This section addresses the requirements of BS5950:2000 5.6.3 (and 2.4.2.7) that indicates when a member is determined to be "sway-sensitive" in an axis, one of two methods can be used to account for the secondary effects. The first method is to use effective lengths for the columns, and the second method is to amplify the sway moments in design. In both these cases only a first-order elastic analysis is required. The engineer can assign which method is to be used for each axis of a member through assignment (refer to BS5950 2.6.6).
For frames more susceptible to second order forces (λcr less than 4.0) a second order (P-Delta) analysis is mandated. This analysis should accurately determine the secondary forces that will result from the deformation of the structure, and will result in member forces following the analysis that incorporate these secondary effects. Where a second-order analysis is performed the members are designed according to the amplified sway method as described below. The engineer can indicate that RAM Frame should perform a first or second order analysis by selecting "Ignore P-delta" or "Consider P-Delta" respectively, in the Criteria-General dialog box in Analysis mode, as described on the Criteria section of the manual.
The BS5950:2000 5.6.3 indicates that for sway-sensitive frames one of two approaches should be taken to perform the code check, these are described below.
| Setting | Description |
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| Effective Length Method (2.4.2.7a) |
Using this method design forces are not directly amplified. Rather the code stipulates that beams need to remain elastic and the columns sway-sensitive effective length factors need to be applied. The user is responsible for ensuring beams remain elastic if this option is used. The user can assign an explicit effective length factor to the column axis or have the program calculate the appropriate value.
- Per 4.8.3.3.4 a lower limit of 0.85 will be placed on the magnitude of mx, my and myx.
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| Amplified Sway Method (2.4.2.7b) |
Using this approach the user can designate one of three methods to determine the amplified sway moments. By performing a second order analysis, by assigning a specific kamp factor or by requesting the program calculate kamp per load combination. | Setting | Description |
|---|
| P-Delta |
The second order analysis method (P-Delta) is available to directly calculate the second order moments (assumed to be equivalent to kamp x Sway Moment). By indicating that a P-delta analysis be performed (see RAM Frame Analysis Criteria) for a sway member, the user is effectively implementing the moment amplification method. The design assumptions are as follows. For each load combination, design moments will be calculated for the gravity (non-sway), and lateral (sway) load cases independently. That is, the sum of all gravity load case moments is stored separately from those of the lateral load cases in the combination.
- Per 4.8.3.3.4 the factors mx, my and myx, are only applied to the non-sway (gravity) moments. That is terms of the form mM are replaced by Msway + mMnonsway. Note that Msway will already be amplified as it is calculated from a second order analysis, and no kamp term is applied.
- If the effective lengths are calculated per Annex E then RAM Frame will use the non-sway mode in-plane effective length factors.
- No lower limit of 0.85 will be placed on the magnitude of mx, my and myx .
Using this approach any kamp assignments made to the member (refer to section 2.6.6) are ignored.
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| Amplified Sway Factor kamp |
If a first order analysis is performed, the user can designate either the effective length method or the amplified sway method be performed. For the amplified sway method either a specific kamp value is assigned or the program can calculate kamp per load combination. Similar to the case of second order design is performed as follows. For each load combination, design moments will be calculated for the gravity (non-sway), and lateral (sway) load cases independently. That is, the sum of all gravity load case moments is stored separately from those of the lateral load cases in the combination.
- Per 4.8.3.3.4 the factors mx, my and myx, are only applied to the non-sway (gravity) moments. That is terms of the form mM are replaced by kamp Msway + m Mnon-sway.
- If the effective lengths are calculated per Annex E then RAM Frame will use the non-sway mode in-plane effective length factors.
- No lower limit of 0.85 will be placed on the magnitude of mx, my and myx.
Where designated to be calculated per load combination the amplified sway factor kamp is calculated assuming cladding, architectural walls etc are not considered in the lateral stiffness and thus:
kamp = λcr / (1.15 λcr - 1.5)
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