Architects have immense responsibility for successful building projects. These include:
- aesthetics of the building
- fire-rated assemblies
- elevating devices
- wall cladding
- environmental separation
This list goes on and on. In addition to this, Item 1.5 of the Canadian Schedule B – Assurance of Professional Design and Commitment for Field Review, includes:
1.5 Performance and physical safety features (guardrails, handrails, etc.)
In addition to all that the Architect is responsible for, the concern for safety features such as guardrails and handrails is added. The architect is trained in details regarding the location, dimension and extent of the safety feature but typically must rely on the manufacturer, their structural engineer, and other supporting registered professionals for the details; particularly the structural details. Because of guardrails’ role in public safety, we at Latera Engineering feel that it is important for the community of Canadian Architects to be aware of several important factors regarding:
- Architect’s Responsibilities for Guardrails and a new APEGBC Professional Practice Guideline
- The History of Guardrail Testing
- Guardrail Installation to Wood Structures
Architect’s Responsibilities for Guardrails
Guardrail manufacturers have typically limited the extent of their responsibility to the railing alone. Because they create a product and cannot always control how it is used or installed, the manufacturers typically rely on the purchaser or supporting registered professionals for details of installation.
The guard manufacturer limits their concern to their product alone. The Structural Engineer of Record typically limits their concern to the primary load supporting members. It then becomes the architect’s responsibility to ensure that the guard load path is continuous from the top rail to the structure.
The Association of Professional Engineers and Geoscientists of BC (APEGBC) published a professional practice guideline, “Designing Guards for Buildings,” in May 2013 APEGBC that outlines the architect’s responsibilities as follows:
3.2 Design of a guard must consider the following:
- Where a guard is required
- Dimensional requirements
- Strength Design including the load path to the primary structure.
- Serviceability (deflection/grasp-ability/climb-ability)
- Relationship to Building Envelope
THE HISTORY OF GUARDRAIL TESTING
Historically, railings have been tested secured to steel frames. This accurately represents attachment to steel or concrete, but typically wood attachment has not been considered by the manufacturer. The reason for this is you cannot test the railing if you are securing it to something weaker than the product. If your mounts keep breaking, you are not really testing the railing.
Also, traditionally, the resistance of the ultimate load was all that was considered; wood crushing, fastener yielding and significant permanent deformation became acceptable as long as the factored ultimate load was resisted.
The new APEGBC practice guideline states that yielding and permanent deformation is no longer acceptable. The recommended Guide for Testing of Guards states that the guard shall resist 1.67 times the code-specified loads without yielding or permanent deformation, a significant departure from the previous practice.
In real number terms, this means that a guardrail designed for a maximum post spacing of 2.0m (6’-6.75”) in a Part 4 building must be able to resist a load of 2.5kN (563 lbs) at the top of each post, then return to its original location without any permanent deformation.
Testing completed prior to the APEGBC guideline or testing that does not reference compliance to the Guideline must be re-done or re-analyzed to determine compliance.
Guardrail Installation to Wood Structures
As noted above, guardrails have been typically tested attached to a steel frame. The result is that the same guard system secured to a wood structure would perform very differently due to the fact that wood has a far lower compressive strength and typical wood connections have a far lower tensile resistance.
The following link to sketches of typical railing attachment methods which highlight some problems with current practice:
BSK-01 The lateral loads on guards cause very high tensile forces at the baseplate. ¼” diameter lag screws do not have sufficient withdrawal resistance for these loads even at 8” long.
BSK-02 For bolted connections, the compression of the wood must be considered. A typical 4”x 4” post base plate does not have sufficient bearing area to spread the load, and wood crushing results.
BSK-03 The proprietary and patented “Breyce” addresses both the withdrawal concerns and the wood crushing by transferring the load directly to the wood joists.
BSK-04 Where the guardrail is secured directly to the fascia, it is important that the fascia have sufficient attachment to the structure to prevent tearing under the guard load.
BSK-05 Where additional attachment of the fascia to the structure is provided, the attachment of the guard to the fascia must still be considered. 6 – ¼” diameter x 4½” long lag screws are not sufficient to resist the lateral loads on the guardrail.
BSK-06 6 – 3/8” diameter x 4 ½” long lag screws are sufficient to resist the lateral loads on the guardrail provided the blocking is Douglas fir. If a less dense species is used for blocking, this detail would fail.
BSK-07 The proprietary and patented “Breyce” transfers the load directly to the wood joists enabling greater post spacing that would be possible with a typical lag screw connection.
It is important to be aware that there are some conditions involving short continuous top rails where the load on the base connection is considerably reduced. However, we recommend considering the linked sketches as valid where the guard length is greater than 12’0”, or the top rail is not continuous across multiple posts.
Recommendation to Architects
In light of the new guard guidelines published by APEGBC, we suggest the following:
- Verify that the testing complies with the APEGBC guidelines (including testing demonstrating no yielding at 1.67 times the specified load). The test report should also state the maximum post spacing of the guardrail system for both Part 4 and Part 9 load conditions.
- Obtain guardrail shop drawings early in the construction process that state the post spacing and applied loads to the structure. This will enable the Structural Engineer of Record to detail the appropriate reinforcement of deck perimeter to resist the guard loads.
- Question any connections involving ¼” diameter lag screw or lighter connection, or details with small base plates.