Exceeding Elastic Limit for A325 Bolts 1

[Montana1]

I have a general question regarding the design of bolted connections: If bolts are tightened beyond thier elastic limit for structural applications, what potential harm will this cause? I know the AASHTO code discusses this topic regarding direct tension indicator washers. (If the washers are completely flat, then the connection is deemed unacceptable.) However, I have also seen studies that claim this is not harmful for shear type connections. (See article in this link

http://www.appliedbolting.com/pdf/over_compress.pdf)

Any input would be appreciated.

 

[BoltMike]

Hi Montana,
Part of my job is to read these types of specs ad nauseum. essentially it depends on what your are building hence what spec you must adhere to. if it is structural steel not related to highway or bridges, most times the RCSC specs are required.


The Research Council on Structural Connections spec states in section 8.2.4 (installation) that "the installer shall verify that the direct tension indicator protrusions have been compressed to a gap that is less than the job inspection gap"

Section 9.2.4 (inspection)states that

"After pretesnioning, it shall be ensured by routine observation that the appropriate feeler gage is refused entry into at least half of the spaces between the protrusions. No further evidence of conformity is required. A pretension that is greater than that specified in table 8.1 shall not be cause for rejection."

a DTI that is completely flat satisfies the condition of >50% refusal as well as injdicating the bolt is tensioned more than specified by the council rules.

As you mentioned, AASHTO does not allow a flat DTI but rather list a maximum number of refusals in the steel so in the case of highway and bridge steel; the DTI cannot be completely flat.

The explanation i was given is that if the DTI is completely flat, then it is not possible to determine how far the bolt has been tightened beyond yield. it may have been tighten just until the DTI flatterned out or well beyond DTI flat state (possibly stretched so far as to be on the verge of stretching into two pieces).

as for which is better depends on who you believe. i have read a study (The Yielding of Fastenings During Tightening by PJ Gill)stating that it is actually preferable to over tension a bolt up to the verge of fracture since doing so allows the friction between the plies to resist shear rather than just the shear area itself.

Mike