We're continuing our 30 day post about structural engineering! Today, we're talking about steel!
STEELER NATION
Steel comes in a variety of strengths, but for beams, you'll often find a very narrow range of steel strengths available for sale, making it easier to determine what a building's steel strength may be.
This wasn't always the case though. In the early 1900s, steel mills would often manufacture their own beams to their own specifications. Some of the most well known steel used in US structures came out of Pennsylvania, which is why their football team is called the Steelers. In fact, their logo is the Steelmark logo with three stars that indicate the three ingredients to make steel. Yellow for coal, orange for iron ore, and blue for scrap steel.
You can read about the logo here :
https://en.wikipedia.org/wiki/Steelmark
Since a standard wasn't in place, one buildings steel strength could be different from another. Often the strength would be specified in the engineering drawings and steel mills would provide steel strength reports for engineers to approve. This means that if the steel strength isnt listed in a drawing then you would have to use the lowest steel strength that was available for purchase at that time, or you have to take a sample to a lab for testing if you want more reliable data.
On my project, we didn't have all of the structural drawings, so our schematic design calculations utilized the lowest steel strength available at the time. This resulted in the majoroty of the steel we were working on needing to be reinforced due to the reuse of the building.
Our team discussed this and decided that testing the steel was worth the cost if we can reliabley determine that our steel strength is stronger than what we assumed.
TESTING STEEL
There's really no way around it, but you need to cut a sample of steel out of a beam in order to get a sample out to the lab for testing. You naturally want to cut the steel out from a portion of the frame where it won't affect the frames capacity. Below are some photos of the samples that we took back to the lab. You'll notice they are fairly large because the lab needs to mill them down to a specific dimension to be tested.
THIS PHOTO SHOWS THE LOCATION WHERE WE DECIDED TO CUT OFF WHAT IS KNOWN AS THE BOTTOM FLANGES OF A I BEAM. THIS LOCATION WAS CHOSEN BECAUSE THE BOTTOM FLANGE OF A BEAM TOWARDS THE END
When we got the results back, we realized that there was quite a bit of variation on what we received however most of the steel was roughly 33ksi, or 33,000 psi. The lowest sample was 27 KSI, and the highest was 33.3 KSI. That's a significant difference. At this point you, as the engineer need to decide which value to use. On the conervative side, you would use the lowest value, 27 KSI. On the more practical side, you would use the average all which is 32 KSI.