AISC bolt pretension

[jp99]

I have noticed something that seems odd regarding bolt required pretension and bolt strength. I'm looking in 2005 AISC manual, ASD.

In Table J3.1 of the AISC manual, a table of minimum bolt pretension values is given, for joints that require pretensioning. For a 1 inch A325 bolt, the minimum pretension is 51 kips.

However, in table 7-2, for a one inch bolt, the available tensile strength is only 35.3 kips.

It looks like the minimum pretension is higher than the allowed bolt load. How is this possible?

 

[swearingen]

Table J3.1 does not depend on load factors, strength reductions or the like. It is simply the minimum tensile strength times 0.70. Now, there appears to be a typo in Table J3.1 because 0.70 * 0.79in^2 * 90ksi = 50k, not 51.

The bolt pre-tension is not considered a load, per se, because the purpose of the pre-tension is to create friction that does actually carry the load. This is why load factors and phi or omegas don't show up here. The tensile "load" on the bolt is controlled and doesn't have the ability to change much over the life of the structure.

The phi and omega DO show up in Table 7-2 because you are actually loading the bolt in tension there. For the ASD case it is 0.79in^2 * 90ksi / 2 = 35.6k and for LRFD it is 0.79in^2 * 90ksi * 0.75(phi) = 53.3k. Yes, it's a little off of what Table 7-2 says, so I guess they round somewhere that I don't or vice versa. But hey, they made a typo in this table as well by calling the factors omega-v and phi-v; they should be omega-t and phi-t. The v is for shear from Table 7-1...



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