Hierarchy

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Good home design entails a lot of categorizing. The categories we use are determined by function. In organizing a home, everything that is used to prepare food would, for example, most likely go into the "kitchen” category. If something in the kitchen category functions primarily to wash dishes, it would probably be placed into the subcategory of "kitchen sink area.” The categories proposed by our predecessors usually serve as pretty good tools for organizing a home. Ideas like "kitchen,” "bathroom,” and "bedroom” stick around because they generally work. But these ideas cannot be allowed to dictate the ultimate form of a dwelling; that is for necessity alone to decide.

Sacred Geometry

Organizing the tops of windows and doors along a horizontal axis and deliberately spacing porch posts in a row are examples of the ways alignment and proportion can be consciously used to create a structure that makes visual sense. Less obvious examples become apparent when regulating lines are drawn on photos of a building’s fa­cade. These lines are stretched between significant elements, like from the peak of the roof to the cornerstones, or from a keystone to the baseplates. When geom­etry has been allowed to dictate the rest of the design, the lines will almost invariably intersect or align with other crucial parts of the build­ing. The intersections are often unexpected, their appearance the unintended biproduct of the cre­ative process described on these pages.

Do not think that, just because our shared idea of "bathroom” includes a bath, a sink and a toilet, that these things must always be grouped together behind the same door. The needs of a particular household may determine that each be kept separate so that more than one can be used at a time. What is more, if the kitchen sink is just outside the door to the toilet, then a separate basin may not be necessary at all. The distinctions made between the categories of "living room,” "family room” and "dining room” might well be combined into the single category of "great room” for further consolidation.

Vernacular designers do not thoughtlessly mimic the form of other buildings. They pay close attention to them, use what works in their area, and improve upon what does not.

Along with all the categorizing that goes on during the design process, there is a lot of prioritizing that has to be done as well. The relative importance of a room and the things in it can be underscored by size and placement. The most important room in a small house, in both the practical and the symbolic sense, is almost always the great room or its farmhouse kitchen equivalent. To make its importance all the more clear, this area should occupy the largest share of the home and should be prominently located. In a small dwelling, it is generally best to position this space near the home’s center, so that small­er, less significant rooms can be arranged around its periphery as alcoves.

Arranging the rooms and objects in a house according to their relative impor­tance is essential to making any space readable. Presenting such a hierar­chy may require that some doorways be enlarged to exaggerate one room’s significance, or that a ceiling be lowered to downplay another’s. As always, necessity will determine these things inasmuch as it is allowed to.

Finishes

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Introduction

Finishes include all surface materials and treatments in the home. They are what is seen on a daily basis and, along with furnishings, constitute the personal signature of the owner. Finishes are the predominant source of odors in a new home. They can introduce a multi­tude of toxic volatile organic compounds into the air and will continue to volatilize, or out – gas, for years after the home is completed. However, when chosen carefully, finishes can enhance health and well-being as well as add to the aesthetic value of the home.

Until recently, nonpolluting finishing products were considered specialty items. Fortunately, healthier products are now regu­larly appearing on the market. Many of these are easily accessible, cost competitive, and comparable in performance to their more toxic counterparts. Some even have the ability to improve air quality by sealing in toxins that may be present in underlying materials.

In some regions, traditional nontoxic fin­ish materials are readily available and widely used. In the Southwest, for example, tile, stone, natural wood, and plaster are commonplace, whereas in many regions of the country they have been replaced even in custom homes by wall-to-wall carpeting, vinyl wall coatings, laminate cabinetry, and other synthetic sub­stitutes. When you build a healthy home, we encourage you to take full advantage of the traditional materials native to your region.

Plaster

Plaster generally provides a healthful interior wall finish. Because of the labor and skill in­volved in its application it is a more expensive finish, but it is much sought after for its beauty. Plaster has the ability to block the small amount of VOCs present in the gypsum lath and taped joints that it covers in frame con­struction. Although this dense material works well as an air barrier, plaster will develop gaps due to shrinkage and on occasion will develop

CASE STUDY 9.1

Sign Supports

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Roadway signs include overhead signs, large roadside signs (area over 50 ft2 or 5 m2), and small roadside signs.

Overhead signs include sign bridges and cantilevered signs. Their supports are generally too large to adapt to a breakaway design. When possible, install overhead signs on existing bridges or other structures. Otherwise, supports within the clear zone should be shielded with a traffic barrier.

Large roadside signs typically have two or more supports, each of which is of the break­away type. Figures 6.9 and 6.10 show the loading conditions and the breakaway features.

