Aren’t all tie designs the same?
Masonry ties are an important part of a cavity wall system, their main function is to transfer all the lateral loads applied 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 (and yes, they should be designed!). Wind and earthquakes are not exactly the same across Canada, as well as exposure conditions and other project specific factors.
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.
Part of our Masonry Specification Series
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Disclaimer
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.
Recommendations
In some cases, ties may be designed by following prescriptive requirements (i.e. for empirical in accordance with Annexe F of CSA S304-14 and prescriptive ties complying with CSA A370-14); however, in many cases the tying of masonry veneer to a structural backing requires engineering analysis and careful selection of appropriate components.
Designers are cautioned against specifying the maximum spacing of ties permitted by the standard without engineering analysis demonstrating that it provides sufficient resistance.
Similarly, specifying the installation of ties, other than prescriptive ties, in accordance with CSA A370-14 or CSA A371-14 will not ensure that the spacing will meet the requirements of CSA S304-14 to resist the applied loads.
Discussion
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 meets the strict criteria which permit empirical design. In addition to the strict criteria of the empirical design approach, using Annex F limits the designer to the use of prescriptive ties complying with CSA A370-14: Connectors for masonry. 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. A maximum spacing of the ties will result in the minimum number of ties resisting the applied loads. This minimum strength may or may not be sufficient to resist the design loads for a particular project.
Limitations on Unit Dimension
Design requirements for masonry veneers can be found in CSA S304-14 Design of Masonry Structures, Clause 9. Within the requirements, the Standard provides limits on both the unit material and unit dimensions (Clause 9.1.2) for Unit Masonry Veneer.
9.1.2 Unit material and dimension limitations
Unit masonry veneer shall be construction using clay (shale) masonry units, calcium silicate (sand-lime) masonry units, or concrete masonry units; the individual units shall be limited in height to not more than 200 mm, limited in length to not more than 400 mm, and limited in thickness to not less than 75 mm.
Note: Masonry units exceeding the specified maximum size limits may be considered to satisfy the requirements for unit masonry veneer, provided that independent testing confirms suitability for unit masonry construction methods, tolerances, and load transfer to structural backing.
The size limitations of masonry units have been harmonized throughout all of the material, construction, and design CSA Masonry Standards. Units within those size limitations are suitable for unit masonry construction methods and tolerances due to the extensive research and field experience that has been conducted on units within this size range as well as decades of satisfactory performance of structures featuring these materials. Units exceeding these size limitations are not covered by the standards and therefore the minimum requirements, performance, and tolerances may not be applicable.
Manufacturers are responsible for providing adequate testing to ensure that units which exceed the dimensional limits to unit masonry may still be designed and installed using masonry CSA standards. Test data, analysis, and relevant specifications shall be provided to the designer and masonry contractor to confirm unit suitability for unit masonry construction methods, tolerances, and load transfer to structural backing. This article provides some of the possible issues that should be considered for testing but does not represent a comprehensive list.
What the standards say about ties
Each masonry standard serves a specific purpose. More information about the landscape of masonry-related codes and standards is available here.
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 the following: 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 is 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.
Making use of Annex F (Empirical Design)
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. This includes restrictions on the height of the masonry and the seismic hazard index.
- 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.
Where to look when Annex F does not apply
For cases where Annex F does not apply, ties for the masonry veneer must be designed according to Clause 9 of the CSA S304-14 with their spacing based on the factored load and the 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.
Other points to consider
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.
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Part of our Masonry Specification Series
Offering recommendations accompanied with background explanatory material to explain how these recommendations were formed. Click here to see the full series.
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As part of our overall effort to educate the design community and our contractor members, CMDC has compiled a collection of articles meant to address common areas of misunderstanding or confusion that have resulted in issues in past projects.