Avoid Combination of combinations in Etabs Design according to ACI/IBC
Current time: 11-28-2021, 06:10 PM
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Author: LiviuM
Last Post: yy2011
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Avoid Combination of combinations in Etabs Design according to ACI/IBC
Found this while browsing, I don't use design option from etabs so it's for those that use it.
If in the same ETABS model a load combination is defined in two methods:
Load combination U1= 1.2DEAD+1.6LL , where DEAD is defined as a combination of SW+WALL+SDL
Load combination U2= 1.2SW+1.2WALL+1.2SDL+1.6LL (Loads are individually added)
Then the design results of a column in both the cases are different. Column capacity ratio exceeds the limit under load combination U2.

Design Using load combination U1
[Image: 29797483850225711093.jpg]
Design Using load combination U2
[Image: 92276901944438315165.jpg]

The difference can be attributed to the fact that U1 is a combination of a combination.
The minor axis bending moment is zero. So the minimum eccentricity moment is considered for both the load combos U1 and U2. In both cases the minimum eccentricity moment is 35.952 kN-m (M2,min). Then for the second order effect this moment is amplified by the Delta_ns factor. For the case of U1, this factor is 1.585. This leads to a design moment of 35.952 kN-m X 1.585 = 56.965 kN-m. For the case of U2, this factor is 3.145. This leads to a design moment of 35.952 kN-m X 4.145 = 113.070 kN-m.

Now the question arises, why the Delta_ns factor is different for cases U1 and U2 even though both have apparently the same original loading. U1 is a combo of combo. It has 1.2DEAD+1.6LL in it, where DEAD = Combo(SW+WALL+SDL). The DEAD consists of some permanent dead load and some superimposed dead load. On the other hand, U2=1.2(SW+WALL+SDL) + 1.6LL. The total moment diagrams and axial force diagram for U1 are the same as those for U2. However, for case U1 program cannot distinguish between the permanent dead load and superimposed load correctly. This causes the beta_d factors to be different for the two cases resulting in different EI and Pe values. Eventually the Delta_ns factors are different.

It is recommended to avoid using a combination of combinations for the purpose of design. While a combination of combinations is not a problem for enveloping analysis results, it is not appropriate for design. This is because some of the design parameters are invariably dependent on the type of loading and this information is lost when combos are used within a combination.

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I tried for this example in etabs but as far as we dont perform dynamic p delta analysis..the results are not affected...so it this particular case only applicable to p delta analysis..if so most of the times we dont perform this analysis on typical structures.
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The difference is how the program calculate β. the β factor= axial load due to all dead load cases in combination(1.2SW+1.2WALL+1.2SDL / total axial load in combination (1.2SW+1.2WALL+1.2SDL+1.6LL).
IN first case you put all dead load inside combination named DEAD so the program consider β=0 because it find no dead load case (you replace the load case by combination)
In second case it calculate β=0.4504 .I2= 0.4/(1+β)*I2 gross=0.275785 Gross.
I2=.275785*200^3*900/12=165471595 mm4
Pc=π2*EI/L2=3309090 N
δns= cm/1-(pu/0.75 pc)=3.145

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