Current Revision posted to Structural Analysis and Design - Wiki by kjgullerud on 11/6/2014 6:34:01 PM
| Applies To | |||
| Product(s): | RAM Structural System, RAM Concept | ||
| Version(s): | Any | ||
| Environment: | N/A | ||
| Area: | Analysis | ||
| Subarea: | N/A | ||
| Original Author: | Bentley Technical Support Group | ||
The Different Analysis Types in RAM Structural System
Throughout the RAM Structural System there are distinct types of analysis performed in each module. The results of the different analysis types may differ slightly because of the different assumptions within each method. Here is a breakdown of the different analysis types.
RAM Steel
When a model is designed using either of the RAM Steel modules (Steel Beam or Steel Column), the program performs a simple tributary analysis which we refer to as the “framing tables”. To determine the loads, the program calculates the tributary areas and attributes gravity loads to each member based on the applied surface, line and point loads in the model. The reactions of a beam framing to another beam, or a beam framing to a column are calculated based on the assumption that all members are simply-supported, determinate members with exactly two supports. Live loads are reduced according to the code rules and the particular member tributary area. Specifically, the things that are not considered in this type of analysis are:
- No end fixity, except for beam cantilevers (all simple span members are pinned ended).
- No braces
- No lateral loads
- No deformation of the supports
- No loads applied to 2-way decks
- Walls will transfer gravity loads straight down to supporting members, but the loads are not distributed or spread out in any way.
RAM Frame
Here the program performs a full 3-dimensional finite element analysis. The reactions of gravity members being supported by lateral members are calculated as in the Steel modules. Then these reactions are applied to the lateral members as point loads. Otherwise, the gravity members are completely ignored in the RAM Frame analysis. Rigid diaphragms are assumed to connect the vertical frames together, but the user has control over this. The program can also consider rigid end zones and second-order, P-Delta effects. Lateral static loads can be applied in addition to the gravity loads. These lateral loads can be applied as diaphragm forces when rigid diaphragms are used, or they may be applied as nodal loads. Dynamic response spectra analysis can also be performed. This is the only module that investigates lateral load cases. Specifically, the things that are not considered in this type of analysis are:
- Gravity members are not considered in the stiffness matrix (Gravity member reactions on frame members are treated as external loads.Gravity columns and walls supporting 2-way decks, are considered in the stiffness matrix to support the gravity loads).
- No skip loading, except for the separation of negative (upward) live loads from downward (positive) forces.
RAM Concrete
Here the program is attempting to perform a more elaborate analysis of the concrete structure for gravity loads. Every single member is considered in the analysis, and concrete gravity members will now be analyzed using the assigned end fixity (presumably fixed-ended). The program has the option to skip load the live loads to account for the worst pattern of load for the design of each member. Another analysis option which differs from RAM Frame is to reduce the torsional stiffness of the concrete members.
Because this method is much more involved, the program analyzes only one floor at a time. The column and wall reactions of the top floor are determined and then added to the lower floor analysis as loads. Consequently, some overall 3-dimensional effects (Vierendeel truss behavior) may be neglected in this form of analysis. Consider the example where an upper level frame has one column supported by a lower level beam, and the other column continues to the ground. In reality, the continuous column will attract somewhat more than of the gravity load, but the RAM Concrete program, when analyzing the top level will recognize the two columns to have the same stiffness and will attribute the same load to each. Only the full 3D analysis of RAM Frame can capture these overall effects, and this is why the program has the option to consider either the gravity load results from the RAM Frame analysis or the gravity load results from the RAM Concrete analysis when designing lateral concrete members. Specifically, the things that are not considered in this type of analysis are:
- No overall 3-D effects
- No braces
- No lateral loads
RAM Foundation
The RAM Foundation program doesn’t actually perform a separate analysis, but there is an option within the program to choose which gravity load results should be considered for the design of foundations supporting concrete, gravity columns. The foundation design forces may utilize either the RAM Steel (simple tributary analysis) or the RAM Concrete results. The founation module always uses forces from RAM Frame to design foundations supporting frame members. The foundation module can use forces from RAM Concept to design foundation supporting gravity members, provided that the reactions are exported from RAM Concept to RAM Concrete and RAM Concept forces are selected to be used for design in the RAM Concrete Analysis mode.
RAM Concept
RAM Concept is a separate application which also performs a finite element analysis of a single floor of a concrete structure. It is mentioned here because there is an option in RAM Concrete to substitute the RAM Concept column reactions from dead loads, live loads and balance loads (i.e. post tensioning effects) in the RAM Concrete Column design routines in lieu of the RAM Concrete analysis results.
See Also
Structural Product TechNotes And FAQs
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Tags: analysis, RAM Structural System