Structurally Speaking

The city rejected the garage plans.  The ceiling was too high.  Placing wide windows above a service door using standard construction practices, resulted in the top plate being above the city limit of 10-feet, zero-inches.

I wasn’t ready to give up on my high windows. I have learned what an impact natural light makes in a room, having converted a former chimney into a skylight that transformed the space in our upstairs kitchen and living area. 

But standard construction practice is based on conservative, rule of thumb materials and techniques, developed over the years to assure adequate structural integrity with a minimum of expense, including design expense. It works well for the general construction industry. It is also an interesting concept integrated into the engineering of buildings and structures in general.  There is what’s called a “margin of ignorance” factored into the design. This is for safety reasons. Material properties and environmental conditions are not always what was assumed during the design, and so a margin is provided.

The margin of ignorance is contained in the spirit of the residential building code conventions, but can become an unnecessary constraint. Take, for example, my case. I would like to install windows that have six-foot spans. Normally, this would demand a 12-inch header beam above it, to distribute the load around it to the sides of the window. And this is what the conventional building code would require.

But in this case, there is no load. The side walls of the garage are not load-bearing. There is no roof weight, and consequently no snow-load weight on these walls.  A 12-inch header is way overkill. Five inches will do fine. Other beams could be downsized as well. And I completed the calculations to confirm it. I was able to fit my windows in over the door and meet the ten-foot maximum ceiling requirement.

My design calculations are not sufficient for the city however. It struck me as a bit odd that the city building code officials were not structural engineers; isn’t that what they are there for? To review the designs that are submitted for structural integrity and the safety of our citizens?

Evidently not. But that doesn’t preclude them from approving, or not, the structural calculations of others, and so my design for snow loads, torques, shears, and stresses would not be accepted.  It had to come from a certified professional structural engineer.

I attempted to get my nephew, a civil engineer furiously studying for his PE (professional engineer) exam to use my garage as a practice problem. He wisely chose to defer the exercise. It wouldn’t have worked anyway. He would have reached the same results, but he didn’t yet have the credentials to convince the city.

Instead (via Angies List!) I was fortunate to find a local structural engineering firm that takes on both large and small projects. I submitted my design for a proper structural review, and with a few modifications, obtained a plan that should satisfy the most critical city planner.

I will be able to enjoy the clerestory windows after all!

Garage Framing Review Package – 14152

 

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