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Soldier Beam Design Methodology

Soldier Beam Design Methodology

(OP)
I've been working on updating my soldier beam spreadsheet and have a few questions. I had previously posted a version that used the "simplified" approach but have now retailored it to the "conventional" approach shown in USS Sheet Pile Design Manuel (modified for soldier beams) and Caltrans Trenching & Shoring Manual. Attached is print out of input / output from a typical temporary shoring condition. This would be used for a relatively shallow excavation for a basement in a single-family residential neighborhood. The adjacent structure in this example is far enough away to avoid surcharging the wall.

The values used in the attached design include:

Surcharge - 72 PSF min (or actual surcharge if higher). 72 PSF is recommended by Caltrans Trenching & Shoring Manual
Active / Passive - as prescribed by geotech
Factor of Safety - 1.3 (recommended by Caltrans Trenching & Shoring Manual)
Neglected Depth - 1 ft (typically recommended by geotech)
Starting Passive Pressure - Passive Pressure x Neglected Depth to provide trapezoidal distribution
Effective Passive Factor - 2-3 as recommended by geotech
Effective Active Pressure - 1x pier diameter below dredge line
Cover on soldier beam - 1" min for temporary condition

These values often tend to produce 1.5:1 embed to retained height.

Some questions...

1) most of the examples for soldier beam calculations I've seen apply the active pressure the entire length of the pier including depth below the dredge line. I believe the thought is that to engage passive you need to have fairly significant movement so you will have active on the backside of the pier. Some calculations I've seen don't do this (Retain Pro module as an example) and instead utilize a form of the IBC cantilevered pole equation. Site soils will likely play into the appropriate approach, but what do you see as typical approach? When do you use one vs the other?

2) The cantilevered pole equation appear to have stemmed from Pole Building Design by Donald Patterson. In this assumptions, it looks like the moment arm couple to provide fixity for the pier has a longer lever than it assume with this approach which results in slightly shallower embedments (say 10-20%). Have you tried to use a similar philosophy for these embedments?

3) if you apply the active times the full depth of the pier, do you use the same effective width as used for the passive pressure or do you only use the width of the pier? Caltrans Trenching & Shoring Manual uses the same for both active and passive and so does other references (Army Corps I believe does as well based on f factor to convert sheet pile to soldier beam) but Civiltechs software manual appears to only use the pier diameter (I don't have the software just read the manual). Other shoring drawings I've seen from engineers also only use the pier diameter. The geotech I discussed this with didn't seem to agree with the concept of not using the same effective width for both active and passive.

4) Do you typically apply minimum surcharge for these conditions? The retaining side does not have traffic but is neighboring property where we cannot dictate to them not to do certain things in their yard during construction. Most similar shoring calculations in our area do not include any surcharge except where undermining existing structures.

5) What factor of safety do you use? I've seen other people use values as low as 1. 1.25-1.3 seems to be commonly recommended for temporary. Some geotechs include factors of safety in the passive values they give us. We try and parse our an ultimate gross passive value (some also provide net instead) for our design purposes.

6) What cover do you use on soldier beam for temporary conditions? Smaller cover allows us to provide deeper soldier beams with same size hole and therefore limit deflections a little with same steel tonnage.

7) Most geotechs require that we neglect the top 1'-0" in our design. While I understand the concept, it seems since we are so far below the natural ground surface this may be a bit conservative. Thoughts?

RE: Soldier Beam Design Methodology

Answers
1. For temporary soldier beam walls, I take the active pressure down to the excavation subgrade only. For permanent soldier beam walls, I take the active pressure to the bottom of the embedded soldier beam and apply it to the width of the driven beam or to the diameter of the concreted drill hole.

2. I would never use a pole design method for soldier beams.

3. See answer in 1. Older AASHTO earth pressure diagrams for cantilevered beams had an error where it mistakenly multiplied the active below subgrade by 3b instead of 1b. The older AASHTO also had commentary saying that for temporary walls, the active pressure for soldier beam walls had to go only to the subgrade. Newer AASHTO manuals seem to make this less obvious.