Sign Supports

Note the hinge joint with fuse plate just below the sign and the breakaway base (shear

plate). The supports must resist ice and wind loads and also meet the following criteria:

• The hinge must be at least 7 ft (2.1 m) above ground to prevent windshield penetration.

• A single post 7 ft (2.1 m) or more from another post should have a mass less than 45 lb/ft (65 kg/m); total mass below the hinge but above the shear plate should not exceed 600 lb (270 kg). Two posts spaced less than 7 ft (2.1 m) apart should have a mass less than 18 lb/ft (27 kg/m).

•

Sign Supports Sign Supports
Sign Supports

Supplementary signs should generally not be placed below the hinges.

FIGURE 6.10 Impact performance of large roadside sign. (From Roadside Design Guide, AASHTO, Washington, D. C., 2002 and 2006, with permission)

Breakaway mechanisms for large sign supports may be either a fracture or a slip – base type. The fracture type includes couplers that fracture, or in the case of wood posts, simply posts with reduced cross-sections. Slip-base-type mechanisms activate when two parallel plates slide apart as bolts are pushed out under impact. As shown in Fig. 6.11, the designs may be of the unidirectional or multidirectional type. The upper hinge design includes a saw cut through the front flange and web of the plate, and a fuse plate on the front flange (impact side). The fuse plate has slotted bolt holes, and the bolts must be torqued to specified values for proper functioning. Alternatively, the fuse plate may have a line of open holes at the cut line, with the plate designed to rupture at the required load, negating the need for the specific values of bolt torque. Even with the breakaway design feature, it is good practice to locate large signs outside the clear zone where feasible.

Small roadside signs may be driven directly into the soil, set in drilled earth holes, or mounted on a base. U-shaped steel posts driven into the ground can generally bend and yield at the base without special devices. Splicing the posts is not usually recom­mended, because performance is not predictable. Wood posts set in drilled holes can fracture at the base, as well as steel pipes connected to anchors driven into the ground. Also, small sign supports may be mounted on fracture bases or slip bases of the unidi­rectional or multidirectional type. A typical unidirectional design uses a four-bolt slot­ted slip base, inclined in the direction of traffic by 10° to 20°. This angle allows the sign to move up so the impacting vehicle can pass underneath. A hinge in the top of the post is not needed. Multidirectional bases are usually triangular and release when struck in any direction. They are often used in medians and at ends of ramps and similar locations. Because torque requirements for slip base bolts are low, wind vibrations have caused supports to “walk” from the slots under wind vibrations, but this can be prevented by using a sheet metal keeper plate. Overtorquing must be prevented, because this causes high friction between the slip base elements and prevents the support from releasing as intended.

Repairable Systems

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For repairable hydrosystems, such as pipe networks, pump stations, and storm runoff drainage structures, failed components within the system can be re­paired or replaced so that the system can be put back into service. The time required to have the failed system repaired is uncertain, and consequently, the total time required to restore the system from its failure state to an operational state is a random variable.

Repairable Systems

Repairable Systems

20

 

20

 

15

 

Repairable Systems

TABLE 5.2 Reliability and Maintainability of Water Distribution Subsystems by Generic Group

Subsystem

MTBF* (x 106 hours)

MTTR* (hours)

Pumps

Centrifugal, open impeller

0.021660

7.825

Axial flow, propeller

0.074191

16.780

Power transmission

Concentric reducer

0.122640

2.000

Parallel shaft

0.710910

32.000

Right angle shaft

0.019480

1.400

Vertical shaft

0.031470

2.023

Variable speed, hydraulic

0.349500

—

Variable speed, other

0.014200

2.500

Gear box

0.045780

3.530

Chain drive

0.017850

8.000

Belt drive

0.091210

1.800

Motors

Multiphase

0.068000

6.853

Variable speed, ac

0.114820

8.000

Gas engine

0.023800

24.000

Valves

Gate

0.008930

3.636

Ball

0.011460

—

Butterfly

0.032590

1.000

Plug

0.028520

—

Controls

Electrical

0.100640

2.893

Mechanical

0.031230

8.000

Pressure (fluid)

0.035780

8.236

Pressure (air)

0.018690

3.556

*MTBF = mean time between failure; MTTR = mean time to repair; MTBF = MTTF + MTTR.

SOURCE : From Schultz and Parr (1981).

