Lateral Earth Pressure Question 2

[Rabbit12]

I'm doing a preliminary analysis on a buried structure from the early 1960's.

Reviewing a Geotech report from the same site and they are recommending a lateral at-rest pressure of 60 pcf above the GWT and 100 pcf below the groundwater table. Then they have this tidbit: "The values calculated for the above parameters would provide ultimate values. We recommend a minimum factor of safety of at least 1.5 be applied to the calculated lateral values."

Help me interpret that. If I apply the loads (60 and 100 pcf) to a wall and I'm interested in the forces in the concrete would I need to add the 1.6 load factor per ASCE 7 or would that be double dipping?

 

[JAE]

Don't confuse strength reduction factors (Φ factors) with safety load factors...two completely different things that are focused and based on different variabilities.

Φ factors are based on the variability of the material properties as well as the variability of the equations that we use to derive strength (resistance) of our structures.
Load factors (sometimes called safety factors) deal with variabilities in the predicted loads.

Load factors turn actual/service level loads into ultimate loads....unless the load is already an ultimate load (i.e. seismic loads).

The geotech here, I think, is trying to say something but turned it completely around.
1. They call their 60/100 load an "ultimate load" but,
2. They then say that you need to kick UP the ultimate load by a load factor, to make it....more ultimate?

They should have instead said: "The values calculated for the above parameters would provide [red]actual service level [/red]values. We recommend a minimum factor of safety of at least 1.5 be applied to the calculated lateral values."

 

[BridgeSmith]

JAE, the approach you outline is what would be done for a new structure under LRFD design, but if the geotech is using the approach we do in structural design when dealing with an existing structure (evaluate or design under the original design criteria), then the given values would be those for ASD. For buried structures under ASD, it's fairly common to have ultimate (expected) loads compared to expected capacities, and the capacity must be greater than the load by a certain margin (FOS). In this case, the recommended FOS was a factor of 1.5. That's how it looks to me, anyhow.

Rod Smith, P.E., The artist formerly known as HotRod10

 

[Rabbit12]

All, I should clarify. The Geotech report I'm using for these lateral values is a much newer report and not specifically for this structure. There are several structures on this site built at different times. Since I have no information as to what the existing structure was designed to, I'm starting by using the values in that report since it's reasonably close to this structure.

JAE, the Geotech report isn't saying kick UP their loads specifically, but if I'm following section 2.3.2 of ASCE 7-10, the load factor for an H load is 1.6 for most load cases. I have always considered the maximum forces/reactions/ etc. from LRFD load cases ultimate. When I was getting moments well above what the existing rebar could handle I started to question that statement because I found it odd.

Retired13, there is an existing 4' square hole that has a spiral stair that possible could have been used to get those beams in, but getting them rotated into place would have been difficult. And yes, they are only in one direction and at least one has been almost completely cut (just one flange remains) because it was evidently in the way of a pipe. There was at one time a platform that at some point was mostly taken out (some random pieces still remain). You're comment about the usage of the same word across disciplines is funny, trying to explain to a mechanical pipe stress engineer why he can't just simple add everything together and give me loads is trying at times.

BridgeSmith, I agree the values seemed reasonable to me given the soil types (CL and SP mostly).

 

[MTNClimber]

The geotech here, I think, is trying to say something but turned it completely around...
2. They then say that you need to kick UP the ultimate load by a load factor, to make it....more ultimate?

That would result in "double dipping" and creating an overly conservative design. If that is what they are saying, I would challenge that recommendation.

 

[PEinc]

Don't confuse load factors with factors of safety. The geotech is saying to use a safety factor of 1.5. Therefore, this would be for allowable stress design. ASD requires no load factors and no resistance factors. If the at-rest equivalent fluid pressure is 60 or 100 pcf, these seem like service load earth pressure or combined earth and water pressures. Use the service loads and compare the driving forces to the resisting forces to see if you have the 1.5 safety factor for sliding and overturning. For designing the actual structure, factor up the 60 and 100 pcf appropriately to perform the LRFD or LFD design. Remember that the 100 pcf pressure includes water pressure. The earth pressure and the water pressure usually have different load factors. Therefore, the 100 pcf may need to be broken up into the soil portion and the water portion so that the different load fctors can be properly applied.

http://www.PeirceEngineering.com

 

[r13]

Rabbit12,

It looks like the beam were got in before the cover was casted. It could have been utilized to hoist equipment, or materials, and left there. But it puzzles me that the beams do not look aged, and there are no anchors at the ends (couldn't tell from the photo). So I can only think they are for temporary purpose. Well, since they are only a few feet from the cover, unless you are positive they are not for support, you may want to excavate a few feet below the level of the beams to relieve the pressure, then carefully remove the beams. Who knows, the beam might just drop to the bottom during excavation

For concrete check, the lateral pressures are reasonable, you should use ASD, not USD, although the result should be satisfactory using either method. Keep in mind, this structure has been there a little more than half of a century, we have to ask "why it is no good now?".

