ETABS Enveloped Base Reactions

[mcc202]

Hello,

I am curious how other engineers are obtaining base reaction output for foundation designs. As far as I can tell, ETABS will not provide you with an enveloped base reaction and only considers live load reduction in the design phase. What I have done is isolated my joints at the base level and exported the joint reactions by load case to excel. I was also able to export the live load reduction factors to excel from ETABS. I then did my own load combinations (including the live load reduction factors) in excel, then enveloped these combinations using the "min" and "max" functions from excel. I feel like with a program as advanced as ETABS there has got to be an easier way. I am new to the program and just wondering if there is a feature that I am missing.

Thanks!

 

[sponton]

Well you can define your own load combinations or envelope at Define -> Load combination menu -> ADD -> Envelop eand select which load cases you want included [you may even include combinations]. I'm not entirely sure about the live load reduction, i typically don't use it, but I wasn't aware that it's only taken into account in the design phase of the program, i would be surprised if this was true, perhaps you can make two analyses, one where you ask it to perform the live load reduction and one in which you don't perform it, or perhaps refer to documentation.

 

[MrHershey]

OP is correct. No live load reduction until actual design is run. And even then the reduction will only show up in your member design forces, ETABS won't output reactions with live load reduction.

I typically will reduce live loads in Excel later. I'm using Excel anyways to post-process the analysis results into something that is actually readable, so it's typically not a big deal. I won't run it individually for each supporting element, but will do a couple rough hand checks to get an idea of general live load reduction levels. Since I'm typically doing heavier concrete structures where live load reduction of 40 vs. 50 vs. 60 doesn't make a huge difference, I'll usually take a little bit less of a reduction than my rough hand calculations resulted in. Just for a bit of wiggle room. Lighter steel structures would of course need a little bit more attention to detail. Also have to watch out for areas where you either can't reduce or can't take the full reduction like heavy loads or parking.

Workarounds for ETABS would be to do exactly what I do but with custom load combinations instead. You could also try running design with your allowable combinations and just look at the axial design load in columns. Doesn't help as much for walls or braced frames, but is a start anyways.

 

[mcc202]

Thanks! I am getting the impression that most engineers use excel to determine the base reactions. I am accustomed to using RAM SS where most of that output is readily accessible within the program. Another interesting item is that ETABS will not calculate a gust factor when determining wind loads....not really an issue for rigid structures but can have a big effect on flexible structures.