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Differences between NBCC and ASCE 7 in Load Combinations

The National Building Code of Canada (NBCC) and the American Society of Civil Engineers' ASCE 7 Standard are two widely adopted guidelines for seismic design. Both standards aim to ensure the safety and performance of structures during seismic events, but their approaches to load combinations differ. This article discusses the distinctions between NBCC and ASCE 7 in terms of load combinations, focusing on the fundamental principles that guide the design and construction of structures.

Basic Load Combinations

NBCC: The NBCC provides basic load combinations that include dead loads, live loads, snow loads, wind loads, and earthquake loads. These load combinations consider the primary load effects on the structure, but they do not explicitly address the combined effects of multiple loads acting simultaneously.

ASCE 7: ASCE 7 provides a more comprehensive set of basic load combinations that consider the simultaneous effects of multiple loads, such as dead loads, live loads, snow loads, wind loads, and earthquake loads. The standard also includes load combinations with temperature, soil, and hydrostatic loads, providing a more accurate representation of the structure's response to various loading conditions.

Load Factors and Importance Factors

NBCC: The NBCC assigns load factors and importance factors to various loads in the load combinations. These factors are used to adjust the nominal loads to account for uncertainties in the design and construction process, as well as the structure's importance and expected performance during seismic events.

ASCE 7: ASCE 7 provides a more detailed approach to load factors and importance factors, assigning different values to loads based on factors such as the structure's performance category, seismic design category, and importance factor. This approach results in more accurate estimates of the structure's response to various loads and ensures that the structure can withstand the demands of seismic events without compromising safety or performance.

Seismic Load Combinations

NBCC: The NBCC provides seismic load combinations that include the effects of dead loads, live loads, and earthquake loads. These combinations consider the primary load effects during seismic events, but they do not explicitly address the combined effects of multiple loads acting simultaneously.

ASCE 7: ASCE 7 provides a more comprehensive set of seismic load combinations that consider the simultaneous effects of multiple loads, such as dead loads, live loads, snow loads, wind loads, and earthquake loads. The standard also includes load combinations with temperature, soil, and hydrostatic loads, providing a more accurate representation of the structure's response to various loading conditions during seismic events.

ASCE 7 seismic load combinations:

NBCC seismic load combinations:

  1. 1.4D

  2. 1.2D + 1.6L + 0.5(Lr or S or R)

  3. 1.2D + 1.6(Lr or S or R) + ωQ

  4. 1.2D + 1.0W + 0.5(Lr or S or R)

  5. 1.2D + 1.0E + 0.5(Lr or S or R)

  6. 0.9D + 1.0W

  7. 0.9D + 1.0E

Where:

  • D: Dead load

  • L: Live load

  • Lr: Roof live load

  • S: Snow load

  • R: Rain load

  • W: Wind load

  • E: Earthquake (seismic) load

  • Q: Fluid loads with well-defined pressures and maximum heights (e.g., hydrostatic pressure, soil pressure)

  • ω: Load factor for Q, typically 1.0

  1. 1.25D + 1.5L + 1.0S

  2. 1.25D + 1.5(Lr or S or R)

  3. 1.25D + 1.5W

  4. 1.25D + 1.5S + 1.0W

  5. 1.25D + 1.5E

  6. 0.9D + 1.5W

  7. 0.9D + 1.5E

Where:

  • D: Dead load

  • L: Live load

  • Lr: Roof live load

  • S: Snow load

  • R: Rain load

  • W: Wind load

  • E: Earthquake (seismic) load


Overstrength Factors and Allowable Stress Design

NBCC: The NBCC incorporates overstrength factors in the load combinations to account for the additional capacity of structural elements beyond their nominal strength. The standard primarily relies on the limit states design approach, which does not explicitly address allowable stress design.

ASCE 7: ASCE 7 provides overstrength factors for various structural elements and systems, ensuring that the structure can withstand the demands of seismic events without compromising safety or performance. The standard also includes load combinations for both limit states design and allowable stress design, providing engineers with the flexibility to choose the most appropriate design approach for their projects.

The American Society of Civil Engineers (ASCE) provides guidelines for designing structures to withstand various loads, including seismic forces, through its publication ASCE 7: Minimum Design Loads for Buildings and Other Structures. To account for uncertainties in the actual strength of materials and construction quality, the code introduces overstrength factors. Overstrength factors are used to increase the design forces in certain structural elements, ensuring they have additional capacity beyond the nominal loads.

The overstrength factor (Ω₀) varies depending on the type of structural system or component. Here are some common overstrength factors from ASCE 7-16 (Table 12.2-1):

The National Building Code of Canada (NBCC) provides guidelines for designing structures to withstand various loads, including seismic forces. Overstrength factors, also known as Rd factors (ductility-related force modification factors), are used to account for the inherent overstrength in structures and their ability to undergo inelastic deformations during seismic events.

The overstrength factors in NBCC vary depending on the type of structural system or component. Here are some common Rd factors from NBCC 2015 (Table 4.1.8.4):

  • Ordinary moment frames (OMF) of steel or concrete: Ω₀ = 3.0

  • Special moment frames (SMF) of steel: Ω₀ = 2.0

  • Special reinforced concrete shear walls: Ω₀ = 2.0

  • Special structural steel systems not specifically detailed for seismic resistance: Ω₀ = 2.0

  • Structural steel systems detailed for seismic resistance: Ω₀ = 1.5

  • Reinforced concrete structural walls and coupling beams designed in accordance with special provisions: Ω₀ = 1.25

  • Cantilevered column systems: Ω₀ = 2.0

  • Ordinary steel concentrically braced frames (OCBF): Ω₀ = 2.0

  • Steel moment-resisting frames - special (SMRF): Rd = 8.0

  • Steel moment-resisting frames - intermediate (IMRF): Rd = 5.5

  • Steel moment-resisting frames - ordinary (OMRF): Rd = 3.5

  • Steel concentrically braced frames - special (SCBF): Rd = 6.0

  • Steel concentrically braced frames - ordinary (OCBF): Rd = 4.0

  • Reinforced concrete shear walls - ductile (D): Rd = 6.0

  • Reinforced concrete shear walls - moderately ductile (MD): Rd = 4.0

  • Reinforced concrete shear walls - conventional (C): Rd = 2.5

  • Reinforced concrete moment-resisting frames - ductile (D): Rd = 5.0

  • Reinforced concrete moment-resisting frames - moderately ductile (MD): Rd = 3.5

  • Reinforced concrete moment-resisting frames - conventional (C): Rd = 2.0


Conclusion

Understanding the differences between NBCC and ASCE 7 in load combinations is essential for engineers working on seismic design projects. The distinctions in basic load combinations, load factors and importance factors, seismic load combinations, and overstrength factors and allowable stress design can significantly impact the design and performance of structures during seismic events. By recognizing these differences, engineers can select the most appropriate standard for their projects and ensure the safety and performance of structures in earthquake-prone regions.


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