calculating pivot point at excavation wall and tierod tensile force for bulkhead
calculating pivot point at excavation wall and tierod tensile force for bulkhead
(OP)
Hello,
In a question I tried to solve when preparing for PE exam, there is an anchored bulkhead supporting an excavation wall. I can not construct the geometry of the problem in my head.. so i couldnt understand the solution.
The question says, an anchored flexible bulkhead, which is fixed from rotation and horizontal movement, is supporting a large excavation. the bulkhead is installed 35 feet deep. The excavation is 20 feet deep. The tie rod will be installed at 5 feet depth, and at a sufficient distance behind the active wedge. it asks for the tensile force in the tierod per unit width of bulkhead. the question gives parameters for the soil also.
Can someone help me to establish the geometry of this problem, or if possible draw a sketch of it and show it? because of not understanding the geometry, i also didnt understand how the pivot point is calculated in the solution. i know what bulkhead means but i dont know what they mean by anchored flexible bulkhead in the first place.
In a question I tried to solve when preparing for PE exam, there is an anchored bulkhead supporting an excavation wall. I can not construct the geometry of the problem in my head.. so i couldnt understand the solution.
The question says, an anchored flexible bulkhead, which is fixed from rotation and horizontal movement, is supporting a large excavation. the bulkhead is installed 35 feet deep. The excavation is 20 feet deep. The tie rod will be installed at 5 feet depth, and at a sufficient distance behind the active wedge. it asks for the tensile force in the tierod per unit width of bulkhead. the question gives parameters for the soil also.
Can someone help me to establish the geometry of this problem, or if possible draw a sketch of it and show it? because of not understanding the geometry, i also didnt understand how the pivot point is calculated in the solution. i know what bulkhead means but i dont know what they mean by anchored flexible bulkhead in the first place.





RE: calculating pivot point at excavation wall and tierod tensile force for bulkhead
Here is a reference with plenty of good stuff for use when you are on the job doing these.
http://www.wbdg.org/ccb/NAVFAC/DMMHNAV/1025_4.pdf
RE: calculating pivot point at excavation wall and tierod tensile force for bulkhead
http://portal.tugraz.at/portal/page/portal/Files/i...
These may answer many of your questions.
RE: calculating pivot point at excavation wall and tierod tensile force for bulkhead
RE: calculating pivot point at excavation wall and tierod tensile force for bulkhead
RE: calculating pivot point at excavation wall and tierod tensile force for bulkhead
based on that understanding, i looked at the solution again. this time i saw that they subtracted the active pressure from passive pressure at the portion below the dredge line.
so they made
Kp x gama soil x height below dredge line (minus) Ka x gama soil x total h (as active pressure starts from the top of wall, not dredge line)
so they found a passive net pressure at the BASE level of the wall. (passive is bigger as its coefficient is much bigger, although its height is smaller)
to this point i am ok
but then , in order to calculate the pivot point, for a reason that i can not understand, they said "using static equilibrium, the pivot point can be calculated by comparing areas" and
the made passive force below the dredge line equal to active force below the dredge line, separated at the pivot point height. so essentially, they divided the section below dredge line into a passive and active part, separated at pivot point, and made those parts equal to each other, to determine the height of pivot point.
why did they make the active and passive pressure below dredge line equal to each other separated at the pivot point? this wall is bigger than that portion below the dredge line. it is continuing above the dredge line. above the dredge line, there are active forces and the force from tie rod. to use static equilibrium, by common sense, you should take the whole thing, or introduce a moment where they cut it, in this case the dredge line shouldnt they? so how can we say active and passive pressure is equal to each other below the dredge line at the pivot point and find pivot point like that?
RE: calculating pivot point at excavation wall and tierod tensile force for bulkhead