Like most builders, Ralph did not have a clear grasp of the above structural concepts. “Unfortunately,” I told him, “What you don’t know can cost you. For example, take stirrups (Stirrups are “U”- or “S”-shaped rebar placed vertically and closely spaced — usually four or six inches apart — in concrete beams/lintels.) You’ve got way more in this job than you need. They’re in the wrong places, and you’ve got a ton where none are required at all.”
“Great,” he said, kicking some dirt. “They’re a pain to bend and worse to snake into the forms. And with the price of rebar going sky-high…”
“Here’s the deal, Ralph,” I said. “Stirrups do one thing — they resist shear stress.”
He wrinkled his forehead.
To help him understand the concept, I said, “Imagine a stack of 10 1x4s, say 15-feet long. If you and a friend picked them up, a guy on each end, they would sag tremendously under their own weight. But, if you glued them all together, then picked them up, there would be no sag at all. Now they’re acting as a 4x10 beam. The glue is taking the shear stress. A beam like that, supported at each end, is called a simply supported beam.”
A light flickered behind his eyes. I continued. “Where is the shear stress in a simply supported beam the greatest? Go back to the 1x4s before glue is applied. Pick them up and notice where the 1x4’s slide the greatest distance relative to each other. It is at the ends. In fact, if you look closely at the middle of the sagging stack, you’ll see that the 1x4s don’t slide at all there. The farther from the middle you get, the more the 1x4s slide relative to each other. Thus the shear stress in a simply supported beam is greatest at the ends, and goes to zero in the middle.”
“What does this have to do with stirrups?” he said.
“Stirrups resist shear,” I replied. “Are they needed in the middle of a simply supported beam? Almost never. I say almost, because if there is a heavy point load on your beam, there may be high shear stresses at or near that load. But for most beams and lintels, where no big girder truss or other beam brings a point load, you only have to worry about stirrups near the beam ends.”
“But I’m putting stirrups all the way across most lintels, as well as in the walls beyond the ends of the lintels! Are you saying all those stirrups are worthless?”
“Most of them, yes,” I replied. “Many are serving no purpose, other than costing the owner money –- and bringing you grief. Further, concrete itself is good at resisting shear. It is, after all, a ‘glue.’ Many lintels need no stirrups at all because the concrete resists the shear by itself.”
Then we discussed why rebar goes in the bottom of all lintels and in the tops of some.
I wrapped up our conversation with this advice: “Understand the theory of your construction — then be sure to question your engineer as to whether all those stirrups, bond beams, holdowns or whatever are really necessary. If you don’t get a straight, understandable answer, find an engineer who will give you one. Remember, it doesn’t cost the engineer a penny to over-design. But ultimately someone foots the bill.”
The views expressed in this article represent the personal views, statements and opinions of the author and do not necessarily represent the views, statements, opinions or policies of the National Association of Home Builders. NAHB does not necessarily endorse any of the views expressed by the author and NAHB is not responsible for any direct or indirect consequences arising out of the views expressed in this article.
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