Blog Archives

Inspection of Flexible Structures

Posted by on 18/ 11/ 15

A flexible structure should be checked to ensure that the cross-sectional shape it was designed for is intact. If the flexible culvert, whether it is a round pipe, a pipe arch, an arch, a horizontal ellipse, or any other structural shape, deflects from its design shape, it is not receiving the required support from the backfill. It is assumed in the design of flexible structures that moment in the structure is negligible and that due to the thrust forces, the structure is in compression throughout. If the deflection is large enough to cause a flattening of the structure, these assumptions will not hold true and the struc­ture may collapse...

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Failure Characteristics

Posted by on 18/ 11/ 15

Any system will fail eventually; it is just a matter of time. Owing to the presence of many uncertainties that affect the operation of a physical system, the time the system fails to perform its intended function satisfactorily is random.

5.1.1 Failure density function

The probability distribution governing the time occurrence of failure is called the failure density function. This failure density function serves as the common thread in the reliability assessments by TTF analysis. Referring to Fig. 5.1, the reliability of a system or a component within a specified time interval (0, t], can be expressed, assuming that the system is operational initially at t = 0, as

/

TO

ft (t ) dr (5.1a)

in which the TTF is a random variable having ft(t) as the failure density func­tion...

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Time-to-Failure Analysis

Posted by on 18/ 11/ 15

5.1 Basic Concept

In preceding chapters, evaluations of reliability were based on analysis of the interaction between loads on the system and the resistance of the system. A system would perform its intended function satisfactorily within a specified time period if its capacity exceeds the load. Instead of considering detailed in­teractions of resistance and load over time, in a time-to-failure (TTF) analysis, a system or its components can be treated as a black box or a lumped-parameter system, and their performances are observed over time. This reduces the relia­bility analysis to a one-dimensional problem involving time as the only variable describable by the TTF of a system or a component of the system...

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Gram-Schmid Ortho-normalization

Posted by on 18/ 11/ 15

Consider a vector x 1 in an K-dimensional space to be used as one of the basis vectors. It is desirable to find the additional vectors, along with x 1, so that they would form K orthonormal basis vectors for the K-dimensional space. To do that, one can arbitrarily select K – 1 vectors in the K-dimensional space as x 2, x3, …, xK.

The first basis vector can be obtained as u1 = x 1/|x1|. Referring to Fig. 4D.1, a second basis vector (not necessarily normalized) that will be orthogonal to the first basis vector u1 can be derived as

У 2 = x2 – У2 = x2 – (x2«0«1

Therefore, the second normalized basis vector u2, that is perpendicular to u2 can be determined as u2 = y2/|y2|.

Note that the third basis vector must be orthogonal to the previously deter­mined basis vectors (u1, u2) or (y 1, y 2)...

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Elements of Inspection

Posted by on 18/ 11/ 15

An inspection of the culvert should include the approach roadway, the embankment, the headwalls and wingwalls, the waterway, and the culvert barrel.

Roadway. The roadway over the culvert should be inspected for sags and cracks in the pavement that are the result of settlement. These may be evident in both the road­way itself and adjacent guiderail. The settlement may be the result of poorly compacted material adjacent to the culvert piping (infiltration or transportation of fines by water flowing through the backfill), or settlement of the culvert itself. The structural integrity of the culvert itself may or may not have been compromised. An inspection of the culvert must be made.

Embankment, Headwalls, and Wingwalls...

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INSPECTION

Posted by on 18/ 11/ 15

Many storm drains and highway culvert systems have in the past been and are presently designed for a 50-year life span. The local roadway and state highway and interstate systems have in large part reached this age or soon will. Consequently, rehabilitation and repair of existing storm sewers and highway drainage culverts are presently requiring more and more attention and resources from the responsible agencies. It is generally less expensive to rehabilitate or repair an existing underground structure than to replace it. In addition, the cost of repair to the facility after a catastrophic failure

greatly exceeds the cost of rehabilitating the structure and preventing that failure. The key, of course, is being able to identify those structures that are in jeopardy of failing.

5.11...

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Camber Requirements

Posted by on 18/ 11/ 15

Where high embankments are placed on original ground, the fill may compress and consolidate the foundation soil. Thus, culverts constructed on or near the original ground surface tend to undergo some settlement. The amount of settlement varies with fill height and the consolidation characteristics of the foundation soil. Because the amount of settlement varies with the fill height, the culvert will tend to settle more toward the center than at the ends. If the culvert is built upon a straight grade between the inlet and outlet elevations, a sag will develop. The sag may create a low point in the culvert, or may cause accumulation of debris and silt and opening and leaking of joints...

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Reading a Cable

Posted by on 18/ 11/ 15

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...

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Install ridge shingles and the ridge vent

Posted by on 18/ 11/ 15

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...

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Mechanical Spreading

Posted by on 18/ 11/ 15

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.

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