column at corner of footing qmax formula

[pattontom]

The above is very typical in our plans. Where there is any column at edge, it comes hand in hand with column at corner of footing (see right lower picture above).

For column at edge (beyond the kern) of footing but not at corner, the formula to get qmax is:

qmax = 2P/(3bm)

b comes from rectangular footings of size l x b
so if the P is say 730kn. And b is 3 meter and m is 0.2 meter, the qmax is:

qmax = 2P/(3bm) = 811 kpa

Now I'd like to know the formula for column located at corner of footing. What is the corresponding formula? I can't find it in any books. Or how do you derive the qmax=2P/3bm for column at edge so we can derive the column at corner of footing formula. Thanks.

 

[pattontom]

What can you say about footings rotational restraints? The combined footings width, depth were designed such that rotational restraints in all directions were controlled. However, isn't it the moments have to transfer elsewhere. Meaning, when the period of a building decrease, the elements have to resist strong seismic forces than when it is flexible. So do you think isolated spread footings with more rotational behavior can make the building more flexible and hence increasing the period and make it more seismic resistant? Or do you believe rotational restraints better but where will the moments go... to the column-beam joints.. no?

 

[ishvaaag]

Now you are talking of soil structure interaction and if you focus only or mainly in gaining some longer period for a lesser input you "may" be right, since we may expect "some" rotation on the footings. But even that may prove not to be the case always ... seating brusquely on an edge of the footing in a competent soil might cause even bigger moments in the column than those coming from the vibration with a shorter period; plus less uniformity, bigger likelihood of local failure etc. So quite likely those wanting reliability would choose better a mat over a set of footings for seismic problems. Yet everything is subject to analysis, that's the part of art we vie to understand and practice.

 

[pattontom]

Thanks, ishvaaag. One last concept I needed to know before getting the confidence of letting them handle it. In normal beams above columns (near ceiling). We are concerned about joint shear. For example. If the reinforcement percentage is high like 2.5% or the column small. There would be more bars penetrating the column-beam joints. This can cause more joint shear effect during seismic activity. How about in foundation to column joints. I don't hear about such footing to column joint shear. Remember they also bend like normal column-beam joints. They have different concepts of 2-way punching shear and one way. So there is this difference between the concepts of beams above columns and combined footings acting as beam contrary to what others are saying that they are similar. I want to know what you think of ground level footing to column joint shear and how it differs to column-beam joint shear. Others too who can comment on this pls do. This is my last question. Thanks.