There are frequently different approaches to the same problem. In engineering, we frequently see different approaches, due to the time of training, place, history, or whatever. Friction in soils, for example, relative to the shear stress, may be expressed as a change in friction angle or as an absolute ratio. The numbers to get the same effect are not the same.
In cohesionless soils, a change in friction angle is more traditional, but when it comes to soil acting on a surface, it is easier to understand and work with a absolute number. This absolute term of reference make life easier, but more confusing to those who training has only included friction as a change in angle.
Neither is more correct than the other, as long as one is used, and the meaning is clear. Using alpha or delta is not clear, as the nomenclature changes textbook to textbook.
The problem with university is there is too much to learn in the allotted time. Additional time would and is just filled with the professors choice of material. It takes time to absorb and learn to the point of understanding, and there is just to much material in some fields. Specialization is forced on us, even if we know a bit about many subjects.
Some of us have worked our entire lives around the field we wanted to be in, only occasionally doing the sort of projects we really wanted to do. We were forced to learn more and more around our chosen field, until we find we are generalists, no expertise in any one subfield, but generalist in the area, doing the work, and having opinions on the various methods. These opinions put us at odds with the academics.
Oh well, life goes on. We are all just learning. It is the first time through life.
Friday, October 5, 2012
Monday, October 1, 2012
Hand Calculated vs Computer
It has become apparent to me that use of the computer has become a impediment to good engineering. I started in this business with a slide rule and a Curtiss, and if you know both, you will know how I was trained. Cancel out, do the simple, and then approximate, and finally work out the mantissa. Then those expensive hand held calculators... worth a weeks pay.
We drew graphs by hand, and remembered the values of the results.
Now the young cannot divide by three and multiply by ten on paper. Draw a graph, one minutes work, without a computer, forget it.
I feel for the next generation, when a problem arises, the young will flail for days to find a solution.
For some of these problems, I have simple numerical solution, which I learned at Uni, that produce an approximate solution. Some of them are as reliable as modern computer solutions. In one case, one of the new wonderful computer programs actually uses crappy approximate method, but provides "calibration" questions to "evaluate" parameters that are listed in the formula found in a 1923 publication. Mind you, the computer is quick at raising a number to the 0.564.
We drew graphs by hand, and remembered the values of the results.
Now the young cannot divide by three and multiply by ten on paper. Draw a graph, one minutes work, without a computer, forget it.
I feel for the next generation, when a problem arises, the young will flail for days to find a solution.
For some of these problems, I have simple numerical solution, which I learned at Uni, that produce an approximate solution. Some of them are as reliable as modern computer solutions. In one case, one of the new wonderful computer programs actually uses crappy approximate method, but provides "calibration" questions to "evaluate" parameters that are listed in the formula found in a 1923 publication. Mind you, the computer is quick at raising a number to the 0.564.
Friday, September 28, 2012
Residential Engineered fill
Engineered fill is used to create housing lots from land that was previously unsuitable. Out here, in the high plastic mud plane flats, there are slough areas that hold water most (all) years. The small ones are filled with engineered fill and turned into housing lots. No problems, usually. Bigger sloughs are usually reshaped to fit the lots better. The topsoil and/or marginal material is used to create a damn to control the water and fill is used to bring up the lots.
The primary difference between engineered fill and clean common fill is the amount of compaction and the documentation of the compaction. There is an issue with the extent of the engineered filled provided in the lots, and as the design, size, and location of the houses are not known at the time of lot construction, errors frequently occur as to the required lateral extent of the engineered fill.
After the house is put on the lot, the question becomes, was the rear of the house placed in engineered fill or into common fill? Where settlement of the back wall of the house occurs, it is likely that the engineered portion did not extend far enough into the lot.
But how do we prove the boundary of the engineered fill when the difference is compaction at the time of placement? If they used organic or marginal fill, the difference becomes apparent. But if it is good fill, now what?
The primary difference between engineered fill and clean common fill is the amount of compaction and the documentation of the compaction. There is an issue with the extent of the engineered filled provided in the lots, and as the design, size, and location of the houses are not known at the time of lot construction, errors frequently occur as to the required lateral extent of the engineered fill.
After the house is put on the lot, the question becomes, was the rear of the house placed in engineered fill or into common fill? Where settlement of the back wall of the house occurs, it is likely that the engineered portion did not extend far enough into the lot.
But how do we prove the boundary of the engineered fill when the difference is compaction at the time of placement? If they used organic or marginal fill, the difference becomes apparent. But if it is good fill, now what?
Wednesday, September 26, 2012
Assumed Bearing Values
You may save a few dollars by using an assumed bearing value, but you take on the liability and when problem arises, and it all get dumped in the court, you become neglect. If you assume a low value, it costs the client, and if you assume a high value, you get movements.
