Reading a Cable

Cables provide a lot of information in the abbreviations stamped into their sheathing; for example, NM indicates nonmetallic sheathing, and UF (underground feeder) can be buried. The size and number of individual conductors inside a cable are also noted: 12/2 w/grd or 12-2 W/G, for example, indicates two insulated 12AWG wires plus a ground wire. Cable stamped 14/3 W/G has three 14AWG wires plus a ground wire. (The higher the number, the smaller the wire diameter.) The maximum voltage, as in 600V, may also be indicated.

Individual wires within cable have codes, too. T (thermoplastic) wire is intended for dry, indoor use, and W means "wet"; thus TW wire can be used in dry and wet locations. H stands for heat-resistant. N, for nylon jacketed, indi­cates a tough wire than can be drawn through conduit without being damaged.

Finally, make sure the cable is marked NM-B. Cable without the final "-B" has an old-style insu­lation that is not as heat resistant as NM-B cable.

CABLE, CONDUIT, AND WIRING

Most house wiring is flexible cable, but you may find any—or all—of the wiring types described here. Inside cables or conduits are individual wires, or conductors, that vary in thickness according to the amperage of the current.

► Nonmetallic sheathed cable (NM or Romex) is by far the most common flexible cable. Covered with a flexible thermoplastic sheathing, Romex is easy to route, cut, and attach.

► Metal-clad cable (MC) is often specified where wiring is exposed. Note: Some codes still allow armored cable (AC), but that’s increasingly rare.

► Conduit may be specified to protect exposed wiring; it is commonly thin-wall steel (EMT), aluminum, or PVC plastic. Metal conduit serves as its own ground. Apart from service entrances, conduit is seldom used in home wiring. When connected with weather – tight fittings and boxes, conduit can be installed outdoors.

► Knob-and-tube wiring (see the top photo on p. 15) is no longer installed, but there’s still plenty of it in older houses. If its sheathing is

Подпись: There are hundreds of options for outlet boxes. The sample of blue plastic boxes at right shows [from top) one-gang cut-in, one-gang adjustable, two-gang nail-on, two-gang cut-in, two-gang adjustable, and three-gang boxes. The multicolored boxes in the middle are ceiling boxes. Подпись: Г~Iimage478

intact and not cracked, it may still be serviceable. You may even be able to extend it, but have an electrician do the work. Knob-and – tube is eccen­tric, requiring experience and a skilled hand.

Install ridge shingles and the ridge vent

At the ridge, many builders install ridge shin­gles on the roof ends with a ridge vent between them. An alternative is to install the ridge vent across the entire roof, even though the ends of the vent (located over the gable-end overhangs) are not functional. Some ridge vents do not require a cap of roof shingles, but others do. No matter which type of ridge vent you use, follow the manufacturer’s instructions regarding its installation (see the top right photo on p. 143).

Ridge shingles are easy to make—simply cut regular three-tab shingles into three pieces, as shown in the bottom right photo on p. 143. Lay the shingle upside-down on a piece of ply­wood and cut it with a utility knife. Start at the top of a slot and angle inward slightly in both directions toward the top of the shingle, cutting out a small triangle of waste. These shingles cover the ridge at both ends of the roof and are overlapped to show a 5-in. reveal, just like regular shingles. The angled portion of each ridge shingle is covered by the exposed part of the next shingle.

To ensure that both the ridge shingles and the ridge vent are installed straight, I like to snap a blue chalkline about 5 in. to 6 in. down one side of the ridge. No one but the eagles may see this, but it only takes a couple of minutes to do it correctly, and it’s important to develop good habits. Fasten the ridge shingles securely with L/2-in. roofing nails. Some ridge vents must also be installed with long nails. Cover exposed nail heads with a good-size dab of roofing tar.

Nice work! This peak experience gives the house its most important protection from the elements. When a roof is installed properly, you don’t have to worry about it for a long time.

Подпись: Photo cour tesy HFHI Подпись: Photo courtesy HFHIInstall ridge shingles and the ridge ventHabitat

"111 for Humanity®

PLAY US A TUNE

Before Katrina hit, Habitat affiliates in the Gulf were building 60 houses a year. After the storm, those same groups were building that many houses in a month! Mobilizing some 70,000 volunteers in the storm’s aftermath, Habitat for Humanity has completed or begun construction on more than 1,200 homes as of this writing in the Gulf Coast, and there’s no end in sight.

