Blog Archives

The great libraries of the Abbasids

Posted by on 22/ 11/ 15

Two grand libraries flourished under the Abbasids. They were motivated by the same goals as the ancient libraries of Alexandria and Pergamon: the prestige of the sovereign and the attraction to scholars from everywhere. The grand library of Baghdad, the Bayt al-hikma was developed under the reign of Haroun al-Rashid (786 – 809). It benefited from the latter’s acquisition of ancient Greek works from the court of Constantinople, a practice continued by his successor al-Mamun (813 – 833). A strong memory of the belle epoch of these rich libraries persisted in the 15th century:

“The caliphs and the sovereigns had a lively interest in the grand libraries and paid close atten­tion to them, enabling them to acquire beautiful and numerous collections...

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Finite Difference Method

Posted by on 22/ 11/ 15

The finite difference method doesn’t postulate explicitly any specific shape of the unknown field. As we are concerned with partial differential equations, exact deriva­tives are replaced by an approximation based on neighbouring values of the un­known (still denoted as p):

P,+1 – Pi-1

W/, 2L

where the subscript i denotes the cell number and L denotes the cell size. For an orthogonal mesh, such derivatives are easily generalised to variable cell dimen­sions. However, non-orthogonal meshes pose problems that are highly difficult to solve and are generally not used. Boundary conditions have then to be modelled by the juxtaposition of orthogonal cells, giving a kind of stepped edge for oblique or curved boundaries...

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Patching Holes and Punctures

Posted by on 22/ 11/ 15

It is not necessary to repair each hole in a sign. When a hole does not damage the mes­sage or symbol and does not create the impression of a sloppy sign, then repair may not be needed. The following procedures can be used to make field repairs on signs.

Retroreflective Aluminum Sign Panels

• Remove all damaged background sheeting and legend. Usually this means about 1 in (25 mm) from the edge of the hole. A retractable-blade knife is a useful tool for this.

• Straighten the sign (flatten out the hole puncture nipple area) using a ball peen hammer and a flat surface (truck bed, trailer bed, or a fender dolly).

• Remove any additional sheeting damaged during straightening.

• Clean the entire area with xylol; then apply varnish maker’s and painter’s (VM&P) naphtha.

• Patch the h...

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Space-Saving Walls

Posted by on 22/ 11/ 15

To conserve space, builders often attach 1×3 furring strips to basement walls. These strips help secure insulation and provide a base for attaching drywall. But furring strips are viable only if the foundation walls are dry and relatively plumb and flat. If foundation walls are lumpy and off-plumb, a stud wall erected inside the foun­dation is the only way to create a flat plane for finish materials.

Подпись: of the rim joists. Then insulate the rim joists with rigid insulation or encapsulated (poly- wrapped) fiberglass batts. ► Lumber in direct contact with masonry walls or floors should be pressure-treated; relatively benign biocides in pressure-treated wood include alkaline copper quat (ACQ) and copper boron azole (CBA). If you use non- treated furring strips, prevent rotted ends by keeping the bottoms of the vertical strips 1/2 in. above concrete floors. ► Don’t place fiberglass or cellulose next to masonry walls because fibrous materials will retain moisture and foster mold. In basements and crawl spaces, closed-cell XPS panels are a better choice because they are impermeable to and unaffected by moisture. Open-cell XPS is Fortunately, not all stud walls guzzle space. Lightweight steel studs are only 15/ in. deep; but, as noted in Chapter 4, they can be quirky to work with. The third option, flat-framing, is a winner: Rotate 2×4 studs 90° so that their broad side faces the foundation wall and use 2×2 plates at top and bottom. Because modern 2x4s are actually 1V2 in. by 31/ in...

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Triaxial Dynamic Compression Test

Posted by on 22/ 11/ 15

A concept similar to side supported creep tests applies to the triaxial dynamic com­pression test. In this method, a specimen is subjected to compression with lateral support. This method has been widely regarded as one of the most accurate, reflect­ing the state of stress in a loaded pavement. Moreover, it enables the measurement of parameters of an asphalt mixture used for the analysis of pavement viscoelasticity (Huurman, 2000). Undoubtedly, it is a recommended method for testing rut resis­tance. It is described in EN 12697-25 as method B.

