Mass Transport in Unsaturated Soil

Posted by on 16/ 11/ 15

Mass transport in the unsaturated part of the road construction (the sub-base and upper part of the subgrade) strongly depends on the soil moisture distribution inside the pores. Where the mass transport is principally by advection then the water move­ment direction will control the contaminant flux direction. As the principal fluxes in the vadose zone are those due to evaporation and percolation, it follows that the
direction of the mass transport will then be essentially vertical, upwards or towards the lower part of the subgrade.

Soil moisture travelling through the unsaturated part of the road construction moves at different velocities in different pores due to the fact that saturated pores through which the moisture moves have different-sized pore throats and differ­ent thickness of the water film on the mineral grains of the soil. The theory of mass transport inside of unsaturated soil is much more complicated than in satu­rated media. The processes are described in standard textbooks (e. g. Fetter, 1993; Hillel, 2004).

In general, the structure of the equations for mass transport in unsaturated soil is similar to the equations for saturated soil. They differ in that the diffusion and dispersion coefficients and flow velocities for unsaturated soil depend on the water content.

The rate of change of the total pollutant mass present inside the unsaturated part of the road construction must be equal to the difference between the pollutant flux going into the road pavement and that leaving it and going into the saturated subgrade. Due to the complex processes inside the unsaturated road layers, several sources and sinks of pollutants can exist. These processes can be associated with biological decay (for organic contaminants) as well as chemical transformations and precipitation.

If occurring in large quantities, organic compounds originating in petroleum products form a special case of great concern in connection with roads. Spillages of petroleum products from traffic accidents or from petrol-filling stations, often situated adjacent to roads, may result in large quantities of organic compounds en­tering the road surface of roadside soils. The different interaction of these organic fluids with the soil’s chemistry will frequently increase the effective permeability and enhance the flux of the contaminants through the soil – behaviour referred to as incompatibility. Such situations are largely undesirable from an environmental and from other points of view.

Diffusion

The steady-state diffusion of solute in soil moisture is given by

„ dC

F = -D* (в) (6.10)

dx

where F = mass flux of solute (units of M/L2T); D*(e) = soil diffusion coefficient which is a function of the water content, the tortuosity of the soil, and other factors related to the water film on grains (units of L2/T); C = the concentration of the contaminant (units of M/L3), x = the distance in the direction of travel (units of L) and dC/dx = the concentration gradient in the soil moisture.

The second-order diffusion equation for transient diffusion of solutes in soil wa­ter is defined as

Advection

In road aggregates that are between saturated and residual saturation, some advec­tion can occur with higher saturation allowing advective transport to increase. In terms of contaminant destination, the influence of advection is important. Diffusion will occur evenly in all directions in which the differences in contaminant con­centrations exist, while advection will transport contaminants wherever the water is draining, either into a fin drain, or down the vadose zone towards the phreatic surface.

In an unsaturated soil, some of the void space is filled with gas. Due to evapora­tion, some contaminants will pass from the liquid phase into the gas phase both by volatilisation and by transport in water vapour. Contaminants within the gas will then be transported by diffusion and advection within the gas phase. Exchange processes transferring contaminants between the gas and liquid phases in the road construction are very complex. The transport in the gas phase inside a road construc­tion can be substantial, especially when incidental spills appear on the road surface. However, the extent to which these processes occur inside the road construction is not well known and may be small for most metals. Certainly, as a soil or aggregate becomes less saturated the opportunity for advective transport reduces markedly, particularly because of the substantial reduction in permeability as described in Chapter 2, Section 2.8.

Dispersion

Rather as in Eq. 6.7, the soil-moisture dispersion coefficient, D(e), is defined as the sum of the mechanical and diffusion mixing and is now expressed as:

D* (в)

D (в) = £ M + D* (в)

where £ = an empirical dispersivity measurement (units L) that depends on the soil moisture and v = the average linear soil moisture velocity. This definition of D(e) may be contrasted with that for Dt given above for saturated conditions (see Eq. 6.7) which includes a soil tortuosity term, a, in place of the £ term which is also controlled by the water content.

