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high moments at column/base

high moments at column/base

high moments at column/base

(OP)
hi, when modelling column loads on a foundation slab and the column loads include a moment the model will show high moments local to this applied load. How do you cater for this? I have had a few suggestions, some seem resonable, some i've used in the past but would like to hear the experts view (you guys).

thanks.  

RE: high moments at column/base

This is why most frame bases are modelled as pinned it is often difficult to manage the fully fixed loads..

RE: high moments at column/base

(OP)
i agree that the frame should be pinned ideally. However this is not our design (the frame) and there are high loads on the kind of frames which I guess (and would hope) that the steel designer is designing economically. I have also on occasions seen details which are certainly fixed when looking at upgrade work etc.

RE: high moments at column/base

The Steel Construction Institute used to have a method whereby you model dummy members at the column base as a method of taking account of a partial fixity.

RE: high moments at column/base

I am not sure what you are asking. How to analyze; How to detail base plate, anchor bolts or foundation; steel of concrete frame?

RE: high moments at column/base

(OP)
sorry. I am looking at the foundation itself. Usually piled. In a model I would get very high moment local to load. i.e. the moment applied at the column position is twisting the plate in the model significantly.

RE: high moments at column/base

Are you looking at frame columns sitting on a mat foundation?  A large pier/pile cap?  If you are modeling a mat/pile cap in SAFE, for example, how much do the moment magnitudes depend upon the size and aspect ratio of your elements?

(slightly off topic) When you have a "pinned" baseplate connection in a moment frame, how do you avoid a "soft story" at your first story of a multi story building?

RE: high moments at column/base

Ok I take it your talking about the baseplate:

so assuming that the load from the column is uniformly ditributed and completely distributed through the baseplate to the foundation. It would be helpful if you were to state what the column section size was, the moment at the base, the axial load coming down the column and the orientation.

I could help you as far as designing the baseplate, holding down bolts and pad foundation if you could suply me with the information.

You will probably need to create a moment resisting baseplate using side plates or a concrete encasing.

Also what is the bearing capacity of the soil, the cohesivness of the soil or if it is on top of a pile any details on the existing pile and pile cap

R. Murphy
MICE MIStructE MASCE

RE: high moments at column/base

herewegothen,

Are you talking about the concrete footing/pile cap under the concentrated moment?

To take care of this, bring your anchor bolts down to below the horizontal reinforcement and put a plate at the bottom.
You then have a truss with anchor bolt verticals, reinforcing bar horizontals and a diagonal concrete compression strut.

http://www.gostructural.com/userfiles/file/pdhse1007.pdf

RE: high moments at column/base

(OP)
there seems to be some confusion here. I have received column loads from steelwork designers. these include moments. When using a model for a piled or even ground bearing the moment will cause a massive moment in the top of the element. Caused i would think by the fact the element in model is a thin plate. How do others rationlize this moment, otherwise you'd have massive reinforcement in top. this is not job specific at the moment. Just occured a number of times in past.

RE: high moments at column/base

Dont treat it as a moment, treat it as a push and a pull on your anchor bolts. This will reduce the effect somewhat.

Refer my previous post, sorry this is not to UK standards but the principal still applies.

RE: high moments at column/base

(OP)
yes I will have a look through that. Like you say, principle of forces would still apply. Thats one approach I've used in past, up down on bolts. Even then I think the forces generated seem high. although maybe its just a good example of why we dont 'fix' columns.

RE: high moments at column/base

If the moment is applied about your major axis, then you can do the push/pull (through flange action) on the bolts with bending on the baseplate. If it is on the weak axis then it can design the baseplate for bending (prying action).

You have a choice to put the thickest baseplate (through cantilever projection and limiting stresses) and transfer all the moment to the bolts or, go for a thin plate and allow for elastic deformations which tends to the pinned base approach.

The problems start when you 'design' for a pinned base and you actually have a fixed detail. The questions are, when is a pin a real pin? And is a fixed base really fixed?

Robert Mote
www.motagg.com

RE: high moments at column/base

(OP)
im not designing base plate im designing foundations. i think the steel designer is probably covering both bases maybe

RE: high moments at column/base

For slab foundations design, if pile are pinned and simple, I often model piles as compression-only springs. The details for 'fixed' embedded piles are difficult to achieve in practice, to the satisfaction of the geotechnical experts.

Do you have horizontal loads as well as moments? One reduces the available capacity for the other.

Robert Mote
www.motagg.com

RE: high moments at column/base

(OP)
I'm struggling to understand why people are not understanding. I am designing the foundations. I am not fixing piles they are 'pinned' supports. The column loads onto the pile cap/foundation are given by others. This includes a moment. When applied to a FEA model. High moments are shown in the top of the elements at the 'column/load' position. This I believe is due to the program not fully recognising the thickness and stiffness of the element. I am aware how to model the piles themselves. This is a question of designing the structural element of the foundation slab.

