.6D + W for footing bearing pressure? 3

[jfrost5]

When designing a footing with moment, do you consider the .6D + W combination from IBC when comparing to the allowable soil bearing pressure as provided in a geotech report? I understand the need to use that combo (.6D + W) when considering sliding & overturning of the particular footing, but am unclear about it's use when it comes to the soil pressure itself. My understanding was that soil pressures are compared to true service loads (1.0 factor on everything).

 

[RFreund]

Yes, I do.

Service load does not mean 1.0 for everything. It just means that it is based on the 're-occurrence interval' associated with service level loads. Therefore I would say the 0.6D + 1.0W should apply.

EIT

http://www.HowToEngineer.com

 

[JAE]

We sometimes ignore 0.6D+W for cases where that condition results in uplift on the footing heel...as long as the structure can remain stable under that combination.

For all other load combinations we try to avoid any net uplift on a footing edge.

 

[EngineeringEric]

I do as well, i look at it as the fact we always over calculate our dead loads from actual dead load and the 0.6 is trying to eliminate this overage. When doing uplift calcs for a shear wall i will use the same 0.6D + 1.0W. For a footing, i would agree with JAE.

 

[msquared48]

0.6 to me is a reduction overestimation of our dead weight assumptions.

If you are just making sure the soil pressure is not exceeded, I would use the old 0.9D, as the pressure due to dead load will be more, and may be controlling.

Mike McCann
MMC Engineering

 

[jfrost5]

The specific condition I'm dealing with is a cantilevered pole. Because of various project/site specific conditions, the best option is to have the steel pole anchored to a footing that is near the surface (ie - no overburden soil available). The moment at the footing comes from wind load against the pole & netting. There is basically no dead load other than the footing wt itself. The wind moment results in a high bearing pressure on one edge of the footing, especially when only 60% of the footing wt is considered to resist that moment. We also have a low allowable bearing (1000 psf). For .6DL + WL, a 8.5 sq footing is ok for overturning & sliding, but has an unacceptable bearing pressure (>2000 psf). However DL + WL combo is < 1000 psf bearing.

Separate but related - 1807.2 in the IBC allows retaining walls to be designed without the load combinations of 1605 (use 1.0 for all nominal loads except E) but requires a SF of 1.5 for overturning & sliding. What do you guys use for checking bearing pressure at a retaining wall? If you are using the 1.0 loads, do you apply a SF of 1.5 against allowable bearing? If not, why would it not be similar condition for my footing - use the extra safety (either with 1.5 sf or .6 DL) for OT & sliding check but not for bearing check?

 

[msquared48]

If you know the pole weight and the weight of the footing, I would use the 0.9 to figure the maximum soil stress as the margin for error of the estimated dead load is very controlled. Make sure your FS for overturning is 1.5 or higher. More than likely, the FS will be greater than 2 in order not to overstress the soil.

Mike McCann
MMC Engineering

 

[RFreund]

I was thinking of uplift for some reason when I posted earlier so I am sorta contradicting myself here...I do use FS= 1.5 for sliding / overturning and I agree with the above posts in regards to uplift on footing or even the 0.9 for DL. For a pole type application I would be tempted to use lateral bearing pressure (passive resistance) and use a shallow pier depending on the application. I hear "netting" so this seems like it would apply.

EIT

http://www.HowToEngineer.com

 

[msquared48]

If this is a pole footing design, then yes, the passive pressure would be the main consideration to resist overturning. However, remember that in order to generate the allowable passive pressures in the code, the soil matrix must be undisturbed, as in not backfilled. So, the use of a spread footing at the bottom is really not possible with a true pole footing design due to the over-excavation necessary to install it.

The exception to this would be to pour a small footing the diameter of the shaft at the bottom after the auger is pulled out, and then either install the pole, or pour it to the top and make a moment connection just above the surface of the soil.

Mike McCann
MMC Engineering

 

[jfrost5]

sorry if I added more confusion with my previous post. It is a steel pole with base plate and anchors into a conventional conc square footing. It is cantilevered in terms of fixity at the base of the column which creates the moment in the footing. My main point was that there is no DL other than the footing itself and the only other load acting is horizontal wind on the netting & pole.