In a road aggregate where pores are only partially saturated, especially during dry periods, the capillary suction of the aggregate can increase very significantly, mov­ing the aggregate towards its residual saturation condition and hindering advective contaminant transport. In these conditions, contaminant transport will, therefore, be very slow and primarily occur by diffusion.

SAGGING STAIRS

Posted by on 16/ 11/ 15

If the staircase has several of the ailments described in preceding sections, it may also have major troubles underneath. Investigate further. If the stairs tilt to one side, the carriage on the low side is having difficulty: That is, nails or screws holding it to the wall may be pulling out, the wood may be rotting or splitting, or the carriage may be pulling free from the stringer. Sagging on the open side of a stairway is common, for there’s no wall to bolt its carriage to. If there are large cracks or gaps at the top and bottom of the stairs, you’re seeing symptoms of a falling carriage.

To learn more, remove the finish surfaces from the underside of the staircase. But before cutting into anything, rent a Dumpster for the rubble and confine the mess by sealing off the stairwell with sheet plastic. When you cut, set your circular saw just to the depth of the finish materials so that you don’t cut into carriages. Wear goggles and use a Carborundum™ blade to cut out the surface in 2-ft. squares.

You can probably save any decorative plaster molding along the staircase by cutting parallel to it—about 1 in. from its edge, thus isolating the section of lath nailed to the underside of the outer carriage. Leaving a 1-in. strip will also make it easier to disguise the seam when you reattach the ornamental border after repairing the stairs.

With the underside of the stairs exposed, you can see exactly what the problem is. If the car­riages have pulled loose from adjacent walls, you’ll see a definite gap. Replace wood that is rot­ted or badly cracked, especially wood cracked across the grain. If the wood sags or is otherwise distorted, bolster it with additional lumber; it may also need to be reattached. All of these repairs are big ones. To do them right, you’ll need complete access to the substructure, from one end of the carriages to the other.

Starting at the top, remove all nosing, balus­ters, treads, and risers. You could theoretically bolster undersize carriages without removing all the treads, risers, and balusters, but it’s better to

image365Подпись: If the center carriage isn't notched, it may not support treads well. In this case, screw plywood tread supports to alternate sides. Also, if the center carriage was only nailed to the face of a header originally, it may have slipped down. If so, jack it up and reattach with a 1/4-in. steel angle plate.remove them. Otherwise, misaligned or distorted carriages will be held askew by all the pieces nailed to them. So remove the treads and risers, and jack up the distorted carriages to realign them. (You may want to stretch taut strings as an alignment aid.)

Number all parts as you remove them, group­ing pieces according to the step number.

Timbers from Your Own Land

Posted by on 16/ 11/ 15

Some owner-builders may harvest tall straight trees on their own property to make their own timbers for their frame. Hardwoods, in general, are stronger than softwoods, but its best to compare individual species, as there is considerable overlap in strength characteristics between a list of hardwoods and softwoods. Being generally harder, the hardwoods are more difficult to nail into. You may have to drill holes and use screws to make connections. Also, hardwoods tend to shrink quite a bit more than softwoods. Here are some common woods, beginning with so-called hardwoods — which are actually deciduous or broad – leafed trees.

Hardwoods

• Ash. Quite strong and usually straight, without a lot of knots. A favorite for baseball bats and hockey sticks. Can develop large checks. The wood is a creamy white.

• Beech. Heavy, very strong and quite beautiful. However, beech has a high rate of shrinkage and can suffer from powder post beetles and carpenter ants, so it should be avoided as sill material or as posts. Save it for timbers from the first story upwards. In old timber frame work in the northern hardwood forest — before the current beech blight — beech was often the wood of choice.

• Birch, White (or Paper). Even-grained, medium strength. As a “pioneer species” in the forest, it is not long-lived, so you might have difficulty finding large diameter trees in good condition. Works quite well with hand tools or a chainsaw, when green. [1]

• Butternut. Fairly strong for its light weight. The wood is straight-grained and has fairly low shrinkage for a hardwood.

• Cherry, Black. Rot-resistant and strong. In some parts of the northeast, it is fairly common. Black cherry is a pretty wood, but, if you have it, you may want to reserve it for furniture, cabinets, or special detailing, to make boards instead of beams.

