Moment Base Plate w/ Tension
Moment Base Plate w/ Tension
(OP)
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?






RE: Moment Base Plate w/ Tension
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
RE: Moment Base Plate w/ Tension
thanks, ted
RE: Moment Base Plate w/ Tension
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
RE: Moment Base Plate w/ Tension
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?
RE: Moment Base Plate w/ Tension
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
RE: Moment Base Plate w/ Tension
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?
RE: Moment Base Plate w/ Tension
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
RE: Moment Base Plate w/ Tension
RE: Moment Base Plate w/ Tension
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.
RE: Moment Base Plate w/ Tension
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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I have been called "A storehouse of worthless information" many times.
RE: Moment Base Plate w/ Tension
For ungrouted plates on levelling nuts, AASHTO's "Standard Specification for Structural Supports for Highway Signs, Luminaires and Traffic Signals", bending stresses in anchor bolts should be considered when the clear distance from the bottom of the levelling nut to the top of the concrete equals or exceeds one bolt diameter. Also, when the base plate thickness is less than the bolt diameter, it can contribute significantly to bolt bending stresses.
RE: Moment Base Plate w/ Tension
For the tension, all six anchor bolts share the force equally.
DaveAtkins
RE: Moment Base Plate w/ Tension
your method of superposition seems like a very reasonable approach.
galambos
RE: Moment Base Plate w/ Tension
Am I starting out right, or not?
Regards,
-Mike
RE: Moment Base Plate w/ Tension
DaveAtkins
RE: Moment Base Plate w/ Tension
This has been discussed quite a bit, but I figured I'd throw my hat in the ring. I would do an analysis of the base plate with the moment only. Then I would evenly distribute the tension into each bolt (meaning the bolts that take the tension from the moment have an even higher tension force in them).
RE: Moment Base Plate w/ Tension
As I see it now the answer seems to be, it depends. When the force in an anchor bolt due the moment is greater than the tension force due to the uplift, then clearly there is going to be bearing on the concrete. So when there is uplift on a column some of the moment (maybe all of it) will be carried by the bolts, and some could be carried by bearing on the concrete and a force in the tension anchor bolt. It depends on the magnitude of each component. This might be a little hard to program.
Thanks,
-Mike
RE: Moment Base Plate w/ Tension
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I have been called "A storehouse of worthless information" many times.
RE: Moment Base Plate w/ Tension
Regards,
-Mike
RE: Moment Base Plate w/ Tension
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I have been called "A storehouse of worthless information" many times.
RE: Moment Base Plate w/ Tension
Indeed. You would need a lot of uplift to eliminate the bending tension.
RE: Moment Base Plate w/ Tension