.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).

 

[steellion]

ASCE 7-10 2.4.1: "Loads listed herein shall be considered to act in the following combinations; whichever produces the most unfavorable effect in the building, foundation, or structural member being considered."

I don't understand what the debate is. It's written in plain English: use the load combination when designing foundation elements.

(ASCE 7-10 has changed the load combination to 0.6D + 0.6W because wind has been changed to a strength case. But it is the same combination as the 0.6D + 1.0W we are talking about)

 

[jfrost5]

"use the load combination when designing foundation elements"

Except that retaining walls are given an exception (in IBC 1807.2.3) to revert to the old 1.0 factors and the 1.5 SF. It would appear this also means that bearing pressure calcs for a retaining wall only require the 1.0 factors.

Why then would other footings that have moment conditions require .6D to be used for the bearing check? The inconsistency creates the debate.

 

[CANPRO]

If your only source of dead load is a single concrete footing, it doesn't seem right to reduce that weight by 0.6 regardless of what the code says. Its not like you have a large tributary area with multiple materials included in your dead load estimate...leading to low confidence in the dead load. If your concrete footing ends up being 60% of the weight that you had expected it to be, I would say you have more immediate problems than a potential bearing failure in a high wind event.

 

[RFreund]

Solid debate.

Here's how I see it.

This is in regards to checking the bearing pressure:

If you are designing a building footing, where you have large trib areas and dead load can vary (aka almost all buildings). Then yes you use the 0.6DL + W and as previously noted some uplift on the footing is probably acceptable.

If you are analyzing a retaining wall condition the concrete and soil could be taken as 1.0 for dead load. However if you have other dead loads applied, say to the stem wall or on the heel then I would analyze both conditions - one with 1.0DL' and one with 0.6DL' where DL' is the weight applied to the stem or heel (not the soil/concrete). Compare the results to the allowable soil bearing values. Once again some uplift is probably acceptable for the 0.6 case.

Side note:
When you go to do the concrete design then I would start to use the load factors and even more debate in my mind lies here (should you factor the applied bearing pressure found with ASD loads, should you apply LRFD and reanalyze to find new applied bearing pressure, what happens if your eccentricity is now too high to perform the analysis, etc.)

EIT

http://www.HowToEngineer.com

 

[nutte]

I don't understand what the debate is. It's written in plain English: use the load combination when designing foundation elements.

I don't think anybody is denying that. When you follow the building code, you have to check the bearing pressure using these 0.6D+W load combinations.

The problem is, bearing pressure has not traditionally been checked this way. The change to the code requiring this was made to address stability concerns (overturning and sliding). This appears to be an unintended side effect, and designers are resisting.