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BLUM THEORY 1
- Thread starter Ash2007
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- #21
My apologies. We are talking about two different programs. SPW2006 is a free program available here
SPW911 is a commercial software program available from PileBuck.
I didn't know about SPW2006, so thanks for the tip.
SPW911 is a commercial software program available from PileBuck.
I didn't know about SPW2006, so thanks for the tip.
- Thread starter
- #23
There sure is a problem regarding high bending moment using SPW 2006. There is another problem encountered is, while following the construciton sequence ( say for example in 5 loading stages) It then while running says equilibirum is not achieved as results do not converge, but on repeated analysis it still gives a bending moment chart. Now I feel this is some sort of an error.
If the software cannot converge to any resultsa it should conclude saying floating point error. But instead it does give the bending moments whioch are exhorbitant ( 6000 kNm) as against MSHEET for the same giving around 3000. can any one explain the SPW error ?
I think I ahve followed the exact sequence of staged construction!
If the software cannot converge to any resultsa it should conclude saying floating point error. But instead it does give the bending moments whioch are exhorbitant ( 6000 kNm) as against MSHEET for the same giving around 3000. can any one explain the SPW error ?
I think I ahve followed the exact sequence of staged construction!
vulcanhammer
Geotechnical
The Fixed Earth method for analysing anchored sheet pile walls is discussed in Sheet Pile Design by Pile Buck, which also discusses application of SPW 911.
I use the term "Fixed Earth method" because, although Hermann Blum came up with the method in the early 1930's, there are many variations in its application. Blum's original idea was to create an equivalent, simply supported beam with two supports: one at the anchor and the other at the point of contraflexure, which generally appears at a distance just below the excavation line. The variations and simplifications begin with estimating that point of contraflexure. The rest of the pile--including the computation of the embedment below the excavation line--is handled separately.
Part of the problem here perhaps is that "Blum's method" should be referred to in the plural. If you look at the literature over the years (Tschebotarioff, ARBED, Pile Buck manuals, etc.) you will see the variations.
The ultimate objective of all of the variations in Blum's method is a pile toe with a) no slope (thus the "fixed end" and b) no moment. There will be a reaction, and that is usually dealt with with the 20% of embedment extension beyond the computed embedment length (although I have seen a variation on that too.)
As far a matching a sheet pile program with Blum's original method, given the variations this can be difficult. Complicating the issue is the lack of user friendliness amongst some of the programs, which can translate into GIGO very quickly.
I use the term "Fixed Earth method" because, although Hermann Blum came up with the method in the early 1930's, there are many variations in its application. Blum's original idea was to create an equivalent, simply supported beam with two supports: one at the anchor and the other at the point of contraflexure, which generally appears at a distance just below the excavation line. The variations and simplifications begin with estimating that point of contraflexure. The rest of the pile--including the computation of the embedment below the excavation line--is handled separately.
Part of the problem here perhaps is that "Blum's method" should be referred to in the plural. If you look at the literature over the years (Tschebotarioff, ARBED, Pile Buck manuals, etc.) you will see the variations.
The ultimate objective of all of the variations in Blum's method is a pile toe with a) no slope (thus the "fixed end" and b) no moment. There will be a reaction, and that is usually dealt with with the 20% of embedment extension beyond the computed embedment length (although I have seen a variation on that too.)
As far a matching a sheet pile program with Blum's original method, given the variations this can be difficult. Complicating the issue is the lack of user friendliness amongst some of the programs, which can translate into GIGO very quickly.
Hello PEinc, i knew that you have delt with sheet piles design. I have i question.
In my case, i am analyzing a cantilever sheet pile.
According to the references i have read, i can do moment about the tip (sum of moments about the tip = 0), in order to find my depth value.
ok, now i get my depth, it;s a reasonable value. But my question is, when i try to check with my Sum of horizontal forces (Passive Vs active forces), they are not ZERO using that value of depth, in order to get them ZERO, i need to find OTHER value of depth.
When i get this new value, then i try to check with my sum of moments (about the tip), but in this case it is not zero,
i think it sounds a little by weird, (they should be zero both at the same value of depth), my question is, what approach should i use, the moments? or the Sum of forces?
in my case, i get a very very low value when i use the sum of forces approach ( 0.5ft Vs 12ft). With moments, makes sense.
am i going to have always an unequilibrium?
Please, reply, thanks
In my case, i am analyzing a cantilever sheet pile.
According to the references i have read, i can do moment about the tip (sum of moments about the tip = 0), in order to find my depth value.
ok, now i get my depth, it;s a reasonable value. But my question is, when i try to check with my Sum of horizontal forces (Passive Vs active forces), they are not ZERO using that value of depth, in order to get them ZERO, i need to find OTHER value of depth.
