Measures for Wind-Induced Vibrations | Wind Engineering

The measures controlling the wind-induced vibrations are classified into aerodynamic and mechanical means.

With increase of light, flexible, low-damping and long-span or high-rise structures, considerations of dynamic effects of wind on these structures have gained more importance. When wind-induced vibrations of a structure or structural member are likely to occur below the design wind speed or when their response amplitude is expected so large as to cause structural damage, functional trouble or human discomfort, some preventive measures have to be taken.

A distinctive feature of wind-induced vibrations of a flexible structure is their variety as shown in Table 1. Although the phenomena to be considered are different for respective structure, there is a possibility of multiple failure or un-serviceability modes for a specific structure at different wind speeds, and some of the phenomena can occur concurrently.

The design of a structure shall be made firstly in accordance with structural and functional requirements under primary loadings, but consideration on aerodynamic stability at preliminary design stage is very essential for such a flexible structure as long span cable-supported bridge and high rise tower located at the site where strong wind is anticipated.

Japanese practice of bridge deck design in these cases, for example, has been to provide with such torsional stiffness that the fundamental natural frequency in torsion exceeds at least twice that in bending and also to select aerodynamically stable cross section.

When the original design is found to be unacceptable in view of the aerodynamic limit states of the structure, some appropriate countermeasures described later shall be taken to meet either of the following requirements:

1. Pushing out the critical wind speed at which the vibration starts, above the design wind speed, or

2. Reducing the maximum response amplitude below the acceptable limit.

The former requirement must be applied to the divergent amplitude responses such as galloping and flutter, whereas the latter is usually considered for the limited amplitude oscillations such as gust response and vortex excitation.

Measures suppressing wind-induced vibrations are classified into aerodynamic and mechanical means as shown in Fig. 1.:

 

1. Aerodynamic Means:

In principle, the aerodynamic countermeasures utilize change of air flow around the structure by geometrical modifications of its cross-sectional shape or by small attachments to the structure.

In almost all of the cable-supported bridges in Japan, wind tunnel tests are conducted and better geometrical cross sections including aerodynamic countermeasures are sought whenever unacceptable behaviors are observed in the test.

In the case of tall buildings, not only the wind-induced structural response but also the change of wind environment around the building are often investigated with wind tunnel model. As an example, the NEC Building in Tokyo was provided with the wide opening at its middle height not to deteriorate local wind condition.

Selection of Basic Cross Section:

The geometrical shape plays an important role in the wind effects on structures. Although the cross-sectional shape and dimensions are primarily determined in accordance with structural, functional and, in some cases, aesthetic requirements, the selection of aerodynamically stable cross section often results in the most economical design for the girder of long span cable-supported bridge.

Streamlining is one of the solutions and approaches to an airfoil-like shape which is free from separated-flow instabilities, including vortex excitation, under horizontal wind. In this extreme case, the classical flutter theory may be applicable and its critical wind velocity is usually high.

As a practical matter, however, the cross section of bridge deck is usually bluff, so some aerodynamic appendages mentioned later may have to be added to the basic structure section. Use of shallow trapezoidal or hexagonal box girder is now popular on long span cable-stayed bridges in Japan. Twin box design gave better performance in some cases. In the case of very long span suspension bridge, air gaps at the center of bridge deck to reduce the pressure difference may be useful.

Another approach to attain wind-resistant design is the use of truss structure with enough opening. Although a truss is generally considered disagreeable from aesthetical viewpoint, it is used even in cable-stayed bridges for other design reasons such as double deck layout.

It is known that the stiffening truss of very long span suspension bridge becomes more stable under wind action when provided with gratings on the roadway as well as the openings between the roadway and chord members. It must borne in mind, however, that existence of solid handrails, thick curb stones, and other small attachment may cause unfavorable effects on overall stability of the basic cross section.

The cross sectional shape of bridge tower is sometimes modified by aerodynamic reason Since stay cables can hardly contribute to the constraint of lateral bending of a cable-stayed bridge tower, the trapezoidal and octagonal (corner cut on rectangular shape) sections are adopted for the tower shafts of the Tsurumi Bridge and the Meikoh-Chuo Bridge, respectively.

