1950's rivet analysis - single v double shear bearing
1950's rivet analysis - single v double shear bearing
(OP)
I have an existing riveted truss, constructed from double angles. The rivets are 3/4" and gussets are all 3/8" pl. After some digging, I found some information for riveted connection design in old AISC specifications. One note has me wondering what I limit the capacity to if I follow what was outlined in the old code. it states "only that portion of a rivet or bolt shall be considered in double shear bearing which lies between two portions which share the reaction therefrom. The remainder of the rivet or bolt shall be considered in single shear bearing". For those unfamiliar, the old code prescribes an allowable bearing stress that is different for single and double shear. So I take this to mean that in my situation, the 3/8" gusset could use the bearing stress value for double shear, but my outer plies, in this case the angles, would use the bearing stress value for single shear? I guess the way the section is worded just has be uncertain if that's the intent. In effect, if the capacity of the connection is limited by the bearing (which I think I will be), its a 25% difference in allowable bearing, and I don't want to be on the wrong side of the coin. Since the loads in the web members are increasing, however, I also want to make sure I am correct, and don't penalize something that doesn't need to be.






RE: 1950's rivet analysis - single v double shear bearing
I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.
RE: 1950's rivet analysis - single v double shear bearing
RE: 1950's rivet analysis - single v double shear bearing
RE: 1950's rivet analysis - single v double shear bearing
RE: 1950's rivet analysis - single v double shear bearing
Mike McCann, PE, SE (WA)
RE: 1950's rivet analysis - single v double shear bearing
Also, for every shop rivet'ed truss that I've checked, the connection was designed based on the member capacity, not member demand. In the age of rivets, material was expensive, members were small, and labor was cheap.
RE: 1950's rivet analysis - single v double shear bearing
However, the examples which Lothers gave indicated that his understanding was that rivets in a two member connection are in "single shear", while rivets in a three member connections are in "double shear", the difference being the eccentricity, not the number of plies. So if his interpretation is correct, the rivets as described in structSU10's joint are all in double shear.
RE: 1950's rivet analysis - single v double shear bearing
RE: 1950's rivet analysis - single v double shear bearing
Maybe I can shed a little light on the subject, although the code edition (5th Ed.) that I think you are referring to was last printed just before my start as a Structural Engineer. I actually learned using the 6th Ed. My take on the matter...
The AISC Spec. 5th Ed. Sec. 22(c) which you quoted, talked about double shear bearing and single shear bearing, and Sec. 22(b) defined bearing area as the rivet dia. times the bearing length (Pl. or matr’l. thickness). Then, Sec. 15, Allowable Unit Stresses; gives an allowable shear stress of 15ksi for rivets; and allowable bearing stresses as 40ksi for double shear bearing and 32ksi for single shear bearing; and a table on allowable rivet loads (pg. 270, my 5th Ed.) shows these same values and tabulated loads for these conditions. And, I believe the difference in these two bearing values is to account for the following: their definition for single shear bearing accounts for the fact that the rivet is much more heavily loaded near the faying surface, some sort of a triangular or trapezoidal bearing stress diagram (block), and yet the bearing area calc. is (rivet dia.)(pl. thick.). So, some lower allowable stress seems appropriate to control the max. bearing stress at the faying surface in the joint, the 32ksi is some average stress on the trapezoidal stress block. A good share of this stress diag. has to do with joint movement and rivet bending (canti. type deflection) in a single shear connection. On the other hand, in a double shear bearing situation, that is, at your 3/8" gusset pl. the rivet is more uniformly bearing across the thickness of the gusset, almost a uniform stress block, so the higher bearing stress more closely matches the max. stress at the faying surface. Still, the bearing cap’y. calc. is (.75" rivet dia.)(.375)(40kis).
We’ve long recognized that allowable bearing stresses can be almost as high as or slightly higher than the basic tensile yield stress Fy in many cases without causing any problems, and the 6th Ed. of AISC allowed that Fp could be 1.35Fy for bolts and rivets. My interpretation of the 5th Ed. RE: that rivet cap’y. would be the 3/4" rivet in double shear (15ksi)(2 x .4418) = 13.25k; or, the 3/8" gusset pl. in double shear bearing (40ksi)(.375 x .75) = 11.25k; or, the two side angles in single shear bearing (32ksi)(.75 times and leg thick.) = ?
Hokie.... My take on the matter is that the rivet is in double shear, but they define rivet bearing conditions in two different ways so as to account for the slightly different way the rivet or bolt act in bearing in an inner pl. hole (inner ply, double shear bearing) vs. an outer pl. hole (outer ply, single shear bearing). Didn’t you ever collect one of those pre 6th Ed. copies of the AISC spec.?
RE: 1950's rivet analysis - single v double shear bearing
Unfortunately, at my wife's insistence, I have culled my collection of references in recent years. In some cases, I have lost some good ones. The earliest American AISC Manual I have retained (at least that I can find) is the 8th edition, 1980.