Effect of age on reliability

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In general, the reliability of a system or a component is strongly dependent on its age. In other words, the probability that a system can be operational to perform its intended function satisfactorily is conditioned by its age. This conditional reliability can be expressed mathematically as

Подпись: Ps(M 11)

P(TTF > t, TTF > t + M)
P(TTF > t)

P(TTF > t + M) Ps(t + M) P (TTF > t) = ps(t)

in which t is the age of the system up to the point that the system has not failed, and ps(M 11) is the reliability over a new mission period M, having successfully operated over a period of (0, t ]. In terms of failure rate, ps(M 11) can be written as

t-t+M

I h(r) dr

 

Ps(M | t) = exp

 

(5.13)

 

Effect of age on reliability

Figure 5.10 Curves for reliability [ps(t)], failure density [ (t)], and fail­ure rate [h(t)] for the 5-mile sandspun cast iron pipe water main in Example 5.3.

 

t-t+f

I h(t) dr

 

f (fit) =-d [ vff1)]

d f

 

 Effect of age on reliability

Ps(f i t)h(t + f)

(5.14)

 

Effect of age on reliability

For a process or component following the bathtub failure rate shown in Fig. 5.8 during the useful-life period, the failure rate is a constant, and the failure density function is an exponential distribution. Thus the failure rate h(t) = X. The conditional reliability, according to Eq. (5.13), is

Подпись: (5.15)Ps(f i t) = e lt

which shows that the conditional reliability depends only on the new mission period f regardless of the length of the previous operational period. Hence the time to failure of a system having an exponential failure density function is memoryless.

Подпись: Ps(f i t) Effect of age on reliability Effect of age on reliability Подпись: a Подпись: (5.16)

However, for nonconstant failure rates during the early-life and wear-out periods, the memoryless characteristics of the exponential failure density func­tion no longer hold. Consider the Weibull failure density with a = 1. Referring to Fig. 5.3, the condition a = 1 precludes having a constant failure rate. According to Table 5.1, the conditional reliability for the Weibull failure density function is

As can be seen, ps(f 11) will not be independent of the previous service period t when a = 1. Consequently, to evaluate the reliability of a system for an addi­tional service period in the future during the early-life and wear-out stages, it is necessary to know the length of the previous service period.

Example 5.4 Refer to Example 5.3. Derive the expression for the conditional reli­ability and conditional failure density of the 5-mile water main with sandspun cast iron pipe.

Solution Based on the reliability function obtained in Example 5.3, the conditional reliability of the 5-mile sandspun cast iron pipe in the water distribution system can be derived, according to Eq. (5.12), as

Подпись:ps (t + f )

Ps (t)

exp[23.25(1 — e00137(t+f 1)]
exp[23.25(1 — e00137t)]

exp[23.25e00137t(1 – e0 0137f)]

ft(§ 11) = 0.3185 x e0 0137(t+^) x exp[23.25e00137t(1 – e00137^)]

Figure 5.11 shows the conditional reliability and conditional failure density of the pipe system for various service periods at different ages. Note that at age t = 0, the curve simply corresponds to the reliability function.

5.2.5 Mean time to failure

Подпись: MTTF = E (TTF) Подпись: T ft(T) dr Подпись: (5.17)

A commonly used reliability measure of system performance is the mean time to failure (MTTF), which is the expected TTF. The MTTF can be defined math­ematically as

Подпись: MTTF Подпись: Ps(t) dt Подпись: (5.18)

Referring to Eq. (2.30), the MTTF alternatively can be expressed in terms of reliability as

0

By Eq. (5.18), the MTTF geometrically is the area underneath the reliability function. The MTTF for some failure density functions are listed in the last col­umn of Table 5.1. For illustration purposes, the MTTFs for some of the compo­nents in water distribution systems can be determined from mean time between failures (MTBF) and mean time to repair (MTTR) data listed in Tables 5.2 and 5.3.

Example 5.5 Refer to Example 5.3. Determine the expected elapsed time that a pipe break would occur in the 5-mile sandspun cast iron pipe water main.

Solution The expected elapsed time over which a pipe break would occur can be com­puted, according to Eq. (5.17), as

f TO f TO

MTTF = ps(t)dt = exp[23.25(1 – e00137t)] dt = 3.015years

00

The main reason for using Eq. (5.18) is purely for computational considerations because the expression for ps(t) is much simpler than ft(t).

FRAMING THE WALLS

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Once the plates are marked up and detailed, you can begin to assemble the walls. When I first started framing walls, carpenters used to toenail a stud in each corner, string the top plate across from stud to stud, and then work off a ladder to nail in the remaining studs. Each wall took a long time to build because we had to nail in one stud, move the ladder, and nail in another. And the fact that we were working on ladders made the work unnecessarily dangerous. It wasn’t long before we learned to frame walls flat on the floor, then raise them into position.

