×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!
  • Students Click Here

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

Jobs

Fixed base on tube steel colum
3

Fixed base on tube steel colum

Fixed base on tube steel colum

(OP)
I am designing a portal frame with tube steel columns that has a fixed base. Right now I have the base connection detailed with a 12"x12"x.75" base plate with 3/8" gussets going up 6" on the tube column and welded to the base plate and four anchor bolts. The numbers show that this should work I am trying to resist a 10ft-k moment. I am just not sure if this connection provides enough rigidity to be considered fixed. I would like to switch to w flange columns but unfortunately this all I have to work with.Comments welcome.

RE: Fixed base on tube steel colum

It is extremely difficult to have steel column with true fixity. Either I will design the frame for both conditions -column fixed & column pinned, which is very conservative and often non-desirable, or try to come up with reasonable column fixity and design the frame with rotational spring at the column bases, quite tedious this way though.

RE: Fixed base on tube steel colum

RE: Fixed base on tube steel colum

In my opinion, if you design everything as fixed (bottom of column, weld of column to base plate, base plate, anchor rods, and footing), then it is fixed.

And I think you can come up with a simpler detail--try a thicker base plate without the gusset plates.

DaveAtkins

RE: Fixed base on tube steel colum

AISC Design Guide 1 covers this

RE: Fixed base on tube steel colum

Dave:

I will do the same as you said if it is for concrete structures. However, I reserve my doubt on most of steel connections, it only needs a little slippage to cause huge loss in capacity. Old school thinking, maybe.

RE: Fixed base on tube steel colum

I'd stay away from fixed supports, altogether.  It's much easier to fix your steel to steel connections than to fix the base.  The main problem is the anchorage. When you get down and dirty with ACI 318, Appendix D, you're likely to have to deal with load factors on your moment (1.6 or 1.7); capacity reductions on your loads (anywhere from .65 to .9) and significant capacity reductions because of spacing and edge distances.  And that's not even considering the stiffness impact of the bolt area stiffness vs. the column stiffness.
Anytime I see one of my co-workers designing frames, I recommend assuming the supports pinned. When they don't do it, they invariably come back and say, "I wish I would of listened."

RE: Fixed base on tube steel colum

It sounds like he is dealing with a flagpole situation. Its hard to get away from a fixed base in that case.
I too avoid moment bases on buildings when ever I can. It complicates the steel desing as well as the foundations. On portal frames I try to use pinned bases with moment conn.'s on the beams.  

RE: Fixed base on tube steel colum

It's true that it's difficult to get true fixity with any steel connection.  A beam to column moment connection is limited by the stiffness of the column or beam (depending on which member is under consideration), but you use them when you need to.  

I have many times had to fix the bases of moment frames just to get drift under control.  That's a lot less expensive than throwing more steel weight at the columns/beams.  

Another thing I've done is grab as many frames as you can with wind moment connections.  They are cheap moment connections that do help laterally, especially with drift.  These are tougher to do with HSS columns.

Your detail (with the stiffeners) should provide plenty of rigidity, but I would make the plate thicker and lose the stiffeners - that's an expensive detail.  A baseplate is cheap, stiffeners and all the extra welding isn't.

Jed,
I don't disagree in principle, but I would say that you always have the ability to lap the anchor rods with pier/footing reinforcement to get away from the tension breakout requirements of App. D.  This is typically only a problem with a pier.  I rarely have a problem with anchorage into a footing with fixed base moment frames.  

RE: Fixed base on tube steel colum

Strength issues aside, "fixed" baseplates often have far more give than a true rigid connection, hence you can end up with more deflection than you bargained for.

RE: Fixed base on tube steel colum

why not design it as a semi-rigid connection and capture the rotation if only approximately.

Arguing with an engineer is like wrestling with a pig in mud. After a while you realize that they like it

RE: Fixed base on tube steel colum

We make many assumptions in design. Most designs are not possible without some assumptions. Base fixity is a basic design assumption. To me, this is just another case of making a science project out of a moment baseplate design which has been laid out in many texts and design guides. In fact, the lighter the columns, the more readily acheiveable base fixity is.

You can knit pick designs as much as you want, but ultimately we are basing much of our design on assummed loading conditions that I cannot believe are any more accurate than, say, assumming a fixed base....or pinned, for that matter.   

RE: Fixed base on tube steel colum

(OP)
Thanks everybody for your input. After listening to some of the previous comments my design approach is such:

I have designed all of the strength requirements for the frame assuming a pinned base and fixed moment connections at the top. I also checked deflection under this assumption taking into account second order effects. The deflections were high and would require the columns to be mush larger to control deflections with this assumption.  

My second analysis assumed a spring constant at the base of the column. I double checked to make sure all strength requirements were met and check deflections taking into account second order effects. The defections were within acceptable range for this application. This is what I used to size the columns for deflection.

