## Tower crane foundation - negative pressure

## Tower crane foundation - negative pressure

(OP)

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

## RE: Tower crane foundation - negative pressure

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.

## RE: Tower crane foundation - negative pressure

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!

## RE: Tower crane foundation - negative pressure

## RE: Tower crane foundation - negative pressure

@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.

## RE: Tower crane foundation - negative pressure

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just call me Lo.

## RE: Tower crane foundation - negative pressure

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.

## RE: Tower crane foundation - negative pressure

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.

## RE: Tower crane foundation - negative pressure

DaveAtkins

## RE: Tower crane foundation - negative pressure

## RE: Tower crane foundation - negative pressure

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.

## RE: Tower crane foundation - negative pressure

## RE: Tower crane foundation - negative pressure

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

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just call me Lo.

## RE: Tower crane foundation - negative pressure

DaveAtkins

## RE: Tower crane foundation - negative pressure

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

## RE: Tower crane foundation - negative pressure

## RE: Tower crane foundation - negative pressure

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just call me Lo.