4. It depends on the job. Is the area accessible to vehicles (fire trucks, ambulances, dump trucks, etc.)? Pedestrian foot traffic only? If you have a nearby building surcharge, you probably shouldn't be supporting the building with sheeting - especially cantilevered sheeting. Consider underpinning or some other exorbitantly expensive, rarely used, support system like a diaphragm wall, tangent pile wall, or secant pile wall, or jet grouting.

5. Usually, FS = 1.5 for the passive resistance. If you divide the passive coefficient, Kp, by 1.5, you will get a less economical design than by using the full Kp and then just increasing the embedment depth by about 20%. Remember, passive resistance is a function of the depth squared. If you multiply the embedment by 1.5, it will multiply the passive resistance by 1.5 x 1.5 = 2.25. That's why most books say to multiply the depth by 1.2. So, 1.2 x 1.2 = 1.44 which is usually close enough to 1.5 safety factor.

6. I have no idea what you are talking about.

7. I never do this unless forced to by specifications. Sometimes for permanent walls, specs require ignoring a certain amount of passive resistance to account for future utility excavation in front of the soldier beam or SSP wall.

Your spreadsheet looks nice but is probably not as useable as a commercially available computer program that allows different soil layers. It is also very "black box." How can a reviewer know if your hidden calcs are correct? You probably would need to give the reviewer verification hand calcs.

Also, why use the unwieldy conventional method when the simplified method is acceptable and much easier. Read Teng's discussion on both methods. Read his discussion on the simplified method exactly as written. Do not read into it what is not specifically written. It says to solve Do for moment equilibrium and, then, "The total depth of penetration D may be taken approximately at about 20 per cent higher than Do calculated by this method." What does "total depth" mean? To me, it means that 20% increased depth is all you need - no additional safety factor. Teng does not say to then add another safety factor on top of the 120%Do.

www.PeirceEngineering.com

RE: Soldier Beam Design Methodology

(OP)
Thanks for your very detailed response PEInc. I really appreciate it, and I understand you are quite the expert in shoring. So I take what you say to heart.

A couple of follow-up questions.

1) Why do you only take the active pressure down to the base of the excavation subgrade for temporary conditions? How is this soil loading fundamentally different than a permanent condition? I see this is in reference to an old AASHTO code, but are there any current codes that specify this? Do you know why the AASHTO was revised to be less clear?

4) That makes sense. In our case the only "near" buildings are generally 1-2 story light residential structures which are generally 10'+ from excavation. This puts it at 1:1 away from excavation. In our area it is extremely common to do these excavations as "stitch piers" (concrete piers without lagging spaced 2-2.5 diameters apart). We don't do that, but if we have something more robust than soldier beam w/wood lagging like tiebacks, we'd need a really good reason. Aside from the weight of the structure which is relatively far away, we would only have loading from a residential backyard so incidental usage. 72 PSF lateral surcharge seems high for these conditions. Would you omit, or use 1' surcharge instead of 2' (37 PSF)

5) We use FS = 1.5 for permanent, but I've seen a lot of references which use either 1.25 or 1.3 for temporary conditions. You use 1.5 for temporary conditions as well? Perhaps this helps offset the active only down to the dredge line. We typically apply this as Kp / FS. I'm confused as to why the depth would be different by using Kp / FS vs multiplying the embedment x 1.2. Shouldn't these be similar or wouldn't the FS not be the same with the two approaches? We resolve the equations using FS = 1.0 for determining bending moments for soldier pier design.

6) Sorry for being unclear. I was referencing the clear cover on the soldier beam within the pier hole. For a permanent application we would require 3" minimum clear cover. For temporary we have reduced that sometimes to allow for larger soldier beam in a pier hole.

7) I would like to avoid this, but our geotechs always require this unfortunately.

I'll take a closer look at the simplified method. I used the conventional method because I wasn't sure of the limitations on the simplified (including creep loading, lateral point loads, etc.) It would have been easier to implement, but the conventional is already complete. I'll also look at commercially available software, but I wanted to have a firm understanding of the process and to QC the software prior to using it. I do have hand calcs to supplement the spreadsheet.

RE: Soldier Beam Design Methodology

(OP)
One more follow-up. Do you know what the rationale is for only using the diameter of the pier to apply active below the dredgeline and not the entire effective pier width used for passive pressure design? The examples I have seen (Caltrans & Army Corps) both use the same effective width for active & pressure.

