| 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?
Although there is currently no direct way to model grade beams in RAM Structural System, they can be simulated by modeling concrete beams 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.