ASME 31E seismic load calculations

Аs ASME 31E said, I can use two variants for choosing seismic load direction: one is three directional excitations such as plant east-west, north-south and vertical, with the combination of square-root sum of the squares (SRSS) or two directional design approach of based on the envelope of SRSS of east-west plus vertical and north-south plus vertical seismic loadings. According to this definitions, the former is as follows if X is east-west direction and Y is vertical with the maximum design acceleration being 0.3 in horizontal and a half in vertical: E1: -0.212 0.15 -0.212. E2: 0.212 -0.15 0.212. Also if the gap or lift-off is to be considered in piping system, you should add positive and negative direction for each X, Y and Z direction with normal operating condition such as W+T1+P1, that means W+T1+P1+E1. Do I correct understand?

  • Hello Mykola,

    Please see the following file for recommended non-linear seismic load combinations.

    communities.bentley.com/.../269425.aspx

    You can modify these recommendations as needed to meet your requirements.

    Regards,

    Mike Dattilio

    Bentley Technical Support Analyst

  • Dear Mike,

    Thank you for your response. I have question regarding option 1 (in pdf file "Non-linear Analysis - Earthquake")

    Will I get the same result in case I enter values for E1 0.6 0.4 0.6?Actually this values (E1) would be combined by SRSS method (are combined as load vectors algebraically). Also when you use combination in your example GR+P+SEIS, what would be direction  of EARTHMAX load?

  • I have the same question. Is there a now a clear method to add EQ loads that meet the requirement of B31E 3.1?

    The example above by Mykola (E1:-0.212 0.15 -0.212. E2: 0.212 -0.15 0.212) seems to be acceleration to opposite directions and nothing perpendicular to this direction.
  • Hi Mike and other forum members,

    B31E allows two options for applying seismic loading

    (1) a three-directional excitation, east–west plus north–south plus vertical, combined by square-root sum of the squares (SRSS)

    (2) a two-directional design approach based on the envelope of the SRSS of the east–west plus vertical and north–south plus vertical seismic loading

    The results will not be the same for either option. (1) is effectively including consideration of a resultant N-S plus E-W earthquake (with a magnitude greater than a N-S or E-W earthquake alone). I think this is given only as an option because it is easier to set up.

    Here is my suggestion to apply loading in accordance with (2), if Y is vertical axis. (Updated 19-Oct-16)

    First set static earthquake loading (inertial loading):

    E1   +x direction

    E2    -x direction

    E3    +z direction

    E4    -z direction

    E5   +y direction

    Apply the support displacements (relative anchor motion) in load cases

    U1, U2 for X-direction displacements

    U3, U4 for Z-direction displacements

    Run the model, then create user defined Code Combination as follows:

    MAX E(X) taking the ABS MAX of E1 and E2

    MAX E(Z) taking the ABS MAX of E3 and E4

    MAX U(X) taking the ABS MAX of U1 and U2

    MAX U(Z) taking the ABS MAX of U3 and U4

    It is necessary in accordance with B31E to calculate the SRSS of the east–west plus vertical, and the SRSS north–south plus vertical seismic loading. It is also necessary to combine the forces and moments due to inertial forces and seismic anchor movement. This can be accomplished in two more user defined Code Combinations:

    SRSS(XY) taking the SRSS of MAX E(X) and MAX U(X) and E5

    SRSS(YZ) taking the SRSS of MAX E(Z) and MAX U(Z) and E5

    Next calculate the “envelope of the SRSS of the east–west plus vertical and north–south plus vertical seismic loading”, by setting up code combination:

    ENVELOPE taking the MAX of SRSS(XY) and SRSS(YZ)

    Finally it is necessary to consider these in combination with sustained loading, so create a final user defined Code Combinations:

    SUS + ENVELOPE taking the ABS SUM of SUS and ENVELOPE.

     All these user Code Combinations are created as Occasional category. In the Load Combinations window, manually enter the appropriate Allowable Stress as defined in B31E.

    It is necessary to run the model twice to find the highest stress, once with E5 defined as a positive Y direction earthquake, and then run the model with E5 defined in the negative Y direction.

    I believe the above method correctly applies the loading combinations in accordance with ASME B31E, and would greatly appreciate any comments or feedback ?

    Regards

    Fergus

  • Thanks Fergus

    We can also note that B31E also allows you to use operating pressure rather than design pressure in the code calculation.

    I have using two operating cases one with design pressure (Pd) and one with operating pressure (Po). Then I have two static analysis sets {1} {2}. {1} is used for Pd, GR and thermal stress and {2} for Po, GR + EQ per B31E. You do have to watch that Pd does not get carried over to {2}, Delete Analysis Results when switching between {1} and {2}.

    Also note when using B31E you need to calculate the EQ loads differently from when using B31.1 B31.3 etc. See B31Ea clause 3.1

    Kevin