RE: Soldier Beam Design Methodology

(OP)

Quote (PEInc)

Also, why use the unwieldy conventional method when the simplified method is acceptable and much easier. Read Teng's discussion on both methods. Read his discussion on the simplified method exactly as written. Do not read into it what is not specifically written. It says to solve Do for moment equilibrium and, then, "The total depth of penetration D may be taken approximately at about 20 per cent higher than Do calculated by this method." What does "total depth" mean? To me, it means that 20% increased depth is all you need - no additional safety factor. Teng does not say to then add another safety factor on top of the 120%Do.

I'm not sure if that is correct. My understanding was that the 20% was not a safety factor but was intended to account for the amount of pier depth that would be required below the pivot point as the simplified method does not include pressures below the pivot point. Any factor of safety used would need to be added to this depth. This is how the approach is interpreted in Caltrans Trenching & Shoring Manual.

RE: Soldier Beam Design Methodology

I recommend anyone using lower load factor/factor of safety for temporary retaining walls read the investigation report of Nicoll Highway collapse that killed four people in Singapore.

RE: Soldier Beam Design Methodology

More Answers

1. The temporary condition is short term. There may be arching behind the soldier beam toe. Usually, soil properties are "estimated." The passive coefficient can be at least 10 times greater than the active coefficient. In addition, you are using 3 times to width for passive. 3 times 10 = at least 30 times more passive resistance than active pressure. A tiny bit more active pressure behind the soldier beam toe is not very significant considering the usually conservative soil properties. How can about 3' width of active soil pressure squeeze onto the narrower rear flange of a soldier beam or on to the back side of the concreted beam encasement? If the soil is very soft (no arching), then you should not be using soldier beams. Using steel sheet piling would consider active pressure to the tip of the SSP.
4. If there is no significant surcharge behind the sheeting wall, I will use either no surcharge (the area is off-limits to vehicles, stored materials, or significant crowds) or a token vertical area surcharge of about 100 psf.
5. You really did not define what safety factor you are talking about. Usually, the safety factor is on the passive resistance. Using Kp/FS will produce higher bending moments, requiring heavier soldier beams. Many references just calculate the embedment without a safety factor and then just increase the embedment length 20 to 40%. The greater % increase is usually recommended by manuals from steel suppliers (big surprise!). Run you spreadsheet using a reduced Kp and then re-run it with a full Kp and then add 20% to the embedment depth. That should show you the difference in bending moments and possibly total beam length.
6. No comment. Except, why are you worrying about beam cover? Corrosion concerns? If so, why aren't you worried about corrosion loss on regular, driven, bearing piles?
7. So, how often do you or the geotechs see an actual problems with this loss of 1' embedment? I doubt lowering the subgrade by 1' will make a wall fail. We aren't building watches. I have designed and or build thousands of walls. I never had a wall problem due to subgrade being only a foot or so deeper.
Follow-up. See previous answer for 1. With respect to the added 20% not being a safety factor, where did Teng say that? He didn't. Check The US Steel Sheet Pile Design Manual, Cantilevered SSP Ex. No. 1, Pages 86 & 88. Check Anchored Ex. No. 1, Pages 95 & 97. It is common to calculate the required embedment length with full Kp and then just add 20% more embedment for a safety factor.

www.PeirceEngineering.com

RE: Soldier Beam Design Methodology

(OP)
Thank you again for your thoughtful response. I think we are talking the same language on most things. Just a couple more questions. Again thanks for your effort. I really respect your responses and approach and I know you have A LOT of experience. I hope you don't take my responses as challenging you, I am just trying to educate myself and have a better understanding of the subject.

1) At the dredge line wouldn't the active pressure be calculated based on the retained height "H" while the passive pressure would be 0? Therefore, doesn't the active pressure have a large "head start" on the passive? Neglecting the active below the dredgeline or including it and multiplying it against the pier depth has a somewhat significant change in depth / moment requirements. For instance, with 35 PCF active, and 350 PCF passive & 9'-0" retaining, I obtain a depth of 7.96' when active is neglected below the dredge line (Moment = 500 k-in), when including active x 1 pier diameter I obtain a depth of 8.81' (Moment = 538 k-in), when applying it against full effective pier width, I obtain a depth of 11.09' (Moment = 655 k-in). Based on the assumptions the depth increases 11% and 39% respectively while the moment increases 7.6% and 31% respectively.

