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“What!” he exclaimed. “Don’t you see that giant hole?! Are you telling me it’s okay to hog out that much meat and not kill the beam?”
“Yes and no,” I said. “There are locations in every beam where stresses are very low. Those are the places to core and notch for plumbing, electrical and HVAC. Of course the trick is knowing exactly where. It looks to me like your pal Hank, understood this concept well. His 3-1/2-inch hole is exactly in a place of zero stress. On the other hand, if a hole is placed in the wrong spot, disaster is sure to follow.”
Hank, who had tagged along, looked over his crossed arms smugly and grinned.
“Blind luck,” Joe retorted. “How can I learn where these zero-stress places are?”
“By attending a ConstructionCalc seminar, or reading my Basic Structural Concepts book,” I answered. “If that’s too much trouble, get any plan set by an engineered I-joist manufacturer and look them over carefully. Usually there are diagrams showing typical places where joists can be cored or drilled. Beware, however; funny things can happen if you’re dealing with cantilevers or multi-span joists. Also, beams with complicated loading don’t always follow the rules. It’s best if you really understand the concepts before you core. Of course, if in doubt, ask your friendly, neighborhood engineer.”
“How is it Hank lucked out on this beam here?”Joe growled. “Explain to me why this beam isn’t ruined.”
“Sure,” I replied. “This beam is simply supported, meaning it doesn’t have a cantilevered end, and it doesn’t have any middle supports. Also, the loading is coming from floor joists only — there are no other beams or columns bringing concentrated, or point loads. So, this means that shear stress is very low in the middle third of the span — where the hole got cored. However, it also means that bending stress is very high in that same region. Had the hole been cored at the bottom of the beam, or at the top, it would have been a gigantic problem. But Hank, wisely, cored the hole at mid-depth, and at mid-span, the exact location where all stresses approach zero.”
Joe shot a glare at Hank, who beamed.“Yep,” Hank said. “Jes’ like I planned.”
“Yeah, right,” Joe retorted. “You put that hole there only because that’s where the toilet above came out, and the fall in that drain line wound up hitting the beam at that particular depth. Pure luck.”
Hank just smiled and turned away. “I got work to do,” he said.
“Luck or not, what’s important here is you two understand there are places it’s okay to core and notch, and there are places where you can’t,” I said to Joe. “Don’t guess, though. The consequences can be very dangerous, expensive, or both.”
Tim K. Garrison P.E. of ConstructionCalc.com has authored books and short courses and lectures on topics relevant to builders. Got a technical or management issue? E-mail buildersengineer@constructioncalc.com. Tim reads every one.
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I was walking a construction site the other day and came upon a fracas. A plumber and a framer had come to fisticuffs.
