Footing Overturning Factor of Safety

[mkrei]

This subject seems to have been discussed on many forums and it appears that the .6D+1.0W will give a O.T. factor of safety of approximately 1.667 compared to the old 1.5. I am looking at a 4'-6' thick concrete footing below elevator shafts which are used as shear walls. I am using the .6D+1.0W combinations. It is a fairly complicated footing with numberous point loads coming down on different corners of two elevator shafts. I have input the D and W reactions into Risa and allowed it to generate the load combinations. I have modeled the footing as plate elements supported on compression springs so as a spring goes into tension it is removed and the solution rerun until a convergence is reached. I have reviewed the results for .6D+1.0W combinations and my soil pressures are less than the allowable soil pressures. My question is have I satisfied the requirements of checking overturning by using these load combinations and checking against allowable bearing capacity. Any thoughts would be appreciated.

 

[mkrei]

Another thought. How do you all hanlde the 150++ load combinations that get developed if quartering winds and eccentric winds are considered. It seems that for moderate height buildings (100' or less) all the load combinations are overkill and my ability to get a feel for all these loads is lost. I have somewhat made it my policy that I design my shear walls above grade to the best of my ability for the multitude of combinations by doing an envelope of results for frame members using Ram. However I have made the simplification of just using wind in the primary directions for my foundation design as I do not I feel that I loose any sense of appropriateness of the loads I am imparting to my foundations. I hate to just be a machine punching numbers from one software into another and having no feel for if the numbers make sense. These are my random thoughts on lowrise buildings. Any additional thoughts would be appreciated thanks

 

[JAE]

It is my understanding that the 0.6D + W load combination is the required check for overturning. The more recent codes have simply formalized this check into the combos and removed the 1.5 SF from the text body of the code.

How do you all handle the 150++ load combinations that get developed if quartering winds and eccentric winds are considered.

Heh....call your local ASCE 7 committee member and complain.

 

[mkrei]

Thanks for the input that was my thoughts also. I have discussed this at seminars with code officials about the complexity of the wind codes and how owners are not willing to pay additional fee to check the additional combinations that were not required 10 years ago. The answer I received was use a high speed idiot(computer). To me that is not the answer and simplification is the answer. I have a feeling this is coming from the academic side as these people feel as if they are being paid to update the code so they need to make it more complex so they look like they are doing a good job....

 

[UcfSE]

Just let the computer do the work. Use the software, or use a spreadsheet or Mathcad. It's only an idiot if the user inputs an id10t error. Otherwise it will do exactly what you tell it to do.

 

[hokie66]

mkrei,

I agree with you completely. Design engineers don't have a very good union, so when the academics tell us to jump, we just ask how high.

 

[csd72]

The US wind code does seem to be more complicated than either the Australian or the UK wind codes and I dont believe that it is that much more accurate.

csd