RAM Frame - Seismic Loads [FAQ]


 Product(s):RAM Structural System; RAM Frame
 Version(s):Any
 Environment: N/A
 Area: Analysis
 Original Author:Bentley Technical Support Group

How are torsional irregularities considered?

In general, the program automatically accounts for any eccentricity in the stiffness of the structure during the finite element analysis. For each structure, there is a center of rigidity (which you can report if you create a special center of rigidity load case). If the load is applied to the diaphragm eccentric to this center of rigidity location, then torsion in the structure develops.

Accidental torsion is also considered based on the percentage set under loads - masses (default is 5% of the diaphragm dimension). Prior to version 14.02, the application of accidental torsion was limited to rigid diaphragm analysis. Our method for incorporating accidental torsion in semi-rigid diaphragm analysis is explained in the Ram Frame Analysis manual.

What the program does NOT do, is amplify these torsion effects according to any specific code provisions (e.g. "Ax" from ASCE 7-02 12.8-14) . It is up to the user to account for additional torsion resulting from plan or vertical irregularities. Most people increase the mass eccentricity under loads - masses from 5% to some larger value to account for the extra torsion required by code, though user defined story forces with a modified location also work well.

For more on the enhancements made in version 17.01 to the Drift report for checking torsional irregularities refer to this wiki https://communities.bentley.com/products/ram-staad/w/structural_analysis_and_design__wiki/6011/asce-7-aisc-360-and-the-direct-analysis-method-in-the-ram-structural-system (Step 4).

Are the seismic results ultimate?

For program generated seismic load cases from any modern code (e.g. ASCE 7-05), the force magnitudes are at an ultimate level.

It's important to note, however, that the drift associated with any static seismic load is the elastic deformation (δxe from ASCE 7-02 Eq 12.8-15). The user should amplify the program drift results to determine design deflection (δx  from ASCE 7-05 Eq. 12.8-15). Rather than factoring the elastic deflections, calculating story drifts, and then comparing against the allowable story drift values in ASCE7-05 Table 12.12-1, a practical approach is to take the applicable coefficient (the story drift ratio) value from the table and modify it so that it can be compared directly with the drift ratio values listed in the Drift report:

Maximum Allowable Drift Ratio  = (Coefficient)(I)/Cd

Also note, the vertical component of the earthquake (Ev) is handled though the generation of load combinations by increasing or decreasing the Dead load factor, it is not part of the individual seismic load cases themselves. Furthermore, increases in the seismic force required by a lack of redundancy (Rho) are only accounted for in the load factors applied to the seismic loads in generated combinations.

Note, recent changes in many building codes now define Wind loads at an ultimate level. Where service level drift due to wind is needed in those cases, a reduction of the program output is required.

What is the difference between seismic loads that use provisions for member forces and provisions for drift?

When creating a seismic load case using the IBC/ASCE7 equivalent lateral force procedure, there is an option to use provision for member forces or provision for drift (see screenshot below).

The difference between these options is the upper limit of the calculated period used to calculate the seismic loads.  When provisions for member forces are used, an upper limit of T = CuTa is used for the calculated period per ASCE 7-05 12.8.2. When provisions for drift are used, the upper limit on the period is not used per ASCE 7-05 12.8.6.2

Do I need to divide the Mass DL by the acceleration of gravity?

No, the program is expecting a weight value for the "Mass DL"  despite the name. Enter the same magnitude as the dead load typically (or dead + some portion of the live depending on the live load type and code requirements - see [[RAMSS Gravity Loads FAQ]]). In the Ram Frame - Loads - Masses dialog the total weight and the equivalent mass of each level and diaphragm are reported for clarity.

On the Loads and Applied Forces report, the "Total Building Weight" is reported. This value is the sum of the Mass Dead Loads considered in the seismic analysis which can include member and deck self-weight. This is also a weight term, not a mass.

How is the period determined?

If a Dynamic Eigen Solution load case is defined, then the program uses the results of that analysis to determine the principal modes. If no eigen solution load case is defined, then the program runs an eigen solution analysis in the background with a default number of modes. The default is 3x number of diaphragms with mass, but can be more when using semi-rigid analysis. We do not verify total mass participation; we only use the mode with the greatest mass participation in the 2 principal directions as the principal modes, which is not always the first 2 modes.

We recommend defining an eigen solution load case explicitly to control the number of modes and to allow for full reporting of the Periods and Modes.

See Also

[[RAMSS Gravity Loads FAQ]]

[[RAM Frame - Masses]]

RAM Frame - Eigenvalue Error [TN]

RAM Frame - Wind Loads FAQ

RAM Frame - Dynamic Modal Analysis FAQ

[[RAM SS Notional Loads]]

[[RAM Frame - Center of Rigidity]]