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1
- #1
ARS97
Structural
- Feb 24, 2010
- 160
I need a sanity check.
I've designed many basic shallow foundations for my structures over the years with, in my opinion, a pretty straight forward and well established procedure (I think). But over the years, I've seen designs from other engineers come through that make me wonder HOW in the world can they can justify the numbers, so to speak. Not only does this now make people ask the question why my footer is typically larger, but it introduces doubt in their minds, not to mention my own.
Let me explain.
The first thing I check with a spread footer is eccentricity. If the resultant of the loads (e = M /P) does not fall within the the middle third of the base, then uplift, on some part of the base, will occur. Also, the traditional elastic theory for finding bearing pressure (q = P/A + MC/I) no longers applies, at least in the sense of using the full base width. From what I understand, meeting this eccentricity criteria is a must, and sound practice. (NOTE - I suppose if you allowed less-than-full base contact that you could use an iterative process to find the actual effective base width, but all my references state that it's not recommended since it leads to high bearing pressures and unnecessary settlement.)
Next, I'll check both sliding and overturn. These are pretty straight forward. Just to note, overturn should NEVER govern if the eccentricity criteria is met.
Lastly it's bearing pressure. Provided that the eccentricity criteria was met, I use q = P/A + MC/I. Obviously q-max must be less than the allowable. q-min should be equal to or greater than zero since eccentricity criteria was met; a negative value (uplift) can't occur.
Lately I've seen some designs come through (from outside sources) that just look unusually small. Most of the time the cases involved are rigid frames where there is a large outward thrust at the base. Typically, there's 2 ways to deal with this outward thrust:
1) Design the footer independantly; the thrust load is resisted simply by the footer's geometry/weight, or
2) Use hairpins (or similar) into a floor slab to tie each side together and eliminate the thrust load on the footer. The slab acts as a tie.
In the recent cases I'm talking about, the engineer used method #1 from above; they're independantly stable....supposedly. However, I'm fairly certain that if I had the column reactions that there's NO WAY that the footers proposed meet the required eccentricity requirement. Granted, I don't have any official involvement in the project, but voicing my opinion makes me look like I'm just being overly critical of someone else's work.
Has anybody ever run into a similar situation? Does anybody else feel that the eccentricity check for spread footers is overlooked frequently?
Or.....am I missing something?
I've designed many basic shallow foundations for my structures over the years with, in my opinion, a pretty straight forward and well established procedure (I think). But over the years, I've seen designs from other engineers come through that make me wonder HOW in the world can they can justify the numbers, so to speak. Not only does this now make people ask the question why my footer is typically larger, but it introduces doubt in their minds, not to mention my own.
Let me explain.
The first thing I check with a spread footer is eccentricity. If the resultant of the loads (e = M /P) does not fall within the the middle third of the base, then uplift, on some part of the base, will occur. Also, the traditional elastic theory for finding bearing pressure (q = P/A + MC/I) no longers applies, at least in the sense of using the full base width. From what I understand, meeting this eccentricity criteria is a must, and sound practice. (NOTE - I suppose if you allowed less-than-full base contact that you could use an iterative process to find the actual effective base width, but all my references state that it's not recommended since it leads to high bearing pressures and unnecessary settlement.)
Next, I'll check both sliding and overturn. These are pretty straight forward. Just to note, overturn should NEVER govern if the eccentricity criteria is met.
Lastly it's bearing pressure. Provided that the eccentricity criteria was met, I use q = P/A + MC/I. Obviously q-max must be less than the allowable. q-min should be equal to or greater than zero since eccentricity criteria was met; a negative value (uplift) can't occur.
Lately I've seen some designs come through (from outside sources) that just look unusually small. Most of the time the cases involved are rigid frames where there is a large outward thrust at the base. Typically, there's 2 ways to deal with this outward thrust:
1) Design the footer independantly; the thrust load is resisted simply by the footer's geometry/weight, or
2) Use hairpins (or similar) into a floor slab to tie each side together and eliminate the thrust load on the footer. The slab acts as a tie.
In the recent cases I'm talking about, the engineer used method #1 from above; they're independantly stable....supposedly. However, I'm fairly certain that if I had the column reactions that there's NO WAY that the footers proposed meet the required eccentricity requirement. Granted, I don't have any official involvement in the project, but voicing my opinion makes me look like I'm just being overly critical of someone else's work.
Has anybody ever run into a similar situation? Does anybody else feel that the eccentricity check for spread footers is overlooked frequently?
Or.....am I missing something?
