November 15, 2017

How to Create Load Combinations with RSA in RISA-3D

The Load Combination Generator in RISA-3D can now include earthquake loads in load combinations to reflect Response Spectra Analysis.

The Load Combination Generator is available when the Load Combination spreadsheet is open. Just click on the ‘LC Generator’ button found at the top of the spreadsheet. Under Seismic tab, select ‘X and Z RSA’ for both Seismic Load Options and Overstrength LC Options.

Select Generate and the load combinations from the relevant code and region will appear in the Load Combination spreadsheet with the Response Spectra Results.

Tags: RISA-3D, Load Combinations, New

June 7, 2012 | RISA-3D, Loads, New

How to Apply Hydrostatic Loads to Circular Tanks in RISA-3D

RISA-3D v10 now has the ability to quickly and easily apply hydrostatic loads to circular tanks using the Cylinder Generator. This generator can be found by clicking the Templates button and choosing the Cylinder - Plates/Members option. This generator provides a quick and easy method of modeling an entire structure, or just giving a starting point on a more complicated model. As you can see when you generate the structure, the program automatically steps your hydrostatic loads to form a triangular distribution over the height…

September 2, 2015 | RISAFloor, New, Link Utilities, Autodesk Revit Structure

How should I model beams in Revit to make them import into RISAFloor…

Revit offers two options for setting the elevation of beams. The way these are used has a significant impact on how those members are round tripped to RISAFloor, so it is important for you to use them correctly. The Level Offsets are available at each end of the beam. They can be modified through the Properties window, or by clicking on the blue text at the end of each beam: These offsets are intended to allow beams to be sloped, as shown below: Because RISAFloor currently only supports sloping on the top floor of a building, this means that the Level Offsets should be set to zero for all of the beams in your Revit model, except for those beams which have their Reference Level set to the topmost level. When round-tripping between RISAFloor and Revit the Level Offsets will be updated based on the slope elevation values in RISAFloor. Also, RISAFloor does not currently support negative Level Offsets, so the reference level for sloped beams should always be set at the bottom of the slope, such that all Level Offsets are positive values. Beams in Revit must often be moved uniformly down in order to account for slab/deck thickness…

April 3, 2012 | Seismic, Dynamics, RISA-3D, RISA-2D, New

Why Don’t My Response Spectra Reactions Satisfy Statics?

The applied loads in a response spectrum analysis (RSA) are the applied masses times the accelerations in the response spectra. Each mode of vibration produces its own set of joint reactions. The reactions for each mode obey statics as shown in the portal frame example shown below: Response Spectrum Analysis involves the statistical combination of the response from multiple modes. A loss of sign results when the individual modal results are combined together to form the total RSA result. Below is an example of the equations used for Square Root Sum of Squares (SRSS) method: The above equation makes it clear that the final results for R1 and R2 will have the same sign despite the fact that individual modal results (R11 and R21) may have had opposite signs. If this were a statically applied lateral load, the reactions would be equal and opposite with a total net upwards reaction of zero. It is possible for the model to approximately match statics if the option to Use Dominant Mode for Signage is selected. This option merely looks at the reactions for the mode with the highest % mass participation and assigns a positive or negative sign to each joint reaction…