12.1.3 Wheel-Tracking Test

The best known direct tests for resistance to deformation are wheel-tracking tests, conducted with special devices...

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A WHOLE-HOUSE EXHAUST FAN

Posted by on 22/ 11/ 15

Although it is not ideal in all climates or seasons, a whole-house fan can be a very attractive alter­native to air-conditioning. A simple fan is more reliable and less expensive than a single window – mounted AC unit, and it can effectively cool an entire house. As shown in the illustration at right, the principle is simple: A single, centrally located fan pulls in fresh air through open windows and blows hot indoor air outside. By turning the fan on in the evening and opening all major windows, it’s possible to quickly cool a house that has become hot during the day.

These fans work best in dry climates, or at least when the air is cool and dry outdoors...

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BLOWING-IN INSULATION

Posted by on 22/ 11/ 15

Loose-fill insulation—usually cellulose—can be blown in at low pressure to supplement existing attic insulation or pumped into wall cavities at high pressures to achieve a dense pack that’s virtually airtight. Before insulating, be sure to review earlier sections of this chapter on sealing air leaks, correcting excess moisture, and block­ing insulation to keep it away from potential ignition sources such as chimneys and unrated recessed lights. Also keep insulation away from knob-and-tube wiring that’s still energized (see Chapter 11 for more information about this old wiring).

Equipment. You can rent insulation blowers and hoses. And some suppliers will loan the equip­ment free if you buy the insulation from them...

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Oversize Units Dehumidify Poorly and Waste Money

Posted by on 22/ 11/ 15

Approximately two-thirds of all residential air conditioners are too large. According to Bruce Harley, an HVAC consultant with Conservation Services Group in Westbor- ough, Mass., these oversize units "will cool your house, but they’re not necessarily
designed to run efficiently." The first prob­lem is that they dehumidify poorly. Oversize units satisfy the temperature at the thermo­stat so quickly that only a little moisture has time to condense on the evaporator coil. This phenomenon is known as short cycling, and it’s more of a problem in humid climates. If cycles are very short, moisture on the coil can evaporate back into the house before it drains away.

Second, air-conditioning units are least efficient when they start up...

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WORKING WITH FIBERGLASS INSULATION

Posted by on 22/ 11/ 15

WORKING WITH FIBERGLASS INSULATIONWORKING WITH FIBERGLASS INSULATION

Glass fibers can irritate your skin and damage your eyes and lungs, so safety precautions are very important when working with fiberglass insulation. Cover your body with a loose-fitting, long-sleeved shirt and long trousers, and wear gloves and a hat, especially while insulating a ceiling (see the photo below). It’s best to wear a pair of quality goggles, too, because eyeglasses alone don’t keep fiberglass par­ticles out of your eyes. Make sure the goggles fit properly; goggles that fit well don’t fog over. Wear a good-quality dust mask or, better yet, get yourself a respirator. Don’t scratch your skin while you’re working (you’ll just embed glass fi­bers), and be sure to wash up well when you are finished. CUTTING BATTS. Cutting fiberglass batts to size is straightforward...

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Sign Cleaning

Posted by on 22/ 11/ 15

Sign legibility can be restored or improved by general cleaning and removal of foreign substances from the sign face. Sign cleaning products are commercially available for use in removing common soil and severe contaminants such as paint and adhesives. In the majority of cases the required cleaning products can be obtained from local hardware stores. Strong solvents and incomplete removal of the contaminant can damage the sign reflectivity. Strong solvents should be trial-tested prior to application on the sign. The following steps can be used to clean signs of common soil and some contaminants [47]:

• General sign cleaning: Mild, nonabrasive cleaners and detergents suitable for painted or enameled surfaces are recommended for removal of common soil...

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