“Back when I was in school, they never…” How cliché is that line, right? That saying was for “old” people, like people in their 30’s! Well, I found there’s something to be said for experience after all. Back when I was in school I never learned about post installed anchors (PIA’s). So after I graduated, like every good engineer, I read! And asked a lot of questions from people who were around the block once or twice. But then I realized something. Those people were only telling me what they learned from the people they asked! This grapevine was good, but a fourth generation grapevine leaves a lot of room for error. My physics professor would have had a canary with the error propagation! The more I heard, the more conflicts I heard in opinions. I realized that I really didn’t trust what people were telling me about PIA’s at all. My license was on the line for crying out loud! And if I couldn’t “prove” it, how could I trust it? So what did I do? I tested it…
And tested. There are a lot of different base materials in NYC you know. Yeah, there’s concrete for starters. But then you have terra cotta, limestone, bedrock, brick of every origin, cmu, cinder blocks, cindercrete, ceramic tile… and oh yeah, steel! Do you think Hilti, Powers, and Simpson test for everything? It is not possible.
Getting back to PIA’s. Are PIA’s important? You betcha! Ask yourself this, “What is the glue (no pun intended) that is holding everything together?” Your connections! Connecting two similar materials is not a big deal. But two dissimilar materials? And old buildings in NYC that have been exposed to ice, snow and wind for hundreds of years… Not to mention water damage and fire (throw in a random tornado too while you’re at it). What kind of capacity can you expect?
Many anchorages, especially for architectural details, are overlooked. The structural engineer is focused on the big picture. The MEP engineers want everything to run smoothly. The architect is focused on the details that make the building unique (and there are A LOT of details!). Something weighing 50, 200, 1,000 pounds is not a huge load in the grand scheme of things. But when that load is hanging from the middle of your ceiling and 100’s of people are walking past it every day, it better be secure.
Now some base materials will surprise you. Some old brick that looks like it would crumble in your hands will give you 1,000 lbs of tension with no problem. Some bedrock that you would think would give you 10,000 lbs easily, will only give you 4,000 lbs if you install it against the grain. And terra cotta? I’ve seen it hold everywhere from 100 lbs to over 5,000 lbs. Cindercrete will give you 2,000 additional pounds of tension capacity with the same diameter anchor, same embedment, same adhesive but a specialized helical anchor rod. There is no way to know exactly what you’ll get unless you test it.
Testing it is easy (at least it should be) if you know what you’re doing. It doesn’t have to take a lot of time (if you plan, prepare and have the right equipment). And it can give you peace of mind knowing you are not guessing… (ahem) I mean estimating the anchor capacity. How do you know when you’re guessing? You think about your design when you get home. You dream about a certain connection. It’s like a splinter in your mind (like the Matrix). Something’s not quite right but you’re not sure exactly what it is.
I’ve had an engineer “assume” the capacity of an anchor set in a stone wall would be at least the minimum of a manufacturer’s allowable load using double the embedment depth and twice the diameter. He was right. The adhesive bonded brilliantly with the stone. And as I pulled the stone right out of the wall during a tension test, the realization that the mortar was like sand had finally occurred to him. Thankfully they were still in preliminary design. They chose to go another direction…
So the moral of the story is “Don’t guess it, Test it”.
Brian Clarke, PE