 

[ATSE]

For lateral demand on a wall, expected pressure = service pressure = ASD.
I would not use "ultimate" as a synonym for expected. For this discussion using ASCE 7 as the basis, I would avoid the term ultimate.
ASCE 7 approach:
For sliding and overturning stability, use service loads (service load combos) and compare with expected resistance provided by the soil (friction, bearing, passive). The factor of safety is the ratio.
For the reinforced concrete design, use factored loads as the demand. Compare with φPn, φMn, φVn...
Do not use factored loads and then compare to resistance with a factor of safety.

 

[Rabbit12]

Retired13, there are anchors on the beams. They are just inside the flanges and barely visible in the photo I attached. There are steel members at two elevations, see attached second photo. These beams also support some platforming that's been butchered over the years.


You're right about this structure it's been working for almost 50 years. However, we are proposing changes including taking out the steel beams and taking the cover off the top (and replacing it with an intermediate floor). I have to make sure our modifications don't damage the existing walls. Leaving the existing steel in place isn't an option since this is a WWTP and they would quickly corrode.

 

[Rabbit12]

I should add, the walls look great. No visible cracking or other signs of distress.

 

[r13]

You are in the correct path - gather information and evaluate the strength of an existing structure. I think result from BOR mono graph will provide the confidence you are seeking. After seeing the new photo, I don't think there is much problem to remove the beams, and take out the cover, provides the structure is properly braced/supported at strategic locations at all time, until completion of the permanent lateral support.

 

[r13]

Instead of waiting for the mono graph, you can model the wet well as plate element and apply the loads to get the results.

 

[JAE]

JAE, the Geotech report isn't saying kick UP their loads specifically...

Yes they are - read what they wrote: [blue]We recommend a minimum factor of safety of at least 1.5 be applied to the calculated lateral values."[/blue] They are saying to kick UP the loads by 1.5.

...when dealing with an existing structure (evaluate or design under the original design criteria)

I don't agree that you should evaluate the current structure based on older codes, methods, etc. You should use the applicable, legally accepted provisions that apply now.

Also - the original geotech called them "ultimate" loads.
So with a factor of safety added either you are double factoring already "ultimate" loads or you are reducing an ultimate load to a service level load - and then you would use ASD with OTHER safety factors to get it right.

The original geotech called them ultimate loads and then said to apply a 1.5 "safety factor" to those ultimate loads. That is simply BS and wrong.

I stand by what I wrote above - and in bold - as to what the geotech should have said.

He obviously intended the 60 pcf to be factored UP to 90 pcf....thus the 60 pcf is a SERVICE load and not an ultimate load.

 

[BridgeSmith]

I don't agree that you should evaluate the current structure based on older codes, methods, etc. You should use the applicable, legally accepted provisions that apply now.

I don't care to debate whether the current code or original design code should be used in the analysis of existing structures, mostly because I don't have a strong opinion on that one way or the other. In our office at the DOT bridge design section, the decision to do analysis and design for rehabilitation under the original design code was made above my level. I was merely stating my supposition that perhaps the geotech had approached it that way in preparing the report, which seems consistent with the terminology used.

I stand by what I wrote above - and in bold - as to what the geotech should have said.

I agree the geotech mixed terminology from different design methods. However, I'm not sure I can agree with your bolded statement. It also seems to mix terminology between design methods. To my knowledge, only LRFD uses 'service load', but it does not use safety factors.

To be consistent with an LRFD approach, the loads should be reported as 'nominal' and the 1.5 as the recommended load factor. Although, my understanding of the AASHTO LRFD design method would indicate the geotech should stay mum on the factors and let the specifications dictate the proper factors to use.

To be consistent with ASD, if my memory serves, the load would have no modifiers - just list the soil loads as 60pcf and 100pcf, and leave safety factor verbiage as is.

Rod Smith, P.E., The artist formerly known as HotRod10

 

[Rabbit12]

JAE, I guess I can see how you could say he's saying that the 60 pcf and 100 pcf are ultimate loads and then adding an additional FOS of 1.5. I guess I interpreted that 1.5 as the FOS for stability checks. Which I guess says we have more ambiguity.

