Fixed base on tube steel colum 3

[dakota99]

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.

 

[paddingtongreen]

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.

 

[BAretired]

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

 

[dhengr]

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.

 

[asixth]

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.

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.

 

[Stillerz]

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.