A horizontal strut was added to the inverse V shape tower of the Meikoh-Higashi Bridge, while a vertical slit was provided on the Sugawara-Shirokita Bridge pylon. Suspension bridge towers have been considered aerodynamically stable after the completion of bridge, but in case of the Akashi-Kaikyo Bridge having the world’s longest span, the corner cut on tower shafts was made to prevent wind-induced vibrations, together with installation of mechanical dampers.

Even on high rise buildings wind-induced vibrations may become the primary concern for structural safety and occupant’s comfort. Care should be taken on the selection of the cross-sectional shape of the super high-rise buildings that will be contemplated in the future, from this viewpoint.

Aerodynamic Appendages:

When the substantial change of the basic cross section is not allowed for design reasons, some aerodynamic control devices are adopted for wind-sensitive structures with bluff cross sections.

Zdravkovich (1981) classified a wide variety of aerodynamic appendages suppressing vortex excitation into the following three categories in accordance with the phenomenological mechanism of vortex shedding:

(1) Surface protrusions (strakes, helical wires, fins and studs, etc.), which destroy or reduce the coherence of the shed vortices by affecting separation lines and/or separated shear layers

(2) Shrouds (perforated, gauze, axial rods and axial slats, etc.), which affect the entraint layers, and

(3) Near wake stabilizers (splitter and saw-tooth plates, guiding plates and vanes, base-bleed, slits cut along the cylinder, etc.), which prevent interaction of entraint layers.

The most typical structure where the above means are applied is a circular cylinder such as steel stacks and light poles Most means in the first two categories mentioned above are effective irrespective of the direction of wind, whereas some means in 1) and all in 3) are unidirectional So it depends on the given case which means is effective. Increase of drag coefficient in some of the above means should be also borne in mind.

Similar investigations have been done on tall buildings. As an example, Kwok and Bailey (1987) compared three different modifications to square tower model, namely those with small fins, vented fins and slotted corners, respectively at the four corners. They reported that the overall performance of the slotted corners was very encouraging. The tall pylons of cable-supported bridges are the structures in this category.

The examples used in Japan are corner deflectors on the Katsushika Harp Bridge and corner cuts on the Higashi-Kobe Bridge and the Akashi suspension bridge Web plate of slender tension members with H-section is sometimes perforated to control vortex excitation. Successful effect of openings in reducing the lift response is also reported on building models.

A wide variety of aerodynamic appendages are used on slender bridge girders. Examples are farings (triangular or semicircular) or wind nose, flaps, splitter plates, corner deflectors, soffit plates, baffle plates and so forth. Sometimes the intermittent arrangement of the appendages is effective due to the reduction of spanwise correlation of exciting force.

The aerodynamic appendages are occasionally provided even on truss girders. Triangular farings are attached to the upper chord of the truss of the Yokohama Bay Bridge, a cable-stayed bridge with a main span of 460m, since this upper chord is a shallow box section which works also as the bridge deck. In case of the Ohnaruto and the Akashi suspension bridges, center barrier was attached to the stiffening truss.

Although preventive measures for the wind-induced vibrations of cables are mostly mechanical, a few aerodynamic means have been employed in cable- stayed bridges The polyethylene sheaths with protuberances along the cable axis were used on the Higashi-Kobe Bridge.

When the aerodynamic appendages are contemplated, attention must be paid on the trade-off with the possibility of increasing cross-wind gust response, which may be also the case of streamlining, and spoiling the appearance of the structure.

Active Control:

For extremely long span or high rise structures it may be necessary to provide special facilities. Thus active aerodynamic control to suppress wind-induced vibrations has recently gained interests. Various means such as actively controlled rotating flaps, farings or rod and use of jet nozzles are being proposed, though not yet realized in civil engineering structures.

As compared with the mechanical active control referred to later, active aerodynamic control may have to rely heavily on the experimental verification, while it seems to be more economical from the viewpoint of required energy.

