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Momentreduktion i spontvägg

Nagy, Rikard (2010) VGTM01 20092
Structural Mechanics
Civil Engineering (M.Sc.Eng.)
Abstract (Swedish)
Examensarbetet behandlar fenomenet momentreduktion. Reduktionen uppstår då en spontvägg deformeras och krafterna i jorden omfördelas.

I arbetet har fyra olika spontdimensioneringsmetoder behandlats som på olika sätt leder till ett dimensionerande moment. Tre av dessa är handberäkningar och den fjärde är en jämförande beräkning som görs i finita elementprogrammet PLAXIS.

Två av handberäkningsmetoderna utgår från de diagram som togs fram av Peter Walter Rowe år 1952. Flexibiliteten, det vill säga hur pass styv sponten är i förhållande till dess höjd, avgör tillsammans med lagringen för jorden vilken reduktion man kan göra.

Den första av dessa två dimensioneringsmetoder har Rowe själv utvecklat. Han baserade sina beräkningar på... (More)
Examensarbetet behandlar fenomenet momentreduktion. Reduktionen uppstår då en spontvägg deformeras och krafterna i jorden omfördelas.

I arbetet har fyra olika spontdimensioneringsmetoder behandlats som på olika sätt leder till ett dimensionerande moment. Tre av dessa är handberäkningar och den fjärde är en jämförande beräkning som görs i finita elementprogrammet PLAXIS.

Två av handberäkningsmetoderna utgår från de diagram som togs fram av Peter Walter Rowe år 1952. Flexibiliteten, det vill säga hur pass styv sponten är i förhållande till dess höjd, avgör tillsammans med lagringen för jorden vilken reduktion man kan göra.

Den första av dessa två dimensioneringsmetoder har Rowe själv utvecklat. Han baserade sina beräkningar på antagandet att sponten hade vidhäftning mot jord på den sida där det aktiva jordtrycket verkade. På den sida som det bildas passivt, det vill säga mothållande, jordtryck ansågs väggen helt sakna vidhäftning. Det mothållande jordtryckets storlek reduceras sedan för att få en säkerhetsmarginal på resultatet.
I tillägg till detta kan man tillgodoräkna sig en kraft som uppstår på grund av att det finns friktion mellan spontfot och jord, denna beror på den vertikala last som verkar på spontväggen.

Den andra metoden är enligt Sponthandboken där spontväggen ses som helt glatt på bägge sidor. På grund av detta blir lasteffekten av det aktiva jordtrycket större och bärförmågan mindre än då vidhäftning mellan spont och jord sker. Detta leder till större dimensionerande moment och större nedslagningsdjup.

Den tredje metoden är enligt Jørgen Brinch Hansens teori. Dimensionerande moment fås genom att göra ett antagande om hur spontväggen kommer att gå till brott. I arbetet behandlas beräkningar gjorda med tre olika antaganden om hur spontväggen går till brott, en där brott sker i jord och två där konstruktionen går till brott. Dessa
antaganden kommer att leda till att rörelser och rotationer uppstår i konstruktionen, som i sin tur ger upphov till den tryckfördelning som kommer att verka mot spontväggen. Beräkningar gjorda enligt denna metod ger ett lägre dimensionerande moment och även mindre nedslagningsdjup. Antagandet om att två flytleder bildas i väggen är det som ger bäst överensstämmelse med den jämförande beräkningen.

Vid jämförelse kan man se att den dimensioneringsmetod som används idag, det vill säga den hämtad från Sponthandboken, ger ett nästan dubbelt så stort dimensionerande moment som det som fås av en analys gjord i PLAXIS. Man kan även se att handberäkningar, utom i det fallet där Sponthandboken används, ger ett lägre värde på kraften i förankringen än det som fås vid beräkningar i programmet PLAXIS. (Less)
Abstract
The thesis deals with the phenomenon of moment reduction. The reduction occurs when a sheet pile wall deforms and forces of the soil are redistributed.

Four different methods of designing sheet pile walls are treated that in different ways lead to a design moment. Three out of these design methods are made by hand calculations and the fourth is a comparative assessment carried out in the finite element program PLAXIS...

Two of these calculation methods are based on the moment reduction chart that was developed by Peter Walter Rowe in 1952. The flexibility, that is how stiff the sheet pile wall is in relation to its height, determines, along with packing of the soil, which reduction that may be done.

The first out of these two... (More)
The thesis deals with the phenomenon of moment reduction. The reduction occurs when a sheet pile wall deforms and forces of the soil are redistributed.

Four different methods of designing sheet pile walls are treated that in different ways lead to a design moment. Three out of these design methods are made by hand calculations and the fourth is a comparative assessment carried out in the finite element program PLAXIS...

