The Devil is in the Details
The first day of my Thermodynamics class in college my professor wrote the following statement on the board: “Units Will Eat Your Lunch.” It was especially funny when he said it in his thick Eastern European accent. He was trying to emphasize the need to pay close attention to the details in solving a problem. He, of course, is absolutely right.
I undoubtedly burned myself multiple times in his class. The most poignant lesson I learned on units was in my Statics class though. I took a one question quiz where I was to solve for the moment reaction in a support. I worked it all out and felt very confident in my answer. After class, I asked other students what answer they got and they all got the same answer as me.
When I got my quiz back, however, I got a 50 %. The number I had arrived at was absolutely right. However, when I wrote my units down I just wrote “kips” instead of “kip-ft”. We could discuss the fairness of giving someone 50% (a failing grade) simply for leaving the “-ft” off, but I can tell you with certainty that I learned my lesson.
Isaac's Notes: Units are a killer. Not only in school but on both the FE and PE exams. Make sure you understand what units and how to convert them. The front cover of the CERM is your best friend when it comes to this so become familiar with it and use it! Understanding this can make a huge difference!
I paid close attention to my units from there on out.
I was not going to make that mistake again. For those studying for the PE, SE or FE exams: be advised that units can eat your lunch on these exams as well. There could be answer options that will look right if you mess up your units while solving the problem. It is not just on exams that units and other small details can eat your lunch.
Details Matter in the Real Engineering World
As Engineers, part of our job is to be aware of the latest building codes and design methods. Understanding how to properly use the design method you choose and knowing the proper load combinations, definitions and safety factors is imperative.
For building design, for example, it takes a good working knowledge of the American Society of Civil Engineers: Minimum Design Loads for Buildings and Other Structures (ASCE 7) and the International Building Code (IBC).
Because there are a lot of details, notes and exceptions in each of these, it is easy to make mistakes. This is important for those studying for the PE or SE Exam, of course. You will want to make sure you know how to apply the appropriate design method on the exam so you don’t miss a problem, but there are also real-life consequences on actual designs.
Real-life consequences could include an embarrassing error that someone else catches, a design that does not meet the relevant building codes, or a life-threatening safety concern on a construction site or finished structure.
A Real-Life Example
I work for a company that manufactures structural building products based on specifications that come from the Engineer of Record (EOR). For years it was common knowledge among our estimators, draftsmen and engineers that when a roof live load is specified alongside a snow load you do not combine them per equation 16-10 in IBC 2009, for example.
Instead, you go with the worst case of the two. This became a standard, rote practice. It seemed there was no need to re-consult the building code each time. But, there was a subtle change that took place in the 2012 version of the International Building Code (IBC 2012) and carried over into the 2015 version (IBC 2015) concerning the definitions of live loads (L) and roof live loads (Lr).
I discovered this change when I was training a new engineer we had recently hired. There were some loading diagrams specified for our product so as a learning exercise I had our new engineer run everything through the ASD Load Combinations to come up with the worst case scenario. I did the exercise as well and when we finished we compared notes.
We got different answers.
Since I was more experienced I assumed I was right. So I pulled out the relevant IBC edition, which was IBC 2015, to show him how to do it. When we started digging into it we realized we were both wrong!
The following loads were specified (I have simplified it some to make it easier to illustrate):
- Dead Load: 20 psf
- Live Load (roof): 35 psf
- Snow Load: 40 psf
The total load I came up with was based on Equation 16-10* in IBC 2015: D + (Lr or S or R) which was 60 psf. When we looked it up in IBC 2015 I went to show him the definitions of the loads. I saw something I had not seen before. Notice the following definition changes:
Lr = “Roof live load including any permitted live load reduction.”
L = “Live load, except roof live load, including any permitted live load reduction.”
Lr = “Roof live load of 20 psf (0.96 kN/m2) or less”
L = “Roof live load greater than 20 psf (0.96 kN/m2) and floor live load”
-*I have left fluid loads (F) and earth pressure (H) out of the equation for the sake of clarity since they don’t apply in this situation.
Lessons to Learn
I know this change happened a few years ago, but I hope it serves to illustrate the need to pay attention to the details (and perhaps inform you of the change if you haven’t noticed it). Specifically, to be up-to-date on the latest codes, design methods, definitions, etc. Whether you are gearing up to take the PE, SE or FE exam or designing real-life structures every day there are several lessons to be learned:
- Don’t assume you know what’s in the code without looking.
- Keep a copy of the relevant codes handy and mark frequently used pages and sections.
- Review new editions carefully for changes. You can find summaries of the changes online including videos put on the International Code Counsel (ICC) website.
- “Garbage in, Garbage out”. Don’t just rely on the design programs we use. We need to know what it is doing behind the scenes to ensure our input makes sense. Maybe the structural software would run the correct combinations from IBC 2015 based on the correct definitions, but if I just assume that the lower value between Lr and S is irrelevant because of my lack of understanding of the most recent code and don’t input it then it will still design whatever I am doing incorrectly.
So don’t forget: DETAILS WILL EAT YOUR LUNCH!
Tim Broadwell, PE
I graduated from the University of South Florida in 2010 with a BS in Civil Engineering. After school, I got the job I still have now as a job as a Sales Engineer at Vulcraft of New York, Inc. which is a division of Nucor Corporation.
At Vulcraft we design and manufacture open web steel joists and steel deck. I took the PE Exam in October 2016. I did the Civil Construction exam since my job, along with the structural design, also involves a good bit of project management and occasional estimating. Thanks to a lot of hard work studying and The Ultimate Civil PE Review Course, I passed on the first try!
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