Grade beams 2

[mikhelson]

I'm sizing square shallow footings for column load. The footings will be tied with grade beams. My understanding is that I should size the footings without taking into consideration the grade beams. In other words, when calculating the bearing capacity and settlement, I don't model the footings as strip (continuous) footings. Rather, I model them as square footings. This essentially means that grade beam is not part of foundation, but rather part of superstructure.
Am I correct?

 

[JAE]

In reality they all work together and some of the column load will flow into the adjacent grade beams - the amount depends on load, stiffness of soil, relative stiffnesses of the footings, etc.

The key is to design based on conservative assumptions. So many engineers will design the spread footing as though it is alone and ignore the connecting grade beam. If there is also a wall load or other continuous load along the grade beam, and that load extends over the spread footing, you should include that portion of the line load in the column spread footing design.

 

[kslee1000]

Superstructure, my interpretation is:
Building - Every structural elements above the floor nearst to the exterior grade/street level.
Bridge - Every structural elements above the lowest level abutment, support bents.

The grade beam is part of substructure, though it is not included in determining bearing pressure. In sense of gravity, it carrys and distribute loads as described by JAE, however, most importantly it prevents column pedestals from free lateral movement (drift apart, or come closer). Also, it plays very important role when there is up (lift)- down (excessive settlement) motions due to wind and seismic effects. In such cases, the grade beams would work with the footings as an integral unit to prevent excessive deformations that may cause failure to occur in a brittle manner.

 

[abusementpark]

It seems to me that if you need to consider part of the grade beam on each side of the footing to meet the required bearing capacity, you can. You will just have to prove that the load safely be spread out to these required locations.

Your effective footing will increase in size. You will go from a square footing to a square footing with the required pieces of grade beam on each side. Thus, you are increasing the moment arm for the critical section of the footing in flexure.