ENGINEERING 101_8: STEEL STRENGTHS
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.
wow...never knew how to differentiate steel...now you have given me free knowledge on steel strengths. thanks @motoengineer for this... keep it coming!
you're welcome! Ill keep on writing so as long as people keep enjoying my writing!
Sure..steemit community needs diverse writers for complete knowledge bank...keep it coming 👏
Hello @motoengineer. What about the lowest value to avoid any chance of failure. As an ex machinery mover, I know those supports take a lot of weight at times. If they fail, people get hurt. Unless you were able to match samples to i-beams and use them in specific places. Then that begs the question: did you send in multiple samples from each piece and get different results? Are the samples representative of the entire length of i-beam?
Good stuff. I-beams come in real handy for riggers and heavy equipment movers.
Geoff.
These samples were taken as one piece from different places across the building. These were taken at the end of the beam where the flexural stresses were near zero. We chose based on ease of access because we had limited areas where we could sample. I recorded where I took the sample from within the building, but there wasn't enough of a variation to make any definitive assumptions.
If you take the lowest value, you will absolutely avoid your chance of failure in terms of steel flexural overstress.
This would be the most practical choice, but it is also the most expensive! We went this route and need to reinforced more than 50% of the beams we were loading as a result of the modification to the building.
Yes it's more expensive. That can relate to topics like offshore tankers and oil rigs that don't get the neccessary care due to extra cost. There's no telling the cost down the road should an accident occur, and there's never a way to put a number on the loss of a life. Glad to hear you're choosing the responsible route. :)
Nice. Didn't expect to be reading something like this here!
Not an engineer but life and logic would suggest using the lowest value 27 KSI, to err on the side of caution!
Would love to read about different varieties of steel used in the construction of building bicycle frames. There are a number of 'grades'(?) of chromoly steel used there, priced differently, but no particular reason given for the same.
Thanks :)
https://steemit.com/blog/@motoengineer/engineering-10111-the-different-types-of-steel
I didn't want to jump right into the steel grades without giving a basic overview first! Come back soon for that!
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