If you’ve done a good job marking the plates, framing walls should be easy. Simply follow the marks, nailing in studs where needed. But first, clean the floor. There’s no need to have extra wood or tools around that might trip you up.

Подпись:
Nailing in cripples

Now the puzzle goes together. Begin by nailing on the top and bottom cripples over and under all rough openings. Cripples are nailed on each end of every header and rough sill and at every lay­out mark in between. When I set out cripples, I also like to scatter 921А-іп. king studs at both ends of each window and door along with their trimmers.

To nail a cripple in place on a header, back it with your foot before starting your first two 8d toenails (see the photo above). Make sure the 2x cripple is flush with the sides of the header, then drive the nails home through the cripple and into the header. This process will become easier as you become more skilled. At first you may find that your toenail angle is too high (in which case the nail won’t have enough holding power in the cripple) or too low (in
which case you’ll drive the 2×4 toward your foot). I’ve driven toenails through a cripple and into the sole of my shoe. It’s rather embarrassing to take a step drag­ging a cripple behind.

The nails should drive easily if you started them at the proper angle, at about 60°. After nailing off one side, toenail two more 8d nails into the other side, repeating the process until all the top cripples have been nailed to their headers.

Attaching the bottom cripples to their rough sills is easier, because it involves no toenailing. Move the rough sill to the upper ends of the cripples placed against the plates (see the photo on p. 114). Nail a cripple at both ends of the sill and at every layout mark. Drive two 16d nails in each cripple about 3Л in. from either edge of the sill.

Подпись:Nailing king studs and trimmers

It’s best to nail king studs to the door and window headers and to the rough sills while nailing on the cripples. Assemble the studs in place, exactly as they go in the wall, but flat on the floor. First nail the king studs flush with the top of the upper cripples into the ends of the headers, driving three 16d nails through the king stud into small headers and four 16ds into larger headers.

For window openings, hold the end cripple on the rough sill flush with the bottom of the king stud and drive two 16d nails through the king stud into the end of the rough sill. Then insert a trim­mer under each end of each door and window header, driving just one 16d nail through the center of every trimmer into the king stud. You’ll drive in additional nails later as you prepare to set the door and window frames.

As you nail, keep all the members tight together. Take time to nail the frame together properly. Gaps left between cripples and headers, for example, can mean cracks in the drywall later on as lumber shrinks and the building settles. As you nail, keep both window and door assemblies square with themselves and with the building.

The aqueducts of Lyon

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Lucius Munatius Plancus founds Lugdunum (Lyons) in 43 BC on the Fourviere hill, 130 m above the waters of the Saone and Rhone rivers. Thirty years later, Augustus makes Lyon the capital of Gaul. It is often the case that such cities are initially established on high ground for strategic reasons, and consequently not well supplied with water. Then the cities rise in importance to become regional capitals, necessitating significant efforts to provide adequate water supply. This was the case of Pergamon as we described in Chapter 5.

At Lugdunum, it was necessary to cross a valley some hundred meters deep to reach the Fourviere hill, only its western facade being accessible. As at Pergamon (but sever­al centuries later), large inverted siphons, the largest in all the Roman world, are used to bring water across the deep valley.

The aqueducts of Lyon

Figure 6.11 The four Roman aqueducts of Lyons.

Figure 6.12 Longitudinal profiles of the four Roman aqueducts of Lyon (Burdy, 1979, 1986); the dark squares represent known points.

The aqueducts of Lyon

Four large aqueducts supply water to the Roman Lugdunum. They have been the sub­ject of numerous archaeological investigations[235]. Their remains are still visible, but unfortunately are degrading over time; however some additional remains are surfacing during modern construction projects. The construction chronology of these aqueducts is not known with certainty, indeed it is based on hypotheses. One of these hypotheses is that the Mont-d’Or aqueduct, issuing from the mountain of the same name, was the first to be constructed, about 10 BC by Agrippa, the son-in-law and close collaborator of Augustus. (Agrippa was a great aqueduct builder – Rome owes the Julia and the Virgo to him). The Mont-d’Or aqueduct is 28 km long, with two siphons. It is the smallest of the aqueducts in terms of dimensions and discharge, and only a few traces of it remain today.

