The classification procedures in each of the codes are as follows:
BS8110 (similarly CP65 and HK-2004)
Bracing Classification — In BS8110 columns (and walls in minor axis direction) are considered as braced if lateral stability is provided (predominantly) by walls or other stiffer elements. This classification remains a matter of engineering judgement.
Global P-Delta Effects — it is an inherent assumption in the above that walls provide sufficient lateral stiffness that global sway of the building is small and hence "Big" P-delta effects can be ignored in braced structures. For un-braced structures there is no clear statement on whether or not global P-Delta is also considered ignorable or is simply
considered to be adequately catered for in the amplification of design moments noted below.
Slenderness Classification — this is based on the effective length. In braced structures effective lengths are < 1 and in un-braced structures effective lengths are > 1. It is considerably more likely that a member gets classified as slender when it has been classified as un-braced.
Short (Non-Slender) Members will see no amplification of moment at all, even if they are un-braced.
Slender Members (Members susceptible to P-Delta effects)
• Braced-Slender Elements - additional moments are calculated based on effective length and are considered to be a maximum at around mid height. These moments are not added to the highest end moment so this may or may not end up being a critical design
condition. This additional moment is clearly intended to cater for "little" P-delta effects (strut buckling)
• UnBraced-Slender Elements - additional moments are calculated based on effective length (which is longer and hence additional moments will be greater), and are considered to be a maximum at the member ends. The additional moment is added to the highest end moment so this will always end up being a critical design condition.
It is assumed that this amplification of the critical design condition is intended to cater for both big and little P-delta effects.
The advantage of the above procedure is that moment amplification in each column is related only to the classification and slenderness of that column. Where columns are unbraced this is not entirely logical and cl3.8.3.8 does provide an option where the average slenderness effect for an entire unbraced storey level can be used for all members at that level. Typically this would mean that members which are un-braced but not slender add to an average stiffening effect and so the design should be less conservative. This option is not applied in Orion because the design procedure would become highly iterative (the design of every column would affect the design of every other column at an un-braced level and moment amplification would need to be introduced to non-slender members).
Orion Documentation page 198 Chapter 12 : Overview of Bracing and Sway Sensitivity
ACI 318-02
When the design code is set to BS8110, CP65 or HK-2004; if you uncheck “User Defined Bracing for Columns and Walls”, a facility is made available for assessing the susceptibility of individual storeys to P-Delta effects. This uses the ACI method of classification during the building analysis.
Bracing Classification — using the ACI approach each storey level within a building is classified as sway or non-sway. The code also provides a method allowing analytical assessment of this classification based on deflections arising from a linear analysis of the structure.
Global P-Delta Effects — when a storey is classified as "non-sway" then it can be assumed that global P-Delta effects are small enough to be ignored at that level. When a storey is classified as "sway" then the frame analysis results need to be amplified in some way, options given are:
• A second order analysis (which would inevitably affect all members in the structure)
• Approximate moment magnification methods (cl.10.13.2 appears to indicate that this moment amplification only needs to be applied to the slender members at each floor level (similar to BS8110) is this logical? - or should this amplify the sway moments in all columns and walls on a level by level basis?)
Slenderness Classification — this is based on the effective length. At "Non-sway" levels effective lengths are < 1 and at "sway" levels effective lengths are > 1. It is considerably more likely that a member gets classified as slender when it exists at a "sway" level.
Short (Non-Slender) Members will see no amplification of moment at all even if they are at
"Sway" levels.
Slender Members (Members susceptible to P-Delta effects)
• Slender Elements at Non-Sway Levels - additional moments are calculated based on effective length and are considered to be a maximum at around mid height. These moments are not added to the highest end moment so this may or may not end up being a critical design condition.
In essence the approach here is identical to that used for braced slender members in BS8110.
• Elements at Sway Levels - as noted above the end moments of all members may be amplified to account for Global P-Delta effects. If a member at such a level is classified as slender, the calculation of the magnified moment is not based on the effective length of each individual member, moment magnifiers are based either on the stability index for the floor (cl.10.13.4.2) or an assessment of the average buckling capacity of all members at the floor (cl.10.13.4.3 - similar to the optional method in BS8110).
The additional moment is added to the highest end moment so this will always end up being a critical design condition.
Additional check (cl.10.13.5) - having amplified the end moments there is a requirement to check that intermediate slenderness effects (using effective length = 1.0L) are not more critical.