• Hickory. Probably the strongest of North American woods. I use shagbark hickory for levers when I do megalithic stone work. It shrinks a lot. Mill it fairly soon after cutting to reduce splitting.

• Locust, Black. Very strong and heavy. The only truly decay-resistant hardwood, although this should not be a big issue, unless you want to build a pole-barn building without using pressure-treated posts. Exceptional for sills or where constant weathering can be expected. Can be very difficult to work.

• Maple, Red and Sugar. Straight and non-spiral growth trees are suitable for timber framing if worked green. If the tree grows in a spiral, as it sometimes does, expect twisting in the timbers. Quite a lot of shrinkage can be expected and rot resistance is poor.

• Oak, Red. Moderate shrinkage, strong, works well. Not as decay resistant as the white oak, so keep it off of the sills. Has an attractive grain. [2]

Softwoods

• Balsam Fir. Looks like spruce, but not as strong. Very pitchy. Balsam firs snap off like toothpicks on our property during windstorms, so I presently have a low opinion of them. Still, if you choose the timbers carefully and have the stress load calculations checked over, balsam fir can do the job.

• Cedar, Northern White. This is one of my favorites for both log-ends and as timber frame material here in Northern New York. It is plentiful and inexpensive, very easy to cut and work, and plenty strong enough for the heavy-framing applications I use it for: posts, sills, and plates. I don’t use it for joists, rafters, and unsupported girders, as white pine is stronger and is also plentiful. White cedar has a pleasant aroma, without being overwhelming.

• Cedar, Red. Very rot resistant, so a good choice for sills and exposed applications. It may be hard to find trees large enough to get a quantity of heavy timbers. Great for unmilled (round) porch posts. And aromatic.

• Hemlock, Eastern. Heavy when green. While strong on bending, hem­lock is not very strong on shear. If using hemlock for girders, be sure to have the stress load calculations double-checked for shear. Watch out for “shake,” the term for separations between annual growth rings. This is, I suspect, where the low shear strength originates. Great for posts — and any timber frame needs a lot of these — but watch out for splinters. There is something particularly painful about hemlock slivers. Sobon (1994) says, “I often relegate it to areas where hands won’t touch it.”

• Pine, Eastern White. Soft and lightweight, yet plenty strong for most timber-framing applications. A pleasure to work with. White pine was popular amongst the colonists for all building purposes. Outer layers can be quite sappy. If so, you may have to dry the milled timbers in the sun a couple of weeks before handling, a good idea in any case. [3]

white, and for this reason I now go with the white if given a choice. My local sawyer agrees. Therefore, as joists or rafters, be sure to block red pine members at each end to prevent twisting. Of course, this is always good building practice.

• Spruce, Eastern. Sobon and Schroeder (1984) say, “It is a good choice for timber frames because of its straightness, small knots, light weight, strength, and resistance to splitting.” Good recommendation. My personal experience with spruce is limited to using it as tongue-in-groove flooring and as log-ends in a cordwood wall, where it has served very well for both purposes.

• Tamarack (Eastern Larch). Another log-end favorite, but I have no personal experience with tamarack (also called larch) in timber framing. Sobon (1984) says it is “A strong wood with small knots, it grows straight, is resistant to decay, and has medium shrinkage.” As with red cedar, it may be a problem finding trees large enough.

Mass Transport in Saturated Media

Posted by on 16/ 11/ 15

Transport in saturated soil takes place in that part of soil where pores are completely saturated by water. In the road construction, this usually occurs in the subgrade but rarely in the sub-base. Three principal transport processes are defined:

• Diffusion – pollutants move from compartments with higher concentrations to compartments with lower concentrations, even if the fluid is not moving;

• Advection – pollutants are carried with the flow of the water;

• Dispersion – the pollutants are locally redistributed due to local variations in fluid flow in the pores of the soil or pavement material.

Diffusion

Diffusion will occur as long as a concentration gradient exists. The diffusing mass in the water is proportional to the concentration gradient, which can be expressed as Fick’s first law. In one dimension it is defined as

where F = mass flux of solute (units of M/L2T); Dd = diffusion coefficient (units of L2/T); C = solute concentration (units of M/L3) and dC/dx = concentration gradient (units of M/L4). The negative sign indicates that movement is from areas of higher concentration to areas of lower concentration. In the case where concentra­tions change with time, Fick’s second law applies. In one dimension it is defined as:

dC _ d2C

~dt = Dd dx2

where dC/dt denotes change of concentration with time.

