Expert: Arshad Khan - 4/27/2009

Question
QUESTION: Dear Arshad,
I am trying to design a combined footing (slab and 2 plinths). how do I find the centre of gravity of the applied vertical loads i.e. Vertical load P1 (-400kN) on 1 plinth and vertical load P2 (200kN) on the other plinth. I normally take moments about plinth P2. if the value i get is positive would the CG be to right or left of plinth 2 (assuming P1 on left hand side and P2 on right hand side).

your help is very much appreciated

Kind Regards

Ayman

ANSWER: Hi Ayman,

I don't know if you got the signs right- are you saying that 400kN is downwards and 200 kN is upwards? If that is the case, you will need the foundation extended past P1 (400kN down) to the left of P1, to a sufficient length so that you have a stable base. In that case the CG will be between P1 and P2, but closer to P1

Please visit this site for more info:

www.ieindia.org/pdf/87/jrspl3d.pdf

and this one too:


forum.vtu.ac.in/~edusat/Prog6/ddrc/mcn/MCN_14-21_Combined-footings.ppt


---------- FOLLOW-UP ----------

QUESTION: Dear Arshad,
Thank you very much fo your reply. actually 400 is upwards and 200 downward. you are right the foundation will not be stable, but what i actually do is to find the centre of graviy of these point loads first and find the centre of gravity of the whole foundation,is there a formula that computes the resultant of these 2 point loads.

Kind Regards

Ayman

Answer
Hi Ayman,

Yes, that is a simple case of using first principles by taking moments about the edge of the base and getting equilibrium of moments and forces (actions=reactions). You will need to add in the self weight of the base, acting at the geometrical centre of the base, acting downwards- in many cases this acts as a stabilizing moment/force, if you make the base big/thick enough.