It is very likely that the Craponne aqueduct (sometimes called the aqueduct of the Yzeron) was built next from catchments developed upstream of the village of Yzeron at 700 m altitude, perhaps under Augustus.[236] This canal is noteworthy for its slope of nearly 17 m per kilometer on the average, i. e. five times steeper than that of the Mont – d’Or aqueduct. Vortex drop shafts constitute what amounts to “hydraulic stairways”. Though the Craponne is of similar length and dimensions to the Mont-d’Or, it is distinc-

The aqueducts of Lyon

Figure 6.12b. Comparison of profiles along the four siphons of the Gier aqueduct. The Dureze siphon has the most pronounced slope, which can be explained by its later construction (hypothetical) by the Chagnon loop. Note the slightly rising profile (1 % ca.) of the siphon bridges, which allows evacuation of air pockets downstream – discovered by the work of Burdy (1996).

tive in having a double siphon.

To understand this double siphon, it is useful to reconsider, yet again, the great siphon of the Pergamon aqueduct, whose longitudinal profile shows a high point (Figure 5.10) with a risk of air accumulation. The Craponne includes a free-surface reservoir that stands 15 m above the ground on an intermediate plateau (the Tourillon de Craponne). This reservoir enables the aqueduct to cross two valleys over a total length of 5.5 km, avoiding the problems of a high point. It serves as both an exit basin, or out­let box, for the upstream portion of the siphon and a head tank for the longer downstream portion, which dips through nearly 70 m of elevation.

The Brevenne aqueduct is a very large installation 66 km in length, whose construc­tion could date from the Emperor Claudius in the middle of the 1st century AD. It issues from the Lyon mountains and is buried for the first forty kilometers. Then it follows the valley of the Brevenne and, after some twelve kilometers, has an increased cross section to accommodate intermediate catchments. This aqueduct also has particular distinctive features. It crosses significant elevation changes in short distances at four locations. At Courzieu, it drops nearly 44 m in less than 200 m; then at Chevinay, there is a drop of 87 m in 300 m, and again three other drops of from 30 m to 40 m each. Downstream of the drop of Courzieu, there is a small basin some 45 cm deep and 80 cm long, likely intended to provide energy dissipation as well as to trap transported sand and gravel. Moreover, there is a contraction of the canal at the location of the last drop, at Lentilly; the normal width of 75 cm decreases to 44 cm. It can be shown that in the reaches of steep slope, the flow is supercritical.[237] As for the other aqueducts, the Brevenne terminates at a siphon. It probably merges with the Craponne aqueduct at its entry into Lugdunum.[238]

The fourth aqueduct, Gier, issues from an intake on the river of the same name and is the longest at 74.5 km. The Gier is the aqueduct that was most carefully constructed, and its art features, bridges, and arcades reflect the majesty of the all-powerful Empire at its peak. Ample remains of the Gier are still visible today, in particular its bridges and parts that were supported on arcades. Traditionally the Gier is attributed to the time of Hadrian (beginning of the 2nd century AD), but recent indices (in particular a fountain bearing the name of Claudius) lead us to date the works from the reign of this emperor. The Gier also has two curious features. In parallel with the first siphon, that crosses the Dureze over a length of 900 m, there is a large derivation into a canal of small slope (only 0.5 m/km). This derivation canal goes around the village of Chagnon (the “loop of Chagnon”) for a distance of 11.5 km (making the total length of the aqueduct some 86 km). The purpose of this derivation is not obvious. Along the path of this loop, there is an inscription on a stone noting the rules for riparian use of the aqueducts:

“By order of the Emperor Cesar Trajan Hadrian Augustus, no one has the right to work, har­vest, or plant in this space that is intended to provide protection for the aqueduct.”[239]

The aqueducts of Lyon

There has been much discussion as to the relative age of the loop and the siphon. If indeed the aqueduct dates from the reign of Claudius, the inscription shows that the der­ivation canal, in service under Hadrian, postdates the siphon. This may be because the

siphon had maintenance or operational problems caused by the steep slopes in the val­ley of the Dureze (see Figure 6.12b).

Upstream of the first siphon there is also evidence of an abandoned earlier alignment along 40 km, dug deeply into the rock parallel to the aqueduct, but 8 to 10 m higher. Could this reflect a surveying error?

Further downstream, the Gier aqueduct passes under the village of Mornant in a one – kilometer curved tunnel more than 20 m underground. It has four siphons in all (table 6.4). The third of these siphons crosses the Yzeron valley. 2,660 m long and dropping 122 m, it is the largest siphon of all those in the Lyon complex.

