How to eliminate Excessive moments in fixed base connection plate of Column

[JoeH78]

Dear ALL,

I'm getting for HEB 400 column base point of M2 = 300kNm bending moment V2 = 165kN shear and N = -30kN axial force no matter what I do I'm failing to solve it, base plate is 700mmX700mm with M20 anchor Class 10.9
what will be your suggestion, Your guidance will be appreciated.

 

[jayrod12]

What's the failure mode? Concrete breakout? You could reinforce the pier to resist the breakout frustrum.

 

[GeorgeTheCivilEngineer]

Does it need to be a moment base plate/ foundation? Can the structure be braced/shear wall or made into a moment frame?

 

[JoeH78]

It'd be good if I elabroate it a little bit, actually it is open structure pipe rack column which supports 24 meter span 3D truss, I modeled the foundations and super structure as whole where 3D truss and columns are steel, which rests on concrete columns then to raft foundations. I'm getting those huge moments at the steel column base joints where it connects to concrete column pedestals.

What's the failure mode?

I'm using eurocode and it fails at Bending capacity check. But any applicable steel code can be used AISC.

Does it need to be a moment base plate/ foundation? Can the structure be braced/shear wall or made into a moment frame?

The structure is moment frame but using the shear walls can be quite unusual I believe, bcoz I haven't seen shear walls used in such structures in past.

 

[jayrod12]

Well if your column fails, it needs to be upsized. Doesn't matter how large your baseplate is. If the lateral system is moment frames, then you're kind of stuck.

What happens if you pin your bases in the analysis. The base moment goes away, but is the rack still stable that way? You would need moment resisting connections between beams and columns for that to work, and the lateral displacement under design loads may not be tolerable. But it's a way to eliminate it the moment at the base.

But if you're going to go down the route of re-classifying the base restraint as pinned instead of fixed, you need to be sure the structure is still stable, and you need to ensure that your baseplate and anchor bolt detailing matches the design assumptions.

 

[GeorgeTheCivilEngineer]

The structure is moment frame but using the shear walls can be quite unusual I believe, bcoz I haven't seen shear walls used in such structures in past.

Indeed - bracing would be more likely. If it is a moment frame then it is more usual to nominally pin the base as noted by Jayrod. For an industrial structure a moment baseplate would be unusual.

 

[milkshakelake]

You can add vertical gusset plates to stiffen the base plate and reduce the bending moment arms.

 

[HTURKAK]

You did not supply a lot of info. here..

If the pipe rack is strutted ,(longitudinal struts connecting the columns of the transverse frames) , the common practice is to assume that the base of the column acts as a pinned connection.

Try with pin supported base model and see if the structural steel sections are satisfactory or not..

https://people.fsv.cvut.cz/~wald/CESTRUCO/Texts_of_lessons/07-GB_Column_Bases.pdf

Don't underestimate a nail. A nail saves a horseshoe, a horseshoe saves a horse, a horse saves a commander, a commander saves an army, an army saves a whole country.. GENGHIS KHAN

 

[human909]

A 400HEB is pretty huge. Why so big? What is the maximum compression load?

 

[JoeH78]

Well actually the problem arised from the fact that I petulantly insited on modeling the r.c. foundation+ r.c. column pedestal + steel super structure as whole not spliting them to foundation + superstructure and analyse them sepereately. I have my own reasons, in our codes seperating structures imposes you to use factored loads on foundations and I was planing to avoid that, furthermore it will be built in seismic zone + I thought that it will be more accurate and economic in that way of modeling.

As you all pointed, modeling steel columns as pinned at the bottom eliminates that moments, @jayrod12 there doesn't seems to be any problem this time at the upper structure, what you said was totally I was worrying about (after pining column at the bottom I was expecting that will to contribute negatively to the upper structure as upsizing, cause overall stiffness of structure needs to be stabilized one way or the other). and was grudging to implement it, but this time it didn't cause problem bcoz I slightly overdesigned some of the sections i think, but I'm precautious.


View from steel column in question which receives high moments.
[URL unfurl="true"]https://res.cloudinary.com/engineering-com/image/upload/v1686849229/tips/sample_nj4hz8.bmp[/url]

A 400HEB is pretty huge. Why so big? What is the maximum compression load?

Axial load is not governing case, 24 mt(80 feet) span, seismic zone design forces, HEB 400 column base point of M2 = 300kNm bending moment V2 = 165kN shear and N = -30kN axial force

You can add vertical gusset plates to stiffen the base plate and reduce the bending moment arms.

Sure I was adding, longitidual along flanges parallel to web and additional stiffeners as an extension to flanges base plate with stiffeners, but that was not conforming with code by huge margin

If the pipe rack is strutted ,(longitudinal struts connecting the columns of the transverse frames) , the common practice is to assume that the base of the column acts as a pinned connection.

WE don't have a chance to provide logitidual struts (passing over the road) but I can provide plenty of transversial, and also thanks for valuable docu. sharing

 

[human909]

Axial load is not governing case, 24 mt(80 feet) span, seismic zone design forces, HEB 400 column base point of M2 = 300kNm bending moment V2 = 165kN shear and N = -30kN axial force

But that is my point why are you using a member designed predominantly for high compression loads in a case where you need a member designed fore high bending moments.

AKA why not use an HEAA 500 or similar? You seem to be using a squat section when you should be using a deeper lighter section. Of course I only have half the picture but that is my immediate reaction.

View from steel column in question which receives high moments.

https://res.cloudinary.com/engineering-com/image/upload/v1686849229/tips/sample_nj4hz8.bmp

I don't see how your columns bases could have EVER seen those moment with that foundation. I'd expect that you have applied excessive rigidity in modelling that foundation. It would rotate well before any decent hold down bolts start having issues.