| Product(s): | RAM Structural System; Ram Steel Column | ||
| Version(s): | Any | ||
| Environment: | N/A | ||
| Area: | Design | ||
| Original Author: | Bentley Technical Support Group |
Why don't the column design forces equal the sum of the beam reactions?
There are 2 things that contribute to the situation noted above.
- In determining the worst design conditions as required by code, the program skip loads the live load around the column to create the worst case of axial load and bi-axial bending. When a live load is “skipped” on a side (i.e., not applied), it is not included in the total axial design load which appears on the column design or column summary reports. This means that sometimes the reported design load does not include all beam live load reactions applied simultaneously. Please refer to the RAM Steel Column manual, section 3.4.1 Unbalanced Moments as well as Tables 3-1 and 3-2 for details. If you are looking to print total loads for the design of your foundations, you should print either the Column Load Summary report, or the Foundation Loads report from the RAM Manager Post-Processing menu.
Note, in version 14.07 and later, the Steel Gravity Column Design report was enhanced to include more information. It now includes a section called "Controlling Axial Column loads" which lists the maximum axial demand used in the Pu/phi Pn checks, and another called "Controlling Combined Column Loads" listing the critical combination of axial and moments that gives the highest interaction ratio.
- There are different live load reduction requirements for Beams and Columns. This will lead to different loads being reported in the member designs. To see the live load reduction percentage used in the column design, print the full Loads report.
Why aren't the column reactions from RAM Steel equal to the reactions from RAM Frame?
There are differences in the RAM Steel and RAM Frame program that affect the reactions you see in each program.
In the RAM Steel Column Module, the column loads are determined directly from the reactions of the simply supported beams. Think of it like simple tributary analysis. The total loads are simply added together (and reduced where Live Load reduction applies).
RAM Frame, on the other hand, determines column forces from a Finite Element Analysis. This method takes into account relative stiffness of the elements in the model to determine how loads are distributed. Refer to the Analysis Types article for more.
Consider these simple examples that illustrate how the FEA of RAM Frame produces more accurate results for the lateral frame reactions.
For complex, multi-story models, or structures where the frames are linked by a rigid diaphragm this difference in behavior can be quite significant.
How can I confirm the orientation of a square column?
When reviewing the member forces, reactions or other results it is important to understand the relationship between column orientation, footing orientation and the global axis systems.
The following diagram shows the various orientation options for column as they appear in plan. Orientation angles in the RAM Structural System are always measured counter-clockwise from the positive “x” axis. This same rule applies to deck angle, lateral load angles, etc. Note that for Tube sections or rectangular concrete columns it is more difficult to be sure of the column orientation since it doubly symmetric. For those sections, the long dimension (presumably the “H” dimension for your concrete sections) is parallel to the angle of the member.
The orientation can be confirmed in the Modeler by using the layout – column - show command or in RAM Frame by checking “Orientation” under the View – Members command. Once the orientation of the column is known, it is easy to reconcile the sign convention for member shears and moments as depicted in the following figure:
For beams, the sign convention is similar. The orientation vector for beams always points upward, so positive moments occur when there is compression in the top flange.
When foundations are modeled, they are typically oriented the same as the column. When this is done, the major axis moments in the column result in major axis moments on the footing as well. It is only when the footing is rotated to the axis of the column that the forces get translated. For more information on footing orientation see the foundation manual.
Why are the column eccentricity and moments all zero for some steel columns?
In the gravity steel column design there are a few cases where the eccentricity is automatically set to zero:
- Where eccentricity is specified to be zero by the user in the Modeler
- Where the column supports a beam with a cantilever extension we assume a perfect fulcrum connection for the beam and no moment transfer to the column in this case. Furthermore the program is assuming some sort of cap plate connection and ignoring the eccentric moment in the column in both axes which may be unconservative.
- When the column is a hanger.
- The base of a gravity column is pinned.
- Connection eccentricity is also not considered in the design of lateral steel columns in RAM Frame. The assumption here is that typical brace frame connections act through the column centroid work-point (and moment frame connections are fixed). Where you have a lateral column supporting a beam with an eccentric shear connection it is worthwhile to cross check the column design in Ram Steel Column.
Why are gravity columns spliced at non-splice levels?
In Ram Steel column the user can assign splice locations and override the defaults from the Modeler Story data.
In some special cases the program will automatically enforce a splice. These are identified with a "T" (for temporary), rather than a "Y" in the column View/Update dialog under splice information. The conditions where the program will enforce a splice are:
- A change in material properties (Fy or py)
- A change in Shape
- A change in Orientation
- A change in frame type (lateral vs. gravity)
Lateral columns can be sized story by story independently of the story splice defaults.
How can I see the thrust that results from model a sloped column?
When the columns are modeled as gravity members the thrust component is not considered in the design of the column (or the lateral forces on the structure as a whole).
For this reason it is recommended to model sloped columns using lateral members. In that case the finite elements are correctly formulated in the Ram Frame analysis and any thrust components will automatically be accounted for. Keep in mind, where a diaphragm is rigid, the diaphragm will transfer those thrust component to various vertical frames directly. If you want to see the thrust as an axial force in a beam, the beam will also need to be lateral and the beam column node freed from the diaphragm constraint.
Can lateral column base plates be designed?
The design of base plates in RAM Steel is limited to gravity columns. We offer Ram Connection for the complete design of moment frame or braced frame (as well as pinned) base plates according to the latest codes.