Chall Vibratn in Eng Strc

8/6/2019 Chall Vibratn in Eng Strc

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CHALLENGING VIBRATION IN

ENGINEERED STRUCTURESBrian Breukelman, P.Eng.

Brian Breukelman, P.Eng.

AUTHOR BIOGRAPHY

Mr. Brian Breukelman is General Manager for Motioneering Inc. of Guelph, Ontario Canada, an international

designer and supplier of damping systems. Prior to joining Motioneering, Mr. Breukelman was technical director of

damping systems at Rowan Williams Davies & Irwin Inc. (RWDI). He is a graduate of the University of Western

Ontario and obtained his Masters degree at the Alan G. Davenport Wind Engineering Group focusing on the

behaviour of a highrise office tower with a damping system installed.

ABSTRACT

Vibration is an ongoing and increasingly common concern in the structural engineering community. The culprits

include wind, earthquakes, pedestrians, mechanical systems, advanced materials, efficient engineering tools, novel

and unique architectural design. Many design states can be affected by vibration, including safety, fatigue,

deflection and comfort.

Traditional engineering approaches to reduce vibration often remain cost effective. However, when designers have

exhausted these possibilities, an alternative solution which takes advantage of newer technology or ideas can be

implemented, without significant cost penalties. A primary source of this technology is the field of mechanical and

automobile engineering where much has been developed to solve vibration problems. Many of these approaches arenow making their way into the realm of structural engineering.


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INTRODUCTION

Vibration associated with civil structures continues to be a timely topic, both for the engineering community and for

those involved with constructing and owning such structures. A quick review of any recent conference proceedings

reveals that there are many approaches to solving problems related to vibration. It is the intent of this paper to

discuss vibration in general and outline a more general approach to dealing with vibration.

Structural engineering is seeing a fascinating revival. Super tall buildings, extremely long bridges, ultra-slender

monuments - these all are being designed and constructed, the possibility for which has much to do with recent

advances in computing and materials. However, these unique structures have become much more within a domain

where vibration can be the dominant design issue. This can be compared with historical structures where strength or

deflection was the primary driver for the design.

These developments have not happened overnight. Some advances have had negative consequences for our

community - a few notable projects have had rather severe vibration problems. Understanding these and making

appropriate changes has only served to accelerate the knowledge base and the possibility of pushing the envelope

along even further.

With this in mind we can understand the two fold meaning of the title: Structural vibration can be a difficult

challenge, while at the same time, the engineering community is challenged to deliver appropriate solutions for these

problems. The following overview will attempt to show how structural vibration can be both an annoyance and aboon for advances in our discipline.

THE ISSUE

Vibration is an ongoing and increasingly common concern in the structural engineering community.

THE CULPRITS

From a historical perspective, fluid flow was the first mechanism that caused vibration problems for civil structures.

It could also be said that earthquakes contributed to vibration problems, but in general this has been a more recent

phenomenon from a vibration perspective. There are many historical references to the phenomenon called vortex

shedding - Leonardo Da Vinci, in the 15th

century sketched the vortices behind a pile in a stream they might havelooked something like the vortices pictured in Figure 11. In wind this phenomenon combined with other

aerodynamic process cause a wide variety of vibration problems.

Figure 1: Computer Generated Vortices - "Von Karman" Vortex Street

1Cesareo de La Rosa Siqueira, Vortex Shedding studies using Computational Fluid Dynamics (CFD) in Unstructured Meshes for2D and 3D flows, Website: http://www.mcef.ep.usp.br/staff/jmeneg/cesareo/Cesareo_HomePage.html


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With increasing height of buildings and slenderness of bridges, fluid flow in the form of wind has become a major

contributor to vibration problems in the structural community. In addition, the desire for various architectural

details such as pinnacles, spires and signs or other lightweight elements has also added numerous areas where wind-

induced vibrations can exist.

Earthquakes have caused destruction to civilization ever since man began constructing permanent shelter.