In New Orleans, hit hard by the hurricane, the New Orleans Area Habitat for Humanity is playing a key role in rebuilding their city. It has expanded its operations in many parishes outside the city, including St. Bernard parish, which sustained damage to nearly every structure within its limits,
and is also committed to the development of the celebrated Musicians’ Village.

Seeing hundreds of Crescent City musicians lose their homes and livelihoods because of the storm’s devastation, singer Harry Connick Jr. and jazz saxophonist Branford Marsalis teamed up with Habitat to do something about it.

Designed to foster the sounds and songs that make New Orleans unique, the Musicians’ Village consists of single-family homes and duplexes that will house musicians as well as residents who want to be part of this musically inclined neigh­borhood. To top it off, the Ellis Marsalis Center for Music is being built in the heart of the community. Part performance hall, part teaching facility, the center’s goal is to bring musicians young and old together to celebrate the rich musical heritage of New Orleans.

After so much devastation and upheaval, restoring New Orleans will not happen overnight. But if the joint efforts of Habitat and the city’s citizens are any indication, the spirit of the city is alive and singing. —Dave Culpepper

Install ridge shingles and the ridge vent

і

Mechanical Spreading

Nowadays, mechanical placement is the only reliable method of executing an SMA layer. Requirements for pavers can very rarely be found in the specifications.

Basically, selection, setting, and operation of a paver are the responsibilities of the paving contractor. Most road-engineering companies, after gaining experience with SMA pavements, work out their own procedures for spreading, compacting, and achieving the required parameters. The following information might help to establish or improve such procedures.

PARKING

Automobile parking poses a significant land use problem in subdivision planning. In the recent past, common practice provided for wide local streets, often capable of accommodating a row of parked cars on each side in addition to two lanes of moving traffic. Such parking space has often been provided where there are also private driveways and other off-street parking that can accommodate several cars. Good planning can reduce this heavy commitment of land to parking without sacrificing adequate accommodation of vehicles.

Following are guidelines for parking:

• Provide off-street parking areas whenever possible.

• Use common driveways. ‘

• Design paving thickness to meet actual parking load requirements rather than to general standards.

• Eliminate curbs and gutters in parking areas.

• If curbs must be built, use roll curbs or other alternatives to standard requirements.

• If street parking must be used, limit such parking to one side of the street.

• Use unpaved shoulders for parking to reduce road pavement width.

• Consider traditionally unused space, such as in a cul-de-sac or court, for parking.

Подпись: Off-Street ParkingPARKINGReduction of street width reduces both the direct costs of street construction and maintenance, and the indirect cost of unnecessary land use. Elimination of one or both parking lanes along as many streets as possible through off-street parking makes a major contribution to the achievement of these savings.

Off-street parking can be accom­modated by various types of common parking areas. Townhouses or clusters lend themselves well to these solutions.

Common off-street parking

PARKING

PARKING

Detached units can often share a driveway, eliminating additional curb cuts and their associated costs. The necessary width of a common driveway may vary according to the number of units being served, but should generally be no wider than the usual width of a single driveway.

Подпись: ConstructionTwo significant variables in the construction cost of parking areas are pavement thickness and require­ments for curbs and gutters. Although local requirements for pavement design and curb and gutter construction usually do not apply to private driveways, many do apply to common parking areas. ‘

Pavement thickness should be based on anticipated usage, both with regard to volume and to loadings. Standards that apply to roads and highways are rarely appropriate for residential parking areas.

Typical community standards for residential parking areas specify a minimum base of 4 to 6 inches. However, a 2-inch base of crushed stone is frequently adequate. As is discussed in the section on Streets, the nature and condition of the subsoil must be considered.

Another factor is the question of whether the parking area will be used by heavy vehicles, notably trash trucks. Placement of trash dumpsters and routes for heavier vehicles can be planned to minimize the amount of pavement that such vehicles will traverse, and that must be strength­ened to accommodate them.

Curbs and gutters can be eliminated in parking areas; stormwater can be diverted and drained off by sheet flows and swales. Where curb and gutter requirements exist, relatively inexpensive approaches such as roll curbs, extruded asphalt curbs, wheel stops, and integral curbs and sidewalks can be considered in place of more costly approaches. More detailed information is provided in the sections on Curbs and Gutters, and Stormwater Drainage.

