Allowable Bearing Press VS Structural Load 2

[geotech753]

Anybody know exactly how structural engineers interpret a allowable bearing capacity from a geotech?

It really is whatever the geotech report says, in my case, the allowable bearing pressure (for shallow footings) is dead load plus design live load (or: dead load as is + (design factor)x(live load) ).

It seems I am answering my question, however, on a project that I got feedback, I am seeing that the structural engineer is considering my allowable as only dead plus live. Is this just a judgement call on the structural engineer? - maybe because the difference between (dead + live) and (dead + design live) is small? So I am asking what the general consensus is prior to pointing it out.
Thanks.

 

[msucog]

well, 3ksf for a 5x5 ftg is not necessarily the same as 3ksf for a 15x15 ftg. some soils and scenarios i've run across act very funky at higher loading conditions especially when the stresses are transmitted shallower/deeper. this is the reason why we start running swanky tests and settlement analysis when loads get above certain levels. i'm in the piedmont and there are some particularly nasty bands of soil that look like most other soils but act much differently under higher loads (highly compressible above certain stresses) or largely loaded areas (fill induced settlement).

as far as the mention of the live load reduction by 1/2 above, my philosophy on it is that the likelyhood of all the live loads acting for long periods of time is slim (most of the time) thus the settlement analysis should account for this to some degree to keep from over-compensating. keep all the dead and at least 1/2 of live. if the scenario happened to have live loads experienced for longer periods of time, the reduction would not happen. others may have different opinions.

 

[Mccoy]

Just as a side info:

the recent Eurocodes (particularly Eurocode 7) have thoroughly reviewed the loads issues, building up a pretty interesting system.

There are many "load scenarios" in function of live loads, environmental loads, seismic loads, accident loads and so on.

There are coefficients which make up for the small likelyhood of two or more unfavourable conditions occurring. Many problems discussed above are worked out that way.

The system at first is not straightforward and takes soem time to get familiar with. Similar to LRFD.

 

[civilperson]

My DL category includes sustained live load, permanent equipment, etc. If the minimum dimensions of differnt footings are more than 200% different, then a separate settlement analysis should be done for the different sizes. If similar, then the method seems appropriate.

 

[msucog]

that seems reasonable to take out a few kinks. i have run across a few situations where the same size/load footings within 50-100 feet of each other had too much differential for the structural to handle. i guess it's impossible to have an easy method for every scenario out there. the main reason for this scenario was the residual soil i mention above under one footing and structural fill under the other. since the loads were not overly large (~100kips), it wasn't a completely impossible scenario (lab data suggested about 1-2" differential but was probably just slightly less--still enough that the owner/engineer wanted to fix) but was enough to require undercutting and replacement with densified aggregate for the footings in the area of the cut-fill line. we just spread the settlement out over several of the warehouse bays. if it had been something like a parking deck or several stories of building, the thing would've had serious issues if put on normal spread footings. and the slab area required remediation (removal and replacement of structural fill) over the upper 2-3 feet to reduce issues with the floor loads.

 

[fattdad]

And now we can look at the specific case of how to do this for a building addition where there is an immediately adjacent footing that's already realized its "1-in" of settlement and you are constructing a footing with a new load.

I find these second-generation foundation loads interesting to evaluate.

f-d

¡papá gordo ain’t no madre flaca!

 

[Lion06]

Do any structural guys out there do their own settlement analysis? I don't know anyone at my firm that does or has.
We just design for the allowable pressure given in the geotech - at least I have never had any of the project engineers I work with as me to do anything along those lines. I have only been asked to size the footings not to exceed the allowable pressure.
I have designed some REALLY BIG footings. I am talking about 45'long x 12'wide x 3'thick combined footings with shearwalls, and columns on them. Should any additional analysis have been done on these types of footings?

 

[msucog]

"Should any additional analysis have been done on these types of footings?"--i say it depends on the site specific conditions & loading. the larger the area of the footing (and larger loads) or the larger the area loaded (say for large fill areas), the deeper the stresses go.
i recently looked at a 30' tall CIP wall. the wall itself didn't scare me but the 30' of new fill going to be placed behind the wall after it would be built did. to make it more complicated, the ends of the wall stopped at the toe of the slope (the wall made sort of a Z shape with the end of the leg being at the toe of the slope). did some borings, lab testing, settlement analysis, etc...came out with 9" at the worst spot and <1" at the toe of the slope less than 100' away. the area was going to have to be surcharged. the architect kept telling me that he didn't have time to wait for surcharging. i told him to either follow our recommendations, don't follow our recommendations with possible severe consequences, design the thing to withstand bigtime movement, or put it on piles. so the owner made the wall go away all together since their schedule could not accomodate the surcharging/settlement monitoring program that was necessary...problem solved.
for "normal" circumstances: for a long continuous footing, we typically look at the stresses to 4B. for spread footings, we look to 2.5B. i'd call the 45'x12' closer to the spread footing or mass fill scenario. for large area mass fill scenarios, i usually end up look at the entire soil profile thickness.

