, using the „Shear Analogy“ or „Extended Gamma“ methods permitted by the NDS:
- Bending Moment (M’n): Checks positive and negative moment capacity of the major strength axis.
- Shear Capacity (V’n): Verifies vertical shear capacity.
- Rolling Shear (Fs,rolling): The critical check for CLT — shear stress in the cross-layers, often governing design for short spans and heavy loads.
- Deflection & Creep: Calculates immediate and long-term deflection, applying the NDS time-dependent factor (Kcr).
- Vibration (Serviceability): Rigorous analysis of floor vibration for occupant comfort.
Design of CLT Floors
Supplier & Code Integration — integrates supplier data with AWC NDS requirements and ASCE 7 loading provisions. Structural properties based on ANSI/APA PRG 320 standardized CLT material properties.
Analytical Methods for CLT Stiffness:
- Supplier-Provided Values: strength/stiffness extracted directly from supplier technical product data sheets.
- Standard Reference Design Values: taken from ANSI/APA PRG 320, defining standardized mechanical properties for designated CLT grades and layups under standard dry service conditions (moisture content below 16%).
Capacity evaluated across Major Strength Axis (parallel to outer layer grain) and Minor Strength Axis (perpendicular to outer layer grain). Reference values include effective bending stiffness, bending moment capacity, shear stiffness, shear capacity.
High-Performance Vibration Design — includes Hamm et al. (2010) and U.S. CLT Handbook guidance. Vibration Methods: Hamm et al. (2010), U.S. CLT Handbook recommendations, or prEN 1995-1-1:2023. Performance Levels: floor categories (e.g., Category I or Category II) matching building occupancy and comfort requirements. Analyzes Fundamental Frequency and dynamic response, tuning floor mass, damping, and participating width.
Design of CLT Fire
*CLT Fire Design Calculators are under development and will be available soon.*
Design of CLT Connections
*CLT Fire Design Calculators are under development and will be available soon.*
Design of CLT Shear Walls
*CLT Fire Design Calculators are under development and will be available soon.*
CLT Design, NDS
Does this check Fire Design (Char Rate)?
Can I use LRFD or ASD?
Does it support Southern Pine?
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