Diffusion in pores cannot proceed as fast as it can in open water because the ions must follow longer pathways as they travel around grains of road material. To account for this, an effective diffusion coefficient D* is introduced. It is defined as

D* = ш Dd

where w is a dimensionless coefficient that is related to the tortuosity. Tortuosity is defined as the ratio between the linear distance between the starting and ending points of particle flow and the actual flow path of the flowing water particle through the pore space. The value of w is always less than 1 and is usually defined by diffu­sion experiments.

Advection

In the road construction, a dissolved contaminant may be carried along with flowing water in pores. This process is called advective transport, or convection. The amount of solute that is being transported is a function of the solute concentration in the wa­ter and the flux of water infiltrating from the pavement surface. For one-dimensional flow normal to a unit area of the porous media, the quantity of flowing water is equal to the average linear velocity times the effective porosity and is defined as

K dh ne dl

where v = average linear velocity (L/T); K = coefficient of permeability (i. e. hy­draulic conductivity) (L/T); ne = effective porosity (no units) and dh/dl = hydraulic gradient (no units).

Due to advection, the one-dimensional mass flux, F, is equal to the quantity of water flowing times the concentration of dissolved solids and is given as

F = v neC

One-dimensional advection in the x-direction is, then, defined as

dC _ dC

~dt = ~Vx ~dx

where vx is the velocity of flow in the x – direction. According to this one-dimensional advection equation, the mass transport in homogeneous porous media is represented with a sharp front.

Dispersion

Water in porous media is moving at rates that are both greater and less than the average linear velocity. In a sufficient volume where individual pores are averaged, three phenomena of mass transport in pores are present:

• Asa fluid moves through the pores, it will move faster in the centre of pores than along the edges;

• In porous media, some of the particles in the fluid will travel along longer flow paths than other particles to travel the same linear distance;

• Some pores are larger than others, allowing faster movement.

Due to different velocities of water inside the pores, the invading pollutant dissolved in the water does not travel at the same velocity, and mixing will occur along the flow path. This mixing is called mechanical dispersion, and it results in a dilution of the solute at the advancing edge of flow. The mixing that occurs along the direction of the flow path is called longitudinal dispersion. An advancing solute front will also tend to spread in directions normal to the direction of flow because at the pore
scale the flow paths can diverge. The result is transverse dispersion which is mixing in the direction normal to the flow path. In the road environment, the dispersal of a pollutant having penetrated into the sub-base will usually occur perpendicularly to the road course.

If we assume that mechanical dispersion can be described by Fick’s laws for diffusion and the amount of mechanical dispersion is a function of the average linear velocity, a coefficient of mechanical dispersion can be introduced. This is equal to a property of the medium called dynamic dispersivity, being a times the average linear velocity, vx.

In water flowing through porous media, the process of molecular diffusion can­not be separated from mechanical dispersion. The two are combined to define a parameter called the hydrodynamic dispersion coefficient, D:

Di = ai v + D* (6.7)

Dt = at v + D* (6.8)

where Dl = hydrodynamic dispersion coefficient parallel to the principal direc­tion of flow (longitudinal) (with units of L2/T) and Dt = hydrodynamic dispersion coefficient perpendicular to the principal direction of flow (transversal) (also units of L2/T). ai = longitudinal dynamic dispersivity and at = transversal dynamic dispersivity (both with units of L).

By the combination of the equations above and with proper initial and boundary conditions, the total mass transport of a non-reactive pollutant in two-dimensional saturated porous media can be described by an advection-dispersion equation de­fined as follows with vx being the velocity of flow in the x-direction, as above:

Often the dispersion and diffusion terms are combined with a “hydrodynamic dis­persion coefficient”, Dh=Di + Dt, being used to combine the effects of diffusion and dispersion. Various analytical and numerical solutions of the equation are possible (see, e. g., Fetter, 1993) dependent on the boundary conditions, but will generally involve a distribution of contaminants, with distance from the source and with time, according to a probability function. In practice, the advection-dispersion equation is usually solved by numerical or analytical computer methods such as Hydrus or Stanmod.