Two of these calculation methods are based on the moment reduction chart that was developed by Peter Walter Rowe in 1952. The flexibility, that is how stiff the sheet pile wall is in relation to its height, determines, along with packing of the soil, which reduction that may be done.

The first out of these two methods was made by Rowe himself. He based his calculations on the assumption that the sheet pile wall had adhesion to the soil on the side of the wall where the active soil pressure appeared. On the other side of the sheet pile wall, where passive pressure is mobilized, there was no adhesion between wall and soil, and also a reduction was made of the calculated load capacity in order to obtain a safety margin. In addition to this, a force caused by friction between the foot of the sheet pile wall and soil, can be added. This force depends on the vertical loading that acts on the sheet pile wall.

The second method is according to the Swedish book Sponthandboken where the calculations are carried out with the assumption that there is no adhesion at all between the sheet pile wall and the soil. This leads to bigger loading effects on the side of the wall where one has active soil pressure and the resistance offered by the passive soil pressure becomes less than when one has adhesion between the wall and soil. This leads to design moments that are overvalued and it also leads to overestimating the dept that the sheet pile wall needs to be a stable construction.

The third method is according to Jørgen Brinch Hansen's theory. The design moments are obtained by making an assumption about how the sheet pile wall will collapse. These assumptions will lead to displacements and rotations that will occur in the construction, which in return shows how the pressure distribution will work against the sheet pile wall. A calculation made by using this method gives a lower value of
the design moment and will also get a smaller depth needed to get a stable sheet pile wall. The calculation made with the assumption that the sheet pile wall will have two points of rotation is the one which gives the best conformity with the comparative calculation.

For comparison, we see that the design method used today, the one presented in Sponthandboken, provides almost twice as large design moments than those obtained by an analysis made in PLAXIS. One can also see that the handmade calculations, except the one where Sponthandboken is used, give a lower value of the force on the anchorage compared to the ones obtained from calculations made by the program PLAXIS. (Less)
Please use this url to cite or link to this publication:
author
Nagy, Rikard
supervisor
organization
course
VGTM01 20092
year
type
H3 - Professional qualifications (4 Years - )
subject
keywords
konstruktioner, spont
report number
TVSM-5166
ISSN
0281-6679
language
Swedish
id
3566947
date added to LUP
2013-08-01 14:18:30
date last changed
2013-10-07 13:20:21
@misc{3566947,
  abstract     = {The thesis deals with the phenomenon of moment reduction. The reduction occurs when a sheet pile wall deforms and forces of the soil are redistributed. 
 
Four different methods of designing sheet pile walls are treated that in different ways lead to a design moment. Three out of these design methods are made by hand calculations and the fourth is a comparative assessment carried out in the finite element program PLAXIS...

Two of these calculation methods are based on the moment reduction chart that was developed by Peter Walter Rowe in 1952. The flexibility, that is how stiff the sheet pile wall is in relation to its height, determines, along with packing of the soil, which reduction that may be done.

The first out of these two methods was made by Rowe himself. He based his calculations on the assumption that the sheet pile wall had adhesion to the soil on the side of the wall where the active soil pressure appeared. On the other side of the sheet pile wall, where passive pressure is mobilized, there was no adhesion between wall and soil, and also a reduction was made of the calculated load capacity in order to obtain a safety margin. In addition to this, a force caused by friction between the foot of the sheet pile wall and soil, can be added. This force depends on the vertical loading that acts on the sheet pile wall.

The second method is according to the Swedish book Sponthandboken where the calculations are carried out with the assumption that there is no adhesion at all between the sheet pile wall and the soil. This leads to bigger loading effects on the side of the wall where one has active soil pressure and the resistance offered by the passive soil pressure becomes less than when one has adhesion between the wall and soil. This leads to design moments that are overvalued and it also leads to overestimating the dept that the sheet pile wall needs to be a stable construction.

The third method is according to Jørgen Brinch Hansen's theory. The design moments are obtained by making an assumption about how the sheet pile wall will collapse. These assumptions will lead to displacements and rotations that will occur in the construction, which in return shows how the pressure distribution will work against the sheet pile wall. A calculation made by using this method gives a lower value of
the design moment and will also get a smaller depth needed to get a stable sheet pile wall. The calculation made with the assumption that the sheet pile wall will have two points of rotation is the one which gives the best conformity with the comparative calculation.

For comparison, we see that the design method used today, the one presented in Sponthandboken, provides almost twice as large design moments than those obtained by an analysis made in PLAXIS. One can also see that the handmade calculations, except the one where Sponthandboken is used, give a lower value of the force on the anchorage compared to the ones obtained from calculations made by the program PLAXIS.},
  author       = {Nagy, Rikard},
  issn         = {0281-6679},
  keyword      = {konstruktioner,spont},
  language     = {swe},
  note         = {Student Paper},
  title        = {Momentreduktion i spontvägg},
  year         = {2010},
}