There are eight siphons in all in the four aqueducts of Lyon. Following the specifi­cations in the manual of Vitruvius, each siphon has an exit basin, a head tank, and at the bottom of the valley, a bridge-siphon. The Tourillon de Craponne comprises what

Table 6.4 Approximate characteristics of the eight siphons of the Lyon aqueducts (the siphons of the Gier aqueduct are the best known; see Burdy, 1996).

Aqueduct

Mont-d’Or

Craponne (double siphon)

Brevenne

Gier

Valley

Rochecardon Planches

Corvelet Charbonnieres

Planches

Dureze

Garon

Yzeron

Trion

crossed

Length

500 m?

3,500 m

2,200 m 3,600 m

3,500 m

700 m

1,210 m

2,660 m

575 m

Drop

25 m?

66 m?

33 m? 91 m

91 m

78.5 m

93 m

122.3 m

38 m?

Head loss[240]

4 m?

11 m

7 m?[241] 33 m

15 m

5.8 m

8.8 m

7.9 m

2.3 m

Number

?

?

6 or 7

?

8 or 9

10

11

9

of pipes?

Vitruvius might call a colliviaria. Whereas at Pergamon the Hellenistic siphon has a sin­gle pipe, here we see the use of veritable batteries of parallel lead pipes. There are no less than nine pipes of average exterior diameter 23 cm (likely 18 cm interior diameter) side by side for the Dureze siphon of the Gier aqueduct, and ten for the Garon siphon (Figure 6.13). There are no remains to indicate the number of parallel conduits for the Yzeron siphon; but taking into account the large size of its reservoir, there were likely 11 or 12 similar pipes (Figure 6.40). The bridge-siphons were particularly wide at around seven meters, to support numerous parallel pipes.

The spring water conveyed by the siphons of Lyon is generally not calcareous, and therefore there is not much encrustation. An exception is the Mont-d’Or aqueduct, which does show evidence of some deposits. The aqueducts arrive into the city of Lugdunum at different elevations. It is thought that the Mont-d’Or aqueduct supplied the thermal baths whose remains have been found in the Minimes area. No traces remain of the water distribution system in the city itself, but it is likely that the conduits passing

under the Saone through siphons supplied the peninsula between the two rivers.

INSTALLING A GFCI RECEPTACLE

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Important: Read earlier sections about installing standard receptacles before installing a GFCI receptacle. As already mentioned, the NEC requires GFCI protection in the following loca­tions: all bathroom receptacles; all kitchen counter receptacles; kitchen receptacles within 4 ft. of a sink; all outdoor receptacles; accessible basement or garage receptacles; and receptacles near pools, hot tubs, and the like.

О Shut off power to the outlet and test with a voltage tester to be sure it’s off. Carefully read the instructions that come with your GFCI. Devices vary, and your warranty—and your safety — depends on your installing the device correctly. GFCI receptacles are amperage rated (15 amp and 20 amp) and should match the cable wire size and be correctly sized for the circuit load.

GFCIs are wired in essentially the same man­ner as standard receptacles—hot wires to gold screws, neutral wires to silver screws, ground wire to grounding screw—except here you’ll need to know which incoming cables are line wires (from the power source) and which are load wires (running on to the next outlet). So disconnect the old receptacle while the power is off and separate wires from each other. Temporarily turn the power back on and—being careful not to touch the bare wire ends—use a voltage tester to determine which pair of wires are "line.” That done, turn the power off again and use white tape and a felt – tipped pen to denote "LINE” and "LOAD” wires.

Attach the wires running from the power source to the screw terminals or wire leaders marked "LINE,” attach wires continuing to the next outlet to the terminals or leaders marked "LOAD,” and attach ground wires to the terminal or leader marked "GROUND” and (if it’s a metal box) to the pigtail that grounds the box. If there are no outlets downstream, use wire nuts to cap the two load leaders.

When connections are completed, gently press the device into the outlet box, screw the recepta­cle to the outlet box, install the cover plate, and turn the power back on. Then test the GFCI by pressing its "on”or "test” button, per the manufac­turer’s instructions. If the device won’t stay on or

Подпись: PROTIP Back-wired receptacles have holes in the back, into which you insert stripped wire ends. They're quicker to wire than screw terminals, but the receptacles' internal tension clamps can fatigue, leading to loose wires, flickering lights, and arcing. Moreover, each time you insert and remove plugs, the receptacles move slightly, which increases likelihood of clamp failure. Manufacturers have stopped making back-wired 20-amp receptacles, so think twice about installing 15-amp receptacles. Back-wired switches, on the other hand, rarely fail. 1111

Подпись: Use the stripper to close wire loops around the screw shanks. It’s acceptable to attach wires directly to receptacles at the end of a circuit.

devices downstream don’t work, you have either a short circuit or a defective GFCI. Most makers recommend testing the devices once a month. All GFCI receptacles used outdoors must be housed in gasketed, weatherproof boxes with covers.