I then checked the same frame with a fixed base. I made sure all of my strength requirements were met and checked deflections taking into account second order effects. The deflections were much less from a pinned base and probably over designed with this assumption.
 

RE: Fixed base on tube steel colum

Stillerz:

You are on both sides of the fence :) While I align with you more on the previous one, I do not disagree totally on the second, as long as one has looked a problem thoroughly, thus the outcomes would live up/come close to one's expectations, that were based on sound engineering judgement.

I am still wary on assuming full regidity on steel connections, unless a rigorous analysis was done. Avoidance is a good advice.

RE: Fixed base on tube steel colum

cntw1953:
Wary why, can I ask? Do you use your own entirely scientific design approaches? I am not trying to be coy here, but bear with me.
We (I, anyway) do code-based design. We are not designing space ships or even pianos. We make very wide-based assumptions in our designs.
To me, it is very analogous to significant digits inasmuch as you cannot miraculously come up with a perfect design given a starting point of wide-based assumptions. I am not a scientist, I simply practice an applied science...structural engineering. Since there are so many things we don't know and probably can never know, we use judgment.
You can't make chicken salad from chicken...you know the saying.
Does anyone really expect that the EOR do an analysis of base fixity on every structure he might design when the texts and codes he is using to design the structure make assumptions in their theory ? Any engineer doing this is fooling himself.
Finally, i will say, full rigidity is something that can be assumed given due regard to relative stiffness.
For an extreme example consider a very slender column full pen welded to an infinitely large plate, infinitely thick.  
 

RE: Fixed base on tube steel colum

I'm personally a fan of making assumptions that I can sleep with and not have dog problem. While a fixed connection can be assumed for ultimate limit state situations and comfortably be implemented assuming the components used are able to handle the strain requirements. To ensure this you must back up this with calculations showing such. That means the plate, bolts (both strength and embedment/edge distance), grout, footing ect must be check for strength and strain requirements.  

"For an extreme example consider a very slender column full pen welded to an infinitely large plate, infinitely thick", my query would be what about the footing, soil conditions?
 
Now let's get serious, how many time have you been called out to site because an element of a design (by others would hope) has moved beyond the service requirements of the client. Now how many times have you been called out because the element has failed, for me it is about 99% service problems (sometimes by engineers making assumptions that cannot be backup). Thus I put forward, "strength is essential but otherwise doesn't matter" (one of my favourite quotes).

Given all the calculations you have to do for the ultimate limit state there are only a few more that would be required to design as a semi-rigid connection, Why not take the extra step?

I am not meaning to pick on Stillerz but I would want more information from the OP before I suggested the fixed based assumption is appropriate. Even thou i am sure that Stillerz was NOT in anyway implying this.

I sure if dhengr was to post it would be twice as long as mine reminding people to get all the information before setting people off on course. What if the OP used two bolts located at the centre of the plate with two at the top and bottom, this would surely cause the problem to have to much deflection for our theories to be applicable, if the bolts and plte were design useing linear theroms. While I assume the OP has more sense than this I would like more info myself before I my approval of a fixed plate design assumption.

I would also point out that there are very few articles dealing with shear for fixed or semi-rigid baseplate with CHS columns, with less than 10 bolts.  

Arguing with an engineer is like wrestling with a pig in mud. After a while you realize that they like it

RE: Fixed base on tube steel colum

rowing shook some load off my mind.
Happy new year, everyone.

RE: Fixed base on tube steel colum

How do you design it as semi-rigid?  How do you determine the moment rotation characteristics of the connection?

RE: Fixed base on tube steel colum

Quite tedious for each case/material involved. Usually start with looking into characteristics of deformation under load. It may require to throw a lot of assumptions based on experience and engineering judgement. The effort is trying to catch the worst scenario cloest to the actual, but exact.

Would a steel building fail (total collapse) for no other weakness but the assumption on support? It's not likely, but some localized defects could be the result.

Be prudent while work on frames with fixity, go over the details before make claim. I think there are quite many examples on moment connections could be used as a guide.

RE: Fixed base on tube steel colum

StructuralEIT
I prefer to use the component method for semi-rigid connections for portal frame base plates, is a bit conservative compared to moment rotation curves comparisons or FEA. However it is effective in getting a number. We had a bit of a discussion in the AS/NZS area and I posted an article I found fairly helpful thread744-260012: Base Stiffness and AS4100 you may wish to read (I would post it again but I'm at home). The article doesn't cover foundations; however there are large amounts of information on stiffness of foundations.  As for the OP's problem, we need a lot more information before we can suggest the spring stiffness.
 

Arguing with an engineer is like wrestling with a pig in mud. After a while you realize that they like it

RE: Fixed base on tube steel colum

As far as I am concerned, except for the "flagpoles" with no effective vertical load, there is no such thing as a fixed foundation, they are all significantly flexible. Back in the days of Moment Distribution, I found that it was reasonable to accept only 50% of the moment; made trial analyses with dummy members to simulate the flexibility of the foundation and soil, then, later, with some computer programs, I could include the foundation and ground springs in the model.