2. Mechanical Means:

When any aerodynamic countermeasures are not effective, when aerodynamic appendages spoil the appearance of the structure, or when the countermeasures are required only during erection stage, mechanical control means are considered to be used. Mechanical means aim at controlling the dynamic response of the structure to wind by adjusting the mass distribution, increasing the stiffness, adding the structural damping, or applying the control force to the structure (cf. Fig 1).

Different from the aerodynamic countermeasures, the mechanical control means may be doubly beneficial as the preventive measures for the vibrations induced by other sources such as earthquakes and vehicle traffic, if they are properly designed.

Table 2 shows the examples of mechanical means used on various structures. Although the data are not necessarily exhaustive, one can recognize several interesting trends in the use of the mechanical means.

Some of the facts are as follows:

1. The tuned mass dampers (TMD) are widely used on a variety of structures.

2. Various alternative means are used particularly for towers and cables of cable- stayed bridges.

3. Mechanical control means have been not so often used on the girders of flexible bridges because aerodynamic countermeasures seem to be given priority so far.

4. Active control means have started to come into practical use in recent years.

Added Mass and Stiffness:

The increase in mass and rigidities of the structure, respectively reduces the wind induced response when other parameters are kept constant. However, the increase in mass results in the increase in the natural period and the dead weight of the structure, so the application seems very limited to such a case that the steel pipe posts of a deck type arch bridge are partially filled with concrete or gravel.

On the other hand, the increase in rigidity of the structure is doubly beneficial in the case of vortex excitation in particular, because the response amplitude reduces and the resonance wind speed increases by increasing the natural frequency. In general, however, the increase of rigidities results in uneconomical design. It may be not wise to put too much materials into the structure for wind design alone.

A simple way of increasing structural rigidity is to enlarge the cross sectional dimensions, and in the case of vortex excitation this brings on a double benefit again; that is, the increase in resonance wind speed can be attained by the increase of both natural frequencies and cross sectional size of the structure.

Alternative way to contribute to stiffness is to provide additional members or to give constraint to the members susceptible to wind-induced vibrations. The increase of torsional stiffness in particular is very effective to raise the critical wind velocity for flutter Tie wires sometimes used on multi-stay cables of cable stayed bridges or the vertical hangers of through-type arch bridges.

These tie wires contribute to both stiffness and damping of the original members, but it has been shown that the tie wires on stay cables are subject to high repeating stress and fatigue failures at the connection occurred in some of the bridges.

Passive Damping Devices:

Direct energy absorbers and energy dissipaters are most prevailing to damp the response of the structure, prevent resonant energy buildup and sometimes augment the critical wind velocity of dynamic instability. As seen in Table 2, various damping devices have been developed and put to practical use. These “dampers” are generally classified into passive and active types.

As compared with active control devices that will be described later, passive systems rely only on behavior of their materials or their mechanism and, given proper design and adequate quality of fabrication, they will act reliably with little dependence on maintenance activities or external systems. The passive dampers are further assorted by whether tuned or not.

The most simple idea of artificial damper is realized by inserting viscous or viscoelastic materials to the members concerned or their connections or by connecting such members with tie ropes or inserting spacers. Although the latter may primarily aim at increasing constraint or keeping the positions of the members, they also result in increase of the structural damping of connected members due to the constraint given to the member and the interaction between the motions of the members connected each other.

The use of tie ropes in cable stayed bridges and the associated problem have been just mentioned in the previous section. On the other hand, the shear type viscous dampers are provided near the ends of stay cables of the Sakitama Bridge in Japan. Other examples of the adoption of viscoelastic damper are on the frame of tall buildings such as the World Trade Center Towers in New York and the Columbia Seafirst Center in Seattle. Care should be given to the selection or the material properties, in particular their temperature dependence and aging effects.

During the period from 1960’s to the mid-1980’s, sliding blocks or viscous dampers connected to the top of suspension bridge towers at free standing stage were widely employed Submerged block or mass spring system may be substituted for the sliding block. But since the tuned mass damper (TMD) of pendulum type was used on the Meikoh-Nishi cable stayed bridge tower in 1984, a variety of TMDs have been developed and employed to suppress the wind- induced vibrations of slender bridge towers during erection.