5) I think we are on the same page here. To obtain pier embedment my spreadsheet uses Kp/FS. It sounds like this should come up with essentially the same depth as using FS = 1.0 and multiplying embedments by 1.2. (I ran one quick check and they were relatively close but not the same -- off by ~10%). When calculating moment, my spreadsheet does another embedment calculation but uses the full unreduced version of Kp (FS = 1.0). Therefore, the bending moments should be the same as your approach.

6) Yes, the concern is beam cover for corrosion. To be honest your question is a good one and I don't have the answer except that we don't do driven bearing piles in our practice (generally light weight structures in residential settings). I understand for corrosion to occur there needs to be suitable oxygen available which may not be present a few feet below grade. Perhaps, the concrete pile cap is deep enough to provide coverage for the bearing piles in the area that may be subjected to corrosion? For a temporary condition we wouldn't be concerned with corrosion and it sounds like you also don't have a concern.

7) I agree with you. I don't think it is a problem, but it is a common design requirement we see from all of our geotechs in the area. I agree with your take on it.

Follow-up I don't have access to the Teng text. Can you scan a copy of the relevant page so I can review? To be clear, we are talking about the simplified method vs conventional method? It is my understanding that to convert simplified depth to approximately what would be obtained by conventional you multiply the depth by 1.2. To then add in the SF you would increase depth by 1.2-1.4 as you mentioned above (or use Kp / FS). Therefore, for the depth based on simplified approach you would increase the depth by 1.2 x 1.2 = 1.44. Is this not your understanding? The example on page 86 of USS Sheet Pile Manual uses conventional method so I don't think the comparison is correct as they are not using the simplified approach. The example on page 95 of USS sheet pile manual also doesn't seem like a fair comparison as they are using the Free-Earth method and therefore assuming the pile is not "fixed" at the embedment (embedment is not past the pivot point). Therefore, you wouldn't need to multiply to convert to conventional approach to account for embedment below the pivot point.

RE: Soldier Beam Design Methodology

More Answers
1. Yes, but it would have taken me too long to get into that by typing. Remember, if the soil is average or better, you will have arching. What happens when you drive a 4x4 vertically into the ground and then push it laterally? Does the soil on the back side of the 4x4 follow the back face of the 4x4? Not unless the soil is extremely soft or loose and below the water table. Otherwise, you will just see a gap that can stay stable for quite a while.
5. If you reduce the Kp for a safety factor, the resistance will be significantly less and the beam will bend more. Theoretically, that would be the more correct way to apply the safety factor; BUT, many thousands of walls have been built over many, many years using full Kp and then increasing by at least 20% the embedment required for moment equilibrium. If it always works, why are we doing it more conservatively?
6. For a temporary soldier beams, corrosion should not normally be a concern. For a permanent soldier beam wall, some agencies or specifications require coated beams or beams with extra sacrificial thickness. Generally, soldier beam corrosion is not a problem unless the soldier beams are exposed to air and water or some other corrosive substance, and that is not normally seen for the embedded toe. If you drill-in the soldier beams, you should be backfilling the drill hole with, at least, low strength, cementitious material. This can prevent corrosion.
7. Tell the geotech that the contractor, your client, will maintain the subgrade where it is supposed to be. That is cheaper than overdesigning the wall just in case the subgrade gets sloppy. Does the geotech want to pay for the extra steel, drilling or driving, or the backfill material? You should not have to overdesign in case someone might do something stupid in the field during construction.
Follow-up. I'd rather not duplicate and send copyrighted publications. Check on-line for a copy of Wayne C. Teng's 1962 Foundation Design. Your "understanding" is that you increase embedment by 20% and then increase it again. That's not how Teng described the method. Over the years, others have conservatively "tweaked" the Teng simplified method. Why not do what Teng wrote without reading anything more into it? Even the US Steel Sheet Pile Design Manual doesn't do that and it is universally(?) considered a leading SSP design reference (which has to be extrapolated for use with soldier beams). Isn't Teng's simplified method also a free-earth method?