PARKING

Подпись: On-Street ParkingWhere it is not practical to accom­modate part or all of residential parking by off-street facilities, the street must be used. However, the need for street parking must be evaluated on an individual basis. Consideration shpuld be given to confining such parking to one side or to parking on road shoulders, reducing street pavement width.

The center of a court or bulb cul-de – sac can accommodate additional parking without increasing street dimensions. A cjuick and relatively simple method is to "stripe” or paint additional parking spaces in the center of the bulb.

Mixture Temperature during Compaction

Almost all publications on SMA underline the necessity of carefully observing the temperature of the mixture during placement and rolling. The expected range of mixture temperature is determined in different ways; it chiefly depends on the kind of binder, but such factors as the layer thickness and weather conditions are impor­tant, too. However, the most important factor is the temperature of the mixture deliv­ered to the construction site and the temperature at the end of effective compaction, below which further rolling becomes ineffective and even harmful.

Minimum temperatures for mixture supplied to a work site according to the European standard EN 13108-5 (which applies only to selected [unmodified] bitu­mens after EN 12591) are as follows:

• 160°C for paving grade bitumen 40/60

• 150°C for paving grade bitumen 50/70

• 140°C for paving grade bitumen 70/100

In the German DAV SMA handbook (Druschner and Schafer, 2000), the afore­mentioned temperatures are presented in a more general way; the suggested tem­perature of an SMA mixture in a paver hopper should not be lower than 150°C. The same rule is presented by Bellin (1997).

Different temperatures at the end of the compaction time have been assumed in various publications, from 80-100°C for ordinary binder, and from 120-138°C for modified ones. The U. S. NAPA SMA Guidelines QIS 122 stipulates no rolling when the temperature of a layer drops below 116°C. A temperature of about 100°C has been stated in German documents as the point at which to stop rolling. The mini­mum temperature at the end of the compacting time may be roughly calculated by adding 50°C to the Ring and Ball (R&B) softening point of the binder used in the mix (Daines, 1985; Read and Whiteoak, 2003).

Other relevant points include the following:

• Problems related to a mix temperature that is either too low or too high are elaborated on in Chapter 11. Additional comments may be found in Section 10.4.2.5, which deals with rolling time.

• Optimum compaction temperatures are related to the viscosity of the added binder. That implies the significance of not only the lower temperature limit of rolling but also the initial temperature of rolling (already described while discussing SMA laydown on a hot underlying layer). A mixture that is too hot also causes problems at placement.

• Remember that spreading mixtures with substantial temperature differ­ences (e. g., from a truck with a hot mixture alternated with another truck with a cool mixture) cause changes in the resistance offered by such mix­tures at spreading.

• Appearing here and there in a layer being placed, pieces of a cool mixture may cause the development of an increased content of air voids and hence decrease the pavement’s lifespan (Pierce et al., 2002).

All these remarks concerning temperatures do not apply to cases that involve the use of special additives for lowering mixture temperatures that create the so-called warm mixes.

Embankment Construction

Where a pipe is required as part of an embankment construction, it may be installed by compacting layers of fill uniformly on either side. It is important to bring the layers up uniformly on either side of the pipe. After a sufficient layer is compacted over the top of the pipe, ordinary embankment construction may proceed. Alternatively, some agencies require that the embankment be constructed first, then a trench dug for the installation of the pipe.

5.10.1 Trench Construction

The open-trench method is commonly used for culvert construction. It is more cost – effective than tunneling except when a pipe must be constructed in an existing high fill. Shoring may be necessary, particularly if the installation is under a traveled way. This will keep the limits of excavation to a minimum and, by the use of steel cover plates, allow the roadway to remain open during nonworking hours. Where it is necessary to use an open-trench method of construction in urban areas, it is wise for the designer to make available to the contractor options for the type of structure to be placed. For example, if a box culvert is deemed necessary by the engineer because of hydraulic considerations and physical constraints, a precast concrete or a prefabricated metal box, as alternatives to cast-in-place construction, should be permitted. In this manner, the traveling public expe­riences a minimum of disruption of service when open-trench construction is used. AASHTO recommends a trench width equal to 1.25 times the outside diameter of the pipe plus 1 ft (300 mm) for concrete pipe and a width to provide for 2 ft (600 mm) mini­mum on each side of the pipe for flexible culverts. However, some states simply recom­mend a constant clearance between the outside of the pipe and the trench wall to ensure that there is room for compaction and compaction-testing equipment.