Michael.
Timing has a lot to do with the outcome of a rain dance.

RE: Fixed base on tube steel colum

Engineering judgment is the key element in this discussion.  Pure fixity cannot be attained, but the rotational restraint at the base of each column can be ascertained approximately by applying sensible considerations.

BA

RE: Fixed base on tube steel colum

rowingengineer,   you're right on the money with your advice.  And, I would not have said it any better, just longer, so your right again, not that you need my blessing.  The younger fellows should listen to your sage advice and give it serious consideration.  We make lots of assumptions, in structural design, but they must be sound, experience based, and defendable if needs be.  There are lots of fixed moments, but few perfect fixed moments, without some rotation, yielding, etc., and thus your semi-rigid design approach.

paddingtongreen,   Moment Distribution, what the hell is that?  We both know that went out shortly after the abacus and slide rule.  But it sure gave you a better feel for how a structure  reacts to loading.  I'll bet you went above 50% of the moment at times, but still slept well.  And, you saved the rigor of semi-rigid design and dissimilar materials, steel to conc. found. to soil.  We used to do in two pages of calcs. what it now takes ten pages of hemming-an-hawwing, a forum, and two hours on the computer to accomplish, and we still slept well, and our building stood up under load.

BA,   I absolutely agree.  I think it's for us older fellows to keep heckling the younger guys to gain the experience, over and above just plugging it into the computer, so that they understand how real world structures work, so they develop good engineering judgement.  I'll bet you didn't have many structure failures with your approach.

I suspect that if the four of us sat down, with a sketch pad, and a beer to lub. our tongues and minds, we could communicate much better than we do in writing, in this forum format.  First of all I could stop you or visa-versa, and ask 'what did you say,' 'isn't this what you meant,' 'what experience leads you to believe that,' 'please explain further, I don't understand,' 'no, this is why that's wrong.'  And, we could draw a sketch of what we mean, instead of 15 intervening conversation, and no sketch, or difficulty in drawing the other person out without causing hurt feelings.  I have had a number of really good mentoring experiences in my career, from both sides of that relationship.  And, it seems that the important thing was that we were there together looking at each other and at the same drawings, specs.,  calcs., etc. and there could be immediate interaction, correction, direction about where to look for the answer, etc.

This forum thing bugs me a little because we seldom have the full picture before we are asked to opine on a solution or bless an approach.  And then, it seems so difficult to get the OP'er to give up that sketch or offer more, or even enough, info. so a meaningful answer can be given.  We don't know who we're talking too or their experience level, although lots of times that's pretty evident by the OP.  There really seems to be a bunch of experienced older fellows here, who have a wealth of knowledge that they would love to pass on to the up-and-comers, but this forum seems a difficult place to do that in a meaningful way and without hurting feelings because voice inflection or body language, etc. can't be appreciated.  I wish there were a better way for us to connect with each other on some of these problems and questions.  I think these forums are an interesting format for exchange of info., but they can not take the place of a real, immediately available mentor.

You young guys/gals, it's dangerous for this forum to be your first or only source for advice.  It's dangerous for some of you to be doing what you claim to be doing without someone in your own office looking over your shoulder and being your primary advisor.  Find a mentor in your own office or community or local engineering association who you trust and can get help and advice from, obviously they should be much more experienced than you.  Don't be afraid to ask for help and advice, let them get to know your experience level, this relationship can be very rewarding, both ways.  The above is particularly true if you are in a consulting, infrastructure, design environment, but also true if you're working within a large corp. on product design, if you take pride in your profession.  Then, this forum is a wonderful way to get a second opinion, or an alternative approach to a difficult problem, but it should not be your only alternative.  And, for goodness sakes look at your own strength of materials and structural design texts before you even ask a question, that might answer the question or at least help you ask a complete question.  Our questions and discussions do not lend themselves well to a twitter mentality, at least not for me.

RE: Fixed base on tube steel colum

For service and stability checks, I would assume a rotational base-fixity of 0.9*4*E*I/l, for a determinate structure like a cantilever, this rotation allowed at the baseplate will amplify the deflection at the tip of the cantilever while maintaining the same moment at the baseplate (if you are designing a cantilever). Whilst for ultimate design checks, I would assumed a fully restrained rotational fixity at the baseplate. This was following the discussion that was had in thread744-260012: Base Stiffness and AS4100.

If you go to http://www.steelbiz.org/ and type in "base stiffness" to the search box it will return plenty of good documents.

Agree with dhengr and their epic spiel on the value of a sketch. We should try to use more sketches on this forum. I noticed in my travel that the better engineers have great sketching ability.

RE: Fixed base on tube steel colum

I guess my only point was that although your base may not be truly fixed, you are only looking for a design approach for the baseplate that yeilds reasonable results.   

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members!


Resources