Lightweight tents and canopies are usually immune to the effects of earthquakes. During the 20th century, with its

notable seismic disasters, we saw a huge effort to understand the behaviour of earthquakes and how damage occurs

in structures. Obviously for very short structures, amplification of the ground motion in upper levels is not likely,

but for the significant buildings, bridges and dams that are constructed today, this is not the case. The study of the

dynamics of civil structures under seismic excitation has become relatively mature, especially now that alternative

design approaches are being taken, including the use of base isolation and energy dissipation.

Pedestrians have likely always caused vibration problems, especially for bridges and more recently for long span

floors. The old tenet of requiring the troops to break step prior to crossing a bridge comes to mind here; this adage

supposedly came from a number of actual experiences in Europe in the first part of the 19 th century. Another source

of human caused vibration comes from coordinated fitness activities such as aerobics or dancing in multi-use

structures, this also being a relatively new phenomenon.

A variety of mechanical systems are connected to and supported by civil structures. This would include obvious

systems such as HVAC units, generators or other motors. There is no limit, however, in the type of mechanicalsystem that might cause vibration any system that involves rotating components has the possibility of causing

vibrations in a structure. Historically, most mechanical installations performed acceptably, but now due to the

lightweight and efficient design and construction in the structural world, problems continue to crop up.

Advanced materials, from exotic carbon fibre composites and even more typical higher strength steel and

concrete, have been a real boon to the construction industry. Structures that in the past could not have been built can

now be cost-effectively constructed. In most instances this has led to more slender and lighter structures. Only from

a seismic vibration perspective has this been helpful as the tendency to reduce the mass reduces the possible seismic

forces. However, for other vibration sources, especially wind, vibration issues continue to grow in number and

complexity.

In some situations relating to advanced materials, the manufacture of these requires extremely low levels of

vibration. The quality and possibility of a variety of micro-electronic components and future nanotechnologydepend on next-to-impossible requirements for manufacturing precision. The presence of even very low levels of

vibration in these facilities, perhaps from someone walking down a corridor, would seriously affect the outcome of

the manufacturing processes.

Tied to the advent of more advanced materials is the fact that engineers have better design tools. A variety of FEA

(Finite Element Analysis) packages allow engineers to accurately determine the load distribution in a structure and

optimize the overall demand on the structural system. This and the presence of advanced materials has brought

about a revolution in the architectural design practise as well, leading to the final culprit.

Significant strides have been made by the architectural community, due in part to these advances in the structural

community. It is not a coincidence that as these materials and techniques have advanced, the envelope of what was

possible aesthetically also sees growth. Numerous architects are leading the way with increasingly complex and

ground-breaking designs. The Guggenheim Bilbao, the Quadracci Pavillion at the Milwaukee Museum of Art or theGlasgow Science Centre Wing are some examples of this leading edge.

THE OUTCOME

Many design states can be affected by vibration, including safety, fatigue, deflection and comfort. Historically the

primary consideration for the structural engineer and architect was the safety and deflection of the building or

bridge. Recently however, almost every design state is being challenged by vibration, including comfort and

fatigue. This trend has forced the structural engineering community to adapt its historical approach to include a

significant focus on vibration in general and on methods of reducing the impact that these vibrations have.


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THE SOLUTION

Traditional engineering approaches to reduce vibration often remain cost effective. This would include making

stiffness or mass changes, or investigating modifying the shape of a bridge or building section. However, when

designers have exhausted these, it is possible that a novel solution exists using newer technology or ideas, one that

can also be price competitive. A primary source of this newer technology is the field of mechanical and automobile

engineering where much has been developed to solve vibration problems. Many of these approaches are now

making their way into the sphere of structural engineering.

Case Studies

1. Dublin Spire, Dublin, IrelandDesign issues: safety and aerodynamic stability

Figure 2: Early evening view of the Spire of Dublin

A fascinating new monument was commissioned in July, 2003 on the site of the former Nelson Pillar in Dublin,

Ireland. Shown in Figure 2, it is likely the most slender structure to have been constructed to date. Its height soars

to 120m from a 3m diameter base which also gives it the distinction as the worlds tallest sculpture.

