Masonry ties are an important part of a cavity wall system, their main function is transfer all the lateral load placed on the veneer back to the building structure while also providing much needed lateral support. There are many important considerations to be aware of while designing the ties for a masonry veneer, and the wording of the specifications is important to ensure the ties are installed in the same manner they were designed.
The purpose of this page is to highlight a number of aspects to take under consideration when specifying ties to appropriately tailor the job or project being constructed to the detailed tie design.
This page is part of a larger series of specification examples meant to highlight cases where vague or poorly written items have lead to issues on masonry jobs across Canada.
The information contained here is intended to serve as educational content for designers, specifiers, or contractors. It is not to be relied upon for formal technical advice, as masonry projects may have details and considerations that are unique to a particular project and may be beyond the scope of the content of this page.
If there are any questions or concerns regarding masonry design or construction, CMDC offers technical assistance to our supporting members, as well as the design community.
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How the specification typically appears:
X.Y Tie masonry veneer to structural backing in accordance with CSA A371.
Discussion points to consider when specifying ties:
Masonry veneer walls are “non-loadbearing” due to the fact that they do not support loads from the structure, however, they must still support their own self-weight. An unreinforced 90 mm (3.5 in.) thick veneer with a height of 9 m (30 ft.) would be considered to be an extremely slender wall if it were to be constructed as a free-standing element. Acting alone, it would not be structurally stable when exposed to even the smallest lateral wind loads. For that reason, it is necessary to place ties over the veneer’s height and length to connect it to a structural backup in order to provide lateral support.
Ties are a structural lateral support and they must be designed using CSA S304-14. The designer may be able to choose between using either the main body of CSA S304-14 (Clause 9), or using the Empirical Design in Annex F, if the structure is able to meet the strict criteria which permits empirical design. In addition to the strict criteria of the empirical design approach, using Annex F limits the designer to using prescriptive ties only. With contemporary requirements for continuous insulation, many of the prescriptive tie options listed in CSA A370-14 are not applicable.
A common source of confusion is mistaking the Maximum Tie Spacing given by CSA A370-14 and CSA A371-14 as a minimum, and conservative, approach to masonry tie design.
Each masonry standard serves a specific purpose. CSA A370-14 is a material standard. It provides material requirements for the manufacturing of masonry ties, anchors and fasteners (collectively termed connectors). CSA A370-14 provides material properties such as minimum connector strength, maximum connector free play, and maximum spacing limits. CSA A371-14 is a construction standard. It provides requirements for the construction of masonry. CSA A371-14 outlines guidance for many different aspects of construction, including but not limited to: tolerances of construction, reinforcement cover and tying requirements, mortar and grout placement, and maximum spacing of ties. The design of masonry ties are solely covered by the CSA S304-14. Masonry ties must be designed to transmit lateral loads applied to the veneer from wind or earthquakes, the design of the masonry ties are subject to material properties such as the strength of the tie, expected differential movement, and construction properties such as the material used for the back-up.
Use of Annex F
For a masonry building that qualifies for design using Annex F, the masonry veneer may be designed using empirical methods. To design masonry ties using Annex F there are several important things to consider:
- The building must adhere to all of the requirements of Clause F.1.1
- Veneers must be supported by a structural backing of concrete or masonry and must have a slenderness ratio, kh/t, less than or equal to 20. Design of veneers on wood or steel stud back-up walls are not permitted through Annex F and are required to be designed to satisfy the requirements of Clause 9 of CSA S304-14.
- Only prescriptive connectors may be used with Annex F empirical design. CSA A370-14 by default requires connectors to be designed for factored loads according to the CSA S304-14. While CSA A370-14 does also provide manufacturing and design requirements for prescriptive ties, the use of these provisions are limited to the same criteria that also apply to empirical design (low seismicity, limited wind loads, reduced building height, etc.). Prescriptive connectors must also be spaced at no more than the maximum spacing limits provided in Clause 10 of the CSA A370-14 as well as follow all of the requirements listed in CSA A370 for the specific connector used.
For all other cases: (when Annex F does not apply)
Ties in masonry veneer must be designed according to Clause 9 of the CSA S304-14 with their spacing based on the factored load and factored resistance of the tie. Maximum spacing requirements are provided by CSA A370-14 and CSA S304-14. These do not represent a conservative solution without accompanying engineering analysis.The intent of this maximum spacing requirement is to limit cracking of the masonry veneer and has no relevance to strength or performance of the tie itself.
Finally, it is important that the most recent editions of relevant CSA standards are referenced. In the case of uncertainty, one can find the relevant edition of the standard in the list of referenced documents accompanying the building code in effect for any specific area. For the current edition of the national model code, National Building Code of Canada 2015, the relevant editions of the CSA standards are circa 2014.
Suggested changes to the specification:
It is not possible to provide specific recommendations to the language of the specifications without conducting a complete design analysis. In this case it would be pertinent for the design team to recognize this conflict and resolve it.
An example of how a spec may look can be seen as follows:
Masonry ties shall be installed as per the requirements of CSA A371-14 with maximum spacings not to exceed 400 mm vertically and 400 mm horizontally. Installed ties shall be XX-XXX or approved equivalent.
In the case of veneers that meet the requirements of Annex F in the CSA S304-14 and for which prescriptive connectors are being used then the maximum spacing requirements of CSA A370-14 (10, and 7), and the installation requirements in CSA A371-14 (10.2.3 and 10.2.4) shall be followed. The specific references to the CSA A370-14 would have to be based on the type of connector and location within the veneer.
Otherwise, for all other cases the spacing of masonry veneer ties must be determined from engineering analysis and specified by the designer based on the requirements of CSA S304-14 (9) and within the maximum spacing permitted by the CSA S304-14 (126.96.36.199).
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