 

[msucog]

lets look at a very simplified scenario. 5x5 ftg vs a 12x12 footing with both have a contact pressure of 3ksf.
3ksfx5x5=75 kips
12x45x3ksf=1620 kips (i'm guessing it probably would not be this high for most cases since there would likely be other reasons for such a large footing size--resisting uplift, fuel tank, etc).
let's just say that the settlement analysis shows settlement on the order of 1% of the 2.5B depth.
5x2.5x.01x12=1.5"
12x2.5x.01x12=3.6"
say take 1/2 of each and you're looking at 3/4" to 1 3/4" settlement between the two scenarios. if you used the 200% size rule mentioned above, then a red flag would have gone up for this particular scenario if your footings were sized as mentioned.
there's a million different scenarios that could happen--so get the geotech to look at it.

 

[Lion06]

So the process you recommend is to size the footings not to exceed the allowable soil pressure given by the geotech in the report and then in addition to that either do your own settlement analysis or have the geotech look at your footing design?

 

[msucog]

provide the geotech with all the details. column loads and any "non-typical" scenario". let them check it out. with the very large footing scenario, the actual loads are probably much smaller so that even though the stresses extend deeper in the ground, the actual settlement across those layers is less than for the regular sized footing.
for example, i recently provided a subsurface for a client to check a small building addition out in the parking lot. they provided maximum column loads of 60k. they (the architect and engineer) specified 10 borings to x-number feet deep. the depths seemed appropriate for the structure and nothing was suspect during the drilling. after completing the drilling, i was passing along preliminary information that everything looked ok for shallow foundation support with 3000psf bearing capacity. then he happened to mention the large generator (actually said "largest that CAT makes"). i told him i'd need to recheck my recommendations and asked for loaded. he also then mentioned the very large fuel tank that would accompany the generator. he made note of a meeting the following week and i invited myself because i had seen nothing of what he had mentioned and suspected i still did not know the big picture. since the structure was going to be critical to the 24/hr-365days/yr business, we began asking lots of questions. turns out, there are 5 of these generators and fuel tanks. the loading areas were large and heavy. so as it turns out, i didn't settlement analysis about eighteen different ways before he finally gave us all the information (as far as we know). we asked for this stuff up front but they provided only very minimal information. we were very close to having to re-mobilize the drill rig to do additional sampling for lab testing. luckily, we had a few samples from the project site from two previous projects. also, since this turned out to be a very critical structure to the company that also happens to have lots of money, they were going to put GAB in the upper 7 feet of the building anyway. with that, we felt comfortable with our recommendations without additional sampling.
moral of the story, help the geotech help you by providing as much information regarding the project as you can.

 

[BigH]

If the geotechnical engineer has given you an allowable bearing pressure, then I wouldn't exceed it. muscog is correct in saying that the allowable bearing pressure will really depend on the size of the footing so if you have larger than 'normal" footings - say 3m x 3m, then you should question your geotechnical engineer as to whether his recommendations are "size sensitive". If they are, then the geotechnical engineer should have stated it - or given you a chart of allowable bearing pressures vs size of footing for a specified maximum settlement. (Note, you might have a couple of curves - one for 25 mm and another for 40 mm, say). As a structural engineer, I don't think that it is your "business" to do settlement analysis - but as a matter of interest you might wish to do a rough estimation to confirm in your own mind the order of magnitude to be expected.

 

[Lion06]

BigH-
Thanks!! That was a very helpful explanation.

 

[geotech753]

What I end up doing is:
Ask or figure out the maximum and typical loads.
Run settlement analysis for the different scenarios.
Provide table with one of the columns being Max Load.
This limits the footing size.
(MaxLoad)/(All.Brng.Cap) = Max Ftng Area
Sqrt(MaxFtngArea)= B
Similar deal with continuous ftngs.

 

[Lion06]

It's not just about max gravity loads. The buildings we design use almost every column as a lateral column - we do a lot of type II with wind connection buildings. To top it off, this particular building had over (15) combined footings - most with (2) lateral columns, some with (2) lateral columns and a shearwall. I have never done a settlement analysis, and I know if we went back to the geotech with these crazy footings asking him to do an additional settlement analysis that would certainly be "additional services".

 

[fattdad]

I'd like to add more, but you guys have covered it all. Just to reinforce an earlier point. If you give the geotechnical engineer the column loads then you will get a report that you can use to size your footings. Somewhere behind the scenes, the geotechnical engineer should be looking at the range of likely footing sizes, the depths of influence and the consequence of these loadings on the soils.

There is a reason that geotechnical reports should state limitations to the effect that, "This report is intended for the _________ project and if plans change the recommendations should be reviewed" (or something like this). If the original development is for a gas station and the proposed construction really turns out to be a multi-story office building, then the gas station recommendations may be just wrong.

f-d

¡papá gordo ain’t no madre flaca!

 

[darkwing888]

For every building foundation that I designed while working for the COE, I used DL + 1/2LL as recommended by policy. Maybe the given allowable bearing pressures had higher safety factors: I don't know. For buildings it seems reasonable (because LL is transient) and I still do this to this day.

"Clever of me to use my spine to break my fall like that."