AISC Code Confusion Table 7-2 vs. Table J3-1

[JrStructuralUCSD]

Hello all,

I have a few concerns regarding table J3-1 and Table 7-2 of AISC 2010. From what I understand and calc'd, table 7-2 is in accordance to section J3.6 (Tensile and Shear Strength of bolts and threaded parts). Table 7-2 is basically calculating the different possible bolts and strengths. If you have slip-critical connections it is common (I think) to pre-tension the bolts to a nominal strength that comes from table 3-1. For instance, if you have ASTM F1852 3/4'' diameter bolts, you are to pretension the bolts to a value of 28 kips in accordance to table J3-1. But, the confusion comes from here...if you are to pre-tension the bolts to 28 kips and the tensile strength of the bolts is less than the pretension value, won't you be yielding the bolt? In accordance to ASD, the allowable stress for 3/4'' diameter bolts is only 19.88 kips, way less than what the pre-tension value of the bolts are set at. The bolt will in essence yield. I presume that I am totally misinterpreting this so I ask one of you experts to help clarify this for me!

Greatly appreciated

 

[ToadJones]

Pretension values in Table 3.1 are based on 70% of the minimum tensile strength of the bolt (Fu) which in this case is 90 ksi. The nominal area of a 3/4" bolt is 0.44 in2

So the min bolt pretension is 0.44 in2 x 90 ksi x 0.70 = 27.8kips.

The available strength in tension is 0.44 in2 x 90 ksi divided by Omega which in this case is 2.0.

So the available strength of in this case is (0.44 in2 x 90ksi) / 2.0 = 19.8 kips.

when you say:
"if you are to pre-tension the bolts to 28 kips and the tensile strength of the bolts is less than the pretension value, won't you be yielding the bolt?"

Tensile strength implies using "Fu", not "Fy" as is implied when you say "yielding the bolt".
To answer your question more directly, preloading the bolt to 70% of Fu ensures that the bolt is stretched elastically, but not yielded.

 

[spats]

The bolt will not yield simly because the pretension exceeds the ALLOWABLE load. The load that would cause the bolt to yeild is much higher.

 

[JrStructuralUCSD]

ToadJones, thanks for the response...seems like I am mixing apples and oranges in my thoughts.


Spats, if I may ask you a follow up question...how do you calculate the load that would cause the bolt to yield then? Would that simply be Fy*Ab?


Thanks

 

[ToadJones]

Actually, I believe this statement I made is incorrect:

"To answer your question more directly, preloading the bolt to 70% of Fu ensures that the bolt is stretched elastically, but not yielded"

I believe it actually ensures that the bolt is in fact yielded.

 

[JrStructuralUCSD]

Toadjones,

Why would you want the bolt to yield?

 

[ToadJones]

It is worth downloading the RCSC for a read as well as AISC Design Guide 17 on High Strength Bolts.
Some of it can be a bit counter intuitive (as I have displayed by posting incorrect information).

I think the idea with pretensioning of bolts is that you can stretch the bolt past the proportional limit and begin to yield the bolt in the inelastic range, however as you continue to stretch the bolt the stress remains relatively flat (think of a normal stress-strain diagram). Hitting 70% of the minimum tensile strength puts you comfortably in the range above yield and below the ultimate strength.

 

[ToadJones]

It is also worth noting that the minimum tensile strength, Fu, of A325 and F1852 bolts is 120 ksi.
The value of 90 ksi found in the Specification is based a 0.75 factor used to account for the reduction in area for the threaded portion of the fastener.

 

[dhengr]

Fy*Ab should give you the tension at which the bolt starts to yield, assuming the bolt material is exactly on spec. The bolt has started to yield when it is stretched to 28k, but it is still working on the upward slope/part of the stress/strain curve of its material strength, so it has not failed, ultimately. It will work up or down at the same slope as the original elastic stress/strain curve slope until it is stressed to the point that it starts downhill on the stress/strain curve to ultimate failure. Look up topics like strain hardening, strain aging, and study the the stress/strain curve for bolt material to better understand this matter, Also, look at the Bolted joint design guides, such as RCSC.

 

[JrStructuralUCSD]

Thank you all for the responses