| Product(s): | RAM Concept | ||
| Version(s): | 4 or greater | ||
| Environment: | N/A | ||
| Area: | Analysis | ||
| Original Author: | Bentley Technical Support Group |
Why does the load history deflection increase when reinforcement is added?
In some situations adding additional user reinforcement can unexpectedly increase long term deflection predicted from Load History Analysis.
There are several factors that should be considered, but one potential cause for this is “shrinkage warping” behavior. Adding reinforcement on the flexural tension face tends to increase cracked moment of inertia and reduce cracked curvature due to loading, but also increases the curvatures due to shrinkage. Think about what happens as shrinkage occurs on a cross section with reinforcement on only the flexural tension face. The flexural tension face is restrained by the reinforcement, while the compression face is free to deform. The result is a curvature and resulting deflection, even in the absence of loading. There are also stresses induced on both the reinforcement and concrete by this behavior (which RAM Concept also accounts for).
A test to isolate this is to set shrinkage strain in the Load History Calc Options to zero.
Why does the load history deflection increase when the column depth is increased?
In some situations, increasing the depth of the columns can result in higher load history deflections than a model with smaller columns. This behavior is associated with the placement of cross sections and the treatment of column elements as point supports.
In general, the program will place the first or last cross section at the face of the support. For small columns, there will be negative moment at this location with top reinforcement available on the tension face. For large columns, the cross section may be pushed past the inflection point and into the positive moment region. If the reinforcement and/or PT tendons at this cross section location are on the top face, much more cracking will be predicted than the model with small columns. This releases the fixity between the slab and column and lead to larger long term deflections.
This is not real behavior but a function of the column element supporting the slab at a single point, the location of the cross section at the face of the support, and the influence of cracking at the cross section on deflections at other locations along the span. The problem can be rectified by drawing a small slab region over each column shape, using the “custom” slab behavior, and changing the stiffness factors for Mr, Ms, and Mrs to 10. This is analogous to placing a rigid link between the column center and the nodes at the corner of the columns. Adding the additional slab over the column is recommend whenever deflections are sensitive.