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?

 

[r13]

I believe those values were un-factored from soil test results, so required a safety factor of 1.5 in performing analysis for bearing, sliding, and overturning.

Are you checking the stability of an existing structure/retaining wall, or with the intent to perform modification/rebuilt?

 

[Rabbit12]

No, I'm trying to check the existing reinforcing in the walls as we are making some changes to the structure. The structure is a buried wet well so there really are no unbalanced forces that would create overturning or sliding.

I've created a FEM and applied the 60 and 100 pcf loads and then applied the 1.6 load factor per ASCE7. The moments produced exceed the capacity using the rebar size and spacing from the existing plans. I'm in the process of sharpening my pencil and that "ultimate value" statement stuck out to me.

 

[r13]

I believe the original design was done using 90 pcf and 150 pcf, respectively, and by Allowable Stress Design method. Does the wet well has concrete cover on top? And have you checked the wall as edges supported plate - a common practice at that time using Bureau of Reclamation engineer mono plates.

 

[Rabbit12]

It does have a concrete top. We are looking at taking that off. There are also some intermediate steel beams that I'm just not sure what their purpose is. We'd also like to get rid of those.

I'm in the process of checking it with the Bureau of Rec tables.

 

[r13]

I made mistake on stating 90/150, which shall be 60 and 100 respectively. As the safety factor concerns stability analysis only. Consider a soil weights 120 pcf, Ko = 0.5, the 60 pcf makes sense, as well as the 100 total weight below water table.

 

[Rabbit12]

So then you're thinking for strength design of concrete a 1.6 load factor would be appropriate? And the 1.5 FS would be for checking stability...?

 

[r13]

I'll start with checking a propped beam spans vertically, using ASD and fy = 40ksi. How the steel beams were attached, structurally speaking?

 

[MTNClimber]

Rabbit12 - When in doubt, check with the author of the report. You don't want to accidentally over or under apply the factor of safety.

Usually its a FOS of 1.5 on sliding, 2-3 on overturning, and 2-3 on bearing capacity. But if they gave you an allowable bearing pressure you can just use that instead of calculating the ultimate and factoring.

 

[Rabbit12]

retired13, attached is a picture of the steel beams. I'm not sure they are original as they don't show up on the original plans. The existing well is 15' square and 28' deep so really mostly one way action horizontally.

MTNClimber, I plan to try to contact the report author but it's two days after xmas and no one is around. I'm just doing a preliminary design so thought someone could possible shed some light on the "ultimate value" they reference. Again, I'm not concerned about a stability analysis or bearing since it's buried on 4 sides and we aren't adding any gravity loads. I'm concerned about the concrete walls since we want to tear the lid off and pull those intermediate beams out. I don't know if the original designer intended for those elements to help redistribute some moment in the concrete.

 

[MTNClimber]

I only factor lateral earth pressures if I'm working in LRFD. I, personally, wouldn't factor loads using ASD. I'm having trouble as to why they would say to factor those loads by 1.5 (which is how I'm interpreting the language). Sounds like its typical language for them. If its urgent, maybe any senior engineer present in the office can help?

 

[Rabbit12]

MTNClimber, I guess I'm kind of interpreting it differently. I think they are saying the lateral earth pressures they are giving are ultimate. Normally that would mean I wouldn't factor the loads when checking the concrete using LRFD.

It's not particularly urgent. I thought maybe someone on here would have seen that language before since I'm the only one here today. I've been doing this for awhile (actually am a senior engineer which means I must be sounding dumb......) and I can't recall ever seeing that verbiage.

 

[MTNClimber]

I 100% agree that the language is abnormal and confusing. Those values line up for a soil with a friction angle of about 32 degrees. Unless the retained soil is very dense sand and gravel I wouldn't consider using a lower value. I would factor those loads for LRFD but not ASD, as factoring lateral earth pressures in ASD doesn't make sense to me (unless its concrete design).

 

[r13]

The steel beams are quite heavily built, don't looks like scaffold for repair works. Is there a large opening to get the beam in, or the beam is spliced inside? I would carefully exam the wall and slab to see if there any indication of large cracks and old repair works, also bulging. For the beams were not shown on the drawing, and obviously not supporting any equipment, I can only guess the wall had experienced excessive load (could be unexpected flood, or construction load), and the beams were there to prevent the walls from cave in. But why only one way?! I am puzzled.

 

[r13]

The aspect ratio is approximately 1:2, so two way action is possible. For one way action, make sure to check both fixed-fixed and pinned-pinned to get maximum moments for both faces.

 

[BridgeSmith]

The loads seems fairly straightforward. The 60pcf and 100pcf are fairly typical nominal (expected, unfactored, sometimes termed ultimate) loads (see below). In the original design, it appears that the FOS was directly 1.5 (equivalent to an ASD reduction factor of 0.67). How you provide a FOS depends on what design method you're going to use.

ko*gamma_soil = 0.5*120pcf = 60pcf
ko_submerged*(gamma_soil-gamma_water)+gamma_water = 0.6*(120pcf-62.4pcf)+62.4pcf = 97pcf

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

 

[MTNClimber]

The OP said the geotechnical report stated "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.". I'm guessing that has nothing to do with the original design. It's such a vague statement that it needs further clarification. My honest guess is that the geotech doesn't know what they are talking about either.

 

[r13]

Isn't that the geotechnical guy used to get soil strength at failure, divide by a safety factor, then hand to the foundation guy to check soil reaction due to service load?! Note, that often times, the same word but its usage and meaning are quite different among disciplines. And that's where confusion roots.

 

[MTNClimber]

Retired13 - I'm not quite following what you're trying to say but I hope this addresses your question/statement. In order to develop the lateral earth pressure recommendations, the geotechnical engineer would look at the soil classification and blow counts to estimate a soil strength or use strength testing in order to develop lateral earth pressures. Those values, which are not factored (but can be, see next paragraph), are provided to the designer. The designer would then design the retaining structure using the appropriate factor of safeties using ASD or factor the loads and resistance values using LRFD. The geotechnical report should not be listing an arbitrary factor safety. If it is going to provide a factor of safety, it should be clear as to what the factor of safety is being applied to or, what is typically done, not list one at all.

In my experience, geotechnical engineers try to be slightly conservative when providing lateral earth pressures. Typically budgets on jobs don't allow for a large strength testing program which can give a clear indication of the soil strength across the site for each soil type. We generally look at the soil type, blow counts, and past experience to estimate the soil's strength. I'm sure a lot of the soil I provide recommendations for could have a higher strength, but unless the client wants to pay to find out, I'm not going to give the higher values to the designer.

 

[r13]

MTNClimber,

I agree with you that, in this case of lateral earth pressure concern, the geotechnical guy was not likely to factor the load up or down, but to emphasis the resulting structural reactions need to maintain a safety factor of 1.5, which in turn resulted in a material strength capacity reduction factor of 1/1.5 = 0.67, that was close to 0.66 required by code, using ASD method. I believe the word "ultimate" was meant soil state at test, nothing to do with "ultimate strength design" (USD). So, same word, two definitions.

Also, I don't think LRFD is a proper term for reinforced concrete design, though the efforts and results are comparable. Maybe I am outdate though.