ETABS And SAFE issue

[Juli]

1. Hello.. can you please explain simply that when I will assign pinned support and when fixed in ETABS? Please do not mention huge geotechnical issues here as they just sometime make it much more complex, you please explain it simply with soil condition basing or footing based. 

2. Does etabs automatic count for long columns, i mean if a column is slender column, will etabs will design it considering slender column design process and length factors? or will I have to do it manually and if so, how? 

3. If I assign fixed support, then what would be for baseplate design consideration? biaxial moment should be considered or fixed isn't ok for baseplate? 

4. When exporting slab from etabs to safe, will i need to export lateral load combos also for moment consideration? Same for footing in Safe, I am exporting etabs base reactions for footing design in Safe (isolated or combined), now will i need the al load combos like for horizontal forces or moments also? Or just like only gravity  load combination reactions need to be exported? 

Thanks  

[Hira Malik]

1. Consider the post below.

2. Yes, ETABS itself accounts for a column's slenderness.

3. See same post as above.

4. When selecting combos, it is better to select all combos so that you may check SAFE results for any load combo if you want. However, for design purpose you have to consider the WORKING COMBO only i.e. the one with gravity loads. If you consider the results of combos that include earthquake, make sure to increase your bearing capacity by 33%. 

 

[Waqas Haider]

On ‎4‎/‎2‎/‎2016 at 4:40 AM, Juli said:

When exporting slab from etabs to safe, will i need to export lateral load combos also for moment consideration? Same for footing in Safe, I am exporting etabs base reactions for footing design in Safe (isolated or combined), now will i need the al load combos like for horizontal forces or moments also? Or just like only gravity  load combination reactions need to be exported

You must export combos related to all loads i.e. gravity and lateral loads because ultimately effects due to all the loads is going to be transferred to foundation. Designing foundation only for gravity loading is not correct approach. But remember, since mostly bearing capacity provided by Geotechnical consultant is safe bearing capacity instead of ultimate bearing capacity (safe bearing capacity means full factor of safety has been applied to bearing capacity as we do in ASD approach). Hence if we use this bearing capacity for design of foundations, loading combos of ASD or unfactored loading combos must be used. UBC and IBC provides these laod combos and footing must be designed for all this loading combos instead of autogenerated loading combos of ETABS which are factored. Either define these loading combos in etabs and export to safe or directly define then in safe.

 

On ‎6‎/‎23‎/‎2016 at 3:35 PM, Hira Malik said:

When selecting combos, it is better to select all combos so that you may check SAFE results for any load combo if you want. However, for design purpose you have to consider the WORKING COMBO only i.e. the one with gravity loads. If you consider the results of combos that include earthquake, make sure to increase your bearing capacity by 33%

Increase of bearing capacity upto 33% is not allowed in all the cases. It can only be used where in loading combos having two or more variable/Transient loads togather, we have not reduced the intensity of transient loads. If we have reduced these loads by 0.75 factor then bearing capacity can not be increased by 33%. Basically it is not the case with bearing capacity only. Any strength determined by using ASD approach (i.e. full factor of safety applied on strength of member, ) can be increased by this approach but only if in loading combos transient loads have not been reduced. UBC and IBC proposes two types of loading combos for ASD. in first type of combos you may find for example a combo D+L+W and in 2nd type you can find the same combo as D+0.75L+0.75W. if you use first type of combo you can increase by 33% and if u r using 2nd type of combo, note that the loads have been reduced by 1/1.33 = 0.75 times instead of increasing strength. so same factor of 1.33 has been applied on load side so in this case no increase can be made in strength.

For a detailed study refer to following document.

1-3rd stress increase AISC.pdf