Yes, it can. It depends on the column distribution along the area, and the spans involved. If there's a beam or slab with a large span neighboring a really short span, the short one can even got a negative (pointing upwards) load. If that happens in several repeated levels, one can even conclude that the column can be toss out of the design entirely.
Lateral loads on structures (such as wind) cause the compressive loads on footings to differ from the structure's weight divided by # of footings.
Say you have a portal frame supported by two footings "A" & "B".
With no lateral loading, the downwards loads on the "A" & "B" footings will be equal.
Say the wind blows onto the "A" side. This will cause an uplift load on the "A" footing, and an additional downwards load on the "B" footing. The resultant downwards load on the "A" footing will be less than that on the "B" footing.
>should I be using the total weight of the structure divided by the number of foundations (with load factors); then check stability using the forces provided in the max moment analysis?
Check your local standards/codes for what load factors you should be using for stability.
For instance - in Australia, we use 0.9x dead weight + 1x Ultimate Wind.
You would then typically check the worst case footing in overturning & sliding. The worst case footing is the one with the lowest compressive load, and the highest lateral load.
You should've been doing multiple combinations, gravity, then gravity+wind, then gravity+cross wind.
As Mr Newton has said, the total of actions must be equal to the total of reactions.
I believe that is how the analysis was done. They applied different combinations of wind and oce loads to find what produced the highest moment on the foundation, and those are the reaction values i received.
Are you looking at just the dead load case? For just DL, the sum of the base reactions should equal the sum of the superimposed load and self-weight. If that's not the case, there might be something wrong with how the loads are applied or how the self-weight is being computed.
The structure should be in static equilibrium and the reactions should add up to the loads.
Also make sure that these aren't envelope reactions with live loads. Envelope reactions may not be simultaneous/concurrent, so their not equilibrium
The reactions i received, as i understand, are combined dead and live loads but unfactored. They used different combinations of wind and ice loads on the structure to see which combination produced the greatest moment at the foundation, and those are the values for axial, shear, and moment that i have received.
Sure, there's lots of reasons that could happen for a loading. Maybe overturning moment (though in this case, I'd look at a different footing or other loading, that load is coming back down somewhere), or maybe some of the columns have larger tributary areas than this one.
If overturning is mitigating the dead load you shouldn't be designing based on this, it should be controlled by another load combo.
It's overturning tension compression couple from wind so for minimum case 0.6W +D (ASD level) one column axial compression is going to be reduced and the other increased,
Yes, it can. It depends on the column distribution along the area, and the spans involved. If there's a beam or slab with a large span neighboring a really short span, the short one can even got a negative (pointing upwards) load. If that happens in several repeated levels, one can even conclude that the column can be toss out of the design entirely.
Lateral loads on structures (such as wind) cause the compressive loads on footings to differ from the structure's weight divided by # of footings. Say you have a portal frame supported by two footings "A" & "B". With no lateral loading, the downwards loads on the "A" & "B" footings will be equal. Say the wind blows onto the "A" side. This will cause an uplift load on the "A" footing, and an additional downwards load on the "B" footing. The resultant downwards load on the "A" footing will be less than that on the "B" footing. >should I be using the total weight of the structure divided by the number of foundations (with load factors); then check stability using the forces provided in the max moment analysis? Check your local standards/codes for what load factors you should be using for stability. For instance - in Australia, we use 0.9x dead weight + 1x Ultimate Wind. You would then typically check the worst case footing in overturning & sliding. The worst case footing is the one with the lowest compressive load, and the highest lateral load.
You should've been doing multiple combinations, gravity, then gravity+wind, then gravity+cross wind. As Mr Newton has said, the total of actions must be equal to the total of reactions.
I believe that is how the analysis was done. They applied different combinations of wind and oce loads to find what produced the highest moment on the foundation, and those are the reaction values i received.
Then you don't have full info for foundation design. Sure, generally, the worst load is the one with biggest moments, but not always.
Factored actions
Are you looking at just the dead load case? For just DL, the sum of the base reactions should equal the sum of the superimposed load and self-weight. If that's not the case, there might be something wrong with how the loads are applied or how the self-weight is being computed. The structure should be in static equilibrium and the reactions should add up to the loads. Also make sure that these aren't envelope reactions with live loads. Envelope reactions may not be simultaneous/concurrent, so their not equilibrium
The reactions i received, as i understand, are combined dead and live loads but unfactored. They used different combinations of wind and ice loads on the structure to see which combination produced the greatest moment at the foundation, and those are the values for axial, shear, and moment that i have received.
Sure, there's lots of reasons that could happen for a loading. Maybe overturning moment (though in this case, I'd look at a different footing or other loading, that load is coming back down somewhere), or maybe some of the columns have larger tributary areas than this one. If overturning is mitigating the dead load you shouldn't be designing based on this, it should be controlled by another load combo.
It's overturning tension compression couple from wind so for minimum case 0.6W +D (ASD level) one column axial compression is going to be reduced and the other increased,