CONSTRUCTION METHODS

Underground structures may be built by a variety of means including embankment construction, open-trench construction, jacking, tunneling, and microtunneling.

The proper design and installation of the foundation, bedding, and backfill for embankment and trench installations are critical to the performance of underground structures. They are also essential factors for achieving an accurate structural analysis of the system. The foundation preparation, bedding, and backfill of underground structures should be done in accordance with standards established by local and state transportation agencies. These standards vary from region to region, but the important aspects of typical practices are reviewed below.

Regardless of whether the pipe is installed in an embankment or a trench, the foun­dation must provide relatively uniform resistance to loads. If rock is encountered, it should be excavated and replaced with soil. If soft material is encountered, it should be removed for a width of three pipe spans and replaced with suitable material. Care must be taken to ensure that the foundation under the pipe is not stiffer than the adjacent zones, because this will attract additional load on the pipe.

The bedding is then placed above the foundation. Bedding thickness and material is contingent upon the type of pipe and the quality of the installation required. Pipe-arch structures require excellent soil support at the corners, because pressures are higher there. For most applications 3 to 6 in (75 to 150 mm) of bedding is sufficient. Some agencies require a shaped bedding for all pipe because of the difficulties in compacting the backfill in the haunch area. More recently, for most round pipes, in lieu of a shaped bedding, specifications call for the bedding under the middle one-third of the pipe diameter to be left uncompacted. This is so that the pipe can properly seat itself in the bedding, resulting in a greater length of support along the bottom circumference of the pipe. Pipe arches and large span structures should always be placed on a shaped bedding.

The backfill should be placed in 6- to 8-in (150- to 200-mm) compacted layers around the structure. Each backfill layer must be compacted to the minimum density required in the construction specifications. Densities less than 90 percent standard Proctor density should not be permitted. The backfill must be kept in balance on each side of the pipe. A granular material free of organic content and with little or no plasticity makes good backfill.

Complete installation requirements for the various pipe materials can be found in AASHTO, ASTM, and state DOT specifications.

TIP

In some very old houses, you may find that the neutral wires were attached to a switch— rather than the hot wires, as required by codes today. Thus, when working on old switches or fixtures, test all wires for current. Even if you’ve flipped a fixture switch off, there could still be a hot conductor in the fixture outlet box.

llll

image476
Подпись: TIP
Подпись: Miscellaneous tools. From left:two slot-head screwdrivers, tapper (to cut threads in metal box holes), offset screwdriver, nut driver, utility knife, small pry bar/nail puller, plaster chisel, drywall saw, and hacksaw.

Подпись:

when an outlet is too distant. Cordless tools now have all the power you could want. Besides, they don’t need an extension cord and won’t electrocute you if you inadvertently drill or cut into a live wire. Cordless reciprocating saws can cut anything from plaster lath to studs; but use

a cordless jigsaw if you want to preserve the plaster around a cut-in box opening.

Miscellaneous tools. Other necessary tools include a hammer, tape measure, Speed Square, hacksaw, plaster chisel, drywall saw, nut driver, small pry bar, and spirit level.

Electrical cable. From top:type NM (Romex), type UF (underground), armor clad (AC), and metal clad (MC). Note: The silver wire in the AC cable is a bonding wire, not a ground. In the MC cable, the green wire is ground, the white is neutral, and the red and black are hot.

 

image477

Design Considerations

For a waterway crossing, the designer must consider the backwater elevation and flow velocity for both the proposed and existing structures. It is recommended that the same hydraulic model be utilized for both the existing and proposed structure. Any increase in backwater elevation or stream velocity must be thoroughly analyzed and the upstream and downstream effects considered. For a grade separation structure the designer must consider both horizontal and vertical clearances. The shape of the replacement structure must be considered when determining the minimum clearances.

It is imperative that an accurate and complete survey of the existing structure be conducted. This will aid the designer in determining the maximum prefabricated struc­ture size that can be installed at a particular site.

