Strategies to Reduce Lateral Forces on High-Rise Buildings That Use Diagrid Structural System

8/2/2019 Strategies to Reduce Lateral Forces on High-Rise Buildings That Use Diagrid Structural System

1/4

Strategies to Reduce Lateral Forces on High-rise

Buildings that Use Diagrid Structural System

Binh K. Nguyen and Hasim Altan

School of Architecture

The University of Sheffield

Sheffield, United Kingdom

Email: [email protected] and [email protected]

Abstract The study focuses on strategies to enhance the

performance of a building against lateral forces and earthquake.

The strategies range from aerodynamic modifications to

structure reinforcement to adopting auxiliary damping

devices/systems. The studys direction was toward Diagridstructural systems the latest trend in high-rise projects.

Keywords-component; lateral forces; tall-buildings; high-rise;

diagrid, damping strategies.

I. INTRODUCTIONThe evolution of tall-building structural systems, based on

new structural concepts with newly adopted high-strengthmaterials and construction methods, has been towardsstiffness and lightness. Structural systems are becoming

stiffer and lighter. Diagrid1, acknowledged worldwide as a

very light structure and one of the best when it comes towithstanding lateral forces, has been leading the trend.

The lighter a structure is, the higher it can rise. On the otherhand, it is also easier to blow away a light subject than a heavyone. Diagrid can save from 20% to 30% the amount ofstructural steel in a high-rise building. Moreover, high-strengthmaterial technology has come a long way since the invention ofmodern high-rise building in 1930s. Materials themselves arestronger and lighter.

It is common knowledge that, rather than directly standingthe forces, it is better to reduce them and dissipate themagnitude of vibrations. A high-rise structure needs bothstiffness and damping characteristics. The strategies to enhancetall-buildings lateral performance can be divided into 3categories: (1) Aerodynamic Modifications; (2) Structural

Reinforcements; and (3) Using Auxiliary DampingDevices/Systems. The strategies presented below are the onesthat can be used for that use Diagrid structure. However, mostof the strategies can be applied to other types of tall-buildingstructures.

1Diagrid (ordiagonal grid) is a design for constructing large buildings with

steel that creates triangular structures with diagonal support beams. It requires

less structural steel than a conventional steel frame. It also obviates the need

for large corner columns and provides a better distribution of load in the case

of a compromised building.

II. AERODYNAMIC MODIFICATIONSA. Aerodynamic Shapes

The form of a tall-building is usually limited to rectangularprisms. From geometrical point of view, this form is rather

susceptible to lateral drift. Other building shape such ascylindrical, elliptical, crescent, triangular and like, offer betterlateral performance due to inherent strength in theirgeometrical form. They provide higher structural efficiencyand allow greater building height at lower cost. Building codes

permit a reduction of the wind pressure design loads forcircular or elliptical buildings by 20%-40% of the usual valuesfor comparably sized rectangular building [1].

B. Corner ModificationInvestigations have established that corner modifications

such as chamfered corners, horizontal slots, and slotted cornerscan significantly reduce the along wind and across wind

responses when compared to a basic perpendicular buildingshape [2]. Fig. 1 shows some types of modification to buildingcorner.

Chamfers of the order of 10% of the building width makes40% reduction in the along wind response and 30% reductionin the across wind response [3]. Excessive rounding of cornersof the cross section, approaching a circular shape in the crosssection, significantly improves the response against wind. Witha building of roughly 70 stories, peak deflection of the modelin circular cross section was about half of the one with squarecross section [3].

Figure 1. Corner modification types

C. Tapering and SetbacksReducing floor areas gradually toward the top is a good

strategy to enhance lateral performance of a building. Thisway, the mass of the building is concentrated in the lowerfloors. More importantly, when hitting a building using

Fin VentedFin

Slotted

CornersChamfered

CornersStair-step

Corners


2/4

tapering and setbacks, the vortices will try to shed at differentfrequencies at different heights. In other words, the winds

become confused and incoherent, which can dramaticallyreduce the associated fluctuating forces [4].

Examples of tapering strategy are: John Hancock Center(Chicago), Millennium Tower (Tokyo) (see Fig. 2), andTransamerica Pyramid (San Francisco). The Jin Mao Building(Shanghai) (see Fig. 3) and the Petronas Tower (KualaLumpur) are good examples of using setbacks to slightlytaper the building shape.

