help - a490 bolts loosening all by themselves

huangyhg

help - a490 bolts loosening all by themselves
we have a job where the contractor and testing agency is telling us that the a490 bolts are loosening all by themselves.  the bolts in question are 7/8" diameter x 2-3" long, and are being used in top & bottom plate moment connections on floor beams, so loaded primarily in shear.  (non-seismic zone)  we only are requiring snug tight connection.  
they are telling us that within a few hours of tightening them to (supposedly) snug tight, that they can go back and run the nut off by hand.
none of us here have ever seen this before.  there is only three things that i cna think of that would cause this:
1.  flange and moment plate are not in full contact, so the plate is relaxing under the load, and thus the tension on the bolts is also relaxing.
2.  tightening method is not reliable, in that they are not going back over the bolts after they have brought the plies together to ensure that they are all still tight.
3.  the bolts are poor quality steel, and are relaxing under the tension from the initial tightening.
has anyone seen this before?  do you have any suggestions for what the problem is, or how to solve it?
i am ready to tell them to test a random sampling of the bolts for ultimate strength and for creep/relaxation properties.  does anyone know if there is an astm test method for this?  i have not found one yet in my web searching.
thanks very much.
as you tighten one bolt, the adjacent ones could become loose. i would suggest re-tightening all bolts before re-checking.
i have always used slip critical bolts in oversized holes for this application. see asd 9th edition, pg. 4-110 and 111, toward the bottom of each page.  
isn't it required to have slip critical bolts in moment connections?
rc
all that is necessary for the triumph of evil is that good men do nothing.
    edmund burke

quote:
as you tighten one bolt, the adjacent ones could become loose. i would suggest re-tightening all bolts before re-checking.
we have asked them that, and they claim to be doing it right.  this has been happening for a while now, some areas have been inspected and failed 4 times now.
quote:
i have always used slip critical bolts in oversized holes for this application. see asd 9th edition, pg. 4-110 and 111, toward the bottom of each page.  
i would too, but in this case, the engineer that designed the connection specified standard holes with bolts in bearing.   also, we just found out that they are using a490 bolts instead of the a325 bolts that were specified.  
all of this is just coming to our attention, even though the contractor and testing agency have been dealing with it for some time. now that it is affecting their schedule (they were supposed to start fireproofing this week), they are getting us involved.

quote:
isn't it required to have slip critical bolts in moment connections?
in this case, clip critical is not required.  these are wind frames with no seismic requirement.  the code generally recognizes that mwfrs components are not subject to "significant load reversal", as the wind is generally considered quasi-static loading, so typically slip critical is not required.
also, they are not sharing load with welds, so no requirement in that case for slip critical.

sounds like a problem with bolt quality.  are the bolts of reputable manufacture or from some unknown foreign source?
is there enough extention on the anchor bolts to double nut?
i would be surprised if you can have all these holes line up in the field for bearing without drifting or enlarging some holes. how could bolts in bearing possibly work in this case? have you ask the ironworkers about this? i assume you also have finger shims or strip shims (which are not welded), therefore you still need slip critical bolts.
the flange plates tend not to sit flat against the beam flange; they often tilt upward from welding.
can you make these connections work with slip critical values since they provided a490's in lieu of a325's and you already have an acceptable faying surface since you are fireproofing. if so, have them fully torque the bolts. if not, you may have to weld.
wrong on the slip critical comments.  bearing bolts are allowed for these applications, even in seismic connections.  (in seismic, the surface has to be prepped for slip critical, and the bolts fully torqued, but bearing is still ok for analysis.)  even with shims, bearing bolts are allowed.  there is a strength reduction factor to be used if the shim is thicker than 1/4".
i have no idea about the bolt quality.  that is why i am thinking of having them submit the bolt cut sheets, and even making them test the bolts.
i don't know about bolt extension right now.  i have one of my junior engineers down at the job site now to look at some of these things, including holes sizes and alignment issues.  and yes there are shims in place, but i am not aware of requirement for slip critical with shim.  can you point out a reference please?
i think we could definitely make them fully tighten the bolts, though i don't know yet (have not yet run the numbers) if the bolts would be adequate in slip ciritcal.  but in reality, they are still in bearing, as they would probably have slipped into bearing condition now, since the slabs have been cast.  pockets in the slab were made at the bolts, to allow the bolts to be installed after the dead load of the slab was in place.  so we may make them fully tighten them, in hopes that they won't loosen once fully pretensioned.
thanks for your help.   
structuresguy,
there are a few reasons why this may happen.
1)are faying surfaces in intimate contact?
2)are bolts being systematically tightened, from the most rigid point out?
3)are washers hardend steel?
4)is there any welding being performed on the connection after snugging the bolts? thermal expansion will almost always cause snugged bolts to loosen. although you state that your issues are occurring within 4 hours of snugging. normal temp differences should not cause problems within 4 hours. but if welding is performed, then this can happen quickly.
a pre installation verification test may be of some help. snugged bolt do not require this. but it may help determine if there are in fact issues with the material. have the bolt snugged in a skidmore and held for a few hours.