In certain situations it may be possible to reuse portions of the existing structure in the design of the replacement structure. The most obvious example is reuse of the existing foundation. If the foundation type is known (i. e., concrete spread footer, con­crete on piling, etc.) standard geotechnical engineering calculations for assessing the suitability of the foundation must be completed. The designer is cautioned against using existing unknown foundation types.

One of the primary benefits of utilizing a prefabricated culvert as a bridge replace­ment is that much of the existing structure can remain in place. This reduces construction time and reduces the work limits required for the structure installation. For single-span structures with vertical wall-type abutments, it is typical to leave the existing abutments in place. It may also be possible to leave the deck in place. For multiple-span structures, existing abutments, piers, foundations, and deck may all be left in place depending on site constraints. The required size of the replacement structure, along with site access will typically control how much of the existing structure can be left in place.

Another consideration for the designer is the void space between the existing and proposed structure. If there is insufficient void space to properly place, compact, and test soil backfill, the use of flowable fill is common. Where flowable fill is utilized, it is recommended that the proposed structure size be maximized. This is because the cost of the additional structure size is typically far less expensive then the cost of the flowable fill.

Lastly the designer must determine the structural capacity of the replacement struc­ture and the existing structure. If the two structures are very close or if the existing deck is left in place, then the composite strength of the two may be considered. The finite element method is well suited for this complex analysis. In the absence of sophisticated computer methods, the designer can conservatively ignore the contribu­tion of the existing structure. However, typical design assumptions regarding sur­rounding soil support must be verified prior to the use of the closed form design methodologies presented in Art. 5.8. The designer must also consider external grout­ing pressures when flowable fill is used as the backfill material.

Orthogonal Transformation Techniques

The orthogonal transformation is an important tool for treating problems with correlated stochastic basic variables. The main objective of the transformation is to map correlated stochastic basic variables from their original space to a new domain in which they become uncorrelated. Hence the analysis is greatly simplified.

Orthogonal Transformation Techniques

Consider K multivariate stochastic basic variables X = (Xі, X2,, XK)t having a mean vector fj, x = (jx1, /г2 …, /гкУ and covariance matrix Cx as

 

011 012 013 021 °22 023

 

01K

02K

 

Cr =

 

0K1 Ok2 0K3

 

okk

 

in which oij = Cov(Xi, Xj), the covariance between stochastic basic variables Xi and Xj. The vector of correlated standardized stochastic basic variables X’ = D-1/2(X – fxx), that is, X’ = (X1, X2,…, XKУ with Xk = (Xk – /xk)/ok, for k = 1,2,…, K, and Dx being an K x K diagonal matrix of variances of stochastic basic variables, that is, D x = diag(o12, o|, …, o^), would have a mean vector of 0 and the covariance matrix equal to the correlation matrix Rx:

 

Orthogonal Transformation Techniques

Y = T-1X ’ (4C.1)

where Y is a vector with the mean vector 0 and covariance matrix I, a K x K identity matrix. Stochastic variables Y are uncorrelated because the off- diagonal elements of the covariance matrix are all zeros. If the original stochas­tic basic variables X are multivariate normal variables, then Y is a vector of uncorrelated standardized normal variables specifically designated as Z’ be­cause the right-hand side of Eq. (4C.1) is a linear transformation of the normal random vector.

It can be shown that from Eq. (4C.1), the transformation matrix T must satisfy

 

Rx = TTt

 

(4C.2)

 

There are several methods that allow one to determine the transformation matrix in Eq. (4C.2). Owing to the fact that Rx is a symmetric and positive – definite matrix, it can be decomposed into

Rx = LLt (4C.3)

in which L is a K x K lower triangular matrix (Young and Gregory, 1973; Golub and Van Loan, 1989):

l11 0

0

. . . 0 ‘

l21 l22

0

. . . 0

Ik 1 Ik2 Ik3

. . . Ikk _

L

which is unique. Comparing Eqs.(4C.2) and (4C.3), the transformation matrix T is the lower triangular matrix L. An efficient algorithm to obtain such a lower triangular matrix for a symmetric and positive-definite matrix is the Cholesky decomposition (or Cholesky factorization) method (see Appendix 4B).

