Wind-induced vibrations

Historically, vessels have tended to have many fewer incidents of wind-induced vibration than stacks. There is a variety of reasons for this ... 

Once a vessel has been designed statically, it is necessary to determine if the vessel is susceptible to wind-induced vibration. Historically, the rule of thumb was to do a dynamic wind check only if the vessel L/D ratio exceeded 15 and the POV was greater than 0.4 seconds. This criterion has proven to be unconservative for a number of applications. In addition, if the critical wind velocity, V,., is greater than 50 mph, then no further investigation is required. Wind speeds in excess of 50 mph always contain gusts that will disrupt uniform vortex shedding. 

In order to effectively control wind-induced vibrations of long span bridges aerod3mamic devices from the viewpoint of required energy are more attractive than mechanical control means. Figure 8.54 shows the simplified model of bridge-section with aerod3mamic devices (surfaces) to flutter-control. 

For other applications TLD are optimal, e. g. to suppress wind induced vibrations of smokestacks... 

The causes of the collapse of an area of vaulting in 1284, twelve years after its construction, have been extensively debated The statics are correct but the buttresses have been made so slender that secondary effects have become important. Iron rods connecting the frames were thought to be superfluous but when removed allowed wind-induced vibrations to damage the masonry... 

ASME STS-1-2006, American Society of Mechanical Engineers, New York, NY, 2006. Findlay, Matt, Improved Screening Criteria for the Prediction of Wind Induced Vibration of Tall Process Vessels Arising from Recent Field Experience, ASME, New York, NY, November 2001. 

Pylons are connected to the bridge foundations and thus are affected by ground motion. However, due to their high vibration modes, they are prone mostly to wind-induced vibrations. Consolidated practices implemented are based on passive control, using different systems. 

This procedure is not valid for vessels that are subject to wind-induced oscillations or that must be designed dynamically. See Procedure 4-8, Vibration of Tall Towers and Stacks and Note 5 of Procedure 3-1 for additional information regarding vessels in this category. 

Reliable modal information can be obtained by output-only dynamic measurements, i.e. accelerations are dne to ambient influences. In the case of bridges, traffic under or on top of the bridge, besides wind pressures can be the cause of the induced vibrations. So closing bridges to apply controlled force excitation is not necessary. This makes ambient vibration monitoring suitable for continuous condition assessment. 

In another civil engineering application, MR fluid dampers have been used to mitigate potentially damaging wind-induced cable vibrations in a cable-... 

Fig. 6.89. MR fluid dampers used control wind-induced cable vibrations on Dong ting Lake cable-stayed bridge in central China...

Tower-like structures are understood, in general, to be slender, tall structures (as television towers, lookout towers, chimneys, masts, and bridge pylons). Usually, gust-induced vibrations in the wind direction predominate, especially those at the fundamental bending frequency. The vibrations connected with vortex shedding that is transverse to the wind direction, however, can be more important. Particularly sensitive in this respect are steel chimneys (of welded construction, not insulated or lined with masonry, and with a fixed base). Vibrations of chimneys, masts and other low-damped tower-like structures lead to structural safety (fatigue) and serviceability problems. The occurrence... 

When the bay water is stratified and has a pycnocline (a layer with a large density gradient), the transfer of motions is prevented. Therefore, a comparatively strong circulation is formed only in the upper layer. Moreover, after the wind stopped, the pycnocline which was inclined by the wind-induced flow may vibrate, and a strong flow may occur. Consequently, even when constant wind blows on a certain bay, the resulting water motions depends on the strength of stratification and its depth. 

W. A. Dalgliesh, Assessment of Wind Loads for Glazing Design, Practical Experiences With Flow>Induced Vibrations. lAHR-IUTAM Symposium, Karlsruhe, Germany, Springer-Verlag, September 3-6, 1979. 

With this, we are now in a position to create the cut-out pattern for each vane in plan view. The straight or bottom half of the cutout pattern is very similar to what engineers sometimes design and attach to tall smokestacks to reduce flow-induced vibration at high wind loadings. The... 

Pipe Supports Loads transmitted by piping to attached equipment and supporting elements include weight, temperature- and pressure-induced effects, vibration, wind, earthquake, shock, and thermal expansion and contraction. The design of supports and restraints is based on concurrently acting loads (if it is assumed that wind and earthquake do not act simultaneously). 

Both the column thickness and the column natural frequency are important in determining the effect of wind on column sway and the stresses induced. To determine the natural frequency of the column, Rayleigh s method, based on harmonic displacement for all elements of the column, is used. The natural vibration frequency formula has been derived as follows ... 

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