Beam and Girder Spacing for Steel Beam and Plate Girder Bridge

Posted by on 16/ 11/ 15

In regard to efficiency in the number of lines of girders in bridges consisting of multiple girders connected by cross frames, cursory cost comparisons almost always conclude that the widest spacing of girders is the most economical. Savings result not only from the reduced number of main members but also from the reduced number of secondary elements (shear connectors, cross frames, stiffeners, and bearings). However, other costs must be considered. Wide girder spacing will generally be accompanied by a wide slab overhang over the outside girders, for a balance of load on interior and exterior girders. This may necessitate extra reinforcing steel in the top of the deck slab beyond the amount required for the slab span between girders, and may require a thicker slab. Use of three lines rather than four or more puts the bridge closer to a nonredundant condi­tion. In some cases, a greater number of girders than the optimum for minimum material cost may be necessary or desirable to permit the bridge to be built in stages, or to have the deck replaced while maintaining traffic on a portion of the deck width. In general, beam and girder spacings of up to 10.0 or 12.5 ft (3 or 4 m) should be investigated for typical bridges. For economy, the size of interior and exterior stringers should be the same.

CUTTING, MARKING, AND DRILLING SILL PLATES

Posted by on 16/ 11/ 15

Just as you must follow rules when play­ing sports, there are rules—or building codes—to follow when building a house. Most codes require that every sill plate have a bolt about 1 ft. from each end and every 4 ft to б ft. between to connect the house frame securely to the foundation. So the edges of the founda­tion and the location of the bolts let you know where cuts in the sill plates should be made.

These cuts can be made by eye if you’re feeling confident in your ability to make a square cut with a power saw.

Remember, sill plates must be placed on the foundation accurately, not perfectly. This is frame carpentry, not finish work.

Sill plates normally are bolted flush to the outside of the foundation. Place the plate directly on the inside of the chalk­line and use a tape measure or a bolt marker to locate where the bolt holes are to be drilled (see the photo below). You can make a simple measuring device to mark bolt holes or buy one from Pairis Enterprises (see Sources on p. 198).

As you become more experienced, try placing the sill plate on top of the bolts and eyeball the edge of the sill flush

CUTTING, MARKING, AND DRILLING SILL PLATES

CUTTING, MARKING, AND DRILLING SILL PLATES

A simple measuring device called a bolt marker can be used to locate the bolt holes. With the sill plate lined up on the inside of the chalkline, hold the end of the marker against the bolt and mark the bolt hole. (Photo by Roe A. Osborn.)

 

CUTTING, MARKING, AND DRILLING SILL PLATES

with the chalkline or the outside of the building. Hit the sill with a hammer right over each bolt to leave a mark for drilling.

Many tasks you do in carpentry can be done by trusting the eye. Marking bolt holes in a sill plate is an example of this. Carpenters call this "eyeball carpentry." Basically, all it takes to train the eye to see accurately is practice.

With the bolt locations marked, set the sill plates on a block of wood and drill the holes. Use a 5/s-in. or n/i6-in. bit with a power drill to make holes for Уг-іп. bolts. Place the sill plates over the bolts, using your hammer to drive boards onto tight-fitting bolts. Put a washer and a nut on each bolt. Now is the time, if needed, to put pressure-treated shims under the sill plates to make sure they are level. Finish by tightening the nuts with a crescent wrench.

In colder parts of the country, you may want to lay down a thin layer of insula­tion between the sill plates and the foundation. This helps prevent air leaks.

Composite Construction for Steel Beam and Plate Girder Bridge

Posted by on 16/ 11/ 15

The concrete deck for steel beam and girder bridges may be designed and constructed on the basis of either composite or noncomposite behavior. With composite construction, the effective area of the slab can be calculated and used in determining the moment resistance of the section in positive moment regions. In negative moment regions, ten­sile stresses can be resisted by the reinforcing steel. The required number of shear connectors must be calculated and furnished. These are generally headed studs that are welded to the top flange (Fig. 4.9). Overall economy depends upon the cost of the installed shear connectors and the reduction in steel weight that can be obtained. However, composite construction is frequently the economical choice.