The orthogonal transformation alternatively can be made using the eigenvalue-eigenvector decomposition or spectral decomposition by which Rx is decomposed as

Rx = Cx = VAVt (4C.4)

where V is a K x K eigenvector matrix consisting of K eigenvectors as V = (v i, v 2,…, vK), with vk being the kth eigenvector of the correlation matrix Rx, and Л = diag(X1, Л2,…, XK) being a diagonal eigenvalues matrix. Frequently, the eigenvectors v’s are normalized such that the norm is equal to unity, that is, vt v = 1. Furthermore, it also should be noted that the eigenvectors are or­thogonal, that is, v t v j = 0, for i = j, and therefore, the eigenvector matrix V obtained from Eq. (4C.4) is an orthogonal matrix satisfying VVt = Vt V = I where I is an identity matrix (Graybill, 1983). The preceding orthogonal trans­form satisfies

Vt Rx V = Л (4C.5)

To achieve the objective of breaking the correlation among the standardized stochastic basic variables X’, the following transformation based on the eigen­vector matrix can be made:

U = VtX’ (4C.6)

The resulting transformed stochastic variables U has the mean and covariance matrix as

E(U) = V tE(X’) = 0 C (U) = VtCx V = Vt RxV = Л

 

(4C.7a)

(4C.7b)

 

and

 

As can be seen, the new vector of stochastic basic variables U obtained by Eq. (4C.6) is uncorrelated because its covariance matrix Cu is a diagonal ma­trix Л. Hence, each new stochastic basic variable Uk has the standard deviation equal to V^k, for all k = 1, 2,…, K.

The vector U can be standardized further as

Y = Л-1/2и (4C.8)

Based on the definitions of the stochastic basic variable vectors X – (vx, Cx), X’ – (0, Rx), U – (0, Л), and Y – (0,1) given earlier, relationships between them can be summarized as the following:

Y = Л-1/2и = Л-1/2 V1X’ (4C.9)

Comparing Eqs.(4C.1) and (4C.9), it is clear that

T-1 = Л-1/2 V1

Applying an inverse operator on both sides of the equality sign, the transfor­mation matrix T alternatively, as opposed to Eq. (4C.3), can be obtained as

T = VЛ1/2 (4C.10)

Using the transformation matrix T as given above, Eq. (4C.1) can be expressed as

X ‘ = TY = VЛ1/2Y (4C.11a)

and the random vector in the original parameter space is

X = vx + D1/2 VЛ1/2Y = vx + D1/2 LY (4C.11b)

Geometrically, the stages involved in orthogonal transformation from the orig­inally correlated parameter space to the standardized uncorrelated parameter space are shown in Fig. 4C.1 for a two-dimensional case.

From Eq. (4C.1), the transformed variables are linear combinations of the standardized original stochastic basic variables. Therefore, if all the original stochastic basic variables X are normally distributed, then the transformed stochastic basic variables, by the reproductive property of the normal random variable described in Sec. 2.6.1, are also independent normal variables. More specifically,

X – N(vx, Cx) X’ – N(0, Rx) U – N(0, Л) and Y = Z – N(0,1)

The advantage of the orthogonal transformation is to transform the correlated stochastic basic variables into uncorrelated ones so that the analysis can be made easier.

Orthogonal Transformation Techniques
The orthogonal transformations described earlier are applied to the stan­dardized parameter space in which the lower triangular matrix and eigenvector matrix of the correlation matrix are computed. In fact, the orthogonal transfor­mation can be applied directly to the variance-covariance matrix Cx. The lower triangular matrix of Cx, L, can be obtained from that of the correlation matrix L by

L = D1/2 L (4C.12)

Following a similar procedure to that described for spectral decomposition, the uncorrelated standardized random vector Y can be obtained as

Y = Л-1/2 Vг (X – цх) = Л-1/2£7 (4C.13)

where V and Л are the eigenvector matrix and diagonal eigenvalue matrix of the covariance matrix Cx satisfying

Cx = УЛ V/1

and U is an uncorrelated vector of the random variables in the eigenspace having a zero mean 0 and covariance matrix Л. Then the original random vector X can be expressed in terms of Y and L:

X = fix + VA1/2Y = fix + L Y (4C.14)

One should be aware that the eigenvectors and eigenvalues associated with the covariance matrix Cx will not be identical to those of the correlation matrix Rx.