Fixtures, and Switches

Posted by on 19/ 11/ 15

О Before working around any electrical device, disconnect the power to it and test with a voltage tester to make sure the power is off.

WIRING RECEPTACLES

Before beginning this section, read the previous sections on grouping and splicing wire. The fol­lowing discussion assumes that each wire group (hot, neutral, ground) in the box is spliced together and has a pigtail that attaches to a receptacle terminal. Connecting wires and devices in this manner ensures that there will be continuous current to outlets downstream, even if a receptacle upstream malfunctions or is tem­porarily removed.

Method one: Using pigtails. First attach the grounding pigtail to the green grounding screw on the receptacle. If the grounding pigtail is insu­lated, strip ‘A in. of insulation from its end; then use needle-nose pliers or a hole in the wire strip-

A New KIND OF NUT

Splicing with twist-on wire nuts can be problematic. It’s impossible to tell if the wires have come loose as you twist on the nuts; and stranded wire tends to slide down solid wire when you join solid wires to twisted-strand fixture leads. Wago Wall-Nuts™ simplify the task: Strip the wire ends the specified amount, and then push them into nut ports that hold the wires fast. The clear plastic housing allows you to see if the wires are connected, and the ports grasp both stranded and solid wire well.

per handle to loop the end of the pigtail clockwise, as shown in the bottom left photo on p. 25′. Fit the looped wire onto the grounding screw, and then tighten it. (Screws tighten clockwise.) Next strip h in. of insulation from the neutral pigtail, loop it clockwise, and attach it to a silver screw on the receptacle. Finally, strip!2 in. of insulation, then loop and attach the hot pigtail to a gold screw on the opposite side of the receptacle.

When all wires are connected, tighten down any unused screws and gently push the wired receptacle into the outlet box, so the wires fold like an accordion. Make sure that the receptacle

Wiring a Receptacle I in Midcircuit

Note: In the wiring schematics on pp. 248-255, thin, solid lines denote ground wires, white wires denote neutral conductors, and black, red, or slashed wires denote hot conductors-unless otherwise indicated on an illustration.

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By splicing like wire groups and running pigtails (short wires) to the receptacle, this conventional method ensures continuous current downstream.

face is flat to the wall plane, not tilted, and the receptacle is centered in the box. Push the recep­tacle into the box by hand: If you use the screws to pull the receptacle into the box, you may strip a screw hole in a plastic box. Later, after the walls have been painted, screw a cover plate to the receptacle.

Method two: Feeding through receptacles.

Rather than splicing like wire groups and run­ning pigtails to the receptacles, you can instead attach hot and neutral wires directly to the recep­tacles. This wiring method—"feeding through receptacles”—is widespread because, on the whole, it is quicker, requires fewer conductors, and results in boxes that are less crowded than those wired with spliced wires and pigtails. It is also acceptable to the NEC.

But feeding wires through receptacles has detractors, who argue that in a circuit so wired, if a receptacle upstream fails or a wire comes loose, receptacles downstream will lose power.

Moreover, there is a voltage drop of about 1 per­cent per receptacle wired in this manner because receptacles offer more resistance to current flow than wire. If you have 6 or 10 receptacles daisy – chained in this manner, those overheated connec­tions can heat up your electricity bill.

However, in older homes with ungrounded outlets, there’s an upside to feeding through receptacles. If you install a GFCI receptacle on the circuit, the GFCI will protect all receptacles downstream. That is, if there’s a ground-fault at a receptacle downstream, the GFCI will cut power to it and therefore make the entire circuit safer.

Подпись: PROTiP If you install three-prong receptacles or GFCI receptacles on an ungrounded circuit, the NEC requires that you label affected receptacles "no equip-ment ground." 1111 End of circuit. A receptacle at the end of a cir­cuit has only one cable entering the box and none going beyond it. In this case, it’s acceptable to attach wires directly to the receptacle terminals, without using pigtails. Attach the ground wire to the green ground screw, the neutral wire to a sil­ver screw, the hot wire to a gold screw.

Fixtures, and Switches

Feeding Wires I through Receptacles

Wiring a Receptacle I at End of a Circuit

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Incoming

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Attaching hot and neutral wires directly to receptacle terminals is quicker and results in a less crowded box. However, with this wiring method, if the receptacle fails, power can be disrupted to downstream outlets. Note: Ground wires are always spliced to ensure continuity.