BridgeSmith, working for a state DOT I would think puts you in a different position when evaluating older structures than myself. It seems reasonable to do what you are describing when you own the structure. We are required to utilize the current design code. I have used the provisions in the IEBC a ton to prove existing structure are adequate because it's virtually impossible (especially in seismic country) to get old structures to work under newer codes when adding loads or modifying.

I also should reiterate that this Geotech report was for a new structure near this existing one on the same site. It's all I have right now to go on so was using it. This report had nothing to do with the existing structure so saying that the language was trying to align to the actual design code isn't correct.

FWIW, I contact the Geotech for clarification on the language. The response verbatim was, "There is not a safety factor added to the equivalent fluid pressures provided.". No attempt to explain the verbiage at all. To add to this, our client likes these guys and is making us use them for the new project we have. I'm pretty stoked to work with them.

 

[MTNClimber]

If they give you the same language in the report for this job, I'd make sure to write up your questions and CC the client as it helps the client understand when one of their engineers are being unclear and/or wasting everyone's time.

 

[BridgeSmith]

...this Geotech report was for a new structure...

Sorry, I seem to have missed that in the previous posts. Well, that theory is shot down, then. It seems to be just poor use of terms by the geotech, then.

We are required to utilize the current design code. I have used the provisions in the IEBC a ton to prove existing structure are adequate because it's virtually impossible (especially in seismic country) to get old structures to work under newer codes when adding loads or modifying.

That's one of the reasons we use the code it was designed under; so that we don't end up in the pickle where the bridge is ok as long as we don't look at it, but if we look at it per the new code, it's substandard. The other main reason is that having different components of a bridge designed under different codes adds unnecessary cost and creates difficulties in load rating.

Anyway, back to the OP. The way I read it, and still understand it, the 60pcf and 100pcf are what would normally be termed "nominal", or to use one of the words they used, "calculated" loading (i.e., no factors applied).

If it was me, I would ignore the 1.5 FOS they "recommend", and apply the load and resistance factors per the governing design code.

Rod Smith, P.E., The artist formerly known as HotRod10

 

[r13]

I don't know when the argument of the lateral loads are "ultimate" or "service" started. To me it is clear, as usually sandy material has a unit weight of 120 pcf, water 62.4 pcf, ko (at rest) = 0.5, thus the soil above water table, ϒ_s= 0.5*120 = 60 pcf; and below water table, ϒ_TOT= ϒ_b+ ϒ_w= 0.5*(120-62.4)+62.4 = 91.2 ≅ 100 pcf (cal done by Rod previously). Since no factors were applied, thus the loads are "service loads".

I could be wrong, but I am holding my believe, that given the same load, ASD will yield a result more conservative than USD, in another word, design done by ASD shall have no problem to meet USD strength requirements.

 

[r13]

Also, the reason I suggested to use ASD to check the existing structure is, because if done correctly, you shall get the exact capacity as the original design. Otherwise, you may have done something wrong. USD can't tell this, unless you are willing to accept the "not that close" capacity literally.

 

[Rabbit12]

retired13, the argument started because of the language in the report. They explicitly state, "The values calculated for the above parameters would provide ultimate values." The only values above that statement are the lateral earth pressures. I asked the question because the statement is odd to me and doesn't make sense because the loads (60 pcf and 100 pcf) seem reasonable to me.

As far as analyzing the concrete, I did check it using the Bureau of Rec tables and a FEM. Both show the concrete failing, but I'm assuming rebar strength (40 ksi), concrete strength (3 ksi), and the loading is likely not what was used for original design. The unity levels are over 1.5. Given the fact this structure is 50 years old and is in great shape, I'm confident my conservative assumptions are the culprit.

So I've actually resorted to doing a comparison between current condition and our proposed condition. In certain elements, our forces increase around 1-1.5%. I'm ok with that.

 

[PEinc]

In my experience, geotechs usually recommend minimum service loads for design and then leave the actual design to others. Geotech are not usually known for their structural design expertise. How can someone specify an "ultimate" load if the structure has not yet been designed? The geotech for this project needs to define "ultimate." If the geotech thinks that "ultimate" means "factored," then what design code (AASHTO, IBC, etc.) and load factor were used in the recommendation? I never see geotechs recommend factored design loads. At the risk of starting a heated debate here with geotechs, I don't usually pay too much attention to what most geotechs say when they "recommend" design procedures or specify how to do structural design. I'm sorry if I insulted any geotechs.

http://www.PeirceEngineering.com