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Posted by on in Uncategorized
For folks who aren’t architects (like me), we don’t give much thought to building design.  We blissfully carry out our lives in and around structures.  I’ve had the privilege recently to try to understand how a building takes shape during the design process.  I’ve discovered that there seem to be 2 methods at either ends the same spectrum.  Top-down and bottom-up.  Each has an profound affect on not only the designs an architect produces but also how they run their business, engage with clients and consultants.
 

Top Down

This approach is often driven by a strong personality (usually a director) with a creative vision.  That creative vision is influenced predominantly by that individual’s style and personal tastes.  Clients select these architects because there is often something in the design that resonates with the client.  In top down architecture, aesthetics reign supreme over most other design factors.  With top down there is often a piece of the architects personality in the design.  So any factor that threatens that design is often taken personally.
 
The advantage to top down is that clients know what they’re getting.  There is often an obvious design language that shines through no matter what the context of the project.  Clients are drawn to it and will select an architect because of it.  The disadvantage is that the architects desire can become a risk factor for the client too.  If they’re too stubborn to challenge their own design language for the good of the project, clients can feel like they’re not getting the outcome they want.  Top down architecture is a challenging balance between client needs and aesthetic form.  It often assumes that you can’t have both - its either/or.
 
Eureka Tower by Fender Katsalidis
Eureka Tower by Fender Katsalidis
 

Bottom Up

This seems to be an emerging practice, particularly in Melbourne.  Bottom up doesn’t start with any defined design language.  The design starts with client needs first and foremost aswell as site context.  Its a far more fluid process where both artistic and scientific inputs are related to the design during various iterations.  Aesthetics are still important, but they’re more responsive than oppressive.  This can result in some really creative designs that clients love.  The type of client that selects this approach is a little more adventurous.  Yet they still want to drive the design process - they want a piece of their personality in the design - not just the architect.  They want to use the design as a way to improve project outcomes.
 
Bottom up assumes that you can have striking form and meet the needs of clients.  It is a both/and approach compared to top down.  Architects that master the bottom up approach become known not for design language or style, but for project outcomes.  Bottom up also requires architects to really push boundaries of construction methods, planning regulations and engineering assumptions.  These things are not for the feint hearted.
 
 Abode 318 by Elenberg Fraser
Abode 318 by Elenberg Fraser

 

Which is best?

I think its about personal preference really.  I tend to sway towards bottom up, mainly because I believe in this approach to other areas in business (like The Responsive Organisation movement).  Of course there is also another way to look at this.  That is top down and bottom up are really a spectrum.  Different firms would sit at different places on this spectrum and perhaps change depending on the market conditions.  In any case, I find the complexity behind the process fascinating.  It makes me look at and experience buildings entirely differently.  Observing them and wondering - how did they arrive at that design outcome?
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Join our online Wind Engineering Q&A session on Google Hangouts.  This will be your chance to talk about wind loads with our Principal Wind Engineer, Dr Seifu Bekele.  Our aim is to help engineers and architects understand wind and use it to their advantage in the design process.  There will be no agenda, however here are the typical topics we tend to come across with our work:

  1. Damper design and building motion.  Which standard should we use?  What return period is appropriate?  How do we minimise or eliminate the need for an active damper system?
  2. Internal pressures.  How do we model and design for internal pressures that may change with respect to building openings?
  3. What type of wind tunnel test method is appropriate for different building designs?  Aerolastic, high-frequency force balance, surface pressure etc.

These discussions allow us to share our current areas of research which we do in partnership with the Monash University.  We want to teach more people about wind, so we're offering a Google Hangout session for anyone to call into for free.  If you're interested, please fill in the form below and we'll send you the information.  Date and time will be determined based on the time zones of people who register.  We'll try to pick a time that works for everyone.

 

Click here to Register

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Posted by on in Structural Wind Load

The increasing height of tall buildings, accompanied with the use of light materials, means that modern day buildings are becoming more sensitive to wind. These flexible buildings may be subjected to excessive levels of vibration under the action of wind. Excessive levels of vibration adversely affect serviceability and occupants’ comfort in a building. In response to this challenge, to achieve design objectives, there are number of motion control methods suggested and implemented by building designers.

The wind structure interaction is mainly characterized by the along wind and across wind responses. The across wind dynamical responses are initiated by vortex shedding. When wind blows perpendicular to the face of the building, vortices are shed alternately from opposite sides of the building. When a vortex is formed in one side of the building, the wind speed is increased on the other side (which correspond to low pressure). These fluctuating vortices induce fluctuating forces perpendicular to the wind direction. This fluctuating load is responsible for most of the dynamical responses of buildings. Alternating vortices from five different building shapes are shown in Figure 1.

 

Figure 1 Vortices generated behind a building

The vortex shedding induced lift forces can lead to prohibitively large across wind motions and their mitigation becomes a principal design concern [1]. The following are some of the suggested methods to reduce the dynamic response created by vortex shedding-induced forces.

Building Layouts: Research into vortex induced excitation show that the peak response corresponds to a critical reduced velocity of 10 and above [2]. This reduced velocity is a function of wind speed at roof height (VH), building frequency (f) and building dimension of the small side (L) and is determined by the following equation:

By increasing the building dimension, thereby reducing the aspect ratio of the two sides, it is possible to reduce the vortex induced responses.

