A real world seismic response of the piping system is probably somewhere between a linear and non-linear behavior since pipes may jump off supports and friction is no longer present
So running a non-linear and a linear analysis then finding the maximum loads between the 2 results may yield the most real world results.
When using "ASCE 2010" Seismic Static Load Generator,
The component response factor Rp is an important factor for calculating the seismic ground acceleration.
Rp = 12 is common for stress, but ASCE-2010 paragraph 13.4.1. states that for supports/structure reactions to use a value NOT higher than 6:
One Recommendation is to run 2 sets of seismic cases
Rp = 6.0 (Support & Anchor Reactions)Rp = 12.0 (Pipe stresses)
Depending on your type of analysis (i.e. Linear or Non-linear) there are options for load case combinations:
On the Combinations dialog, select the "Combination Options" button, and place a check mark next to "Add Negative Occasional Cases", From the online help:
If this option is selected, the program will generate default Non-Code combinations for negative occasional load cases in addition to the default Non-Code combinations for positive occasional load cases of seismic and wind only, i.e. GRTP1+W1, GRTP1-W1, GRTP1+E1, GRTP1-E1, etc. The default state is unchecked. This option will be disabled (grayed out) if the "Add default Non-Code combination" option is unchecked or if the static analysis = non-linear.
Some behaviors:
• Disabled when 'Disable Auto Non-Code Combination' checked
• Always Enabled for B31.8, B31.4 Offset, B31.8 Offset, CSA Z662 Offset
• KHK (Enabled only for level 2)
• All other codes (Enabled when one or more load sets are linear)
For static seismic cases we recommend a minimum of 8 cases as shown here which will provide the accurate non-linear support load combinations and occasional code stresses.
Define minimum of 8 static seismic cases:
+/- X +/- Y (4) and +/- Z +/- Y (4) where Y = Vertical
Note: You would typically use OP1 as the initial state. Seismic loads at the load case level e.g. E1, E2 etc are combined as load vectors algebraically to get a resultant loading.
Horizontal X and Z seismic loads are typically not combined in the same load case. Seismic guidelines recommend analyzing two non-concurrent horizontal seismic loadings. It is common to consider separate loadings for X and Z directions in different load cases E1 and E2 both with vertical Y component.
So only cases SUS +E1 to SUS +E8 need to be considered for occasional stress compliance.
For most tall structures, the greater seismic accelerations occur at the top of the structure / building.
We can use the AutoPIPE's Insert> Xtra Data> "Member Static Earthquake factors" or "Point Static Earthquake factors" commands to model the increased seismic accelerations upward through the building / supporting steel structure.
Note:
The Static G load can be varied at different locations by factoring. To factor all the weight you must use both the member and the point Earthquake factors.
- Member Earthquake Factor (Insert > Xtra Data > Member Earthquake Factor) are applied to weights of pipe, pipe contents, pipe insulation etc...
- Point Earthquake Factor (Insert > Xtra Data > Point Earthquake Factor) are applied to point loads: valve, flange, and added weight.
Typically component amplification between 1 and 2.5
The static earthquake "g" force, or Seismic Coefficients, are dimensionless coefficients which represent the (maximum) earthquake acceleration as a fraction of the acceleration due to gravity. Typical values are in the range of 0.1 to 0.3. The static seismic gravity factor is multiplied by deadweight in the system to provide a lateral seismic loading typically in the X & Z directions applied to mass points or to the nodes points of the pipe, not the supports. This type of loading is classified as an occasional loading by the piping codes (301.5 Dynamic effects: 301.5.3: Earth quake. Piping shall be designed for earthquake-induced horizontal forces. The method of analysis may be described in ASCE 7).
A full response spectrum analysis can also be performed. See AutoPIPE/ Help/ Contents/ Reference Information/ Analysis Considerations/ Response Spectrum Analysis
"Static Earthquake" Load Case
Bentley AutoPIPE