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moment base plate w/ tension
does anyone know if a moment base plate with tension is taboo? my thought is to use a six bolt pattern and consider the two middle bolts taking all tension and the outer fiber bolts taking the flexural tension. thoughts, please?
galombos,
blodgett's "design of welded structures" has a good explanation and examples of column base plates in my opinion. when eccentricity of the load exceeds 1/6 of the plate uplift will occur which will cause tension in the anchor bolts. there is a long way to calculate this and a short way. if you can't get a copy of blodgett, perhaps someone here can help with another idea, or maybe i could even find my copy of blodgett. i know it's here somewhere.
regards,
-mike
mike, perhaps you misunderstood me. i have column end forces that consist of net uplift and moment
thanks, ted
galambos,
well that makes it more interesting. what i've done in the past is to determine the cross section area and moment of inertia of the bolt group and then calculate p/a and mc/i. it makes sense to me but i've never seen how anyone else does this. i too would be interested in comments or suggestions by others.
regards,
-mike
galambos,
i would apply a portion of the tension force to the outer bolts.
mrmikee,
for bending, where do you take your moment of inertia about?
whilst we are on the subject of baseplates, how do you guys consider prying forces?
tomfh,
the moment of inertia would be about the center of the bolt group which should be at the center of the column to keep things simple. if two bolts of a six bolt pattern are on the na then they could be ignored for bending and load would be just p/6. outside bolts would be p/6 plus or minus force due to bending. i'd check the shear on these too.
as far as prying is concerned; i have never checked base plates for this type of force. in theory it could be a factor but i think it would depend on the details of the layout. maybe next time.
-mike
mrmikee
blodgett treats the baseplate like a concrete beam. if that were the case, then the centroid would not coincide with the baseplate moment of inertia, thus the middle two bolts would in probability take some force, whether it be compressive (ineffective, of course) or tensile. could you agree for the sake of discussion?
galambos,
if there is a force down on the base plate i think the compression in the concrete and tensile forces in the anchor bolts are calculated as if it were a concrete beam, so i think you are right. blodgett has a cubic equation to solve for the neutral axis, which is ok if you have it set up on something like mathcad. if the triangular stress load on the concrete goes out to the middle of the base plate or farther the middle two bolts won't see any load.
on the other hand, if the column force is up there won't be any compression on the concrete, and all the uplift and moment needs to be resisted by the elastic properties of the anchor bolts. like i said previously i have never seen a discussion of this topic anywhere else.
well it seems to make sense now. if anyone else has any questions, comments, or more technical info please post.
-mike
what are your forces galambos? how does the moment induced tension compare with the extra tension?
with a symmetrical bolt pattern, the outer four bolts will carry all the moment. in that case, assuming the center two bolts carry the tension is unconservative. i would assume the tension was evenly divided, so the bolts loaded in tension by the moment would be highest stressed. ie, it would be mc/i +/- p/a.
the issue of stress distribution in an anchor bolt pattern has reared its ugly head before, and i am not aware of a good answer. the idea that one should analyze the concrete section immediately below the surface as a concrete beam sounds fine, except that it is equally reasonable to analyze the steel section right above the surface, and these two analyses will give you different bolt force distributions. the problem is that in either case, designs intended for long beams are being applied to end sections.
do the anchor bolts rest on leveling nuts? or do they rest on concrete? i use the bolt group moment of inertia method to determine the individual bolt loads. then i size the base plate to resist moments created by the bolt loads. the trick is to determine the width of the plate that resists the bending moment.
if the gap between the concrete and the bottom of the base plate is less than 2 bolt diameters, you can neglect the bolt bending according to asce manual 72.
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