Why ASTM A490 in direct tension must be pretensioned but not A325? 1

[IJR]

My friends

I can find no explanation as to why ASTM A490 bolts in direct tension must be fully pretensioned. This is not required for A325. Without understanding the mechanics involved, I am in the dark.

thanks in advance
ijr

 

[ajh1]

Prior to 1985 the RCSC Specification required ALL high strength bolts to be pretensioned, regardless of the loading condition of the bolt. That was relaxed in 1985 to allow bolts subject to shear only to be snug-tight. In 2004 that was further reduced to allow A325 bolts in tension without fatigue to be installed snug-tight, but the pretensioning requirement remained for A490. 2004 was right at the beginning of my time on RCSC so I don't recall the actual balloting discussion that resulted in that change, but looking at the Guide to Bolted Joints which is available on the RCSC website, it would appear that the distinction may be in regard to the level of ductility provided by the bolt. Higher strengths result in less ductility. As it has been shown that repeated application of tensile loads may be problematic on bolt longevity, the requirement for pretensioning reduces the impact on the bolt from that occurrence. Even though pure tension is not inherently fatigue, or at best very low cycle fatigue, there may well have been concern on the committee regarding relaxing that provision for the less ductile A490 bolts.

 

[dik]

You might look at ASTM F3125 specification.

Dik