HOW FAR IS IT RIGHT? 1

[trilinga]

Recently, I and my colleague had an argument.

I am designing a foundation for a framed structure. The soil is good with Safe bearing capacity of 30T/Sq.m. for 25 mm. settlement as per the soil investigation report.

I have modelled the frame with fixed base and obtained the axial load and Moment at the base. I design the footing such that the actual base pressure is considerably lower than the allowable Safe bearing capacity.

My colleague argues that the assumption of fixed base is not correct. He says that when you model the structure with soil represented as springs, the foundation will show rotation and hence the frame should be modelled as pinned at bottom.

I feel that the assumption of fixed base is valid as long as the bearing pressure does not exceed the SBC for the prescribed settlement. Physically the footing can not undergo excessive rotation unless the soil yields below the footing.

Can anyone throw some light on the validity of the assumption?

Thanks

 

[dbuzz]

I agree withn your colleague, unless of course you intend to design the footings and foundation for the column base moments from your "fixed base" analysis.

I would usually design a braced frame structure assuming pinned column bases and then design the footings and foundations for a combination of axial load and column base shear.

 

[UcfSE]

The footing with moment and axial load causes soil pressure to develop. It does this by compressing the soil. In order for soil to be compressed the footing must settle. If the pressure on one side of the footing is greater than the pressure on the other side the footing will settle more on that side, meaning it rotates. Whether this is excessive or not is up to your judgment. Yes the footing rotates, maybe not enough to act like a pin but it rotates none the less. A better model might have a pinned base with a rotational spring but that's a lot of extra effort to go through to evaluate that spring stiffness.

If you keep the soil reaction within the kern distance and the pressures are all very low, your assumption is probably reasonable. I would design some "fat" into the frame since the base isn't actually fixed and the frame will take a little more load than assumed. Bowles has a method for checking footing rotation is I am not mistaken. You must of course design the column connection, base plate and anchor bolts for that moment as well as designing the footing for eccentric load.

If you can make your frame work without having a fixed base connection by all means do so imho. The footing will be much smaller and the connection detail easier to deal with.

 

[jike]

I generally agree with the above posts.

Footing rotation, however, is only one issue. The other components that will also allow base rotation are the elongation of the anchor bolts and the bending of the base plate.

If a fixed base is necessary, I would prefer to support it on a footing stiffened by a foundation wall, grade beam, strap or combined footing to minimize the footing rotation. If it is an isolated footing where a tie to an adjacent footing is not possible, then I would use a rectangular footing to try to minimize footing rotation.

Th anchor bolt elongation can be minimized by the size and spread of the anchor bolts. The base plate should be sufficiently stiffened to minimize rotation also.

I believe in the PCI manual, there is a section on how to evaluate each component for rotation.

I hope this helps!

 

[trilinga]

Thanks to all for the valuable responses.

jike, sorry, I forgot to mention that the frame is RCC. Hence, the rotation due to the anchor bolt deformation may not be the criterion.

My structure is about 17m. tall and the decision on having a fixed or pinned base has a definite impact on the member sizes to contain the storey deflection within the allowable limits.

I referred to Bowles. There is a relationship between the Modulus of subgrade reaction and the Soil bearing capacity given by

k_s = 40(q_ult)

For a SBC of 30t/Sq.m.,with a factor of 3 for q_ult.,is it not reasonable to assume the footing as fixed since the k_s value will be high?

 

[Ron]

In order for rotation to occur, you must develop moment from the structure to the footing, therefore by definition, the connection is fixed. If there is no rotation of the footing at its base, then the footing either has enough internal stiffness and mass to accommodate the moment or it is a pinned connection.

 

[haynewp]

There is no such thing as a fixed connection. Your colleague is not completely correct in saying it is a pinned connection either. Springs are the most accurate way to model it, but not necessarily the most conservative.

I usually try to avoid using a "fixed" column base.

 

[Ron]

I agree with haynewp, except that I do use "fixed" conditions for ease of analysis. It is usually conservative from a bearing capacity standpoint assuming you include the moment transferred to the footing in your bearing pressure calcs.

Remember, when we're dealing with dirt (soil, excuse me Professor Sowers), we tend to measure it with a micrometer, mark it with a crayon and cut it with an axe. Don't try to be accurate to the nth degree with something that is inherently inaccurate to begin.

 

[trilinga]

Thanks Ron and heynewp.

Ron, I am designing the joint between the column and foundation for the moment and the connection is rigid so that in case of any rotation due to soil settlement, there will be no relative rotation between the column and footing. However, if the footing-column joint rotates, there will be an upward flow of moment in the frame making the footing a pinned end.

I would definitely think twice before going for a fixed support if my soil is weak.

My point is that if the soil is strong with a good SBC, the equivalent soil spring will be stiff enough to make the rotation less.

Does this not justify a non-rotating (Fixed) support modeling?

Trilinga

 

[LPPE]

I'd model it as pinned.

Ron- The soil comment made me laugh. My Soil's professor favorite line was "Plus or minus 200%, sure."

 

[LittleWheels]

I lost interest in being a geotechnical engineer when my professor mentioned, in the first lecture, that getting the right order of magnitude was close enough to the 'real' answer

 

[pba]

The question boils down to - is the ground stiff enough to allow the frame to behave as if fixed at the ground?

Given that you have a frame analysis programme - Model the bases as springs. We all know that the 'real' answer lies somewhere between fixed and pinned. Trial different spring values and see how sensitive your model is.

I'm guessing that your model IS sensitive to base stiffness otherwise I don't think you would have posted the question. Remember that this thing has to be built by morons who will get everything wrong which they possibly can. Your most sensitive pad base excavation is the one which they will use as the sump when they de-water the site!

Good sensible engineering judgement indicates that the assumption of pinned is more conservative, but you have the means to provide further 'comfort' that your less conservative assumption is not reckless...

 

[trilinga]

Sorry for my delayed response. My PC was down for a few days.

Thanks for the enthusiastic response of all.

I infer from these posts that it is desirable to design the footing as pinned.

Maybe, I need to revisit my concept of fixity for a footing.

However, I think we should not forget the impact on the design due to any fixity (partial or full)developing in reality vis-a-vis the assumtion of pinned bottom.

Particularely in a RCC beam, the assumption of pinned bottom leads to underdesign of the column at the bottom and also the midspan of the frame beam at the top. Hence I feel that we should build some additional moment capacity at these locations.

Trilinga

 

[VoyageofDiscovery]

Hi trilinga,

I was always told by my mentors, design it that way and it will act that way.

Stiffness attracts load, so if you intend to add moment resistance, your analysis should account for this with rotational springs.

Regards

VOD