The outcome of a competition for the project, called the O'Connell Street Monument, is the spire designed by the

British architectural firm, Ian Ritchie. The structural engineer was Ove Arup and Partners of London, UK.


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Figure 3: Spire Section showing TMDs

Of particular concern for the stainless steel structure was its aerodynamic stability. For some structures with very

low levels of damping, and especially those that are very slender, the stability during certain wind events can

become a dominant design issue. For this structure, it was expected that the inherent damping levels (rate of energy

dissipation) could be low, even down to 0.2% which if this were to happen would cause concern for the safety of thespire. As a reference most steel high-rise structures are assumed to have about 1.0% inherent damping.

Traditional engineering approaches might have included such ideas as reducing the height, increasing the mass, or

even changing the shape. However, as this project was intended to be a sculpture, the architecture prevented most of

these changes. Only increasing the mass would have had a beneficial effect, but would have come at significant

increases in fabrication and supply costs, as well as impacting the foundation design.


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Figure 4: Internal view of spire

The solution to this potential vibration problem was to install two Tuned Mass Dampers (TMD) which dissipate

dynamic energy from the two modes of vibration that were the cause of concern to the structural engineer. The two

TMD masses, weighing 800kg (1760lb) and 1250kg (2750lb) were suspended as natural pendulums at

approximately two-thirds the height of the spire as shown in Figure 3. Combined with appropriately specified

viscous dampers, these TMDs increase the equivalent damping to well above 1%, ensuring the aerodynamic stability

of the spire in all wind conditions.

The TMDs were constructed entirely of stainless steel components to match the materials of the spire itself. This

was done to ensure that corrosion would not be a problem through the design life of the spire. A specially designed

monitoring system is being installed to ensure that the TMDs are continuously operational. Figure 4 is an internal

view looking up within the spire showing the TMDs with a variety of mechanical items and drains as well as a

ladder allowing access to the top portion of the spire.

2. Bloomberg Building, New York, NYDesign Issue: occupant comfort

Figure 5: Artist Rendering - Bloomberg Tower


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Designing a mixed use building is a challenge in most environments designing a high-rise variety in New York

City can be nightmarish. The Bloomberg building which has been under construction since 2001 is a case in point.

Designed by architect Cesar Pelli, the lower floors for retail and office occupancy are constructed of steel. The

upper portion, a high-rise luxury condominium development, is constructed of reinforced concrete.

Wind engineering studies performed by Rowan Williams Davies & Irwin Inc. demonstrated to the structural

engineers, Thornton Tomasetti, that lateral accelerations on the top levels of the building would be higher than that

desirable for luxury condominiums. A number of structural iterations were performed in an attempt to optimize the

structure to reduce the predicted motion. Consideration was also given by the developer for a building with shorter

height this being a very effective method of reducing vibration, but it also seriously impacts the viability of the

development project as the most valuable real estate is the top portion of the tower.

The structural optimization studies performed indicated that due to the multiple structure types and materials, the

motion at the top of the building was difficult to control from a purely structural approach. As the development

program required a less than ideal structural system, it was decided to implement a damping system solution to

reduce the vibration for the top portion of the building.

Two different types of damping systems were considered: a TMD and Tuned Liquid Column Damper (TLCD). A

TLCD works similarly to a TMD except it dissipates energy internally in the liquid (water in this case) and the mass

is a large U shaped tank of water. Due to the lower density of water compared to steel, a TLCD takes up

substantially more space. The space required for an appropriate TLCD installation for the Bloomberg building wasbeyond what was available. As a result a TMD solution was chosen a birds eye view of the system is shown in

Figure 6. The TMD weighs 545 tonnes (600 tons) and will be installed by Motioneering in early February 2004.

Figure 6: Bird's Eye view of Bloomberg Building TMD


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3. Las Vegas FootbridgesDesign Issue: pedestrian comfort and deflection

While investigating the expected accelerations of proposed pedestrian bridges in Las Vegas, it became clear that

under certain occupant/event conditions, the bridges could have vibrations of a magnitude that would cause concern

for the structural performance.

