Friday, May 15, 2020

Little details add up

The thick black, vertical line is an I-beam that supports one end of the trusses beneath the new roof. The trusses are the thinner, horizontal lines.

I have a friend who does some engineering in his retirement.

He may be reconsidering that.

Please understand that some details have been changed to protect his privacy and the privacy of his client.

The client is a restaurant that decided that they wanted to expand their floor-space. They live in an area with lots of tourist traffic. Since the peak customer counts are when the weather is nice, the client decided to convert the flat roof to a space where he could serve customers. How hard could it be? Throw some umbrellas and beach umbrellas on the deck and you are good-to-go.

Unfortunately, changing the roof from a roof to a retail/restaurant drives some changes in how the code looks at it. That, in turn necessitated some changes to the structure beneath the roof.

First, the asphalt-and-gravel roof would not do. The builder advised the client to go to a pre-cast pier and modular deck panel system. The modular panels are a much better choice for high-heels than the more typical decking a home-owner might choose.

Then, the space needed railing that was compliant with code. The client's wife decided that she wanted a transparent partition made from tempered glass.

None of those decisions sound like a big deal, right?

The modular pier-and-panel flooring added 110 pounds per square foot. A typical, residential deck runs about 10 pounds per square foot.

The live-load for commercial floor space is 100 pounds per square foot vs. the 40 used for residential.
Snow load in pounds per square foot to be used in code calculations.

The snow load calculations were impacted because the solid railing will not allow the snow to be cleared by the wind. The code specifies that the snow-load be calculated as if the  volume enclosed by the railing was filled with snow (42" deep) in addition to the snow-load specified for that particular county in Michigan. That penciled out to a snow load of approximately 90 pounds per square foot.

Would the entire roof ever see that much snow? No. But the portion supported by the I-beam might if the wind blew the snow off one leg of the roof and stacked it up in that corner.

Flat commercial roofs are also required to be able to support the ponding effect of water that cannot drain due to clogged drains. That adds about 25 pounds per square foot...about three inches of standing water.

The weight of the actual roof adds about another 30 pounds per square foot.

So, what are we up to...100 + 110 + 90 + 25 + 30 = 355 pounds per square foot. There are a few other minor loads but these are the biggest ones.

Then the code requires a factor of safety of 4, so the I beam beneath the trusses must be sized to support a load of 1420 pounds per square foot.

The I-beam bounds one side of an area of 900 square-feet. 1420 Times 900 is 1,280,000 pounds.

"Joey, it is the same as asking the roof to be able to support a swimming pool that measures 30'-by-30' and is over twenty feet deep." my friend said.

A good thing steel beams are cheap.


  1. A few days ago I was watching a documentary about the collapse of the skywalks in the atrium of the Kansas City Hyatt Recency in 1981 that killed 114 people. Long story short, the welded box beams that the skywalks were suspended from were not nearly as strong as they were thought to be. They would have had to be at least three times stronger to carry the load.

    As a direct result of this disaster, civil engineering standards were revamped nationwide, with new standards often requiring that load bearing structural steel beams be three to four times as strong as before. Also, the engineering firm that signed off on the design at first tried to pass the buck by saying the contractor that built the welded box beams did not build them right. The new rules said that if you as the engineer sign off on the design, then YOU are responsible for making sure everything is done right. So these are probably some of the reasons that your friend is being required to have it built strong enough to support a 20 foot deep swimming pool.

    As an aside, I worked at that Hyatt for two years from 1991 to 1993. Some of my co-workers had been there that night, and ten years later were still haunted by the memory of that experience.

    1. I remember that disaster. The threaded rod was supposed to be continuous and to engage both top-and-bottom surfaces of the box section.

      The fabricator decided to take a shortcut. They welded nuts inside the box section and threaded the rod into the top an inch or so and then had a second rod thread into the bottom one.

      So the top box section not only was carrying its own load with just one nut, but all the load from lower mezzanines also passed through that nut and horizontal section rather than being carried by the rod in tension.

      The box section had been fabricated from a couple of channels that had been welded together flange of two "C" channels to make a box. The nut pulled through flanges.

      There were a half dozen minor modifications that could have been executed to avoid the tragedy but none of them were done. My biggest take-away is that an engineer who comes up with a design that is very, very difficult to execute is culpable when the guys at the job-site take a few, invisible short-cuts.

    2. I remember seeing a documentary about the C channels. The channels were placed flange to flange instead of back to back. Probably to accommodate the change in fastener style.

    3. I found a detailed analysis here:

      and overview here:

      Lots of errors in design going back to the original thru-rod version.

      Yet another teachable moment on why Engineering is HARD!

  2. Yep, and it will be interesting to see if they get it done correctly...

  3. Very high cost for a small amount of additional floorspace for a short season. And not a lot of profit in food service anyway. Not a viable project from the looks of it. ---ken

    1. New restaurants are nature's mechanism for separating heirs from their parent's money.


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