[CCMC 12691-R] CCMC Canadian code compliance evaluation

• Posi-Strut®  Maximum Clear Span Tables (including vibration criteria^{Footnote (2)}, in limit states design for Canada), dated October 2018; and
• MITEK POSI-STRUT Installation Guidelines, dated October 2018.

The product must be installed in accordance with MiTek Canada Inc.’s installation guidelines noted in these documents for those applications falling within the scope of the documents. Applications outside the scope of these installation guidelines require engineering on a case-by-case basis.

MiTek Canada Inc.’s pre-engineered details within the documents outlined in (i) above are limited in scope to building designs where the anticipated loads on the following structural details are not exceeded:

• floor span tables;
• strongback installation details;
• top chord installation details; and
• rim board and ribbon details.

For structural applications beyond the scope/limitations of the above-referenced MiTek Canada Inc. publications, or when required by the AHJ, the drawings or related documents must bear the authorized seal of a professional engineer skilled in wood design and licensed to practice under the appropriate provincial or territorial legislation. Installations beyond the scope/limitations of (i) and (ii) imply, but are not limited to, the following:

• perimeter rim board/ledger resistance;
• higher loads/longer spans than the manufacturer’s pre-engineered details;
• concentrated loads;
• offset bearing walls;
• areas of high wind or high seismicity;
• stair openings;
• design of supporting wall studs/beams when the total load exceeds the NBC 2015 pre-engineered floor/roof joist tables; and
• design of supporting foundation footings when the total load exceeds the NBC 2015 pre-engineered floor/roof joist tables.

The engineer must design in accordance with CSA O86-14, “Engineering Design in Wood,” and may use the Canadian Wood Council’s “Engineering Guide for Wood Frame Construction” as a guide.

Additional installation details include:

• the assembly drawings must show the size of truss, the species and quality of the chord used, the product model number, and the relative location of the different truss types in the building assembly;
• the minimum end-bearing area must be 38 mm;
• the manufacturer’s instructions on required strongbacks and glued subfloor (adhesive conforming to CAN/CGSB-71.26-M88, “Adhesive for Field-Gluing Plywood to Lumber Framing for Floor Systems”) must be followed;
• notching or drilling of the wood chords is not allowed;
• the nailing pattern for the wood subfloor must conform to the NBC 2015; and
• ends of joists must be restrained to resist uplift and rollover. The top chord must be laterally supported at 600 mm on centre.

Fabrication of joists

The scope of this CCMC evaluation is the metal web product and the capacity of joists that may be fabricated by a third-party using these metal webs. When the product is used as floor joists, the truss fabrication details must be followed by the authorized truss fabricator. The fabrication instructions are contained in the document “Manufacturing the Posi-Strut® Metal Web Floor System,” dated October 1, 2018, and are to be used in conjunction with TPIC-2014, “Truss Design Procedures and Specifications for Light Metal Plate Connected Wood Trusses (Limit States Design),” published by the Truss Plate Institute of Canada (TPIC).

The AHJ must request conformity with respect to fabrication of the “Posi-Strut®   Metal Web Joists” in the same manner as requested for light metal-plated roof trusses. The tooth-like projections of the metal webs must be fully pressed into both sides of the chords. The structural chords must be clearly grade-marked as S-P-F No. 2 or MSR grades and must contain less than 19% MC during the fabrication. Knots or wanes under the bearing point of the web plates are not permitted.

MiTek Canada Inc. provides engineering services in conjunction with “Posi-Strut®   Metal Web Joists” specifications and offers the following support contact number for their Canadian offices:

MiTek Technical Support: 800-268-3434

The metal web components must be identified with “CCMC 12691-R” on the shipping box. In the field, the metal webs on the completed joists are identified by the joist depth/model identification (e.g., PS-10V2).

The design values obtained from testing to CCMC’s technical guide for “Posi-Strut^®  Metal Web Joists, following the principles in CAN/CSA-O86, are summarized below.  The manufacturer’s published pre-engineered joist spans were designed in accordance with CAN/CSA-O86.

The CCMC Registry of Product Assessments contains opinions on the suitability-for-use of products intended as structural elements in houses. Although historically there has been no need to regulate the structural fire performance of houses, an inherent intent of the National Building Code of Canada (NBC) is that occupants have sufficient time to escape from a house in the event of a fire. There are many factors that may determine whether that intent is achieved. The fire endurance of structural elements may be one. However, its importance may be minimized by other factors such as combustible content load, early warning devices, smoke movement and toxicity, and fire department response time; all contributing to the overall system performance. Research is underway within the NRC Construction Research Centre to determine the critical factors that affect occupant escape from houses.

Some innovative structural products have been used in the marketplace for several years and have gained the confidence of design professionals, code authorities and users with respect to their performance under typical fire scenarios. Some newer products have not been in service long enough to have gained that confidence and may present a more obvious concern.

The minimum fire performance of innovative structural materials, or alternative solutions, as compared to that of the NBC-specified conventional wood-frame construction, or acceptable solution, has been the subject of analysis and discussion for several years among fire officials, provincial and territorial regulators, and AHJs. In fire tests^{Footnote (2)} conducted between 2002 and 2008 at NRC, the innovative structural joist systems tested, and currently in the marketplace (i.e., I-joists, C-channel steel joists, metal-plated wood trusses and metal-web trusses), had a time-to-collapse below the performance of exposed 38 mm × 235 mm (2 × 10) lumber joists.

The CCMC provides this floor fire performance information to the local AHJs across Canada to aid their decision-making on whether the fire performance of floors (i.e., the time to evacuate before failure occurs) for alternative joist systems performs “as well as” the inherent fire performance of exposed 38 mm × 235 mm (2 × 10) lumber joists. As is the case for all innovative products, designers and authorities should exercise judgment in considering the use of innovative structural products for houses. Unless otherwise stated, innovative structural products for houses have not been evaluated in the context of the NBC intent noted above.

Proposed fire protection options

The manufacturer’s fire protection options for its proprietary joists is presented in below. The CCMC has reviewed this fire-protection option in comparison to the fire performance of unprotected exposed 38 mm × 235 mm (2 × 10) floor joist system.^{Footnote (1)} The proposed fire-protection option performs “as well as” exposed 38 mm × 235 mm (2 × 10) lumber joists. It should be noted that the NBC exempts single-family houses constructed using conventional wood-frame construction, in accordance with Part 9, from requiring a fire-resistance rating (see Article 9.10.8.10. of Division B of the NBC 2015). The proposed fire-protection option for proprietary alternative floor joists is not to be considered in sprinklered single-family houses or where fire-resistance-rated assemblies are required.

Posi-strut® Metal Web Joists fire protection option