Combined footing question 3

[Gus14]

When designing a combined footing, we use the total load on both columns to find the resultant and then we size the footing accordingly to maintain that the center of loads and areas coincide. However, what if depending on the actual loading case one column gets loaded the assumed loads, while the other gets loaded less. This would result in an eccentric combined footing, I know that most of the loading is dead load and only the live load portion is changing but still this idea is bugging me, wondering if other engineers address this situation or if I am thinking too much about the whole situation, and I don't want to overdesign.

 

[SlideRuleEra]

When designing a combined footing... what if... one column gets loaded the assumed loads, while the other gets loaded less.

...most of the loading is dead load and only the live load portion is changing...

Engineering is a compromise. Most problems do NOT have an exact solution, although software can create the illusion that perfection is possible. The high dead loads are going to dampen magnitude of eccentricity caused by variable live load. Size and shape the footing so that soil is not overloaded under the various loading conditions... and accept that some (acceptable) eccentric loading can occur.

http://www.SlideRuleEra.net

 

[JLNJ]

You are not over thinking it at all. Combined footings are often controlled by some sort of patterned or unequal loading.

It will take some judgment to figure out which is your worst case. For example, sometimes we estimate dead load on the high side to be “safe” but this may not be the worst case for a combined footing.

I worked on a project wth a high collateral load (requested by the Owner). Applying the full collateral load “balanced things out” but since the collateral load is not “reliable” we had to make sure that the worst case was created through unbalanced loading.

 

[KootK]

When designing a combined footing, we use the total load on both columns to find the resultant and then we size the footing accordingly to maintain that the center of loads and areas coincide.

My understanding of that approach is that it's an attempt to maintain uniform settlement at the two columns. Like JLNJ said, I believe that it is common, and appropriate to then give consideration to load variation in other aspects of the foundation design.

 

[trilinga]

Gus14
Your question is very valid and needs to be addressed in design.

In all practical designs of combined footings, this problem exists.

First of all, the actual loads may be different from the estimated loads.

Secondly, we design for the worst combination of loads only. The loading (particularly the live loads) may be different at different times causing the imbalance.

Aligning of the CG of the load with the geometric CG of the footing is done to ensure uniform pressure on the soil. But, when there is a number of load combinations, each having its own load CG, it is not possible to satisfy uniform pressure condition under each load combination.

Further, the footings are subjected to eccentric loading not only due to varying vertical loads, but also due to the horizontal loads.

When certain reversible loads are predominant, such as wind or seismic loads, matching the geometric CG of the footing with the CG of the loads will tilt the balance completely when the load reverses.

So, when there are many load combinations or when there are large reversible loads, it is better to align the geometric CG of the footing with the CG of the permanent (non-reversible) loads - such as dead loads – and design the footing with eccentricity due to the other loads/load combinations.

 

[ldeem]

What I typically do is consider a min and max case (possibly more) where max is downward pressure and min is overturning/sliding. So in your case max LL on one pier and min LL on the other.