Analysis Method

RAM Frame currently implements the First-Order Elastic Analysis method considering rigid and simple members according to Section 4 of the specification. All lateral (frame) members of the structure are considered in a full building analysis (no sub structuring) with spans taken as centre-to-centre of supports. Pattern loading is considered in the design of all "gravity steel" beam and columns in their design in RAM Steel, and for all concrete members in RAM Concrete, but is not currently considered for steel frame members in RAM Frame. Note that changes in geometry and effective stiffness due to axial load on members is not considered in the analysis.

Second Order Effects

Per 4.4.3 of the IS 800-07 "Second-Order Elastic Analysis" The analysis shall allow for the effects of the design loads acting on the structure and its members in their displaced and deformed configuration. These second-order effects shall be taken into account by using either:

a first-order elastic analysis with moment amplification in accordance with Clause 4.4.2, provided the moment amplification factors (δy = CmyKy) or (δz = CmzKz) are not greater than 1.4; or
a second-order elastic analysis in accordance with Appendix B"

In RAM Frame, option (a) which accounts for P-δ ( P-small delta) is always performed. The user can choose to consider P-Δ (P-large delta) in the analysis (see RAM Frame Analysis Manual - Analysis Criteria and P-Delta Sections) then this is equivalent to option (b) being considered. Without P-Delta the requirements of 4.4.3.1(a) are effectively considered.

Only design moments are amplified for the secondary affects. Axial and shear forces remain un-adjusted.
P-Delta when selected will be considered for all members both braced and unbraced (cannot have P-Delta only apply to unbraced members in a structure). These secondary forces will be relatively small assuming the member is truly braced against sidesway by other frame members in the model. That is so that there is little lateral displacement of the braced members.
When P-Delta is considered the secondary multiplier (δb) is applied directly to the moment calculated from the analysis. That is the gravity transverse loads are not considered as a simple load that is superimposed with the lateral load case forces, rather all forces are applied to the structure in separate load cases and superimposed in a load combination to produce final design forces.

2nd Order Analysis by Geometric Stiffness Method (P-Delta)

In Criteria > General in RAM Frame Analysis mode there is an option to perform a P-delta analysis. This analysis employs the geometric stiffness method. The masses used in this analysis are those assigned as part of the floor and roof loads, and self-weight if that option was selected. The mass values are listed in the Loads > Masses command in RAM Frame Analysis mode. Those masses are also used in the calculation of building periods and modes and for the generation of seismic story forces. Therefore, those values generally only include the masses associated with Dead Loads. In order to perform an appropriate P-delta analysis the effects of Live Load should be included, therefore a P-delta Factor should be specified such that the factored Mass values are approximately equivalent to the combined Dead and Live Loads. Furthermore, in order for the principle of superposition to be valid, the P-delta effects should be determined at an ultimate value of loads. Thus the P-delta Factor should be additionally increased to account for the load factors.

For example, assume that the Dead Load and Live Load are approximately equal; this means that the Mass value should be factored by 2 for the P-delta analysis in order to account for the effects of both the Dead and Live Load. For IS, the mass should be further factored by the load combination factors, which for the most conservative combination is 1.5 on both the Dead Load and 1.5 on the Live Load, for an average in this example of 1.5. Thus the P-delta factor that should be specified is (1.5)(2.0) = 3.0.

An additional benefit of utilizing the P-delta analysis as described is that the effects of "leaning" columns (e.g., the gravity columns that are leaning on the frames for lateral stability) are automatically accounted for.

Note: IS 800-07 mandates that if d exceeds 1.4 you should consider applying a second order analysis according to the provisions of Appendix B. It is the engineer's responsibility to select P-Delta for the analysis in the event that situation occurs.