Column base plates w/ biaxial moments

[gb156]

Can anyone direct me towards a hand-calculation method for designing column base plates for biaxial bending? So far, the only thing I've found is an equivalent uniaxial method used by STAAD etc. The plate thickness appears reasonable with this method, but the anchor bolt requirements do not. I'm developing a spreadsheet to do column base plate, anchor bolt, pedestal and footing design for up to 25 load cases, otherwise I'd simply purchase existing design programs.

The only other method I can think of would be the Bresler reciprocal method. 1/Pi = 1/Pnx + 1/Pny - 1/Po

Another question, how accurate/valid is the AISC method for designing base plates with uniaxial moment? Aside from a finite element approach, is there a more accurate/valid method?

Any help would be greatly appreciated!

 

[LPPE]

If I'm not mistaken, the LRFD 2nd Ed. Connections (Part II) covers a variety of base plate designs.

 

[gb156]

LRFD 2nd Ed. Connections (Part II) only covers axially loaded base plates. Although, they do make reference to DeWolf and Ricker (1990) with respect to design of base plates subject to moment and tensile loads. Any ideas where I could find that information?

Thanks for the reply.

 

[haynewp]

Roark's book of Stress and Strain has all kinds of plate formulas. Blodgett's method in Design of Welded Structures is what I like to use for base plates. It is well worth the money if you don't have it. I'm not sure if it covers biaxial plate bending though.

 

[LPPE]

I actually studied under DeWolf. All I have are notes. Where to find his work in print I am not sure. I will ask him.

 

[LPPE]

That publication is available from AISC in Chicago.

 

[gb156]

Oops, I didn't realize DeWolf and Ricker had authored that AISC design guide. Thanks for the info. Blodgett's method looks similar upon first glance.

Still, both references only consider uniaxial bending. Anyone have suggestions for biaxial bending? Does the Bresler reciprocal method seem valid here?

 

[peterman722]

I have been confronted the same problem in my design. I believe the base plates of steel frames subjected to biaxial bending are common in practice, so I hope professional engineers will give us a great answer.
Thanks!

 

[Imagineer]

See Steel Design Guide Series #1: Column Base Plates from AISC. You can order it from their website

http://www.aisc.org.

Note:Blodgett is commonly used. However, it is inaccurate for a couple of cases. The design guide has a very good discussion about the theory. In synopsis, different theory applies for the axial load; small eccentricity and large eccentricity cases.

Oops... Just saw "BI-Axial Moments". The design guide does NOT cover bi-axial conditions. Although, you can make some reasonable projection based on their discussion. Imagineer

 

[ishvaaag]

I think to define a reasonable thickness of the base plate is far more difficult a thing than to find reasonable anchor bolts, since it really requires codification of actual strength of the different setups. Even if Yield Lines may give us some understanding of the limit load, the situation around bolts, the heavy concentration of welds, maybe dynamic solicitations make that everything that comes not from experiment is just merely technical guess.

As long the reasonable thickness is accepted won't be ruined nor by the bolts in tension if any or by the bending of the base-plate in compression, one can always find a value for tension in the bolts for Pxy cases as a Reinforced Concrete section in compatibility of deformations. That is, as a rectangular section under Pxy (forfeit the in compression bolts compressive strength). From this an anchor bolt can be derived, then enlarged if in doubt.

 

[lippie]

A true way of doing it is through proper FEM analysis rather than a rigid interpretation. An equivalent uniaxial method is not correct. In the FEM analysis, the maximum principal stress is compared against the allowable stress on the base plate. If the principal stress is higher than the allowable stress, the base plate thickness is increased and then the FEM analysis is repeated. The above process should be repeated until a thickness satisfies the stress condition. Good references for anchor bolt and shear lug design would be

1) AISC Steel Design Guide Series - Column Base Plates by J. T. DeWolf and D. T. Ricker.

2) Analysis and Design of Structural Connection by L.H. Martin

As for software, STAAD.etc is the only one that I know of that does it without having to buy a full FEM package. Enercalc has a base plate module but I am not sure if it handles biaxial bending.

 

There is a program called RISABase which will design baseplates for biaxial conditions using a FEM solution. It can be found at

http://www.risatech.com.

 

[peterman722]

Thank you all verymuch for your helpful reply!
Since I am not in USA, the "AISC Steel Design Guide Series - Column Base Plates by J. T. DeWolf and D. T. Ricker" is not available to me. Where can I found online this great guide which, I am sure, worthy a reading?
Again thank you for reply.

 

[lippie]

Can't find it online. Have to buy it from AISC. It's a hard print to get. Go to Amazon.com or

http://www.aisc.org

and order it from there.

 

[peterman722]

thanks for reply.
I have another question here abou the design methods of tension base plates, which, for example, are used in cable structures as the connections between the cable and the anchors underground. Where can I find the information about that?

 

[flamby]

I dont know how complicated your case is but consider finding out the resultant loading ( resultant of X & Z loads) to give you a uniaxial loading. I know the pains involved in working with rotated sections but you can develop a program for it.