| Product(s): | RAM Structural System; Ram Frame | ||
| Version(s): | All | ||
| Environment: | N/A | ||
| Area: | Analysis; Modeling |
How does changing the ground level affect restraints?
The ground level for Frame analysis can be set through Criteria - Ground level.
The default is for the model base to be the ground level. This is the bottom of the lowest story in the model story data.
The program will automatically provide a support point or restraint at the base level under every lateral column and wall. Additionally, where lateral columns or walls terminate on a higher story with nothing below them, additional support points are provided.
When you set an upper level to be the Ground Level, the program applies lateral restraint in the global X and Y directions to the diaphragm (or multiple diaphragms) at that story. This is analogous to having a vertical roller support on all the nodes attached to those diaphragms. Nodes that are disconnected from the diaphragm are no longer constrained by any ground level restraint. Any subterranean story below the ground level is also treated the same way.
In models with the ground level above the model base, overturning of the structure will be partially resisted by force couples formed by reactions at the ground level and level below (aka prying action). These reactions result in shear reversals and high shear forces in columns and walls on the subterranean levels.
How does changing the ground level affect wind and seismic loads?
Starting in version 17.02.00.130 (January 2021) there is a separate control for the ground level for wind loads and the ground level for support points.
The information below applies where the Wind Generation ground level is the same as the Analysis ground level, and to older versions.
In the calculation of wind and seismic loads, the program assumes the ground level as set under Criteria - Ground level is the zero elevation. All heights using in calculating wind pressures or seismic load distribution are measured up from this ground story.
Can I have the loads calculated based on an elevated ground level, but not have the ground level restrained?
Yes, starting in version 17.02.00.130 there is a separate control for the ground level for wind loads and the ground level for support points.
In older versions, "User Defined Story Forces" were the alternative where the diaphragms are rigid or semi-rigid. We usually recommend to run the model first with an elevated ground level. Then print the Loads and Applied Forces report. Next, create new wind (or seismic) load cases, using the "User Defined Story Force" option under Loads – Load Cases, that are based on that output and delete the original program calculated loads. Finally, reset the ground level to the base.
See Also
RAM Frame - Building and Frame Story Shear
RAM Frame - Seismic Loads [FAQ]