When i get this new value, then i try to check with my sum of moments (about the tip), but in this case it is not zero,
i think it sounds a little by weird, (they should be zero both at the same value of depth), my question is, what approach should i use, the moments? or the Sum of forces?
in my case, i get a very very low value when i use the sum of forces approach ( 0.5ft Vs 12ft). With moments, makes sense.
am i going to have always an unequilibrium?
Please, reply, thanks
The first step in the cantilevered sheeting design is to find an embedment deppth that gives moment equilibrium. Then, I increase the embedment by at least 20% to provide a safety factor for overturning. (Remember, it is just as common to increase the embedment depth to provide a safety factor after calculating the embedment depth as it is to initially divide the passive earth pressure coefficient by a safety factor and then calculate the embedment. The former method will result in a more economical sheet pile size than the latter while giving somewhat similar embedment depths.) Next, references will say to check horizontal force equilibrium. This is where I disagree with the references.
In checking moment equilibrium, because the passive pressure moment arm is much less than the active pressure moment arm, the total passive force must be much greater than the total active force. Therefore, you can never really increase the embedment in an effort to equilize the horizontal forces. The deeper you make the sheeting, the more you increase both the active and passive forces. In addition, as you increase the embedment, you also increase the safety factor for overturning.
Remember, passive earth pressure is not an applied force. It is a reaction. For cantilevered sheeting walls, only enough passive force builds up as necessary to resist overturning. Since the total passive force will always be greater than the total active force, you will automatically have a safety factor against sliding. If a cantilevered sheeting wall wanted to slide backwards toward the higher side, then the lower side would become the active side and the higher side would become the passive side. Then, following that line of thinking, the process would have to reverse and then keep repeating. However, this is impossible.
I know that's not what the books say, but that's how it works.
In checking moment equilibrium, because the passive pressure moment arm is much less than the active pressure moment arm, the total passive force must be much greater than the total active force. Therefore, you can never really increase the embedment in an effort to equilize the horizontal forces. The deeper you make the sheeting, the more you increase both the active and passive forces. In addition, as you increase the embedment, you also increase the safety factor for overturning.
Remember, passive earth pressure is not an applied force. It is a reaction. For cantilevered sheeting walls, only enough passive force builds up as necessary to resist overturning. Since the total passive force will always be greater than the total active force, you will automatically have a safety factor against sliding. If a cantilevered sheeting wall wanted to slide backwards toward the higher side, then the lower side would become the active side and the higher side would become the passive side. Then, following that line of thinking, the process would have to reverse and then keep repeating. However, this is impossible.
I know that's not what the books say, but that's how it works.
Hey PEinc
thanks for your reply. I really don;t know if you have read about the rectilinear diagram theory (similar to blum's theory) which deals with how to find the depth taking into account the passive pressure that is developed behind the wall.
it;s based on a fourth degree equation. But i really don;t know what would be the difference by using that method and the one you use (applying moments about the tip and incresing the depth by 1.2).
Also i have checked the CIRIA report No. 104 which describes the blum's theory (by pathfield and mair 1984) and they do very similar to what you do, but after they get the depth value (d0) they check the new values of passive and active pressures with the increased depth (1.2 do).
The condition is that if the new change in passive pressures - the new change in active pressure, is less than the passsive - active (using d0), (Also they name that R), then is ok.
if not, you gotta increase the depth until this condition satisfies.
But in my case, IT IS NEVER SATIsfied!, Always the R value is very very very high compared to the Change in passice - change in active.
if you have any experience please let me know.
thanks
thanks for your reply. I really don;t know if you have read about the rectilinear diagram theory (similar to blum's theory) which deals with how to find the depth taking into account the passive pressure that is developed behind the wall.
it;s based on a fourth degree equation. But i really don;t know what would be the difference by using that method and the one you use (applying moments about the tip and incresing the depth by 1.2).
Also i have checked the CIRIA report No. 104 which describes the blum's theory (by pathfield and mair 1984) and they do very similar to what you do, but after they get the depth value (d0) they check the new values of passive and active pressures with the increased depth (1.2 do).
The condition is that if the new change in passive pressures - the new change in active pressure, is less than the passsive - active (using d0), (Also they name that R), then is ok.
if not, you gotta increase the depth until this condition satisfies.
But in my case, IT IS NEVER SATIsfied!, Always the R value is very very very high compared to the Change in passice - change in active.
if you have any experience please let me know.
thanks
Hey PEinc, thanks for your reply.
now another question about the sheet pile in Cantilever.
Do you think Prosheet is a reliable software to use?/ does it uses the BLum theory to analysis or it only makes moment about the tip and that;s it? or in other words, does it takes into account that at some depth in the active pressure side, it can be developed passive pressures?
bassically i want to compare my calculations with a software to check if i am on the right path.
