One aspect of engineering we run into often is the difference between practical and professional. We aren’t talking about dress attire in the office; in this conversation, we’re referring to the realities of construction and the difference between competent engineering and aesthetic engineering.
When we’re designing structures, it’s always important to keep the end result in mind so we can provide a design that works in context. Let’s look at an example of why this is important.
In structural design, deflection refers to the displacement of a beam due to a load. Canadian Building Code (Section 9.4.3.) specifies the maximum deflection criteria for structural members. The maximum required deflection is provided as L/360, where L is the length of the beam or span. This means that for a 30ft (9m) beam, the allowable deflection per Code is 1” (25mm). For a more extreme example, if we double the length to 60ft (18m) we are allowed 2” (50mm).
While these aren’t huge numbers from an engineering perspective, aesthetically, they could have a huge impact. Having your floor drop 1” can drastically change the reveal you will see on trim work, or how straight your shelves end up. In addition, when we design per Code, there is no requirement to take in to account the dead load acting on the structural member when determining deflection. This can further impact your deflection!
Resolving the Challenge of Deflection Limits
Unfortunately, there is little guidance outside of the Building Code for what to do to remediate the issue. CSA S16-14 for steel design has a maximum deflection recommendation (not requirement) of L/360 for roofs and floors, and again, this is only based on live loads or snow loading. CSA O86-14 for wood design specifies elastic deflection to not exceed L/180, and permanent deformation of L/360.
With so little outside guidance, it’s important for engineers to have conversations with their clients about the parameters of their structure. If they’re building a shop to repair cars, maybe the beam having a 1” deflection doesn’t matter. However, if they’re going to put their $10,000 Italian marble floor on top, that 1” may matter and should be reduced.
This is an important consideration, as the difference between the beams to get from 1” to ½” could be substantial. At CVL Engineers Inc., we’ve seen cases where the beam has had to more than double in size to achieve this! Despite the challenges, we’re always willing to work with clients to find the right solution.