|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.