{"id":32666,"date":"2026-04-27T12:23:19","date_gmt":"2026-04-27T12:23:19","guid":{"rendered":"https:\/\/protasoftware.com\/technical-manuals\/building-analysis\/sway-sensitivity-second-order-effects\/"},"modified":"2026-06-19T12:37:52","modified_gmt":"2026-06-19T12:37:52","slug":"sway-sensitivity-second-order-effects","status":"publish","type":"wiki","link":"https:\/\/protasoftware.com\/wiki\/sway-sensitivity-second-order-effects\/","title":{"rendered":"Sway Sensitivity Second Order Effects"},"content":{"rendered":"
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Automatic Assessment of Sway Sensitivity<\/span>
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Sway sensitivity is automatically determined in Prota<\/b>Structure <\/b>when the design code is set to EC2.<\/span><\/span><\/p>\n

For other codes as discussed below, an assessment of sway sensitivity <\/span><\/span>can also<\/span><\/span><\/i> be made, however it should be noted that this is based on analytical results and the recommendations of the ACI code.<\/span><\/span><\/p>\n

Whenever sway sensitivity is assessed automatically you are advised to be aware of the limitations that apply, these can be viewed by clicking the \u2018Limitations\u2019<\/i><\/b> button on the Building Analysis\/ Reports tab.<\/span><\/span><\/p>\n

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User Defined Bracing<\/span>
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In both the BS8110 code and also CP65 there are no provisions for assessing sway sensitivity by analytical methods. Therefore, when using BS8110 or CP65 you need to check the option for “<\/b><\/span><\/span>User Defined Bracing for Columns and<\/b><\/span><\/span><\/i> <\/b><\/span><\/span>Walls” <\/b><\/span><\/span><\/i>and then apply the condition (braced or unbraced) that you deem appropriate for the building.<\/span><\/span><\/p>\n

If you do not check this option then <\/i><\/b>ProtaStructure will make an assessment of sway sensitivity based on analytical results and the recommendations of the ACI code.<\/span><\/span><\/p>\n<\/p><\/div>\n

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\n \"Notes\"<\/p>\n
\n However that this assessment is deflection dependent and that the ACI code gives guidance on appropriate adjustments to section\/material properties to be used in the building analysis for the purposes of this assessment. Such adjustments will increase the deflection value used in the checks.<\/span>
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\n \"Warning\"<\/p>\n
\n That the check can result in different classifications for different storeys which is not a condition that is recognized by BS8110. We continue to advise that this assessment is used cautiously and that when the appropriate overall condition is determined that this should be applied as a user-defined classification (in accordance with BS8110).<\/span>\n <\/div>\n

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Classification Requirements of each code<\/span>
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BS8110 (similarly CP65 and HK-2004)<\/span>
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Bracing Classification \u2014 <\/span><\/span><\/i><\/b>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.<\/span><\/span><\/p>\n

Global P-Delta Effects \u2014 <\/span><\/span><\/i><\/b>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.<\/span><\/span><\/p>\n

Slenderness Classification \u2014 <\/span><\/span><\/i><\/b>this is based on the effective length. <\/span><\/span><\/p>\n

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  1. In braced structures effective lengths are < 1 and<\/span><\/span><\/li>\n
  2. in un-braced structures effective lengths are > 1.<\/span><\/span><\/li>\n<\/ol>\n
    \n It is considerably more likely that a member gets classified as slender when it has been classified as un-braced.<\/span><\/span>\n <\/div>\n
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    Short (Non-Slender) Members will see no amplification of moment at all, even if they are un-braced.<\/span><\/span><\/p>\n

    Slender Members (Members susceptible to P-Delta effects)<\/span><\/span><\/p>\n