7:20 PM. Time to go home.

www.PeirceEngineering.com

RE: Soldier Beam Design Methodology

(OP)
Thanks again.

1) Yes, I agree with this argument. One condition to consider though. Say you have 24" diameters piers @ 6'-0" and are using 3x pier diameter for effective passive. Where can your active arch to and not impact passive? You are essentially designing the wall the same as a sheet pile. I suppose if you spaced your soldier beams at 8'-10' you would have areas in between the effective passive that are not being used. However, do you think the active could arch 4-5 pier diameters to access these areas without impacting the effective passive area? I don't have an opinion, just a question.

5) I think we are talking the same thing. My spreadsheet runs the analysis twice. To calculate embedment depth I use factored Kp. To calculate moment I use unfactored Kp, therefore the moments are not more conservative than your approach. The analysis is completed twice instead of once, but as it is automated in a spreadsheet, now that the calculations are written, it's no extra effort on my part.

6) I agree. We use 2,500 psi concrete below dredge line and lean concrete above dredge line. Caltrans Trenching & Shoring Manual states to only use width of soldier beam flange for passive pressures unless at least a 4 sack mix is used. Looks like you are extending this to a lean concrete as well. I'm not educated enough to have an opinion.

7) I agree. I'll see where that goes.

Follow-up -- I understand. Let me see how I can obtain the reference. I do agree that the USS Sheet Pile Design Manual is a universal reference. In the example you referenced the conventional method was used not the simplified method. I understand on page 23 of the USS sheet pile design manual they only state to increase d by 1.2. However, I believe they are intending to imply that by doing this you get the same d you would by the conventional method. If you did not then provide an additional safety factor on the simplified approach wouldn't you obtain much shallower embedments using the simplified approach than the conventional approach? Do you think this was the intent? I don't...

While I don't have Teng's reference, I think it is simplified as a free-earth method but not intended to be a true free-earth approach. In a cantilever pole condition you need a "fixed-earth" condition (i.e. fixed restraint). To avoid the onerous calculations of the conventional method, only the pier above the pivot point is analyzed. The depth is then increased by 1.2 to approximate the additional depth that would be required below the pivot point. This depth has a FS = 1.0 (in my opinion). In the tieback example you reference, a true "free-earth" approach was used as pole fixity was not required as the tieback made the pier a simple pin-pin condition rather than a fixed-fee condition. So the additional embedment to provide a "fixed-earth" approach is not necessary.

RE: Soldier Beam Design Methodology

1. 3b cannot be greater than the spacing of the soldier beams. For example, soldiers at 5' spacing with 24" diameter drill holes. 3b = 6' > 5' So, use 5' maximum passive width. I would not use greater than 3b. Possibly only 2b is softer soil.
5. Like I said, spreadsheets are "black box."
6. If not a CALTRANS job, don't use their specs. Even FHWA does not require 2500 psi fill.
7. No comment needed.
Follow-up. I never use the conventional method. It is too iterative and too theoretical a process. We're talking about soil, usually a non-homogeneous material. Don't get too technical. If you closely follow some of the US Steel examples, you will see that they take some "shortcuts." The conventional method is not clear or easy. That's why computer programs don't use it.

www.PeirceEngineering.com

RE: Soldier Beam Design Methodology

(OP)
Thanks again!

1) I'm not sure my question was completely clear. If you have soldiers at 5' spacing, and you use 5' maximum passive as you state, how can you neglect the active below the dredge line? Where can it arch to and not impact the effective passive?

5) I agree, but so is computer software written by someone else to some extent.

6/7) thanks.

Follow-up -- I agree, I'm just trying to get an understanding. My spreadsheet actually follows the approach of Design Example No. 1 of USS Sheet Pile Design Manual. The adding / subtracting triangular areas shown in the example is actually not too bad to implement in a spreadsheet which does iterative calculations pretty efficiently. I wasn't aware that computer programs didn't use the conventional method, interesting to hear that.

Thanks for the good discussion!!