The crux of the question is whether the DL should be marginalized for bearing pressure checks (regardless of if that is done with a 1.5 SF or by using the .6 DL). I agree that it should be done for overturning & sliding, just not sure it should apply to bearing pressure. With the retaining wall design, you are not required to use the .6DL and nothing indicates using a SF of 1.5 for bearing pressure, so I question whether the .6DL really applies to the bearing pressure check for a footing that has a similar moment loading.

 

[RFreund]

If we are talking about a building isolated pad footing then I would use the 0.6DL + WL and allow for uplift as JAE noted. Even for a retaining wall condition I would do this. I suppose you could use 0.9DL as M^2 suggests as well depending on how 'accurate' you feel the dead load is. In your case of the netting support, it seems like the dead load would be pretty well defined.

However, I think the main question is now - why are you using a rectangular footing for this application. I would think it would be more economical to use a shallow drilled pier. This is probably why you were led to these questions in the first place - cause the footing seems large in order to get your bearing to 'work'. Or maybe not...

EIT

http://www.HowToEngineer.com

 

[jfrost5]

site conditions are getting in the way of preferred design options Contractor says they can't get a drill rig to the location required so we switched to a conventional footing with pier so that we could take advantage of overburden. Now they've identified some piping nearby that limits the depth of excavation that can be done. Discussing the situation in our office led to differing opinions on the .6DL for bearing, so I thought I'd get some additional input from the forum.

 

[msquared48]

So I hear that you are also limited by the footprint of the footing, not just the depth?

If not, limiting the bearing could be achieved with a footing with a larger footprint, up to a point of course. Are you at that point?

Mike McCann
MMC Engineering

 

[jfrost5]

No, i think we have a solution identified that will work, I was more curious to hear generally how others handled the .6DL in relation to design bearing pressure. There is disagreement in our office on the topic.

 

[steellion]

The 0.6 coefficient includes both the factor of safety and a factor for DL overestimation. 1/0.6 = 1.67 > 1.5 which is the old "nominal" minimum FS.

 

[jfrost5]

steellion - correct, but didn't that "nominal" minimum FS apply to stability checks only (overturning & sliding)? Is bearing pressure a check that was subject to the old 1.5 FS?

 

[nac521]

If you search past threads its seems this topic comes up every year with no clear answer. The FS for bearing pressure is built-in to the allowable pressure that is given to us in the geotech report (usually a FS of 2-3). The question is whether to check the 0.6D+H+W load combination for bearing pressure checks. From a code (IBC) standpoint, I don't see how you can neglect that combination for worst case. However, I do not agree with that, especially for isolated retaining walls where the dead load is not variable. Every retaining wall design example/spreadsheet/software I've come across uses 1.0D for soil bearing checks and so do I.

 

[johnbridge231]

use 0.9 instead of 0.6

Analysis and Design of arbitrary cross sections
Reinforcement design to all major codes
Moment Curvature analysis

http://www.engissol.com/cross-section-analysis-design.html

 

[Jerehmy]

I would not worry about allowable bearing pressure with wind loading because most allowable bearing pressures are based on settlement. I'm pretty sure you're not going to get settlement from a wind loading.

I'd only worry about shear failure of the soil from wind so find out what the allowable pressure is when not considering settlement.

 

[nutte]

Some history on the topic:

ASCE 7-95 does not have the 0.6D+W (or 0.6D+0.7E) load combination. For the traditional 1.5 factor of safety against overturning and sliding, they say a couple paragraphs after the load combinations that the overturning moment or sliding force must not be more than two thirds of the stabilizing effects form the dead load. 1/1.5 = 2/3, so this is the traditional way of doing things we’re all familiar with.

ASCE 7-98 does have the 0.6D+W (and 0.6D+0.7E) load combination. Also, the paragraph about overturning and sliding was removed. The Commentary states that these two load combinations were added to remove the inconsistency between ASD and LRFD, and to increase the emphasis on stability checks.

I think an unintended consequence is that the bearing pressure calculated has now been increased by about 1.5. This does not make sense to me. Overturning and sliding are phenomena that occur or don’t. You don’t have “a little” overturning. So you provide a factor of safety of 1.5 to make sure it doesn’t happen.

The allowable soil bearing pressure already had a factor of safety applied to it. There is no need to apply an additional 1.5 to it, like there is for overturning or sliding. A strict reading of the code tells you to compare 0.6D+W to the allowable bearing pressure, but I do not think that was the intent.

It would be nice if the powers that be would clarify this.