Aerodynamic Modification: The organized shedding vortices can be reduced by changing building’s cross-sectional shape. Some of these treatments are chamfered corners [3], building openings [4, 5], tapering, and drop of corners [6]. By using these techniques, considerable reductions of dynamic loads are reported. The tallest building, Burj khalifa utilized this aerodynamic treatment to reduce dynamic wind response effectively [7].

Building Properties: The resonant structural responses of buildings are directly related to building mass, frequency and damping. The damping directly from the building has a limited variability according the type of building (ie. prestressed concrete or steel) and is difficult to predict. However, the building mass and stiffness can be varied to reduce the dynamic response due to vortex shedding. If the building frequency is kept constant and the mass increased, then the predicted building accelerations can be reduced. Similarly, higher frequency also leads to a reduction in the predicted acceleration, as well as predicted dynamic loads.

Misalignment of Excitation and Response Directions: This is based on the idea that vortex shedding induced forces are important only for the wind directions which are approximately normal to the face of a building. Thus, organising the structural system so that its principal axes of stiffness are along the building diagonal is suggested. This leads the generalized forces for these diagonal modes of vibration to become a mixture of drag and lift forces that are more random or broad band in comparison with pure vortex shedding lift forces that results in a narrow band. This arrangement reported by Isyumov [8] resulted in a substantial reduction in the dynamic responses.

Dampers: Recently, using auxiliary dampers to reduce the dynamic responses is becoming more acceptable. Tuned mass, viscous and tuned liquid dampers are commonly used types. The type of damper depends on the available space, budget and level of dynamical reduction desired. Tuned liquid sloshing dampers are becoming more popular due it their ability to be incorporated into the building water reserve. The tuned liquid damper devices achieve a reduction of the dynamic responses by sloshing in water tanks and dissipate energy. Wind tunnel test for 220m residential building resulted with top floor acceleration above the recommended criteria. This high acceleration can be reduced using liquid dampers.

Auxiliary dampers are also used to reduce dynamic loads due to seismic loads. Reducing wind and seismic loads means that less material is required in the construction of the building. Thus, sustainable building design which optimises the structural efficiency uses auxiliary dampers.

Wind tunnel study for structural loads and human comfort study has been in use for decades. The cost of building construction can be reduced substantially by predicting the wind load tailored to the individual building rather than using a general guideline, which leads to conservative load estimations and higher cost or increased risk. The disasters caused by strong winds are minimised due to an increased understanding of wind characteristics, modelling techniques, dedicated wind tunnel studies and the years of experience gained in the field. Advances in the measurement techniques and analysis in wind tunnel testing, as well as in building materials, has opened a door to the construction of taller and more complex buildings and reduce building motion.

To cope with the demand of the present day super high-rise towers in terms of vertical transportation, motion control and sustainable design, researchers and practitioners contribute to the advancement of the knowledge in building construction. The wind tunnel usage, computational fluid dynamics and the related analysis methods are also expected to continue growing to fulfil the future demand. .

 

 

References

[1] Davenport, A.G., “The Dependence of Wind Loading on Meteorological Parameters”, In Proc. Int’I Res. Seminar on Wind Effects on Buildings on Structures, University of Toronto Press, 1968

[2] Lin, N., Letchford, C., Tamura, Y., Liang, B., Nakamura, O., “Characteristics of Wind Forces Acting on Tall Buildings” , J. Wind Eng. Ind. Aerodyn. 93, (2005) 217-242

[3] Kwok, K.C.S, “Effect building shape on Wind-Induced Response of Tall Building”, J. Wind Eng. Ind. Aerodyn. 28, (1988) 381-390

[4] Dutton, R. J., and Isyumov, N., “Reduction of Tall Building Motion by Aerodynamic Treatments”, J. Wind Eng. Ind. Aerodyn. 36(1990) 739-747.

[5] Bekele, S., Putten, K.V., “Tall Buildings Base Openings through Size and Shape Relation to Wind Loads and Local Flow Structure”, CTBUH 7th World Congress, New York, 2005

[6] Kareem, A., and Tamura, Y.,” Mitigation of Wind-Induced Motions of Tall Buildings, Tall Building Structures: A World View”, CTBUH, Lehigh University, 1996

[7] Baker, W.F., Korista, D.S., and Novak, L.C., “Engineering the World Tallest – Burj Dubai”, CTBUH 8th World Congress, Dubai, 2008

[8] Isyumov, N., Steckely, A., Amin, N., and Fatehi, H., “Effects of the orientation of the Principal Axis Stiffness on the Dynamic Response of a Slender Square Building”, J. Wind Eng. Ind. Aerodyn. 36, (1990) 769-778

 

 

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It's as Melbourne as the MCG, meat pies and cold days in summer. It's wind whipping throughDocklands(link is external).

When there is even a breath of wind about, Docklands is like one of those cool machines that simulates sky diving(link is external). OK, maybe that is a little over the top, but you get what I mean. I understand that it was a windy place prior to development, but I do not think that is an excuse.

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