RE: Soldier Beam Design Methodology

1. You are overthinking this. It works; don't worry about it. The 3b is just a method to help account for other things helping support the toe, besides passive resistance. Yes, there is passive resistance on a per linear foot of width basis but there is also shear strength. You just can't push out a 1 to 3 foot wide triangular block of soil without having to also shear that block from its surrounding soil (on the sides and bottom of the block). Don't get hung up on 2b or 3b. It's just an empirical method for determining the passive resistance. In actuality, the "correct" width may be more than 2b or 3b, but I haven't seen anyone try to argue the validity of greater widths.
5. I use CivilTech's Shoring Suite because it performs its calculations just like I have always done by hand, before ever having CivilTech. It is easy to verify the CivilTech Shoring Suite calcs.

www.PeirceEngineering.com

RE: Soldier Beam Design Methodology

(OP)
1. So to be clear why don't you neglect active pressure below the dredgeline for a sheet pile? What makes a soldier beam fundamentally different if your effective passive width is equal to your pier spacing? I can understand the concept of neglecting active pressure below the dredgeline if your effective passive pressure is less than the your soldier beam spacing. If the effective passive width is the same as the pier spacing I don't see how you can neglect the effect of the active pressure on the effective passive width and assume the active arches. There is no where for the active to arch to without impacting the effective passive width. If you therefore rely on the shear strength of the soil to support the active below the dredgeline you are essentially saying that the soil could stand with a vertical cut for the entire depth of the pier. Is that the argument you are making?

On a side note, I was able to read Teng's text. It looks like his section on the simplified method is very similar to the way it is described in the USS Sheet Pile Design Manual. I agree with how he has written it he has no mention of a safety factor and instead says to obtain D solely by multiplying Do by 1.2. I still strongly believe he is making the comparison to the D obtained by the conventional method and that this D would still need to be multiplied by a factor of safety. If you do not multiply by a factor of safety I believe you are using a FS = 1.0 with this approach. See attached excerpts from Caltrans & NYDOT that agree with my assessment. We'll likely have to agree to disagree on this one.

RE: Soldier Beam Design Methodology

1. Because the SSP is continuous, there is no arching possible. (That being said, I have designed many walls without active pressure below subgrade, behind the SSP. I works. In the old days, pre-1990's, you could get away with that but not today.) Quit worrying about the effective passive width being the same as the beam spacing. I already said that the effective passive width is an empirical number; it's not real. It is an allowance that has been proven to work.
Side Note. Everything always gets more stringent because everyone's project is more important than the other guy's. People writing design manuals and specs are also unwilling to be unconservative. Therefore, the requirements snowball. If Teng didn't say or do it in his book for his simplified method, why do others say it and think it needs to be done? Because they figure the safer, the better, despite what the cost is. Specs never get looser. They tighten up every time someone has a problem or is asked to assume a little risk.

www.PeirceEngineering.com

RE: Soldier Beam Design Methodology

(OP)
Thanks for the continued discussion. I suppose we'll disagree on the side note but that's ok. You have your way I'll have mine.

Regarding the active pressure I think we are going in circles somewhat and you are either ignoring my point or not accepting of it. It doesn't matter what the assumed effective width is for passive, BUT if the effective width used is the same as the beam spacing how can you ignore active below the dredgeline? This essentially turns the soldier beam analysis equivalent to a sheet pile. Please tell me in this instance where the active could arch to. There is NO WHERE for it to go aside from the effective passive.

And you and I both know things work in reality that don't always work on paper. Just because it "worked" doesn't mean it met intended levels of factor of safety against collapse.

RE: Soldier Beam Design Methodology

I used the SPW-911 program to actually check some of Teng's examples, and I found that increasing the required embedment by 20% is about the same as using a FS = 1.5.

DaveAtkins

RE: Soldier Beam Design Methodology

(OP)
DaveAtkins -- Thanks for the response. When you say you increased the depth by 20% what are you comparing it to for a FS = 1.5? The same depth determined by Teng's example using Kp / FS or are you comparing it to the depth determined by the conventional approach multiplied by 1.2? If the comparison is just against the depth determined with the Simplified Method is this really a FS = 1.5 as the depth determined by the Simplified Method does not include the depth of the pier required below the pivot point for stability.

My opinion is that the depth of the simplified method should be multiplied by 1.2 to determine an equivalent depth that would have been found with the conventional method. You would then need to multiply this depth by 1.2 to obtain a ~ FS = 1.5.

