Tower crane foundation - negative pressure 3

[ChrisToms]

Hi all.. i am designing a tower crane base. The footing is safe in overturning although it has about 25% of its area in negative pressure for out of service condition. The design is carried out based on ACI code. I am not able to find any clause related to maximum percentage of negative pressure permitted as per the standard. Can anyone provide the clause on limitation on negative pressure below footing as per any American standard? TIA

 

[Agent666]

You foundation shouldn't see any negative pressure, maybe zero, but not negative. Soil is compression only.

Sometimes under service loading you'll need the reaction within the middle third of your footing, this is meant to ensure no uplift of the footing. Not sure if it is codified though, more of a rule of thumb.

 

[MIStructE_IRE]

Agree with Agent’s comments above.

What size base have you got? Can you go bigger? If not, consider tension/compression piles.

Also, don’t forget the 45 degree load case where the jib is skewed diagonally over the base. I’ve seen that missed in my office plenty of times as people have a tendency to think in X and Y directions only here!

 

[Settingsun]

Sounds like a question for the geotechnical engineer.

 

[ChrisToms]

@Agent666 Thanks for your reply. And yeah by negative pressure i mean there is an uplift (P/A-M/Z is a negative value). I guess some codes allow certain percent of uplift but I can find a reference for that in ACI or ASCE. And yeah since P/A-M/Z is negative, the reaction of course does not fall within the middle one-third. Overturning is fine though ; FOS > 2.

@MIStructE_IRE Thanks for the reply. I have a 6x6m footing. They cant increase the size due to site restrictions. And thanks for reminding of the 45 degree boom; checked it.

 

[Lomarandil]

In addition to passing an overturning check, make sure that your footing rotations are within the limits from your tower crane manufacturer. Depending on the height, that can be a big deal.

----
just call me Lo.

 

[JStephen]

If the P/A-M/Z is a negative value, it just means that the P/A - M/Z is no longer a valid measure of soil pressure. Assume a point of uplift, and calculate additional moment on the remaining contact area due to eccentricity, then calculate P/A and M/Z for that reduced area, and adjust the point of uplift where P/A - M/Z is zero at that point. For a rectangular area with moment aligned with a face, I think that works out to a direct solution, with circular foundations, it becomes iterative. As you get near the tipping point, soil pressure on the heavy side increases non-linearly with moment, so check with load/safety factors applied to the load, not just the soil bearing.
One of the tank standards requires with 1.5x overturning moment, bearing on the heavy side is still less than ultimate, or with 1.0x overturning, bearing on the heavy side is less than allowable. For ASCE 7, that would be 1.3 or 1.6 factor on the overturning moment and 0.9 factor on the dead load. For a crane, I assume factors would vary.
You can calculate rotation at the base and include additional moment for P-delta effects in the overturning if desired.

 

[r13]

The footing area with calculated negative pressure is no longer in contact with the ground, the effective footing length/area is reduced to account for the lost contact , and the stress redistributes until reaches the limiting equilibrium - the entire footing base is under compressive stress, and the resultant force fall within middle third of the footing base (maximum stress at one end, and zero at the other).

The failure of tower crane foundation can have grave consequence. I suggest to enlarge the footing, add more weight, or find other stabilizing methods to resist overturning. It seems you are not familiar with this type of analysis/design, you should find help from your company, and source that provide design guide and criteria, before going too far on the task.

 

[DaveAtkins]

If your footing is square or rectangular, and the resultant load is outside the Kern limit, then P/A +/- M/S is no longer valid. The proper formula for bearing pressure when the load is outside the Kern limit is 2P/3aB, where P is the resultant load, a is the distance from the leading edge of the footing to the resultant load, and B is the width of the footing. The bearing pressure is triangular, with the centroid of the triangle directly below the resultant load.

DaveAtkins

 

[JLNJ]

Non-existent negative soil pressure is such a basic foundation concept that it makes me think the OP should be seeking out someone in his or her office who can explain these basic concepts, rather then querying the internet at large. It begs the question about what else has been overlooked or simply not understood.

 

[ChrisToms]

Hi all..thanks all for your valuable replies..Yes I understand that when there is negative pressure (or to re-phrase P/A-M/Z is negative) there is re-distribution of soil pressure since the bearing area will be reduced.
My question was if the overturning FOS >2 and the maximum bearing pressure < SBC, is there any limit on the reduction in bearing area as per American codes?
I was asking about the dimension marked in the below figure.

I can find a reference in Indian code (code for bridges IRC:78-2014) which is as given below:

May be my question was not clear in the first place or it may too basic. But I believe it is a place where engineers of all grades can come and discuss doubts and queries. I have seen many posts in other threads where by budding engineers are offended or discouraged may times for posting their queries. If someone thinks the question is too basic or it is wasting their time, its better they don't participate in the discussion. Those who are kind enough may only reply.

Of course we will be discussing all the calculations with others in the office and none of the calculations go out without review. But its because I would like to get more insight on the matter from engineers around the globe that I posted here.

 

[r13]

ACI is reinforced concrete design code, as its name suggests. ASCE shall have many old papers on stability considerations for design of earth retaining structures. Corp of Engineer offers the best practical design guide on stability for earth retention/hydraulic structures.

 

[Lomarandil]

My question was if the overturning FOS >2 and the maximum bearing pressure < SBC, is there any limit on the reduction in bearing area as per American codes?
I was asking about the dimension marked in the below figure.

I haven't done a tower crane foundation for a few years, but I am not aware of any such restriction in American codes.

----
just call me Lo.

 

[DaveAtkins]

I am not aware of any restrictions that force you to keep the resultant load within the Kern when designing a foundation for a bridge crane. Foundations for such things as dams and spillways should have enough mass to keep the load within the Kern.

DaveAtkins

 

[WinelandV]

My first ever assignment at my first job was to size a spread tower crane foundation. My boss had 15+ years working with them, but wanted me to struggle with it. Even gave me 3 days to work on it. From my Geotech classes at college, you learn that you need to keep the resultant force in the kern. Turns out (as DaveAtkins and Lomarandil noted) that there is no such restriction on a TC foundation. I kept coming up with plan dimensions of 35' x 35' (give or take), while he was looking for plan sizes under 30'. He gave me a book called "Cranes and Derricks" (I can't remember the author) which covers all things cranes and derricks, including their foundations. The big concern is always the wind blowing on the diagonal of your square footing. Thankfully it included a method for calculating the pressure for that scenario.

At any rate, it is common practice to allow the footing to lift off the soil under wind loads.

 

[chicopee]

An excellent reference on tower crane installations is Crane and Derrick by the Shapiro families whose work is I NYC. My 2nd edition( also look for updated editions) chapter 6covers the subject extensively. Chapter 6 with uplifts under self standing tower crane footings. Shapiro also confirmed some statements from your above responders to contact the crane manufacturer for technical details such as support reactions, wind loads, lift consideration, etc... Also pay particular attention when designing the foundation when the crane is in service and out of service.

 

[Lomarandil]

I had forgotten about Shapiro. Definitely a valuable reference.

----
just call me Lo.