4.16.1 Economical Design of Steel Plate Girder Bridge

Suggestions for maximum economy of steel girder bridges may be summarized as follows[5]:

1. Load-and-resistance factor design (LRFD) is the preferred design procedure. Load-factor design (LFD) yields more economical girder designs than does allowable – stress design (ASD).

2. Properly designed for their environment, unpainted weathering-steel bridges are more economical in the long run than those requiring painting. Consider the fol­lowing grades of weathering steels: ASTM A709 grade 50W, 70W, HPS70W, or 100W. Grade 50W is the most often used.

3. The most economical painted design is that for hybrid girders, using 36-kip/in2 (248 MPa) and 50-kip/in2 (345 MPa) steels. Painted homogenous girders of 50-kip/in2 (345 MPa) steel are a close second. The most economical design with high-performance steel (HPS) will also be hybrid, utilizing grade 50W steel for all stiffeners, diaphragm members, and web and flanges, where grade 70W strength is not required. Rolled sections (angles, channels, etc.) are not available in HPS grades.

4. The fewer the girders, the greater the economy. Girder spacing must be compati­ble with deck design, but sometimes other factors, such as maintaining traffic dur­ing a future deck replacement, govern selection of girder spacing. For economy, girder spacing should be 10 ft (3 m) or more.

5. Transverse web stiffeners, except those serving as diaphragm or cross-frame con­nections, should be placed on only one side of a web.

6. Web depth may be several inches larger or smaller than the optimum without sig­nificant cost penalty.

7. A plate girder with a nominally stiffened web—1/16 in (1.6 mm) thinner than an unstiffened web—will be the least costly or very close to it. (Unstiffened webs are generally the most cost-effective for web depths less than 52 in (1320 mm). Nominally stiffened webs are most economical in the 52- to 72-in (1320- to 1830-mm) range. For greater depths, fully stiffened webs may be the most cost-effective.)

8. Web thickness should be changed only where splices occur. (Use standard – plate-thickness increments of 1/16 in (1.6 mm) for plates up to 2 in (51 mm) thick and 1/8-in (3.2-mm) increments for plates over 2 in (51 mm) thick.)

9. Longitudinal stiffeners should be considered for plate girders only for spans over 300 ft (92 m).

10. Not more than three plates should be butt-spliced to form the flanges of field sec­tions up to 130 ft (40 m) long. In some cases, it is advisable to extend a single flange-plate size the full length of a field section.

11. To justify a welded flange splice, about 700 lb (318 kg) of flange steel would have to be eliminated. However, quenched-and-tempered plates are limited to 50-ft (15-m) lengths.

12. A constant flange width should be used between flange field splices. [Flange widths should be selected in 1-in (25-mm) increments.]

13. For most conventional cross sections, haunched girders are not advantageous for spans under 400 ft (122 m).

14. Bottom lateral bracing should be omitted where permitted by AASHTO specifica­tions. Omit intermediate cross-frames where permitted by AASHTO, but indicate on the plans where temporary bracing will be required for girder stability during erection and deck placement. Space permanent intermediate cross frames, if required, at the maximum spacing consistent with final loading conditions.

15. Elastomeric bearings are preferable to custom-fabricated steel bearings.

16. Composite construction may be advantageous in negative moment regions of composite girders.

Designers should bear in mind that such techniques as finite-element analysis, use of

high-strength steels, and load-and-resistance-factor design often lead to better designs.

Consideration should be given to use of 40-in-deep (1016-mm) and 44-in-deep (1118-mm) rolled sections. These may be cost-effective alternatives to welded girders for spans up to 100 ft (30 m) or longer. Economy with these beams may be improved with end-bolted cover-plate details. Equally important is the availability of material, either in the form of rolled beams or plates. Long-lead items may cause schedule delays and contractor claims, which increase the cost of construction. Contract documents that allow either rolled beams or welded girders ensure cost- effective alternatives for owners.

With fabricated girders, designers should ensure that flanges are wide enough to provide lateral stability for the girders during fabrication and erection. Flange width should be at least 12 in (305 mm), but possibly even greater for deeper girders. The AISC recommends that, for shipping, handling, and erection, the ratio of length to width of compression flanges should be about 85 or less.