The issue with foundations is usually to much movement. Often we expect movement, and then you have a choice, build, and allow for the movement, or not build. Some of the time, we can switch foundation types, and reduce movement to an acceptable level.
All foundations move. Movement is a response to changing load, and to often, changing moisture regimes. It is limiting these movements to an amount that is acceptable that is the trick.
Our building codes are out to lunch in some respects. It calls for foundation design utilising ULS, which should never control in soils, SLS will always govern. But the building code is written by a bunch of structural engineers, who do not understand soils. They force us soils types to adapt, to take what is right and bend it to fit a wrong code.
The issue with foundations is usually to much movement. Often we expect movement, and then you have a choice, build, and allow for the movement, or not build. Some of the time, we can switch foundation types, and reduce movement to an acceptable level.
All foundations move. Movement is a response to changing load, and to often, changing moisture regimes. It is limiting these movements to an amount that is acceptable that is the trick.
Our building codes are out to lunch in some respects. It calls for foundation design utilising ULS, which should never control in soils, SLS will always govern. But the building code is written by a bunch of structural engineers, who do not understand soils. They force us soils types to adapt, to take what is right and bend it to fit a wrong code.
Friday, September 21, 2012
Engineered fill
Engineered fill is what we call the process of placing clay to support small buildings such as "traditional" houses. What is being built is far from traditional. And then there is the wet soil issues, where 0.6 metres of dry high plastic clay is placed and crusted over with a packer. The remainder of the fill is placed. Now what?
At the best, with traditional houses, we expect 2 inch of differential movement. Now a few construction problem/errors, tight basement finishes, and what do we have but houses who do not meet the owners expectations, but are within the expectations of the land developer and geotechnical engineers.
Such fun.
At the best, with traditional houses, we expect 2 inch of differential movement. Now a few construction problem/errors, tight basement finishes, and what do we have but houses who do not meet the owners expectations, but are within the expectations of the land developer and geotechnical engineers.
Such fun.
Tele-posts
Tele-posts, adjustable columns in the basement of most modern homes are to account for differential movement between different foundation types, and they are there for adjustment. They should be adjusted as required to keep the main floor straight. The basement walls must be constructed to allow for adjustment, and not to carry any vertical load to the basement slab. This will cause uplift of the floor, and partition cracking at doorways and other points of stress concentration.
Monday, September 10, 2012
Ice Jacking
Ice jacking is a phenomena that occurs when water is frozen, water is drawn to the freezing front through the soil, and freezes. There is also lateral frost jacking where water ingresses into a crack and freezes, producing large lateral forces. The amount of movement can become substantial when partial melting and cyclical freezing occurs.
This can occur in a number of situations. The typical one is in old stucco homes with no insulation and vapour barrier in the joist space over the basement. Water condenses and freezes, expands, and produces lateral force. The sun hit it, it melts, runs down, freezes and expands. You get the picture. Beam pockets over basement have a similar issue. Slabs outside have the same effect happening next to houses.
This week I saw a new occurrence of this, inside a unheated insulated garage. Cars drag in snow and it melts, and condensates on the grade beam. On nice days, this melts, runs down into the space between the grade beam and slab an freezes. This provides force, and in this case, after the slab moved a bit, it pushed the grade beam out of alignment.
The solution is not cheap. Heat, to prevent freezing, is one solution. But just insulation with a vapour barrier may be enough. The thermal gradient and water may be there, but these are separated to a point where the thermal capacity is not sufficient to produce frost in quantity to cause a problem. The vapour barrier must be attached to the concrete to prevent moisture ingress with a mastic or like.
This can occur in a number of situations. The typical one is in old stucco homes with no insulation and vapour barrier in the joist space over the basement. Water condenses and freezes, expands, and produces lateral force. The sun hit it, it melts, runs down, freezes and expands. You get the picture. Beam pockets over basement have a similar issue. Slabs outside have the same effect happening next to houses.
This week I saw a new occurrence of this, inside a unheated insulated garage. Cars drag in snow and it melts, and condensates on the grade beam. On nice days, this melts, runs down into the space between the grade beam and slab an freezes. This provides force, and in this case, after the slab moved a bit, it pushed the grade beam out of alignment.
The solution is not cheap. Heat, to prevent freezing, is one solution. But just insulation with a vapour barrier may be enough. The thermal gradient and water may be there, but these are separated to a point where the thermal capacity is not sufficient to produce frost in quantity to cause a problem. The vapour barrier must be attached to the concrete to prevent moisture ingress with a mastic or like.
Subscribe to:
Posts (Atom)