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.
thanks.






RE: high moments at column/base
RE: high moments at column/base
RE: high moments at column/base
RE: high moments at column/base
RE: high moments at column/base
RE: high moments at column/base
(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
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
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.
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RE: high moments at column/base
RE: high moments at column/base
Refer my previous post, sorry this is not to UK standards but the principal still applies.
RE: high moments at column/base
RE: high moments at column/base
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
RE: high moments at column/base
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
RE: high moments at column/base
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
Engineers should only be allowed to draw
Robert Mote
www.motagg.com
RE: high moments at column/base
RE: high moments at column/base
Maybe you need to define a finer mesh.
RE: high moments at column/base
RE: high moments at column/base
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
RE: high moments at column/base
RE: high moments at column/base
RE: high moments at column/base
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