Designers also should avoid specifying thin flanges that make fabrication difficult. A thin flange is subject to excessive warping during welding of a web to the flange. To reduce warping, a flange should be at least 3/4 in (19 mm) thick.

To minimize fabrication and deck-forming costs when changes in the area of the top flange are required, the width should be held constant and required changes made by thickness transitions.

To get cost-effective results from the many different designs of fabricated girders that can satisfy the requirements of specifications, designers should obtain advice from fabricators and contractors whenever possible.

Evaluating Recycled Timbers

Posted by on 16/ 11/ 15

Recycled timbers should be carefully evaluated before you buy them or agree to dismantle a building, so carry your wish list (more correctly called a timber schedule) with you to the procurement site. Firstly, timbers have to be of sufficient sectional size and length to do the job. Use actual usable length,
allowing for damaged ends or unusable ends due to mortise and tenon joints.

Подпись: Fig. 3.1: The author uses a knife to probe an old timber for soft or deteriorated wood. This one was a reject. Once you’ve established that you’ve got something potentially worthwhile, you need to look at each piece carefully for defects that might compromise their use on your project. With barn beams especially, all deterioration is not necessarily obvious: use a sharp knife to poke all four sides to check for soft wood. Reject any soft or “punky” pieces. Sometimes, you might get a good seven-foot post out of a twelve – foot beam, and that’s the best you can do. Catalog the piece as a good seven – footer. Do you need ten seven-foot posts? Well, here’s one of them.

Watch out for a mortise carved out of the middle of a timber that you want to use as a girder, as the void will greatly diminish bending strength. However, you might be able to use the piece as a post, or as a girt which will eventually be supported along its length by an intermediate post, or cordwood infilling.

Old timbers that have been under cover are likely to be in good condition, as are those fresh from a demolition site. However, timbers left out in the open for a year or two are almost certain to have begun a process of deterioration. It is so sad to see a beautiful old hand-hewn timber on the ground, and then to turn it over and find an inch of rotted wood on the underside. Even here, though, there is an exception. My friend Bob has some old virgin-growth heartwood timbers — twelve-by-twelves and the like — which have been lying on the ground for years and are still in excellent condition. I saw them myself, and was amazed. They just don’t make timbers like these anymore!

The use of recycled timbers may not be allowed in some code enforcement jurisdictions, because the timbers are not “graded” according to the building code — a subject already discussed in Chapter i. You need to know that you’ll be allowed to use the timbers before you spend a lot of time and money on them. As for stress load calculations, it may be hard to even judge the species of a ioo – year-old timber, never mind its grade. If we are planning to use the piece as an

important girder, it is best to use a conservatively low value as the unit stress rating for both shear and bending. If you see close-grained timbers with small knots or no knots, you know that these timbers are likely to be really strong. They’re likely to be really heavy, too.

Greek philosophers and thinkers of the classical era

Posted by on 16/ 11/ 15

The hydraulic works of Greece during the classical era do not measure up to those of the Mycenaen era. The few extracts cited above also show that in Greece, hydraulic know-how lags behind that of the Orient. And yet this classical Greece is known to us as the privileged cradle of development of philosophical and mathematical thought. Two facts are important in this regard. First, Greek science at this period took it as a point of honor to be disconnected from engineering, i. e. not to be associated with prac­tical applications. Second, and it is here that one can effectively and for perhaps the first time refer to science, the “sages” sought to apply reasoning to the explanation of great natural phenomena.

Such explanations of natural phenomena are often incomplete, and sometimes false. Regarding the hydrologic cycle, the brilliant mathematician and astronomer Thales of Miletus (610 – 545 BC?) – who had established that the world is round – postulated that water is the primordial substance and that the land masses float upon it. This is a theo­ry perhaps inspired by the ancient Mesopotamian and Egyptian cosmologists. Later, Anaximandre of Miletus (610 – 545 BC) identifies the origin of precipitation as water torn from the earth’s surface by solar action. Xenophane of Colophon (570 – 475 BC?) writes that the sea, source of all water, is also the origin of clouds and winds, that fresh water comes from evaporation of the sea, that rivers result from rain, and that it is by rinsing the ground that watercourses carry salt to the sea. The hydrologic cycle is there­fore fairly well understood in Greek civilization of the 6th century BC, except for the role of groundwater. Indeed, during this period it was thought that the water in springs and wells comes from the sea.

