Providing vertical support for masonry veneers
Shelf angles play a crucial role in cavity wall systems as they provide support for masonry veneers that cannot be directly supported on foundations or interstorey slabs. Shelf angles can come in many different configurations and may be constructed from miscellaneous steel sections, welded on site, welded off site, have embedded anchoring, or have bolted connections, and they can be manufactured using proprietary techniques.
The purpose of this page is to highlight several aspects to take into consideration when specifying shelf angles.
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
Structural shelf angles always require engineering design. The structural design of a steel shelf angle, following applicable design standards, should consider the support requirements for masonry outlined in CSA S304-14.
Although off-the-shelf and proprietary systems for shelf angles exist, careful attention is always needed to the minimum levels of corrosion protection of the shelf angle and detailing of flashing, insulation, and horizontal and vertical movement joints around the location of shelf angles.
Detail drawings may be necessary to ensure proper execution.
Discussion
Different methods of support and attachment
Shelf angles have traditionally been limited to generic designs and standardized specifications. However, due to the increasing size of cavities and a stronger focus on minimizing thermal bridging effects, there is now a growing demand for shelf angle designs tailored to each project’s requirements.
Typically, shelf angles are attached to the backup wall at the interstorey slab location. Alternatively, they can be directly affixed to the backup wall, provided that proper anchorage and support are ensured and the design of the structural backup accounts for it.
Providing enough support area and limiting masonry overhang
When placing the veneer on the shelf angle, it is recommended that no more than one-third of the width of the veneer extends past the edge of a shelf angle. According to Clause 6.4.2 of CSA A371-14:
6.4.2 Projection of a nonloadbearing wall or veneer
The projection of a nonloadbearing wall or veneer beyond the edge of a supporting member such as a shelf angle or edge of a beam shall not exceed 30 mm or one-third of the width of the wall or veneer, whichever is more. The projection of the units beyond the bearing support shall not contain voids.
Similarly, masonry designed using Part 9 of the National Building Code of Canada are governed by a similar provision in clause 9.20.12:
9.20.12.3. Corbelling for Masonry Veneer
(1) Masonry veneer resting on a bearing support shall not project more than 25 mm beyond the supporting base where the veneer is at least 90 mm thick, and 12 mm beyond the supporting base where the veneer is less than 90 mm thick.
(2) In the case of rough stone veneer, the projection, measured as the average projection of the stone units, shall not exceed one-third the bed width beyond the supporting base.
Shelf angle design
The loads acting on the shelf angle must be determined, and the design of steel sections used for shelf angles must adhere to relevant steel design standards, particularly the CSA S16. Similarly, the design of anchors must conform to the specific material design standards of the structural backup material. Requirements for the support of masonry that are relevant to design of shelf angles, including rigidity requirements, can be found in Clause 4.8 of CSA S304-14.
Corrosion protection requirements
It is important to keep in mind that shelf angles are prone to corrosion as they are left exposed within the cavity’s airspace. While proper installation and overlapping of flashing materials can offer protection against direct moisture, the underside and backside of the steel itself may still be exposed to external air, potentially leading to long-term corrosion risks. Corrosion protection requirements for shelf angles are indicated in Clause 4.11.3.5.1 of CSA S304-14. For conditions wherein CSA A370-14 requires veneer ties to have Level 3 corrosion protection (stainless steel), the shelf angle must be hot-dip galvanized. Otherwise, if ties only require Level 2 corrosion protection (hot-dip galvanized), then the shelf angle may be protected only with a primer coating conforming to CSA S16 as long as it is also protected by a durable flashing or in case the service life of the building is less than 25 years.
Impact on energy performance
The shelf angle serves as a vital structural connection between the exterior veneer and the backup wall. However, this connection also creates a direct pathway for thermal transfer, bypassing the insulation layer. Shelf angles act as thermal bridges in masonry veneer walls, resulting in relatively small but noticeable adverse effects on the overall thermal resistance of the wall system. The larger the area of steel that passes through the insulation layer, the greater the negative impact on the whole-wall R-value.
The trend towards increased cavity sizes in masonry walls has brought about a twofold advantage for thermal performance. Firstly, the wider cavity allows for more insulation to be installed. Secondly, the use of stand-off shelf angles reduces the extent of steel that crosses the insulation layer, when compared to shelf angles connected directly to a floor slab or wall. A fundamental principle in masonry building envelope design is the necessity for continuous insulation over the exterior of the backup wall. Therefore, it is crucial to ensure that insulation is always placed behind the shelf angle, when stand-off supports are utilized, in order to maintain the continuity of insulation and preserve thermal performance.
Differential movement
The location of the shelf angles commonly coincides with the placement of horizontal movement joints. These joints are necessary to prevent the transmission of vertical loads through the veneer from one storey to another. However, it is essential to appropriately size the movement joint to accommodate structural movements, veneer movements, and deflections of the shelf angle itself.
In the case of concrete block masonry and cast-in-place concrete, the superstructure is expected to undergo short-term elastic deformations caused by axial loads, as well as long-term creep and chemical shrinkage due to carbonation. For clay brick veneers, it is important to consider the effects of moisture and temperature on the short-term shrinkage and expansion of the bricks, as well as long-term chemical expansion. Occasionally, shelf angles are used to support a masonry veneer locally above an opening. This will typically happen when the opening spans a length that would be impractical to achieve using a loose lintel. In such cases, the masonry supported by the shelf angle above the opening should be separated from adjacent masonry bearing on a different support (e.g., lower shelf angle or building foundation) by a vertical movement joint in order to avoid the buildup of stresses and cracking from differential movements between the two supports.
When estimating the size of the movement joint, it is necessary to account for a range of potential relative movements that the veneers may experience over their service life. This ensures that the movement joint is appropriately dimensioned to accommodate these potential movements and maintain the integrity of the wall system.
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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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