| Applies To | |||
| Product(s): | RAM Structural System | ||
| Version(s): | Any | ||
| Area: | Analysis; Reports |
How is the Building and Frame story shear calculated?
When the program calculates the shear at a particular level, it does so by adding up all of the horizontal components of the member forces parallel to the global X and Y axes for all of the lateral columns, braces and walls at the given story.
Total Story Shear is calculated based on all of the nodes at a level. The Change in Story Shear for static load cases is simply the total from the current story minus the total of the level above.
Diaphragm Story Shear is calculated based on the subset of the story nodes and members within a specific diaphragm boundary. The total shear at nodes that are disconnected from the diaphragm or outside a diaphragm boundary are reported as Diaphragm # "None" at each level in the Building Story Shear Report. For a chevron braced condition where the braces are all disconnected from the diaphragm, this diaphragm "None" shear can be almost all of the shear in the structure. (Note, it is the diaphragm shear and not the total story shear used in the ASCE 7 Stability Coefficients report)
Frame Story Shear is a subset of the story nodes and members associated with a specific frame number.
For Dynamic Response Spectrum analysis the program uses a modal combination approach to determine the total shear and the change in shear. For more on that see Dynamic Story Shear.
Why is the Building Story Shear slightly larger than the Applied Story Forces?
The use of the Global Stiffness Reduction method of P-Delta analysis results in a stiffness matrix that is slightly softened to account for the additional deflections due to P-Delta analysis. Refer to RAM Frame P-Delta [TN]
When calculating member forces, which determine the building or frame story shear output, the program uses the original member stiffness with the increased strain resulting in an increase. Additional factors are outlined below.
Why does the Frame story shear suddenly change at one level?
Multi-story Braces
When a brace is modeled such that it bypasses a level, it will not have any nodes at that intermediate level. Thus, when the program sums all of the loads for that level, the forces within those braces are not accounted for. The net force given in the report is therefore low, and the change in shear may appear as negative.
As you can see, the story shear is correctly reported for the Roof level and the 2nd level where the braces have nodes. At the mezz. Level, however, the braces have no nodes, so the forces within those braces are not included. The only shear in the frame in that case is the shear in the columns, which in this case is null. So the total shear that the program recognizes at the mezzanine level is 0, which is a loss of 24k from the level above.
Obviously, this information is not helpful. In those cases, it is recommended that you strike out the intermediate levels from the frame story shear report altogether, and recalculate the "change in shear" value manually.
Inconsistent Frame Numbering
A common cause for unexpected frame story shear results is inconsistent frame numbering. For example, if you delete and remodel the braces in the modeler, the new braces are automatically assigned Frame #0. If you do this at one level but not another level and forget to reassign the frame numbers to the new braces then the frame story shear will be confusing.
Generally speaking the best policy is to assign the frame columns and braces of any interconnected frame a single number. Corner or L shaped frame may be an exception to this rule as described on the RAMSS Pseudo Flexible Diaphragms FAQ.
Upper level Foundation nodes
If there is a base node above the very base level of the model, then here will be a reaction at that foundation node and the total shear in the frame below this point will not count the upper reaction.
Diaphragm Connections
In any structure, it's common to see the frame story shear changing and forces moving from one frame to another through the horizontal diaphragm, this can happen whether the diaphragm is rigid or semi-rigid or even in flexible diaphragm situations if there are lateral members tying the various frames together. For a more complete explanation see RAM SS - Rigid Diaphragm Constraints and Frame Shear [TN].
Sloped Framing
Since the beam forces are not considered in the determination of the story shear, having sloped beams will not generally affect the reports, but the discussion of Rigid Diaphragm Constraints noted above should be reviewed in cases where sloped framing is used with rigid diaphragms.
Through what point is the building story shear plotted?
To show the building story shear on screen use Process - Results - Building Story Shears. Display of building story shear - diaphragm shear is through the diaphragm center of mass. Where there is no diaphragm, or for shear associated with diaphragm "None", the plot is through the a somewhat arbitrary location near the geometric center of all framing.
Why can't I access the Diaphragm Section Forces Report?
The diaphragm section forces report in Ram Frame - Analysis mode is used to report the resultant forces acting on a slice through the diaphragm. lt is only for use with semi-rigid diaphragms and only when the option to save diaphragm forces is turned on under criteria - general.
See Also
RAM SS - Rigid Diaphragm Constraints and Frame Shear [TN]