While setback is rather tricky to apply to a Diagridbuilding because of the continuity of the structure, tapering ishighly suitable.

D. Sculptured Building TopsThe sculptured building tops not only highlight the height

of the structure, but also serve the practical aerodynamicpurposes such as reduction in the wind response of thebuilding. The more sculptured a buildings top, the better it canthe along wind and across wind responses. Examples: Jin MaoBuilding, Petronas Towers, etc.

E. Varying Cross-section ShapeVarying cross-section shape with height, e.g. going from

square to round, can force the wind to behave differently,preventing in becoming organised. This results in confusedvortices, which is a similar effect to using tapering and setback[4]. Examples include Buji Khalifa (Dubai) and the SearsTower (Chicago).

F. Rotated and Twisted FormsRotated building forms can minimize loads from prevailing

directions, while twisted forms can avoid simultaneous vortexshedding along buildings height.

G. SpoilersVortex shedding can also be reduced by adding spoilers

to the outside of the building. The most well-known form ofspoilers is the spiral Scruton strake which is widely used oncircular chimney stacks [4]. It is very convenient to use thisstrategy on Diagrid structures due to the geometry benefit ofthe grid. Example: 30 St Mary building (London) (see Fig. 4).

H. Through-building OpeningsIn sustainable design of tall buildings, opening areas are

fundamental. This can be exploited to provide areas whichwind can blow all the way through, thus reduce wind loads.Diagrid, with its unique structure characteristic, can providerandom openings without affecting the structure at all. Multi-openings is also a good strategy. Example: Shanghai WorldFinancial Center (Shanghai) (see Fig. 5)

III. STRUCTURAL REINFORCEMENTSWithout going too deep into the technical aspects, basically,

structural reinforcement strategies that can be applied to high-

rise Diagrid structures include:

- Super-columns addition: Adding super columns or setsof columns to strategic locations of a Diagrid structure

Fig. 6a.

- Coupled buildings: 2 or more buildings are linkedtogether for better structural performance - Fig. 6b.The Petronas Towers are a typical example of thisapproach.

- Core and Outrigger support: Adding a core andoutriggers to the Diagrid structural shell Fig. 6c.

Figure 2. Millennium Tower Figure 3. Model of Jin Mao Building

Figure 4. 30 St Mary building uses

spoilers to defect wind loads

Figure 5. Shanghai World

Financial Center


3/4

Figure 6. Structural reinforcement strategies

IV. AUXILIARY DAMPING DEVICES/SYSTEMSDamping systems for high-rise buildings can be divided

into two categories: (1)Passive systems and (2)Active systems Fig. 7. Passive systems have fixed properties, and, in orderfor them to perform as intended, they do not require energy,while active systems do need an actuator or active controlmechanism relying on an energy source to modify the system

properties against ever-changing loads. Thus, active systemsare more effective in general. However, due to their economyand reliability, passive systems are more commonly used.

Source: [5]

Figure 7. Types of Auxiliary Damping Devices/Systems

A. Passive SystemsPassive damping systems can be divided further into two

sub-categories:

- Energy-dissipating-material-based damping systemssuch as viscous dampers and viscous-elastic dampers

- Auxiliary mass systems to generate counteractinginertia forces such as Tuned Mass Dampers (TMD) andTuned Liquid Dampers (TLD).

1) Energy-dissipating-material-based damping systemsare generally installed as integral parts of primary structuralsystems at strategic points, reducing the dynamic motion oftall buildings. The damping force in a viscous damper orvisco-elastic damper is dependent upon the time rate of changeof the deformation [5]. Damping is accomplished through the

phase shift between the force and displacement.

An example of viscous dampers, installed as an integralpart of the bracing members, can be found in the 55-storyTorre Mayor in Mexico City the tallest building in LatinAmerica at present, and visco-elastic dampers were installedin the destroyed World Trade Center Towers in New York.Other types of damping systems in which the dampingmechanism is through direct dissipation of energy from thesystem include hysteretic damping and friction damping. Fig.8 presents an example of a viscous dampers system applied toa Diagrid high-rise building.

Source: [5]

Figure 8. Example of a viscous dampers system applied to a Diagrid structure

2) A TMD is composed of a counteracting-inertia-force-generating huge mass accompanying relatively complicated

mechanical devices that allow and support the intendedperformance of the mass. The frequency of the TMD mass isgenerally tuned to the fundamental frequency of the primarystructure. Thus, when the fundamental mode of the primarystructure is excited, the TMD mass oscillates out of phase withthe primary structure, generating counteracting inertia force[5]. Fig. 9 shows different types of TMD.

