Footings & Load Factors

[DCBII]

I seem to come across an issue a lot that I don't know what to do with.

Often when checking bearing pressure under service loads I have no uplift across the footing. As soon as I factor my loads though I do get uplift.

In other words, the load factors change the structural support. The concrete design loads are now based on a an altered support condition influenced by load factors. This doesn't seem like a realistic approach to me. Any comments or thoughts?

 

[delagina]

arent factored loads only for strength design of footing and not for stability check.

 

[AVak]

For simplicity I always use ASD load combinations for footing stability and bearing pressure checks. For the design of the reinforcing, I then use a load factor of 1.6 on the bearing pressure. This should prevent the phenomenon you are describing.

Adam Vakiener, P.E.

 

[Lion06]

I've run into this a couple of times. I've typically only run into this if you are just barely inside the kern for 0.6D + W (it will kick you outside the kern for 0.9D + 1.6W), or if you have very little compression for 0.6D + W (0.9D + W could give you a very little amount of uplift). I'll typically try taking advantage of a little more DL when this comes up (soil over the footing comes to mind as a nice one) to get it to work out.

 

[DCBII]

AVak,

That approach seems a little conservative, but reasonable to me. Any other ideas anyone?

 

[delagina]

where did you get the 1.6?

i encountered a similar thing about tension load.
load combination was allowable.
tension reaction was say 20 kips.
another engineer's calcs, he multipied the tension to 1.6 to make it ultimate or factored.

 

[JStephen]

The problem is that you have nonlinear relationships between load and stresses in some cases. I had assumed that the whole point of LRFD was to accomplish the effect which you're describing. Specifically, have the safety factors on the load, rather than on the final stresses. However, as was pointed out to me sometime back, if you look at the definition of the loads, D is the "dead loads OR related internal moments and forces" and depending on which side of that "or" you use, it gives you two different approaches and two different results.

 

[DCBII]

JStephen,

That summarizes my question very well. Am I after the factored loads, or the factored load effects?

 

[BigH]

and they say that by integrating LRFD into geotechnical engineering, we get a more consistent and understandable and elegant solution . . . ha !

 

[Lion06]

You're after the factored load effects, in my opinion. I believe this is clear in the DAM in steel and it makes sense that approach is consistent with the intent of the code. Even in ACI, the moment magnification is based on the factored moment and axial load and is not factoring the manificarion based on a service moment and axial load.

 

[Lion06]

What is a manificarion, you ask?! Good question. It's apparently iphone's correction for magnification.

 

[miecz]

And herein lies the contradiction with using factored loads. In some cases, like steel, it doesn't appear to matter whether one factors the load or the load effect. In others, like soil, it does matter. (I think it matters with concrete, too.) To be consistent, the whole theory should have been based on factored load effects, not factored loads. I propose a whole new system, LEFRD. Pull out your credit cards.

 

[Lion06]

I think it does matter whether you factor the loads before the analysis or factor the load effects after the analysis. This is only true where second order effects are considered. The reason is that the second order effects are larger for the factored loads, so the effects are greater than a similar analysis carried out with service loads and then factored at the end (you're not taking into account the additional second order effects - additional lateral drift from the factored lateral loads * the higher, factored gravity loads). This is the reason that the DAM requires, even if you're using ASD, to factor the loads to do the analysis, then divide the final load effects by the load factor - to ensure that the second order analysis analysis results are consistent no matter which approach you use.

 

[DaveAtkins]

There have been occasions where I had to make a footing a little bigger, because it worked for bearing pressure under 0.6D + W, but the load resultant was off the footing for 0.9D + 1.6W.

And I don't agree with simply multiplying unfactored bearing pressure by 1.6. This can be unconservative sometimes.

DaveAtkins

 

[DCBII]

Thinking about it a little more... the 1.6 factor doesn't solve this problem does it.

 

[DCBII]

Thinking about it a little more... the 1.6 factor doesn't solve this problem does it.

Also, you couldn't be looking for the factored load effects because you'd have a different support condition for dead, live, wind, etc. and you can't just find the effects of each one and add them together. You have to combine the loads somehow before you find the load effects.