Load Combination confusion: horizontal vs vertical loads

[LearnerN]

Newbie question, brushing the dust off my structural engineering. I'm confused about load combinations in relation to horizontal and vertical loads. Take for example the load combination 0.9D + 1.0W on a single footing. Would the engineer then apply a vertical load of 0.9D on the footing, and then a 1.0W horizontal load (inducing a moment) to then size the rebar and such in the footing?

Meaning, the load combination gives the factor for each individual type of load, and then the effects of each of those loads (whether as vertical, shear, or a moment) are each considered on the footing for each respective load combination. It is NOT the case that the values of 0.9D and 1.0W are added directly together to make a resultant vertical load to size the column, right?

Is my understanding above correct? Thank you for your input on a very basic question!

 

[JAE]

Load Combinations are combinations of load effects, not the directional loads themselves.

So I think you are correct in your understanding. The dead load (D) on a footing is a gravity load and thus is vertical.
The wind load (W) on a footing may contain horizontal forces acting on the footing as well as vertical forces (up or down) due to, perhaps, overturning effects on the structure.

For each D and W condition you would apply these loads in the direction and orientation that they occur (either vertical or horizontal) and then apply the safety factors to those loads.
Both the vertical and horizontal wind load components, or effects, would be multiplied by the wind load factor. (which for LRFD is 0.9D + 1.6W depending on your code)

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[manstrom]

reducing the DL is essentially a safety factor when looking at uplift or overturning.

Usually, overestimating DL is conservative unless you are looking at uplift design.

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