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Posted by David Sherman on Saturday, February 28, 2009 at 00:55:12 :
In Reply to: Some other things to keep in mind posted by Kevin in Ohio on Friday, February 27, 2009 at 23:50:50 :
In structural steel (bridges and buildings), bolts have replaced rivets in places where field welding won't do. In those applications, the bolt shanks are stressed in shear and are full-diameter, with the threaded part only threaded far enough to accept the nut. The threaded part of the bolt has only to hold carry the tensile stress caused by tightening the nut, whereas the shear stresses of the structural joint are carried through the full-diameter shank. Although a hot-riveted joints will gain some strength due to the rivet shrinking and pulling the pieces together, where friction will help hold them, you can't count on such friction in design calculations, so it's a moot point.
The main assembly difference between riveted and bolted steel is that an undersized rivet can be put through slightly misaligned holes and will expand to perfectly fill the holes when you set it, whereas to get a bolt to perfectly fill its holes, you have to either drill the holes in the field with the pieces held in position, or drill them undersized in the shop and then ream them together to full size in their assembled position.
If absolute strength in a frame is a concern, these same considerations would apply -- don't count the squeezing effect of the rivets as being worth anything, but do ream or drill misaligned holes to identical size and position and do use a bolt that's only threaded as far down as it absolutely needs to be. A split lock-washer under the nut is a handy way to provide for a little tolerance and for the imperfect ending of the thread, while making sure that only unthreaded metal is within the joint.
I doubt that grade 8 bolts are necessary in an old-fashioned non-tempered steel frame. The compressive stress on the loaded portion of a bolt is the same as that on the adjacent portion of the frame next to the hole, and with a mild steel frame, the frame metal will probably deform before a grade 5 bolt does. However, in any design of such a joint, the engineer will calculate both the shear stress in the bolt itself, and the compressive stress at the edges of the holes, to see which one is weaker. Common joints can fail either way (either by tearing the metal or by shearing the bolt). Crush-resistance goes up directly with the diameter of the bolt, whereas shear resistance goes up with the square of the diameter of the bolt, so if it looks like the joint as designed will fail by shearing the bolts, you can switch to fewer, but larger fasteners, whereas if it looks like it will fail by crushing the steel, you would use more fasteners, but smaller ones. Of course then you also get into the weakening of the structure due to all the holes, which also has to be calculated.