Plato (427 – 347 BC?) adopts the idea, articulated before him by Empedocle of Agrigente, of the four primordial elements: water, air, fire, and earth. His student Aristotle (385 – 322 BC), born in Macedonia, frequents Plato’s Academy until the death of the master, and becomes preceptor of Alexander at the court of Philippe of

Подпись:Macedonia. At Athens he creates the institution called the Lyceum. Aristotle in his turn adopts the theory of the four ele­ments, a theory that will remain a standard reference until the Middle Ages. He believed that each of these elements tends to return to its natural place, e. g. water below the ground, air above water, etc. Thus, for Aristotle, wood floats because it contains air, and it is in the natural order of things that air be above water. To each element is attributed two of the four fun­damental “qualities”: cold, moistness, heat, and dryness (Figure 4.16).

Attempts to explain nature from such a postulate can obviously lead to absurd con­clusions. Here is how Aristotle, reflecting his ignorance of what we now call inertia, shows in his Physics that a vacuum cannot exist, and in so doing sets down an absurd theory of wind resistance. First, in virtue of the natural “position” of each element, as we have seen above, if an object (for example a stone thrown by a man) moves through the air, “outside” of its natural position, there is an action, a “motor” that sustains its movement:

. .earth and all other bodies should remain in their proper places and be moved from them only by violence. J

Then, since “it seems that everything in motion is impelled by something”, the stone that continues its movement through the air must be sustained, in its movement, by the air itself:

”(…) things that are thrown move (although that which gave them their impulse is not touch­ing them, either by reason of mutual replacement, as some maintain, or because the air that has been pushed pushes them with a movement quicker than the natural locomotion of the projectile wherewith it moves to its proper place.”[161] [162]

Aristotle continues his demonstration in postulating that in a vacuum, all movement would be impossible – i. e. in the absence of this air that “maintains” movement. Therefore “there is no vacuum separate from things”.

At about this same time Pytheas (380 – ? BC), of the Phoenician colony of Massalia (Marseilles), sets out on a voyage that leads him to the great north, probably as far as Iceland. During this voyage he establishes a correlation between the periodic phenom­enon of the tides and lunar cycles.

Overall, one can only admire this quest for truth as representing progress and prom­ise for the future. Yet it is also clear that during the Greek classical era, hydraulic the­ories are, at best, rudimentary. It is not until the Hellenistic period and the intellectual movement coming out of the school of Alexandria that mathematics and the faculty of reasoning lead to practical innovation.

Physical Processes

Posted by on 16/ 11/ 15

Many physical processes in the pavement, the embankment and the road environ­ment influence the flow of water away from the road surface. Pollution transport is heavily influenced by the physical and chemical characteristics of the specific pollutants. It is also strongly influenced by the interaction between pollutants, and with materials making up the pavement and embankment. During their transport, pollutants interact with materials in the solid, liquid and gaseous form.

The physical processes of the movement of pollutants in and by fluids in roads and their environment can be described in terms of pollutant mass transport. Mass transport can take place in solution, in suspension or in the form of particulate mat­ter. There are great differences between these processes, and they also vary between the unsaturated and the saturated part of the road construction. The majority of pol­lutant transport from the road surface is via surface runoff towards the soil, surface water and groundwater. Part of the precipitation falling on the pavement surface is infiltrated through the pavement or the soil adjacent to the pavement. In the be­ginning, the infiltrated water flows more or less vertically through the unsaturated zone. The nature of this flow mainly depends on the road geometry and the materials used in the road construction. Once having reached the groundwater, the infiltrated water will follow the direction of the groundwater flow that is usually more or less horizontal.

In porous pavements, in the embankment and in the soil adjacent to the road, the transport of pollutants is usually in water solution. Pavement cracks also allow transport of pollutants in the particulate form. Due to the clogging of pore spaces with particulates, however, this kind of transport is stopped some ten centimetres below the pavement surface (Brencic, 2007 pers. comm.; cf. Khilar & Fogler, 1998).