connection type-differenec in bearing and pretensioned

huangyhg

connection type-differenec in bearing and pretensioned?
what is the difference in a snug-tight connection and a pretensioned connection if they both ultimatly rely on the bearing value of the bolt for load transfer? thanks.
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there are differences in intent, construction, checks to be made by the designer and use.
respect the intent the pretensioned connections are usually expected not to slip up to the service level solicitations, where such thing is not of concern in the others. this is quite useful specially where dynamical solicitation occurs, the pretensioned connections (if well attuned to the true solicitations) forcing a lesser stress and strain excursion range and so lower fatigue and longer life. so it is used for bases of structures holding information panels in roads, for example.
respect the construction, again, you need some measure of the pretension in ones, whereas in the others merely ensuring a conventional installation torque or if you want just proper mechanical interlock is enough.
the checks are to be made following different specs, so little to say here. that both kinds of connections have bearing on a plate, or a shank in shear does not affect this, you will perform the design and have the significant value at hand, which may come from one or other limit behaviour.
respect use, as said in the first item, the intent is no slippage at service level for pretensioned bolts, and no separation if the solicitation is tensile, that is, contact is expected to remain more or less as installed. for bolts in bearing this aspect of the life of the connection doesn't matter and shouldn't have any negative impact in the life of the structure as long the fasteners have been designed for an application that permits their use.
hi tim76
snug tight method requires a certain amount of turn from the snug tight position which is usually marked prior to turning.  this method pretensions the bolt shaft to the required pretension level, barring amounts of friction from oil, galvanizing, dirt, etc.  thus they are one and the same.  to know the exact pretension, special astm specified washers can tell you the point at which pretension is attained, however this is rarely done.
regards
vod