Towards the Harmonization of Canadian and American Masonry Structures Design Standards

Citation

  • Dutrisac, H., Banting, B. (2021). Towards the Harmonization of Canadian and American Masonry Structures Design Standards. Canadian Standards Association, Toronto, ON.

Executive Summary

The Masonry Society (TMS) 402-16 Building Code Requirements for Masonry Structures have been derived from the same pool of research, notable differences have been observed over the years and questions have been raised over the accuracy, degree of conservatism, and overall economy associated with each standard. To study disparities which may exist between these two standards, a Canadian-American research project was initiated to identify requirements within the standards which may in one case be insufficiently rigorous and in other cases be overly conservative. This report highlights the findings of this initiative and summarizes areas that have been identified for improvement in the context of the Canadian standard.

The primary goals of the research project were to conduct a comprehensive comparison of the masonry requirements contained within the CSA S304-14 and TMS 402-16 design standards and establish a collaborative Canadian-American front to strive for better long-term cross-border harmonization between the two standards. The collaborative Canadian-American initiative involved three key activities:

  1. Comparison of the Canadian limit states and the US strength design provisions, including load (NBCC/ASCE 7) and resistance (CSA S304-14/TMS 402-16) provisions;
  2. Parametric studies of reinforced masonry beams and in-plane and out-of-plane bending of reinforced masonry walls; and
  3. Comparison of preliminary archetype building designs.

The comparison of the National Building Code of Canada (NBCC) and the American Society of Civil Engineers, Minimum Design Loads and Associated Criteria for Buildings and Other Structures (ASCE 7) loading criteria revealed similarities with respect to dead loads and live loads due to use and occupancy. Although the approach for calculating snow, wind, and earthquake loads are similar in both codes, diverging results for the archetype designs were observed. Differences in the design loads were primarily attributed to variations in climatic and seismic hazard data.

The review of the CSA S304-14 and TMS 402-16 design standards revealed that, similar to the loading provisions, the methodologies used by both standards for computing reinforced masonry element resistances are generally similar in nature. Key nuances identified include:

  • Lower CSA S304-14 masonry compressive strength, f'm and flexural tensile strength, ft which were noted to be approximately half of those specified by the TMS 402-16 standard;
  • Use of a directionality factor, 𝛘, in CSA S304-14 which impacts the resistance of masonry elements where compressive stresses are applied normal to the head face;
  • A lower CSA effective compressive width of 4t when computing out-of-plane resistance of masonry walls which is triggered much sooner compared to 6t used in TMS 402-16; and
  • Differences in resistance (strength) reduction factors with impacts noted to be more prominent in compression-controlled responses.

Parametric studies were carried out to investigate the nuances identified during the standard comparison. The studies illustrated the following:

  • Lower axial resistance in CSA S304-14 under combined axial load for out-of-plane bending response due to lower masonry compressive strength values and lower masonry material resistance factors;
  • Lower CSA S304-14 squat wall resistances due to reduced moment arm;
  • The CSA S304-14 reduced moment arm provision was shown to overestimate the actual moment for wall aspect ratios near 1.0;
  • Reduction in CSA S304-14 resistance in tension-controlled regions of combined axial and out-of-plane bending response due to smaller effective compression width of 4t versus the TMS 402-16 provision of 6t;
  • Reduced CSA S304-14 beam resistances, nuances attributed mainly to the directionality factor, 𝛘 and to greater compression stress block depth as a consequence of the smaller masonry resistance factor and lower masonry compressive strength; and
  • Overall reduced seismic capacity of shear walls inhibiting their use in regions of high seismicity in Canada.

Preliminary designs of two building archetypes were carried out at two locations along the Canada-US border to identify nuances in location-specific design. In general, the two-storey mixed-use archetype design in the US was achievable with either smaller masonry block units or units of the same size but with significantly less reinforcement. Several differences were noted in the beam designs and highlighted how restrictive the Canadian design provisions are in comparison to the US. On the other hand, the multi-storey residential archetype exercise revealed that designs with a greater number of storeys can be achieved using the Canadian provisions. The number of storeys was restricted in the US due to the maximum reinforcement limit, a provision which is not included in the CSA S304-14 design standard.

Although a number of suggested changes and research needs have been identified, the following three areas have been identified as having the most significant impact on CSA S304-14 masonry behaviour response:

  • The masonry compressive strength, f'm;
  • The effective compressive width, 4t; and
  • The directional factor, 𝛘.