I have a two-part question concerning design strips and cracked/uncracked behavior for vibration analysis.
Design Strips for Vibration Analysis
I am analyzing a one-way concrete slab with post-tensioned concrete girders spanning longitudinally, and concrete joists at 3'-0" on center spanning latitudinally. Is this the correct design strip input? My ultimate goal is to perform a vibration analysis. Do the design strips have an impact on the vibration analysis?
Latitude Cross Sections Perspective:
Longitude Cross Sections Perspective:
Cracked/Uncracked Behavior for Vibration Analysis
By clicking Criteria-Calc Options-Vibrations, there is an option to select a stiffness matrix. If I select Global Stiffness Matrix, it is my understanding that the analysis will be based on gross uncracked cross section properties if the slab or beams is defined with stiffness modifiers equal to 1.0. If I want the analysis to be based on cracked cross section properties, I need to set the Behavior to "Custom" and the flexural stiffness factors to less than 1.0.
However, I noticed in the RAM Concept Manual that I may use the design strips to define a cross section as cracked/uncracked. How do these two forms of input work with each other? Do I need to ensure both match for the program to analyze the slab or beam as cracked/uncracked?
From the RAM Concept V8i (SELECTseries 8) Manual:
Thank you for your help!
You can perform the vibration analysis using the default global stiffness matrix, in which case the design strips don't matter. This is an uncracked behavior unless you have modified the stiffness of the elements intentionally on the Mesh Input layer. Or you can run a model with design strips, perform a load history analysis to establish a modified or cracked stiffness matrix, and then perform the vibration analysis using a specific load step from the that load history analysis.
You don't want to assign cracked factors in the mesh input and also perform load history analysis, that would be like double cracking the concrete.
The PT stress limits are something else.
See also: https://communities.bentley.com/products/ram-staad/f/ram-staad-forum/138587/vibration-analysis-in-ram-concept
Thank you, Seth. I really appreciate your help.
1. To confirm, if I perform a vibration analysis using the default global stiffness matrix, I do not need to create any design strips.
2. If I have PT beams, do I need to specify the tendon information to perform a default global stiffness matrix vibration analysis?
Correct, we don't require design strips for vibration analysis, but you may still model them and that's never a bad idea. Likewise, tendons are needed for others hings but not vibration analysis.