RE: Soldier Beam Design Methodology

(OP)
I've attached two versions of my spreadsheet. The first is calculated using conventional process and the second is calculated using simplified process. I know the spreadsheet is "black box" so you'll have to take my word (or check on your own) that the calculations seem to be correct.

Both processes determine the same bending moment.

The embedment from the conventional pproach is 16.59'
The embedment from the simplified approach is 15.29'

Both of these are with F.S. = 1.0.

If we increase the simplified embedment by 1.2 we get 15.29 x 1.2 = 18.35'. This is larger embedment depth than required by conventional method. This makes sense to me as you are taking approximate "shortcuts" with the simplified approach so it should err on the side of conservatism. For the example you would only need to increase simplified embedments by 8.5% to obtain the conventional embedments. I don't think this ratio holds true for all cases, so it seems like the 1.2 factor would be a likely upper bound on what would be necessary.

If you calculated according to the conventional approach and multiplied by 1.2 to obtain ~ FS = 1.5 you have embedment of 19.91'
Therefore, in this example, if you applied simplified x 1.2 you would only be increasing embedment 1.106 to obtain approximate FS = 1.22.

RE: Soldier Beam Design Methodology

Here is a comparison of the US Steel Ex. No. 1 (conventional analysis of 14' cantilevered SSP wall) and CivilTech's simplified analysis for the same wall. Both use the same soil weights and earth pressure coefficients. The results are reasonably similar. US Steel determined D based on equilibrium and then added a percentage for a safety factor. I adjusted the US Steel pile length for a 20% increase to compare to the same 20% used by my CivilTech analysis. Remember, due to the complexity of the conventional method, US Steel never did finish their design by iteration. US Steel gave about a 6.5% longer embedment but a slightly lower bending moment. Both differences are expected when comparing a fixed earth analysis (longer toe) to a free earth analysis (more moment). Neither method is wrong; both have worked for many years. Just different!

www.PeirceEngineering.com

RE: Soldier Beam Design Methodology

SPW-911 gives the factor of safety against failure of the toe. If you input the exact required embedment, as calculated by Teng, SPW-911 says the FS = 1.0. If you increase the embedment 20%, SPW-911 says the FS is approximately 1.5.

DaveAtkins

RE: Soldier Beam Design Methodology

(OP)
Interesting, thanks for the input here guys. I appreciate all the time and effort in the responses. Especially your last post Pe,Inc. Really above and beyond.

It looks like I am in the minority here, but I remain unconvinced. It's ok, as you all have said there is no one correct way and the soil properties are not an exact science regardless so sharpening the pencil too much is somewhat pointless.

I still firmly believe the conventional method is the traditional way of assuming the soil pressure (assuming rigid pile which is flawed on its own). As this approach is quite onerous the simplified approach was developed to make the process much easier to calculate by hand. The simplification introduces some errors that need to be corrected for.

It is my understanding that the simplified approach will always have smaller embedments than the conventional approach. This is why I think it then needs to be corrected to embedments calculated by the conventional method. However, based on PE,Inc analysis and my spreadsheet comparison it seems like the correlation factor is likely closer to 1.05-1.1 than 1.2. Therefore, the 1.2 factor is a bit conservative (it appears) to be used against the depth and then again as a factor of safety. However, I still believe applying just the 1.2 factor against the D calculated using Teng's approach does not provide the same factor of safety as you would have by applying the 1.2 against the conventional method. Perhaps, the differences are small enough and can be chalked up to falling within the margin of error for design soil properties. I'm not a geo so I'm not an expert on that but I'll defer to you experts.

Thanks again for all the help. I appreciate the discussion. I posted initially on the structural forum and didn't get a single reply. Dang structural engineers get scared when they see dirt instead of steel :)

RE: Soldier Beam Design Methodology

So, tell me what the "errors" are with the simplified system. What is worse - small "errors" in a simple method or elaborate, theoretical assumptions in the conventional method? Essentially, the conventional method nets out the active and passive pressures to give a combined diagram. Then, crazy assumptions are made to try equalizing the horizontal forces. However, passive resistance is not an applied force. It only builds up enough (hopefully) to resist the driving forces. For a cantilevered sheeting wall, the total passive resistance must always be greater than the total driving forces because their respective moment arms are much different. Passive arm is much shorter. Therefore more resistance is needed. IMHO, trying to equalize the horizontal force is unnecessary for a cantilevered wall. The wall cannot move backwards from a passive resistance that is greater than the driving force.