Mine, is a simple problem, i am not using bussinessq loads neither Caquot parameters, only soils properties and that;s it.
or do you know if is there a more reliable software?/ what about plaxis?
thanks and please reply
now another question about the sheet pile in Cantilever.
Do you think Prosheet is a reliable software to use?/ does it uses the BLum theory to analysis or it only makes moment about the tip and that;s it? or in other words, does it takes into account that at some depth in the active pressure side, it can be developed passive pressures?
bassically i want to compare my calculations with a software to check if i am on the right path.
Mine, is a simple problem, i am not using bussinessq loads neither Caquot parameters, only soils properties and that;s it.
or do you know if is there a more reliable software?/ what about plaxis?
thanks and please reply
The best way to learn how to design is to get a copy of Pile Buck (old or new version) For a cantilever wall, passive pressure forms initally on the front face of the wall, then on the back at a lower depth. Imagine holding a rod with boh hands, on a little higher on the rod than the other. If someone pulls on the rod, you will develop forces in your hands to resist. the forces will be in opposite directions to form an oposing couple.
Although I see many pepole do it, you can not put the active force on the back side and the passive on the front.
Te resulting formula is best solved by trial and error. Again see pile buck for detils. Bowles Foundatin design also reviews it.
Blum theroy is only for walls with on level of bracing. It is not a solution method for cantilevered walls.
There are many other programs besides
plaxis for designing sheets. Civiltech and SPW911 are both fairly easy to use & inexpensive.
Although I see many pepole do it, you can not put the active force on the back side and the passive on the front.
Te resulting formula is best solved by trial and error. Again see pile buck for detils. Bowles Foundatin design also reviews it.
Blum theroy is only for walls with on level of bracing. It is not a solution method for cantilevered walls.
There are many other programs besides
plaxis for designing sheets. Civiltech and SPW911 are both fairly easy to use & inexpensive.
I believe that ProSheet lets you choose a cantilevered analysis or, for braced or anchored wall, a fixed earth or free earth analysis method. I almost always use the free earth method. While I believe that Pile Buck's Sheet Pile Design Manual (formerly the US Steel heet Pile Design Manual) has a lot of good information in it, I don't think much of its design examples. At times, they are hard to follow. I don't remember if Pile Buck ever corrected the errors that were in the US Steel design examples.
ProSheet, I believe, also multiplies a Boussinesq surcharge pressure by Ka, therby giving a lower lateral earth pressure than the formula gives. The railroads may not accept this lower surcharge. ProSheet is easy to use for triangular earth loads and sheet piling. It is not easy to use for soldier beams or earth pressure distributions other than triangular. I also have gotten some very high (unreasonable?) sheet pile deflections using ProSheet. The program's strength is that it allows multi-layerd soil profiles and includes cohesion and wall friction.
ProSheet, I believe, also multiplies a Boussinesq surcharge pressure by Ka, therby giving a lower lateral earth pressure than the formula gives. The railroads may not accept this lower surcharge. ProSheet is easy to use for triangular earth loads and sheet piling. It is not easy to use for soldier beams or earth pressure distributions other than triangular. I also have gotten some very high (unreasonable?) sheet pile deflections using ProSheet. The program's strength is that it allows multi-layerd soil profiles and includes cohesion and wall friction.
P.S. Version 8 of CivilTech's Shoring program is good and has fixed the bugs of previous versions. Although I own both programs, I recommend CivilTech over SPW911. I never use SPW911 unless I'm checking a design done with it by another engineer.
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- #33
Hi ,
Just to conclude my previous conversation regarding using SPW 2006 by prof arnold verruijt and not SPW 911, I ahve found the exact soultion for simulating the Blum and would like to share it with ya'll. Since that was my key reason for posting this thread.
The SPW program denotes / asks for the Stroke parameter, which is represented as the difference in passive pressures and Spring stiffness, now usually design following sub grade reaction model is used using the spring stiuffness, but SPW uses the concept of follwoing the stroke parameter. If U enter this stroke parameter as a constant minimal value like 0.001 it exactly simulates the Blum condition.
If anyone still disagrees to this, kindly let me know
Danke schön!
Just to conclude my previous conversation regarding using SPW 2006 by prof arnold verruijt and not SPW 911, I ahve found the exact soultion for simulating the Blum and would like to share it with ya'll. Since that was my key reason for posting this thread.
The SPW program denotes / asks for the Stroke parameter, which is represented as the difference in passive pressures and Spring stiffness, now usually design following sub grade reaction model is used using the spring stiuffness, but SPW uses the concept of follwoing the stroke parameter. If U enter this stroke parameter as a constant minimal value like 0.001 it exactly simulates the Blum condition.
If anyone still disagrees to this, kindly let me know
Danke schön!
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