Repeat Load vs Load Combination . Linear vs Non Linear Analysis in Staad

Often we are confused like when to use the Repeat Load and when to use the Load Combination in Staad.
Repeat Load is for Non linear analysis and this video demonstrates the proper explanation of the same.
#staad #staadproadvance #staadpro #finiteelementanalysis #structuralengineering

[@azmat_waseem]

Very Informative... Thank you...

[@zal4ever123]

Very well explained!

[@azmat_waseem]

Please do a video on eot crane load application for a peb warehouse. Thank you...

[@Torq123]

Thanks. I will try

[@syedarsalanali8506]

A great video... ❤ Thanks. I have a question. There are programs in which an option called "generate load cases using load combinations". Is that same kind as that of repeat load in staad ?? Another question. In uplift analysis for foundation when non linear combos are run then how the tension in soil spring goes to zero ??

[@Torq123]

Thanks , the load combination and repeat are not the same although the main objective is to determine the response against the combined loading action . Now the Load Combination in Staad simply sun them algebraically and hence you can expect the correct result if the structure behaves linearly. However the REPEAT LOAD option actually generates a new load case and the same is resolved not by alzebric summation but by solving stiffness matrix . And hence if there is any non linear condition the Repeat Load exactly captures the same . Regarding your second point, when the compression only springs experiences tension, they get switched off and hence when there is uplift these springs will be switched off if the REPEAT LOAD is used

[@syedarsalanali8506]

@@Torq123 Thanks for your response. I want to know that in some software there is option for generating LOAD CASES from LOAD COMBINATIONS. so is this for the same reason as that making repeat load in staad ?

[@Enfld1122]

If structure is being analysed with only first order(linear) analysis, then also repeat load case can be used ? or the same gives awkward results?

[@Torq123]

It depends on how the structure behaves . If the structure behaves non linearly then Repeat Load is must. Some example are like Pdelta case, Tension Only or Compression Only specifications, Direct analysis or Geometric NonLinear case . These are all generates non linear response in structure. Now one might argue how to know if the pdelta effect is present in the structure or not . Now the PDelta or the 2nd order effect exist is all the structure but in different magnitude. If this effect is very pronounced then only we need to consider its effect or treat the structure as non linear . A general thumb is to check the 2nd order drift to first order drift , if it exceeds more than 5% in deviation, then always consider the Pdelta effect . That means here you must use the Repeat Load . Now you can use Repeat Load feature in any structure irrespective of linear or non linear but you can use load combination only if the structure responds linearly..

[@Enfld1122]

It's okay that if the deviation of 2nd order drift to 1St order drift is exceeds by 5%, then structure is considered as non linear and p delta effect should be considered, but p delta effect or drift deviation is applicable only for building like structures.what about structures other than building like structures? i.e underground tanks, pipe racks, equipment supporting structures, retaining walls etc .
How to determine about non linearity of such structures?

[@Torq123]

@@Enfld1122 Yes , the thumb rule of 5% is based of ASCE prescription for drift mostly for building like structures. Now coming to machine supporting structures( general block or table structure , retaining wall and underground tank , the PDelta effect is insignificant. Secondary moment is most pronounced is tall and slender members . …The structure that you mentioned are mostly compact. but still if you are concerned about to check if second order moment are indeed developed in the those compact structures, you might want to run the model both with Pdelta and Perform Analysis separately and compare the maximum deflection. If the difference is negligible then it indicates second order moment is insignificant..Yes other type of non linearity might existing like Compression Only plate mat support . So if the compression only , tension only specs are involved, use repeat load ..