Spectrum Load
Shock waves (due to a seismic event), where wavelengths could be long, travel through the soil causing ground motion. Every support of a piping system connected to the ground will then experience the same excitation (hence “uniform” excitation at those supports).
On the other hand, if the piping is routed along the wall of a tall building, then the seismic motion at the ground level will cause higher excitations at higher elevations of the building. In this case, the piping supports at higher elevations will be subject to higher excitations. It is fairly common in the industry to “envelope” the excitations at different elevations to arrive at one “uniform” excitation for all supports of a piping system. Such “uniform” excitation is then specified in CAEPIPE using a response spectrum, which is a table of maximum response versus natural frequency (with damping) for single degree-of-freedom systems.
Starting Version 10.50, CAEPIPE can also perform Multi-level Response Spectrum Analysis of piping systems experiencing different spectrum loads at different supports. The supports could be at different elevations or at the same elevation, grouped under different Spectrum level tags as shown below.
The response spectrum analysis can only be initiated after at least one spectrum level is defined (under menu Load > Spectrums). If the pipe is supported on multiple levels, with different levels experiencing different spectrum loads, each of the spectrum levels should be defined here. The same level tag should be assigned to all the supports located at that particular level.
Double click on the row to assign the spectrum load. Then, select the spectrum that can only be input using the drop-down menu from the spectrums defined under menu Misc> Spectrums.
Level Tag
Type an alpha-numeric name (up to 8 characters long) in this field. Example: If you have three elevation levels (ground, 100m and 300m) with different spectrum loads, you could name them as L000, L100 and L300. If a single level is defined for uniform spectrum analysis, the same level tag will be automatically assigned to all supports. For multi-level response spectrum analysis, the level tags defined here need to be assigned to each support individually or assigned to the group of supports using the Layout window > Edit > Change.
X, Y and Z spectrums
sSelect a spectrum from the drop-down combo box, which should have been input in the spectrum table for each global direction. The spectrum tables can be input in Layout Window Menu: Misc > Spectrums.
Factor
The multiplying (scale) factor for the spectrum is input here. The same spectrum may be multiplied by different (Scale) factors to apply spectrum loads for different dynamic events.
Mode Sum
Pick one of three choices, SRSS (square root of sum of squares), Closely spaced or Absolute. See section on Dynamic Analysis in Technical Reference Manual for more information.
Direction Sum
Pick one of two choices, SRSS (square root of the sum of squares) or Absolute. See section on Dynamic Analysis in Technical Reference Manual for more information.
Level Sum
Pick one of two choices, SRSS (square root of the sum of squares) or Absolute. Level Sum is only enabled when multiple spectrum levels are assigned. See section titled Dynamic Analysis in Technical Reference Manual for further details.
Uniform Response Spectrum and Multi-Level Response Spectrum Analysis
CAEPIPE will automatically switch between Uniform Response Spectrum and Multi-level Response Spectrum depending on the loads defined.
1) If a single spectrum level is defined, the spectrum load will be applied simultaneously at all piping supports, and CAEPIPE will perform a “Uniform response spectrum” analysis. It will compute the modal and directional responses (to this uniform excitation), which are further combined in a manner you specify.
2) If more than one level is assigned to supports, then CAEPIPE will further check if the spectrum loads are the same or different. Please refer to the flowchart below for details on the decision-making process by CAEPIPE. The Multi-level Response Spectrum Analysis will be initiated when more than one spectrum load is assigned to the supports. The combination over level contributions will be performed first, followed by interspatial and then intermodal combination.
Note: CAEPIPE will compare only the “Name” of the spectrum and "Factor” in deciding whether Uniform or Multi-level Response Spectrum Analysis is to be performed as shown in the flow chart below.
