July 28, 2005
Let’s suppose that you are building a metal bookcase or cabinet, with prefabricated pieces. The shelves have formed longitudinal flanges to impart rigidity, and smaller flanges, with bolt holes, on the ends, The vertical sides, and perhaps the back, if there is one, have matching holes. All the components, painted with enamel, are smooth. Let’s suppose that the shanks of the bolts are 1/4 inch in diameter. The holes in the pieces are necessarily slightly larger to permit insertion of the bolts. They may be 5/16 or 3/8 inch in diameter.
Suppose you assemble the whole bookcase or cabinet, tightening the bolts up snug tight. Assume for the sake of argument that all the matching holes are perfectly aligned, with the bolts centered in the holes. This, of course, is unlikely to happen precisely that way, but it could.
Now you load the books or other products, and all the shelves slip slightly to the point where the shanks of the bolts bear upon the lower edges of the holes in the supporting elements, while the uppers edges of the holes in the shelves are bear on top of the shanks, so that the shelves can be depressed no more. Now all the weight of the supported shelves is transferred to the vertical elements through the bolts, which are said to be subject to shearing stress or simply shear.
These connections are called bearing-type connections, because some of the elements bear on the bolts, and the bolts bear on some other elements.
Until mid-century, steel structures were largely riveted. After mid-century, connections made in the shop were welded. Those made in the field, where welding was more difficult, were bolted. At first, the rule was to use bearing-type connections. All the beams, girders and trusses were fully painted beforehand, erected on the columns and connected with bolts that were not necessarily tight enough to prevent the slight slippage due to the oversizing of the bolt holes. Girders and beams would slip down till they engaged the shanks of the bolts, and the loads would be withstood by shear in the bolts.
Later, a new and superior approach to bolted structures was adopted.
Returning to the example of the bookcase or cabinet, let us suppose that, instead of having all surfaces smooth and glossy, we omit paint from all the faying (matching) surfaces, scuffing them up with a power wire brush. Then we assemble the pieces in the same way, supposing that the holes are perfectly aligned and the bolts centered in the holes. But this time we tighten the bolts so tight that even after we load the books or other products, the shelves don’t slip, but are held rigidly in place by the friction between the scuffed-up faying surfaces. Such connections are called friction-type connections. Even if slippage should occur occasionally, the net result in a structure would be increased rigidity and stability.
Today, in steel structures, friction-type connections are the rule. Note that the bolts are no longer in shear and that there is no bearing of elements on shanks, or vice versa. The whole load is transferred by friction.
Generally, doubling the weight or force between two steel elements doubles the amount of friction, and so forth. So by determining the coefficient of friction for such connections, with the faying surfaces left unpainted, we can determine the tensile stress that must exist in each bolt to induce the amount of friction in the total area subservient to that bolt until it equals or exceeds the allowable load of the bolt, had it been designed to act in bearing. This load is called the ‘proof load’, and is very high. For example, a 7/8 inch bolt of ASTM A325 steel, whose allowable load in shear is 9,000 pounds, must be tightened to a proof load of 39,000 pounds, which puts it in tension rather than shear, to satisfy the requirements for friction-type connections.
The prescribed method for determining the amount of tension in the bolt is to tighten it snug tight, that is, as tight as a man can tighten it with a wrench, and then to tighten it with an impact wrench through an arc of so-and-so many degrees, depending on the length and diameter of the bolt, usually 240º to 360º.
Bolts have the same allowable load, regardless of whether they are in bearing-type or friction-type connections. In the worst-case scenario, bolts that are or have become loose just revert to bearing-type calculations. But these will be only few in the whole structure.
After the bolts are tightened, touch-up painting is done. Maybe 150 years ago they would just have said, “Tighten it up good and tight, Gallagher!” Now they have it down to a science.
About the author Thomas Keyes: I have written two books: A SOJOURN IN ASIA (non-fiction) and A TALE OF UNG (fiction), neither published so far.
I have studied languages for years and traveled extensively on five continents.
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