| Product(s): | RAM Structural System | ||
| Version(s): | Any | ||
| Environment: | N/A | ||
| Area: | Modeling | ||
| Original Author: | Bentley Technical Support Group |
How Do I Model Grade Beams?
It is common to model grade beams in RAM Structural System using continuous foundations, but it is important to be aware of the assumptions. Since the continuous footings are not in the finite element model, the base of the columns will be rotationally fixed or pinned based on the column fixity. The user can assign rotational springs in Ram Frame, but this is a manual process. There is no automatic accounting of the footing flexural rigidity (nor the soil stiffness) in the analysis using continuous foundations. Furthermore, in the analysis of the continuous footing itself the program considers it as a beam on periodic compression-only springs. This would not be correct for pile or column supported grade beams. For more on this see RAM SS - Foundation [FAQ].
Grade beams that do not confirm to those assumptions can be simulated by modeling them as concrete beams analyzed in Ram Frame and designed in RAM Concrete. These basic steps produce satisfactory results for most configurations.
- Model a grade level that contains the concrete grade beams supported by columns. The top of the columns should be pinned and the bottom fixed.
- Model a slab edge on the grade beam level and assign a noncomposite deck with no self-weight. This is necessary so that there is a diaphragm present on the level in RAM Frame.
- Add the grade level to your story data.
- In RAM Frame, set the ground level to the grade level.
- Run the RAM Frame analysis and RAM Concrete Beam design.
By creating a rigid diaphragm at the grade level with the nodes at the grade beam - column intersections to it, translation is restrained when the ground level is set to the grade level. Therefore, no shear will exist in the column stubs. Since the top of the column stubs are pinned, no moment will be developed in the column stub below the grade beams. Spread footings or pile caps can be modeled at the column stub locations. The foundation loads will only be vertical forces.
There are a couple of important things to note. First, automatic calculation of effective length factors may be inaccurate for this procedure. No boundary condition is assumed at the lower node of the column above the grade beam level. Therefore, the G value for the lower node is a function of the column and grade beam stiffness in the direction being considered. If this is not an accurate assumption, the effective length factor should be explicitly defined. Second, don't specify a story height on the grade beam level that is too small. Using an extremely small story height is not necessary because there will be no translation of the grade beam level and only vertical forces in the stub columns. The only ramification of using a larger story height is an increased self-weight for the stub columns. If you use too small of a story height you might produce a poor mesh if lateral walls are modeled on the grade beam level.
Foundation Springs
Alternatively, foundation springs can be assigned to the bottom of the columns (or walls) in Ram Frame using Assign - Foundation Springs.