Also, I have my own problems with SPW-911, especially for soldier beams; but that's a different thread.

www.PeirceEngineering.com

RE: Soldier Beam Design Methodology

(OP)
PEInc you understand that the simplified method is a simplified version of the conventional method? Any errors in the theoretical assumptions of the conventional method are also errors in the simplified method. The main difference between the two is that the simplified method is the conventional method but stops the analysis at the pivot point and introduces a fictitious reaction at depth Do that is intended to represent the passive pressure below the pivot point. As the location of this reaction does not occur at the pivot point as the simplified method assumes you need to calibrate the depth by multiplying Do x 1.2 Aside from this assumptions of both methods are the same. The main difference is that the conventional method directly determines the length of the embedment required below the pivot point where the simplified method does not and instead estimates this distance as "1.2 x Do". Both methods assume triangular active / passive pressure loading and rigid poles. Both of these assumptions introduce error but the simplified method introduces an additional error of estimating the length of embedment below the pivot point as "1.2 x Do".

Now I know you don't agree that this is what the simplified method is doing so I'm not entirely sure how else to explain it. But that is what it is doing.

Page 20 of the USS Sheet Pile Design Manual -"A cantilevered sheet pile wall may be designed in accordance with the principles and assumptions just discussed or by an approximate method based on further simplifying assumptions shown in Figure 15" Pay close attention to Figure 15. You'll note that 1.2 Do is drawn to the exact same depth as the "D" determined from the conventional method

Page 23 of the USS Sheet Pile Design Manual - "The passive resistances are simplified by assuming a right triangular pressure on the left side of the piling and by substitution of a concentrated force C for the net passive resistance on the right side of the piling This method results in some error but saves greatly in the computations. The distance, Do, must satisfy both the requirements of equilibrium. The calculated value of Do should be increased by 20 to 40 percent to get the total design depth of penetration

RE: Soldier Beam Design Methodology

My last comments:

In my opinion, the entire focus of the conventional method for cantilevered sheeting design is to develop force equilibrium in the horizontal direction after determining a depth of embedment for moment equilibrium. However, a cantilevered sheeting wall is not a free body diagram. The wall cannot push backward as a result of the greater passive "pressure." If the wall tried to push back the soil behind the wall would become passive. Would this now much greater, full height force now try to push the wall forward? No. If properly designed to resist overturning, the wall cannot move forward just because the total passive resistance is greater than the driving force; and the wall cannot move backwards. The simplified method addresses moment equilibrium (overturning) and then puts a safety factor on the embedment and therefore on the overturning. If you are worried about horizontal force equilibrium, why don't you find D for moment equilibrium, increase it by at least 20%, and then divide the new total passive resistance by the total driving forces (active and surcharges) to get a safety factor against sliding?

For over fifty years, people have been tweaking Teng's simplified method, beyond what Teng's clearly wrote, in order to make it more conservative. It's too bad we can't ask Teng to settle this.

www.PeirceEngineering.com

RE: Soldier Beam Design Methodology

(OP)
Thanks for your continued input. I've appreciated the dialog and the different views. While I think we still do not agree I think it was a good discussion and I respect your opinion and vast experience.

I think what you described about the wall pushing back below the pivot point is infact the baseline modeling assumption for these piers. The simplified method uses the same assumption just simplifies the calculation process. If you read Teng's book or USS Sheet Pile both state this as the case. And yes, the passive pressure is assumed to flip to the other side below the dredge line. This provides you a "moment couple" to provide the fixity. It's my understanding that this is how the system would work and if you run a PY analysis I think you will see that it has the same conclusion.

I agree it would be nice to have Teng able to provide clarity to settle this. Having not practiced as long as you, I haven't seen his method be refined over the years with more conservative approaches. Had I witnessed that I my viewpoint may better align with yours.

For now I think we will just have to agree to disagree.

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