A TMD system, located near the top of the building for itsbest performance, is installed in a room that is usually notaccessible to the public, as in the cases of the sliding typeTMDs installed in the John Hancock Building in Boston and

a) Super-columns

addition

b) Coupled buildings c) Core &

Outriggers support


4/4

the Citicorp Building in New York. However, the pendulum-type TMD installed in the Taipei 101 Tower is used as adecorative element in the building interior as well, attractinginterest of visitors Fig. 10.

Source: [5]

Figure 9. Different types of TMD

Figure 10. Taipei 101 Towers TMD system

3) TLD, such as tuned sloshing dampers (TSD), usewaving water mass as a counteracting inertia force generator.Thus, this system can be designed using the existing watersource in tall buildings, such as a pool or water tank locatednear the top of a building. In a TSD, sloshing frequencies aretuned by adjusting the dimensions of the water container and

the depth of water. Another type of TLD is tuned liquidcolumn dampers (TLCD), which uses a U-shaped vessel [6].Fig. 11 shows different types of TLD.

Source: [5]

Figure 11. Different types of TLD

B. Active SystemsAn active structural control system is the one that has the

ability to determine the present state of the structure, decide ona set of actions that will change this state to a more desirableone, and carry out these actions in a controlled manner and ina short period time [7]. While some passive systems, such asTMDs or TSDs, are effective only for a narrow range ofloading conditions, active systems can perform effectively

over a much wider range and they are a more advanced formof functional performance-driven technologies in architecture.Examples are active mass dampers (AMD) and active variablestiffness devices (AVSD).

The AMDs resemble the TMDs in appearance, althoughthe vibration of a building is picked up by a sensor, theoptimum vibration control power calculated by a computer,and the movement of the building is reduced by shifting amoveable mass with an actuator [5]. The AVSDs continuouslyalter the buildings stiffness to keep the frequency of the

building away from that of external forces, such asearthquakes, to avoid a resonance condition. Although theircost-intensiveness and reliability issues are limiting the use of

active systems at present, with more research, they would havegreat potential for future applications.

V. CONCLUSIONAmong various types of structures for tall buildings,

Diagrid is the one which has many advantages such as:aesthetic, structural performance, material saving potential,freedom of space arrangement thanks to the lack of structuralcolumns, etc. Diagrid is becoming the predominant choice forhigh-rise projects. Keeping in mind these features, this papersummarises the strategies to mitigate lateral forces which aresuitable for high-rise Diagrid buildings, but also applicable toother types of structures. The strategies range fromarchitectural design techniques such as modifying buildingform, to structural reinforcement tactics, and using dampingsystems/devices.

REFERENCES

[1] H. E. Ilgin and M. H. Gunel, The Role of Aeroynamic Modifications inthe Form of Tall Buildings against Wind Excitation, METU JFA2007/2 (24:2), pp. 17-25.

[2] A. Kareem, T. Kijewski and Y. Tamura, Mitigation of Motions of TallBuildings with Specific Examples of Recent Applications, Wind and

Structures, Vol. 2, No. 3, 1999, pp. 201-251.

[3] A. G. Davenport, The Response of Six Building Shapes to TurbulentWind, Seria A, Mathematical and Physical Sciences. Vol. 269, No.

1199, A Discussion on Architectural Aerodynamics, 1971, pp. 385-394.

[4] P. Irwin, J. Kilpatrick, J. Robinson and A. Frisque, Wind and TallBuildings: Negatives and Positives, The Structural Design of Tall and

Special Buildings, Vol. 17, (2008), pp. 915-928.

[5] M. M. Ali and K. S. Moon, Structural Developments in Tall Buildings:Current Trends and Future Prospects, Architectural Science Review,

Vol. 50.3, 2007, pp. 205-223.

[6] R. J. Smith and M. R. Willford, The Damped Outrigger Concept ofrTall Building, The Structural Design of Tall and Special Buildings,

Vol. 16, 2007, pp. 501-517.

[7] J. J. Connor, Introduction to Structural Motion Control, New York:Prentice Hall, 2003.

Strategies to Reduce Lateral Forces on High-Rise Buildings That Use Diagrid Structural System

Documents

Transcript of Strategies to Reduce Lateral Forces on High-Rise Buildings That Use Diagrid Structural System