RE: high moments at column/base

herewegothen,
If you are talking about designing a spread footing or pile cap that has an axial load,P and a bending moment, M then you assume a footing size, calculate an Area, A & Moment of Inertia,I of the footing, calc the min & max brg pressure, q from P/A plus or minus Mc/I. Then calc shear & bending moment in the footing. Finally calc the required reinforcing.
You will have to do a few cycles to get the soil bearing pressure to the allowable value that the geotech supplies.
Most reinforced concrete text books have examples of this process. I don't think that there is any way that a FEA program will do this for you.
Regards

RE: high moments at column/base

You nailed it I think. FEA is lmited.

Engineers should only be allowed to draw smile, and on a beermat, with a pint in hand.

Robert Mote
www.motagg.com

RE: high moments at column/base

(OP)
right i want to make this clear here. I am well aware how to design a footing, pile cap etc. Occasionally we have a large raft structure which has a number of piles. We use (as many other companies/engineers do) a FEA program to calculate loading on piles where there can be a high number of piles with columns and other loading that is not in any was uniform. To calculate by hand would be a huge undertaking. Other than to design separate pile caps for each column/loading area which will not be as economic. We would produce a model that has pinned or spring supports to model the piles and input the loads. This is where I think the program falls down in that if the columns have moments (not ideal I know but these are sometimes supplied to us). On the comment on FEA being limited. It would not be used to calculate bearing pressures on a ground bearing slab, only moments shears etc. This is described in amny textbooks and the theory is based on a slab on springs to model subgrade reactions. moments generated then will depend on the stiffness of the soil and the stiffness of the soil relative to the slab.

RE: high moments at column/base

Your FEA program doesn't like it when you apply moments to the madel? What program are you using?

Maybe you need to define a finer mesh.  

RE: high moments at column/base

(OP)
refining the meash doesn't seem to help much. The effect is obviously greatest if the moment is applied as a point load. The other option as someone suggested is use tension or downforce in the bolts. But as bolt spacing will be reletively small even then when we have high moments the plate still 'twists' considerably.

RE: high moments at column/base

Herewegothen,

I've encountered this problem many times with FEA analysis also. When you apply a moment as a point load to a slab you will get very local forces in the slab. Spreading this moment into a couple in anchor bolts will help a bit but not much, as the bolts wont have a big lever arm. Refining the mesh will only make things worse.

Normally the FEA program will give a contour plot of bending moments in kNm/m width of slab, with very high peak moments at the column. These will only act over a very small distance and their magnitude will drop of very quickly as you move away from the column.

What i generally do in such a circumstance is determine (using engineering judgement) what width of slab is effective in resisting the moment that is transferred from the column. Then average the moment over this width and  design the slab/beam for the averaged bending moment.

Hope this helps and also hope that this is how you have dealt with it in the past.

RE: high moments at column/base

(OP)
yes I have done something similar when dealing with very local moments. Sometimes we have very high moments associated with the columns which makes me uneasy doing this at times.

RE: high moments at column/base

I've experienced this as well, and have averaged the local moments over an effective width, based on engineering judgement.  I was hoping you would get a response with a reference to some research.  Seeing none has been posted, looks like we're on our own with this kind of thing.  I believe I have used an effective width of 3 times the thickness of the concrete to each side of the applied force.  I wonder what others have used.

RE: high moments at column/base

(OP)
ive resticted mine to average over a metre really. though I suppose you could go further, effective depth either side of column? so 2 X eff. depth + column width? Looks like its a, as you say 'engineering judgement' call.

RE: high moments at column/base

herewegothen,

I don’t quite understand, you stated a couple of threads ago that you have "pinned supports". How would transfer any moment from the proposed framing (column) into the foundations?

If you do in fact have a fixed base column that is transferring moment then you could go use Steel Structures by Salmon and Johnson. Using the Steel Structures book you would essentially convert the moment into a moment couple (I know it’s the foundation your interested in, not the steel). By designing using this procedure (grouting under the base plate) you basically reduce your loads into a bearing load at the top of the foundation and a tension load through the anchor bolts. You can then go to your ACI code and check the design (anchor bolts for pullout, the footing punching shear etc.)

Typically based on the anchor bolt size, length and as you mentioned moment arm you can transfer a fairly large moment. The only issues that I have run into when your bolts are fairly close together is that the pullout cone overlaps and the concrete does not yield a linear correlation between number of anchor bolts and pullout strength. You take a pretty good hit with groups of anchors.  

I hope this helps.

fas

RE: high moments at column/base

(OP)
pinned or spring supports were referring to piles position not column from superstructure. The approach you state I have used in the past. The fact that bolts are always reletively close together ends up giving the same effect in the top of the slab as just applying the moment. If you run through a FEA program and apply moments you will see the problem.

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