Second-Order Analysis

The requirements to perform a second-order analysis is satisfied by performing a first-order analysis and calculating and applying B1 and B2 factors to the design forces as outlined in Section C2.1b of the Specification. Those factors need to be calculated for each load combination. The implementation details are given in RAM Frame Steel Post-Processors manual. It should be also noted that the engineer is provided two options to consider 2nd order (large P-delta) effects: either the engineer use the current P-delta analysis implemented or the engineer chooses B2 factors.

Notional Loads

Notional loads are specified in the Loads – Load Cases command in RAM Frame. The magnitude of the notional loads that should be specified is dependent upon the value of τ_b used in calculating the reduced stiffness. When the engineer defines notional load cases, two options for gravity loads fractions are provided: 0.002 and 0.003.

Reduced Stiffness

An option to use the AISC 360 stiffness reduction is available (Criteria – General dialog). The engineer should select this option if it is intended to use the AISC 360 design code. The program does not iterate to determine the correct value of τ_b, so the engineer either specifies 1.0 or some other value. Although technically τ_b is distinct for each load combination for each member, the program uses the specified value on all members. Also, the program does not provide an Assign command to assign different τ_b values to different members, nor does it use a different stiffness for each different load combination.

Note that the program uses the reduced stiffness for all load cases. Unfortunately, it is not clear from the Specification whether the reduced stiffness is needed to be used for the drift analysis. Thus, it is up to the engineer to decide whether or not to run the analysis twice, once for drift with the full stiffness, and once for lateral analysis with reduced stiffness.

The reduced flexural stiffness for members are only applied to steel members "whose flexural stiffness are considered to contribute to the lateral stability of the structure". Thus, it is not applied to members that are either pinned at both ends or not steel members.

The reduced axial stiffness is applied to members "whose axial stiffness is considered to contribute to the lateral stability of the structure". All lateral beams, columns and braces are reduced for their axial stiffness.

Load Combinations

It is generally agreed that the Specification does not require that the notional loads need to be considered in all directions (i.e, perpendicular or opposite of the direction of the lateral load case), but only in the same direction as the lateral load case; however it is provided as an option. In the majority of cases the Notional Loads need only be considered with the gravity load cases.

Analysis Validation

It is the engineer's responsibility to ensure that the selection of P-delta, B₁, B₂, fraction of gravity loads for Notional Loads and τ_b used in the stiffness reduction are all consistent and correct. To aid the engineer, a new report is implemented in the Steel Standard Provisions module of RAM Frame that is available when AISC 360 has been selected as the design code. The report examines the selection of P-delta, B₁, B₂, Notional Loads, and τ_b, along with analysis results, and report on the validity of each criteria selection. If the analysis is deemed invalid, it is the responsibility of the engineer to make the necessary corrections to those criteria selections, or in some cases to modify the structure so that limitations are satisfied.