Load combinations for shallow or deep foundation sizing

[NewbieInSE]

Dear Respected Engineers,

ASCE7-05 states:

1. So, for the purpose of finding soil bearing pressure and comparing it with allowable soil bearing capacity (shortly for foundation sizing), are we supposed to use the above-listed combinations?
2. If the above combinations are to use for foundation sizing, then do we apply the SBC overstress of 33% when lateral loads are used with gravity loads?
3. If the above combinations are used for foundation sizing, we have to include the vertical EQ effect with the horizontal EQ effect? That's what ASCE-7 05 says, but I want to know if there is anything else.

4. What about the following combinations? I had always thought these are the only combinations to use for sizing foundations, be it shallow or deep.

5. If the above-mentioned green load combinations are permitted to use for proportioning foundation size or finding pile numbers, do we need to include the vertical EQ effect?

Thanks all. Your valuable response is highly appreciated.

 

[phamENG]

Newbie - I'd say your information is about 20 years old.

1) Yes
2) No
3) Yes
4) No. Use the equations you listed in number 1 (assuming, of course, that ASCE 7-05 is part of your code. There are 2 more current versions, the 2010 and 2016 editions).
5) They're not, so the follow on question is moot.

Take a look at this article: [URL unfurl="true"]https://www.aisc.org/globalassets/modern-steel/steelwise/30722_steelwise.pdf[/url]

 

[slickdeals]

@PhamENG - Maybe I am misunderstanding, but I think you're confusing the material one-third stress increase (which is not permitted) with the soil overstress which is typically allowed by Geotechs for transient loading.

1) Yes
2) Yes, you can use a one-third increase in soil bearing pressure for transient loads (wind and earthquake) if your geotechnical engineer allows for it.
3) Yes, include vertical and horizontal seismic effect. Please also note that a 25% reduction in foundation overturning is permissible if you are using the ELF method though it's not recommended to use it in combination with the one-third increase.
4) For Item ii. ASCE/IBC allow two transient loads to be reduced when used in a combination. You can use 0.75 * L + 0.75 * (W or E), per ASCE combo 6. Item iii. use 0.6D that includes a built-in safety factor. I believe the 0.9D requires a FOS of 1.5.
5) Vertical seismic needs to be included as appropriate.

 

[phamENG]

soil overstress which is typically allowed by Geotechs for transient loading.

Not around these parts. I remember seeing several a few years back that specifically disallowed it, but looking at a few I have right now they don't even bring it up. It's the same concept as the material stresses - the increase was allowed to account for multiple transient loads not being at their peak simultaneously. Now that the load combinations account for that explicitly, the increase in bearing stress is no longer permitted (at least by the geotechs I've worked with recently.) Now if your geotechnical engineers do allow for it, be sure to discuss with them about the load combinations permitted to be used with it.

IBC doesn't allow it with standard ASD load combinations, but does allow it with the "alternative basic load combinations" of Section 1605.3.2. If you notice, those alternative load combinations do not reduce the transient loads as much as the basic ASD load combinations do. (See IBC 1806.1)

 

[slickdeals]

@PhamENG - noted on your point about the alternate basic load combos. We've had one Geotech that I remember who provided that explicitly. Not many of them do.

 

[NewbieInSE]

Thanks, PhamEng, and Slickdeals.

It's clear to me now.

Our code allows the green load combinations for shallow foundations, also allows 33% in soil bearing capacity.
But for deep foundation, it doesn't explicitly say anything. But somewhere it said to use the above listed ASCE-07 combinations. So I became confused.
So my conclusion is:

The ASD load combinations have to be used for deep foundations but soil bearing capacity overstressing is not allowed, and green load combinations can be used for shallow foundations with 33% overstress.

 

[NewbieInSE]

But the combinations including seismic effects, such as
5. D+0.7E this doesn’t include Live load. Looks less conservative.
6. D+0.75*0.7E+0.75L+0.75Lr = D+0.525E+0.75L+0.75Lr, this also looks less conservative since EQ is only 52.5%.
These look to me less conservative.
Any advice or clarification?