Moment Base Plate w/ Tension 3

[Galambos]

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?

 

[mrMikee]

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

 

[Galambos]

Mike, perhaps you misunderstood me. I have column end forces that consist of net uplift and moment

thanks, ted

 

[mrMikee]

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

 

[Tomfh]

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?

 

[mrMikee]

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

 

[Galambos]

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?

 

[mrMikee]

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

 

[Tomfh]

What are your forces galambos? How does the moment induced tension compare with the extra tension?

 

[JStephen]

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.

 

[transmissiontowers]

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.

_____________________________________
I have been called "A storehouse of worthless information" many times.

 

[miecz]

I would put this plate up on levelling nuts to simplify the analysis, for then you can just deal with the bolt group.

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.

 

[DaveAtkins]

For the moment on the base plate, I assume a triangular stress block under one edge of the base plate, and tension in the two anchor bolts at the other edge of the base plate. The stress in this block is assumed to be the maximum allowable concrete stress (0.35 f'c or 0.7 f'c), and a quadratic equation results, with "x" being the depth of the stress block. The tension force in the two anchor bolts is equal to the volume of the triangular stress block.
For the tension, all six anchor bolts share the force equally.

DaveAtkins

 

[Galambos]

Dave,

your method of superposition seems like a very reasonable approach.

galambos

 

[mrMikee]

This is an interesting discussion and I think I am going to incorporate base plate design into one of my calculation programs. The two general cases are loads down and uplift, each requiring a different approach. With uplift I don't see how the bearing pressure on concrete enters into the calculation of forces in the bolts because, well, the column is pulling up.

Am I starting out right, or not?

Regards,
-Mike

 

[DaveAtkins]

If the base plate has moment and uplift, more than likely it will bear on the concrete at one side of the base plate.

DaveAtkins

 

[AggieYank]

mrMikee. When the base plate has a moment on it, one side will "go" up and the other side will "go" down. When this plate has uplift on it as well, whether one side of the plate bears on concrete or not depends on the magnitude of uplift and moment.

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).

 

[mrMikee]

Thanks DaveAtkins and AggieYank.

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

 

[transmissiontowers]

Here is a link to a paper written by Daniel Horn (deceased) about design of base plates on steel monopoles that is 118 pages and covers several methods and theories used in the telecommunications field. Base plate design is sometimes complicated by the fact that some pole designers use propritary methods that they are reluctant to divulge.

http://www.c-conceptsinc.com/files/Technical Manual 1.pdf

_____________________________________
I have been called "A storehouse of worthless information" many times.

 

[mrMikee]

Thanks transmissiontowers. That looks like a very good paper. I'm going to print it out and go through it today.

Regards,
-Mike