The compact TMDs with long natural period are now available. These TMDs are, if necessary, left even after completion of the bridge as permanent fixtures. TMDs are also used on tall buildings to reduce their motions insofar as they affect occupant comfort.

The natural frequency of TMD is tuned to that of the structure. The necessary mass of TMD may be only several percent of that of the structure, whereas the optimal damping of TMD depends on the type of vibration concerned. It should be reminded that TMD is effective only for the vibration mode whose frequency is tuned.

The tuned liquid damper (TLD) which relies on liquid motion in a container is similar to TMD Tuned frequency and appropriate own damping are required. Damping control of the liquid damper is made by utilizing boundary layer on the bottom surface in shallow water case, by generating turbulence through nets or rods placed in the liquid in deep water case, or by other methods.

Another type of liquid damper is the tuned liquid column damper (TLCD) utilizing the motion of liquid in U-shape tube. Frequency adjustment can be done by controlling air pressure in the tube, while the orifice in the tube servers for damping control. TLCD can be applied not only to horizontal motion but to rotating motion of the structure in the vertical plane.

The advantages of the liquid damper are:

(a) Simple and economical,

(b) Applicable to very small amplitude,

(c) Multi directional (in case of TLD),

(d) Reliable, and

(e) Easy to transfer in general, though it may occupy rather large space according to situations.

Very simple and small TMDs represented by the classical Stockbridge damper have been prevailing technique to prevent excessive vortex excitation of transmission power lines It aims at suppressing mainly the last half-wave generated by the cable oscillation. Other vibration problems of long span power lines are associated with full-span galloping and sub-span wake-induced galloping.

TMDs may also be provided at the center of cable span to alleviate the galloping of power lines. Countermeasures for sub-span wake-induced galloping occurring in bundled conductors have included the special spacers to detune the various cables in a bundle from each other and the energy dissipating spacers, though they are not always fully effective.

The similar techniques to the above mentioned have been applied to the vibrations of stay cables or diagonal hangers of cable-supported bridges. Among various preventive measures, use of cable-to-cable ties, viscous dampers and aerodynamic means were already mentioned. Although the Stockbridge dampers were employed in several European bridges, they are not popular for aesthetic reason. The most prevailing recently is the installation of viscous or hydraulic (oil) dampers near the cable anchorage.

The impact damper is generally categorized in TMD. The vibration energy of the structure is absorbed by the impact between the buffer fixed to the structure and the moving block, ball or chain. The block suspended by wires or the suspended chain itself moves as a pendulum which has own natural frequency. They are applied to light, vertical structures such as light pole.

The application of a gyroscope to suspension bridge flutter and building motions was once investigated. It is one of the passive type devices to counter the external action induced by wind, and the experiments proved its effect.

Active Control Means:

Active devices depend on a control loop that provides a force to counter those responses induced by external disturbances. More concretely, the control force estimated from the dynamic response of the structure, which is detected by a sensor, is generated by driving an actuator and applied to the structure to suppress the dynamic response.

The advantage of the active system is that, if the correct control algorithm is developed for the structural system in which it is installed, the precisely correct response of the structure can be commanded by the actuator, while disadvantages lie in the cost, reliability, and maintenance cost.

Active mass dampers which have recently been put into practical use are classified as seen in Fig. 2, the respective features of which are as follows:

(a) Active Type:

The mass is driven by actuator, and its reaction acts controlling force suppressing the response of the structure. The mechanism is simple, but the actuator expends much energy.

(b) Hybrid Type:

Passive type damper (TMD) is driven by actuator, and its reaction acts as controlling force Although the system is more complex, the TMD works even if power failure occurs.

(c) Semi-active Type:

The actuator controls the dynamic characteristics of the structure so that the vibration of the structure is suppressed or the effect of the control system is optimized.

The active mass dampers, in particular hybrid or semi-active type, were employed during the erection of a few cable-supported bridge towers and on some buildings in Japan.