Baseplate thickness

[sedesigner06]

I am designing a column with a fixed base. Service moment is in the 540 k-ft range. The baseplate thickness is getting to be very large in the order of 3 1/2''. I am considering adding stiffners to increase the section modulus of the base plate.

As far as sizing the weld and considering the shear flow,but the base plate connection is almost purly moment so having a hard time sizing the weld of the stiffner to the baseplate.

I have sized the vertical column to stiffner connection length based on the bolt eccentricity and shear with respect to the end column flange.

Does any one have a good reference or on advice on how to make sure the weld between the base plate and stiffner is sufficient to consider the stiffner a part of the section of the plate in order to reduces its thickness.

I have already looked in the AISC design guide and have had no such luck with stiffners being added to reduce baseplate, which is a red flag to me.

Regards

 

[hokie66]

How many bolts will you have? With a W14 column, maybe 6? The 'normal' way of fixing a base is to extend the bolts up to the top of the stiffeners, where they bear on a plate spanning across the stiffeners. A good steel textbook should have an example

 

[Hokie93]

Design of base plates intended to transfer moment, including stiffener design and their connections, is covered in good detail in Section 3.3 of "Design of Welded Structures" by Blodgett.

 

[JAE]

Both "hokies" have good points that I agree with. The method of extending your bolts up to the top of the stiffeners is a good method for such a high moment.


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[sedesigner06]

Thanks for the feedback, i have 12 bolts (2 rows of 6)with and average of about 20 kips per bolt, il have to work out the moment and see what kind of plate thickness will be required to span along the top of the stiffners. I think that plate itself will have to be rather large to support the bolt tension loads.

I was able to find a triangular bracket design in my steel book to help provide some checks on my design.

Thanks,

 

[KootK]

I would expect your stiffeners to be spaced close enough that you're plate wouldn't have to span too far or be too terribly thick. That being said, I believe that a thick plate is more cost effective than extraneous stiffeners.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[ajk1]

I agree with Kootk, that in general a thick plate is more economical than a lot of stiffeners, although I am not certain that is the case when you get to 3.5", but it might be). We used to design the large roof canopies over the pump & drive aisle areas at gas stations for a large oil company, with HSS columns, thick base plates (although perhapsnot 3.5" thick) and full penetration weld with shaped backer bar inside the HSS. But Kootk your spelling of "your" as "you're" is not correct here because "you're" is not the possessive form of "you". "You're means "you are".

 

[KootK]

@ajk: you'll have to excuse me the odd spelling error. I do much of my eng-tipping on my phone on the train. It's a wonder that my posts are even comprehensible sometimes. Please tell me more about those shaped backer bars. Are they literally shaped to match the inner wall radius of the tube? Is it much of an imposition to ask for that?

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[KootK]

With regard to the business of thick plates being preferable to stiffeners, I've always considered the limit of that to be about 2", although I can't really say why.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[sedesigner06]

Thanks your input, as far as the arguement of thick base plate vs stiffners, seems like everybody has their own preference. My orginal though was just to have a thickplate, but then a colleague suggested the stiffners and wanted to get some extra feedback. Seems like it may be six one-way, half a dozen the other.

As far as the final decision I did go with the stiffned base plate but when i took a summation of the volumne of plate used, their is not much savings but it is a stronger connection. I still ending up using a 1 1/2 plate w/ 3/4 stiffners for the final detailed connection.

Regards

 

[cliff234]

Not that there is anything wrong with your design, but why are you using two rows of 6 bolts? That's a lot of bolts on each side of the column. If the bolt tension is only 20k/bolt, why not use a smaller number of larger diameter bolts?

 

[JLNJ]

My first thought is: That's a lot of moment and I hope you can live with a little rotation at the base. Maybe you should pretension the bolts.

Second thoughts: If the base plate gets to be thicker than Koot's 2" inch number, extend the bolts up to the top and make a "chair" out of the stiffeners. That way you can keep the stiffeners close together and keep the top plate to a reasonable size. If you try to just stiffen the base plate, your stiffeners will want to be close together to help the base plate, but too close and you can't get a wrench on the bolts. If you really don't want to go with chairs, you figure out the pressure under the plate and consider the plate as supported on three sides.

Third thought: An older engineer once told me to keep the bolt diameter and the plate thickness close to the same size. That seems to optimize things and keep them "looking right".

 

[TLHS]

Has anyone ever seen anything that talks about what happens when you get into that stupid zone where your base-plate is obviously so thick that it isn't really acting primarily in bending anymore? Like if you've got a 2" thick base plate spanning out 3" past the flange or something wacky like that.

I've roughed up some half-assed strut and tie or tension field type models in this situation before that I'm comfortable were conservative, but I don't think I've ever seen anything written down about this.

 

[KootK]

I stumbled across this paper which contains a yield line method for the design of the top plate in the chair assembly: Link. Maybe that can help to. Keep the thickness to something reasonable.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.