Combined footing quick question,

[engrnmc]

Hi guys,

i was trying to design a combined footing, and i notice that the reaction that im getting due to seismic is positive in one column and negative in the other ( due to the direction of the siesmic, in reverse if the direction is changed). since they have a couple reaction due to siesmic sizing the footing is impossible i'll be getting trapezoidal in siesmic and if checked in static condition i'll be getting rectangular combined. i know that siesmic is instantaneous reaction and im thinking of using both the positive reaction from both the column irregardless of the direction. but i hope someone could in lighten me. i cant find a good sample dealing with siesmic combined footing.

thanks

 

[KootK]

If you've got one column in uplift, I wouldn't be attempting to apply the conventional combined footing methods. I'd look at each load case independently and work out a first principles solution for each. Your uplift case will be quite different from your all downwards load cases with regard to punching shear and mid-span positive moment. Depending on the strategy that you're employing, you might also need significant footing mass just to hold the uplift column down.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[engrnmc]

Then how would you design a footing that causes an uplift reaction on one column? And even if its not an uplift. most one the time reaction from dynamic will cause either a much small reaction at one column or an uplift, in my case an uplift, compared to static which will have a reaction almost the same, if reareaction loads are the same proportionally from the two column.

 

[KootK]

I'd design it for each load case using first principles. Often, a combined footing only works when a substantial portion of one or both column loads is present. If there are load cases where that is not true, or there are load reversals, you need to keep a close eye on them.

If you post some sketches showing the load cases that you're considering and how you're proposing to handle them, I'd be happy to provide comment.



I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[hokie66]

I agree with KootK, I think. I would design the combined footing for the gravity cases, then add enough mass or tiedown resistance at each column to overcome the seismic uplift.

 

[engrnmc]

thats kinda my problem

thanks alot from your inputs, i tried to design the size of the combined footing considering static case and applying the said size to dynamic causing a great amount of eccentricity causing it to failing in the allowable bearing capacity even considering 130% increase in the allowable. Non the less i think i don't have any choice but to satisfy both static and dynamic.

thanks again,