To date, three slender footbridges, similar to that shown in Figure 7, having clear spans ranging from 40m (130ft) to

49m (160ft) have been constructed in Las Vegas. Traditional structural solutions were implemented for other

similar bridges in the area, but for the subject three bridges an alternative approach was required. The depth of the

span was limited to 1.5m (5ft) which led to vertical frequencies in the 1.7Hz to 2.2Hz range. It is in this range that

typical pedestrian vertical excitation is possible and if the damping of the bridge is very low, uncomfortable

vibration levels are possible.

It was also discovered that if pedestrians could coordinate their activities, sufficiently large deflections of the bridges

were possible. The structural engineers, Martin & Peltyn of Las Vegas, considered other structural solutions,

including making the bridges very heavy and increasing the stiffness. However, as the spans were simply supported,

this approach would have been very inefficient and very costly.

The solution for both the pedestrian comfort and the possible deflections was to add damping. By implementing a

system of six TMDs (as shown in Figure 8), weighing approximately 8000 kg (9 tons) in total, the effective damping

of the bridges was increased by more than an order of magnitude.

Figure 7: Pedestrian Bridge from Bellagio to Caesars Palace


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Figure 8: Layout of TMDs on pedestrian bridge

4. Taipei 101 PinnacleDesign Issue: wind-induced fatigue

Late in 2003, the newest entrant in the worlds tallest building list became a reality. The 508m high Taipei 101

located in Taipei, Taiwan with its 60m pinnacle (as seen in Figure 9) surpassed the Petronas Towers which have

held the distinction of worlds tallest since their completion in 1998. The architect for this project is CY Lee &

Partners, while the structural engineer was Evergreen Consulting Engineers in association with Thornton Tomasetti.

Figure 9: Installed Pinnacle - October 2003


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Located in one of the most adverse construction environments due to significant seismic activity and constant

typhoons in season, the Taipei 101 structure required a considerable engineering effort to ensure life safety and

comfort. However, it was the pinnacle structure that demanded the most innovative solution due to the potential for

fatigue damage.

In designing the pinnacle, the first approach was to keep the structure as light as possible. This was due to the desire

to keep the demands due to seismic responses lower, both for the pinnacle itself as well as for the tower structure.

However, with a light pinnacle, the wind-induced vibration and the resultant fatigue from many cycles of this

vibration became the dominant design issue.

The traditional approach of increasing the mass of the pinnacle would have caused serious implications for the

overall tower design with respect to seismic loading. Another option that of changing the shape to reduce the wind

effects was considered by the owner and architect, but for a variety of reasons this was not implemented.

Due to the overall structural system for the building, several of modes of vibration also included motion of the

pinnacle. Three modes (six if counting the perpendicular direction) were found to be affected by vortex induced

vibration; however, only two were found to be significant relating to fatigue damage. Figure 10 shows the bending

moments and total number of vibration cycles generated in the structure at the base of the pinnacle due to wind

excitation.

0

10000

20000

30000

40000

50000

60000

70000

80000

90000

Basedynamicm

oment(kN*m)

1E3 1E4 1E5 1E6

No. accumulated cycles in 100 years

Mode 7 Mode 10 Mode 12 M10 TMD4 M12 TMD4

mode 7 VIO

mode 10 VIO

mode 12 VIO

Figure 10: Vortex Shedding (VIO) responses of Taipei 101 Pinnacle


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To reduce the fatigue damage in the pinnacle, a system of two TMDs was designed and installed by Motioneering.

The TMDs will be tuned so as to provide the most benefit to the structure. With reference to Figure 10, it can be

seen that a significant amplitude reduction in modes 10 and 12 can be obtained by these TMDs (as indicated by the

unfilled squares). Each TMD as seen in Figure 11 weighs 4,500kg (9,900lb) and they are located near the tip of the

pinnacle.

Figure 11: CAD rendering of TMD mass and pinnacle structure

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Chall Vibratn in Eng Strc