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How do I design columns when unbraced length is especially long?

Extended Question: I am modeling a parking structure in which only ramps frame into certain columns. Since there are no beams to provide bracing points, effective length is unrealistically long. How is this resolved?

ANSWER: When columns frame multiple stories, the software only identifies brace points under the following circumstances:

• When bracing objects connect to the column, as with beams framing into the column.

• When columns are part of a diaphragm definition at a given story level.

Parking structures may be of particular concern because diaphragms are not assigned to ramps, but rather, only to horizontal area objects. As a result, when interconnecting beams are not present to serve as bracing, effective column length may become very long.

To design such columns, refer to the lower column segment. The software will apply the appropriate length factor to these segments and correctly calculate the actual design length. The lower column will then govern design of the entire column.

How should I deal with center of mass for ramps?

Extended Question: The diaphragm center keeps shifting floor by floor since ramps are not included in the diaphragm definition. How is this resolved?

ANSWER: Since diaphragms are not assigned to ramps, and only to horizontal area objects, diaphragms are usually assigned to levels above and below each ramp. Center of mass is then calculated according to nodal masses, therefore proper meshing is important for the distribution of mass and the accuracy of the center-of-mass location.

How do I find the ramp area?

ANSWER: Since floor area only applies to horizontal area objects, ramp area should be found using weight, unit weight, and thickness properties.