huangyhg

odd beam to column connection (steel)
i have a 3 bolt connection of a beam to its supporting double angle connection. one of the angles is full length for 3 bolts, but the other angle is only long enough for 2 bolts. therefore, 2 bolts will be in double shear while the third bolt will only be in single shear. this sounds an alarm in my head, but i cant reason why. is there anything that allows 2 bolts in double and 1 in single shear connections? it seems like a combined welded/bolted connection, which should either be all bolts or all welds.
any thoughts?
with the weld/bolt combination, the thought is that the weld is much stiffer than the bolts (they are in 1/16" larger holes and will slip) so that the weld will initially take all the load - then fail - transferring the load to the bolts...thus only one or the other could be counted on.
with a double/shear single/shear bolt condition, i would think that the single and double shear conditions would have similar initial stiffnesses, either that or the bolt would deform slightly re-distributing the load to the others. thus, each bolt would get exactly 1/3 of the load and the weak link would be the single shear taking 1/3 of the load.
does this make sense?
i do agree. i think the whole connection is only as strong as the single shear bolt, with the bolts in double shear only taking what the single can handle.
pylko-
i think looking at the failure of the single bolt might tell you a lot. if the single bolt fails in bearing on the plate, then the single bolt will deflect until the 5 double shear bolts can take up the load. if the bolt fails in shear thru the bolt cross seciton, then it may be a brittle failure, but that the bolt is likely to see some deformation prior to all-out failure. so it also likely to redistribute the load.
if you treated it like a 6 bolt, single shear connection - what is the demand/capacity ratio. this might tell us more.
-for what its worth
most of the times somebody claimed to me that their bolts were in double shear, i pointed out that they were only bearing in a single thickness of material. just be really careful to adequately check all the failure modes of your connection before sounding the double shear cheer.
this connection was for beams framing into both sides of a column web. the connection they claimed was for erection safety. so i looked in osha and they poopooed this type of connection. ironic how a fabricator says its for erection safety and osha wont allow it.
i remember the osha report coming out. i don't have it in front of me, but it states that beams sharing common bolts, such as two beams with double angle connections using common bolts and holes must have a a way of holding one of the beams in place while erecting the other. this can be accomplished in several ways. one way is with an extra non-common bolt to hold the beam in place until the beam on the other side is erected. otherwise you need to remove the bolts and try to hold the beam in place with a drift pin or another crane while placeing the other beam.
if the beam works with only four double shear beams, i wouldn't worry about it. i don't know the loads, beam or the supporting
jcox - osha suggests an offset double connection, so that there are 2 bolts (1 each angle to column) that can secure the beam into place before the second one goes in. the idea being that obviously one bolt for the beam will make the beam roll once the crane hoist is slackened. so you need at least one bolt on each side of the beam web to connect to the column. hence they offset the double angles. i think modern steel construction/may 2001 has an article on this.
khs - any aisc steel manual will have those.
you didn't answer to jcox's question, that is crucial: can you withstand the design loads with 2 double shear bolts?
if the answer is yes, i wouldn't have any more worries.
i usually repeat to myself that wherever steel is added to a structure, this cannot harm the structure (except under very special conditions: high fatigue loads): so if you can take the loads and fulfil code rules after the third bolt is withdrawn, then you are done.
if you need the third single shear bolt, then i would evenly divide the load by 5, but conformance to code is a more delicate question here: i can't comment on this as i don't know your standard.
prex
pylko...your connection is only as strong as the two bolts in double shear, not the single bolt in single shear. i would neglect the capacity of the single bolt as i agree with jae's premise of connection stiffness, though you would get a better opportunity for load sharing in this instance than with a weld/bolt combo.
in its most simplistic state, if all the load were initially taken by the single bolt, it would deflect or fail then transfer load to the two bolt system, which would either resist the load or fail depending on magnitude. conversely if all the load were initially taken by the two bolt system, then failed, the third bolt is irrelevant.
pylko,
i agree with ron; our practice is to treat the single shear bolts as erection bolts only, and do not figure them in the equation when evaluating offset connection capacity. evaluating the connection as has been suggested seems only necessary in the (uncommon) event that there is not enough room in the connection angles for the extra (unaccounted) bolts.
david

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