Where there are no receptacles downstream, attach wires directly to the device.

Подпись: Wiring a Fourplex Receptacle in a Metal BoxПодпись:Подпись: A two-gang box with fourplex (double duplex) receptacles will be crowded. Especially if the box is metal, use insulated ground pigtails, wrap electrical tape around receptacle bodies to prevent screw terminals from shorting out, and always ground the box (see the bottom photo on p. 243).Подпись: Wiring a GFCI ReceptacleПодпись: From power sourceПодпись: Spliced ground wiresПодпись:image502

A GFCI receptacle in midcircuit can protect receptacles downstream, if correctly wired. Attach wires from the power source to terminals marked “LINE." Attach wires continuing to receptacles downstream to terminals marked “LOAD." As with any receptacle, attach hot wires to gold screws, neutral wires to silver screws, and grounding wires to the green ground screw. Note: Here, only ground wires are spliced; hot and neutral wires attach directly to screw terminals.

Ganging. By ganging devices you can make the most of limited wall or counter space. Most often, a switch and a receptacle are housed in a single four-square box (4 in. by 4 in.) or in two smaller metal boxes ganged together. Two recep­tacles will also fit. When in doubt, consult the table "Box Fill Worksheet,” on p. 240, to make sure that the box in question is big enough. When ganging two devices, use two pigtails per wire group so you ensure continuity downstream.

SIGN AND LUMINAIRE SUPPORTS AND SIMILAR FEATURES

Posted by on 19/ 11/ 15

Approximately 15 percent of all fixed-object fatalities involve sign and luminaire supports or utility poles. The options available to the highway engineer to improve on this record
are similar to those presented earlier: remove or redesign, relocate, use a breakaway device, shield, or delineate. Although it is desirable to have an unobstructed roadside, it is not always possible to relocate appurtenances such as signing and lighting supports, because they must remain near the roadway to fulfill their intended purpose. Thus, emphasis is given to the use of breakaway hardware—selection of the most appropriate device and installing it to ensure acceptable performance. (See Chap. 7.) Supports should be designed in accordance with AASHTO’s Standard Specification for Structural Supports for Highway Signs, Luminaires, and Traffic Signals.

Breakaway supports include all types of sign, luminaire, and traffic signal supports designed to yield when hit by a vehicle. Typical release mechanisms include slip planes, plastic hinges, and fracture elements. Criteria for breakaway supports are given in National Cooperative Highway Research Program (NCHRP) Report 350, Recommended Procedures for the Safety Performance Evaluation of Highway Features. The criteria require that a breakaway support fail in a predictable manner when struck head on by a 1800-lb (820-kg) vehicle, or its equivalent, at speeds of 20 and 60 mi/h (35 and 100 km/h). It is desirable to limit the occupant impact velocity to 10 ft/s (3.0 m/s), but values as high as 16 ft/s (5.0 m/s) are acceptable. Also, the maximum stub height is set at 4 in (100 mm) to avoid snagging the undercarriage after impact. The crash vehicle must remain upright with no significant deformation or intrusion of the passenger compartment.

Full-scale crash tests, tests with bogie vehicles (reusable, adjustable surrogate vehicle), and tests with pendulums (having special nose sections to model vehicles) are used for acceptance. Pendulum tests are the least expensive, but are used mostly for luminaire support hardware and are mainly limited to 20 mi/h (35 km/h). NCHRP Report 350 dis­cusses acceptance testing. Tests are run in a standard soil, but weak soil should be used in addition for any feature whose impact performance is sensitive to soil-structure interaction.

Many general practices are similar to those previously discussed. Supports should not be placed in drainage ditches, because vehicles may be channeled into the obstacle and freezing might interfere with proper functioning of the breakaway device. Also, breakaway supports must not be located near ditches or on steep slopes where a vehicle is likely to be partially airborne at impact, because breakaway devices may bind and not function properly when hit in this manner. They have been developed to be struck about 20 in (500 mm) above the ground.

Locate supports where they are least likely to be hit, such as behind roadway barriers (beyond design deflections of the barriers) or on existing structures. In general, only when the use of breakaway supports is not feasible should a traffic barrier or crash cushion be used for shielding. Generally, breakaway supports should be used unless an engineering study indicates otherwise. Concern for pedestrians being struck by falling supports after a crash has led to the use of fixed supports in some urban areas such as near bus shelters or where there are extensive pedestrian concentrations.