Please clarify - do you mean grade beam as in concrete beam spanning between caissons or grade beam as in combined footing supporting multiple columns?
Grade Beam supporting a masonry wall and supported at it's endes by buttress walls! I think analizing as a combined footing sounds the most accurate. What do you think?
We typically design grade beams without considering any soil support. However, what you are talking about sounds more like a continuous footing with unusual end supports. I would recommend designing it as a beam on elastic foundation, either by the classical formulas while ignoring the end supports, or with an analysis program. Either way, you will need the subgrade modulus.
THE POST-TENSIONING INSTITUTE HAS A LINE LOAD ANALYSIS FOR GRADE BEAMS IN THE SLAB ON GROUND PROGRAM. PERSONALLY, FOR THE LAST FORTY YEARS I'VE BEEN DESIGNING THEM LIKE A WALL FOOTING. THE PROOF IS IN THE PUDDING. SO FAR SO GOOD.
To:Legner, your solution was interesting.In my situation, I considered the grade beam to span between buttress walls that are supporting masonry walls above them. The grade beams were designed to limit deflection to 1/8 in. This was to prevent cracking in the masonry walls. I ignored the soil all together because, I was not sure if the the soil was totally activated in bearing.
you must also consider wheather the soil is expansive or not because if it is then it is better to isolate the beam's sofit than to support it directly on soil
I would say that if the grade beam is supported at the ends then it should be designed as the framed beam without the help of the soil below. The settlement in the soil is something very difficult to estimate. The beam might in couple of years end up with the gap betwwen the bottom of the beam and the grade. Thus using the soil to reduce the beam size and/or reinforcement may result in under-designed beam.
The other option is to design the continuous footing under the load. In this case you can use Winkler model to assess the forces and the deformation.
I would like to know the sanctity of having a grade beam resting on the ground without any excavation and supporting RC wall and roof. I would analyse the beam with the Winkler model. Would that be correct? I would like to specify that this type of structure is to be set up in an earthquake prone area.
You are probably talking about a very light structure. To my knowledge statics is only one part of foundation engineering and though I am not an expert here, I know that the other side of the story is protection of your foundation. So placing a footing on level 0.00 (vegetation)grade is not healthy. Please check out stuff like snow depth, vegetation soils, expansive soils etc.
Since your proposition is completely out of question for a large building, where earthquakes hit better and foundations play a significant role, then I will not attempt to comment on placement of foundation at level 0.00