While the method of moment amplification is different for slender members at sway levels, the general principles of moment amplification are the same in BS8110 and ACI and the terms used for classification are interchangeable:
• BS8110 Braced = ACI Non-Sway
• BS8110 Un-Braced = ACI Sway
The ACI has the advantage that the classification is not a matter of engineering judgement and also that it introduces the flexibility to mix both braced and un-braced classifications within one structure.
The ACI amplifications are applied only to lateral load cases - this does not address the fact that sway will occur as a result of vertical loads applied to any unsymmetrical structure and hence ignores the possibility that significant P-delta effects could accrue due to this aspect of sway. However, for the majority of "building" type structures this simplification/assumption is likely to be acceptable.
There does seem to be a question mark relating to the ACI approach for slender columns. If the sway moment amplification is made using the stability index then should the column be taken into design as a braced column using an effective length = 1.0 (because the unbraced (global P-Delta) aspect of slenderness has already been allowed for?). This seems much less conservative than the suggested implementation procedure for EC2 discussed below.
EC2
In EC2 similar terminologies are used but the meanings are different:
• Cl 5.8.1 - Introduces concept of braced and bracing members.
• Cl 5.8.2 - Second Order Effects - this clause distinguishes between global effects (applying to the whole structure) and isolated member effects (slenderness).
Bracing Classification — Bracing members are the members which are assumed to provide the lateral stability of the structure. Columns and walls that are not “bracing members” are classified as “braced”. Unfortunately there is an element of engineering discretion involved in this classification which will be discussed later.
Global P-Delta Effects — there is some guidance on determining if these effects can be ignored (For the purposes of this discussion we will classify structures in which global P-Delta effects cannot be ignored as "sway sensitive"). Cl 5.8.3.3 (1) gives a simple equation that is only applicable in limited circumstances and is actually also difficult to apply. Initial calculations using this equation have suggested that it would be too conservative resulting in too many structures being classified as sway sensitive.
Annex H provides slightly more general guidance. In order to automate the Annex H classification in Orion, the approach has been modified to become similar in principal to the ACI classification method. It is noted that a single classification gets applied to the entire sway resisting structure (the bracing members). If it is determined that global P-Delta effects
Orion Documentation page 200 Chapter 12 : Overview of Bracing and Sway Sensitivity
cannot be ignored (the structure is sway sensitive) then the approach becomes a user driven procedure, in which the sway loads are amplified in accordance with Annex H. This is a relatively simple procedure applied as follows:
1. View the sway sensitivity report to obtain the suggested load amplification factors.
2. Apply this amplification to the existing load combination factors.
3. Re-analyse using the option to over-ride further sway sensitivity assessment and design the structure as if it is not sway sensitive (because the global P-Delta effects are now catered for).
Tests have indicated that the sway sensitivity assessment procedure described above results in a non-sway classification for the vast majority of structures .
Note Although the classification applies to the bracing members, it is impossible to isolate these when analysing the structure, so P-delta forces (introduced by load amplification or P-delta analysis) will accrue in all members (braced or bracing, short or slender).
Slenderness Classification — this is based on the effective length. For braced members effective lengths are < 1 and for bracing members effective lengths are > 1. It is considerably more likely that a member gets classified as slender when it has been classified as a bracing member.
Short (Non-Slender) Members:
• As noted above, if these members exist in a sway sensitive frame then there may have been some amplification of the design forces introduced during the general analysis procedure.
• no other amplification of moments is then applied.
Slender Members (Members susceptible to P-Delta effects)
• Slender Braced Members - additional moments are calculated based on effective length and are considered to be a maximum at around mid height. These moments are not added to the highest end moment so this may or may not end up being a critical design
condition.
In essence the approach here is identical to that used for braced slender members in BS8110 and ACI.
• Slender Bracing Members - as in BS8110 - additional moments are calculated based on effective length (which is longer and hence additional moments will be greater). Un-like BS8110 the additional moment does not have to be added to the highest end moment (because the end moment is already amplified if the structure is sway sensitive). In EC2 additional moments in slender members are introduced in the same way regardless of whether or not the member exists in a sway sensitive frame.
In summary - it seems EC2 maintains a distinction between global P-delta effects and local slenderness effects which potentially results in a 2 stage amplification of moments. Once the sway sensitivity is assessed the global P-Delta effects are introduced in the analysis results as necessary. For the local slenderness effects the general principles of moment amplification in EC2 are very similar to those applied in BS8110:
• EC2 Braced = BS8110 Braced
• EC2 Bracing = BS8110 Un-Braced (but we would expect that the EC2 amplification might be lower since the BS8110 amplification at this point mixes both global and local effects whilst in EC2 any global effects would already have been introduced).