huangyhg

moment connection
does anyone have information on the moment resistance of a single slip critical high strength bolt in torsion?
the direction of the torsion vector is in line with the axis of the bolt.
i have a small moment that must be resisted by the clamping action of a single bolt.
dik
i have not but we can develop some theory for it. for example we can assume the tightening force is applied uniformly on the foorprint of the washer (if not an extended one) and at say 45o decreases to zero. by equilibrating this volume of stresses upon the area we have a radial scheme of stresses sigma(r) . now we need only to integrate the friction forces around the center as
limit torque = integral in the standing compressed radius length of mu·sigma(r)·2·pi·r·r·dr
the first r accounts for the ring being integrated and the second for its "polar" moment strength.
dik...what is the configuration of the force couple? how is the other half of the couple resisted?
to my mind, this is the same question as "how much torque to tension/untension a bolt?". this was discussed a few days ago.
to tension a bolt is approximately
t = 0.2 f d
to undo a bolt is perhaps 70% of that.
as for design moment resistance, i'd say zero.
quote (tomfh):
as for design moment resistance, i'd say zero.
i agree. keep in mind that slip critical bolts are still checked for bearing, so that if they do slip, you don't have a complete failure. one bolt in bearing has zero moment capacity.
the problem is an existing rack that has no bracing and has been in use for a couple of decades. another engineer, outside the firm i'm with, has been asked to check the capacity of this.
the moment resistance for any lateral load has been provided by 1/2" dia a307 bolts and the clamping action of these.
it is not easy to add proper bracing and my initial suggestion was to look at using a325 bolts in slip critical mode. there are in excess of 30 fasteners providing the resistance.
having nothing better to do and the work is interesting, i've decided to take a closer look at it, just to determine what the lateral capacity of it is. to this end, it is necessary to determine the torsional resistance created by the clamping action. it is max at smaller radius and minimal at the larger radius (with the larger moment arm). the distribution is non linear. i don't know what the outer effective radius is or the manner of pressure distribution between the edge of the hole and the outer radius and i didn't want to fem it. i was thinking that it can't be that unique a problem that someone hasn't already encountered it.
i'm also aware of the problems associated with opinions as well as relying on clamping of a single fastener to provide resistance.
dik
ron:
there is no other half to the couple. the moment is resisted solely by the clamping effect of the bolt with the axis of the bolt being the torsional or flectural vector. i'm hoping that the moments from my frame model will be small (only a fraction of the bolt torsional moment). there are a lot of fasteners that must fail (slip) for the rack to become a mechanism.
dik
i don't think you're going to find a solution published anywhere. the party line is that this type of connection has no torsional resistance. i would assume that you can torque the bolt to yield. multiplied by the tensie stress diameter, this gives a tension force. for the friction coefficient between the nut and the washer, i'd use 0.14. for the moment arm, i would ignore the washer and count on the head. assume a reasonable moment arm from the center of the bolt to the clamping force, around 3/8" for a 1/2" bolt.
i agree with miecz.
installing slip critical connections would be a big hassle too.
are you suggesting that the rotational restraint at this joint is from the clamping force on the bolt?
slickdeals... yes... but, the moment is small and there are over 30 bolts... and low strength bolts have functioned for the last 20 years...
dik

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