Wind load on Building

Wind load on building is a complex phenomenon because of the many flow situations arising from the interaction of wind and structures. Wind is composed of a varying sizes of eddies and rotational characteristics carried along in a general stream of air moving relative to eart’s surface. These eddies give wind its gusty or turbulent character. The gustiness of strong winds in the lower levels of the atmosphere largely arises from interaction with surface features. Near the earth surface, the motion is opposed, and the wind speed reduce by the surface friction. At the surface the wind speed reduce to zero and then begins to increase with height and at some height ,  So it could be at a certain altitude conditions, seismic forces acting on the tall building does not become dominant compared to the influence of the wind. Wind load regulation in Indonesia adopted a regulation of ASCE 7-02 (American Society of Civil Engineers Minimum Design Loads for Buildings and Other Structures). ASCE 7-02    provides three procedures for calculating wind loads for buildings and other structures, including the main wind-force-resisting systems and all components thereof. The designer can use method 1, the simplified procedure, to select wind pressures directly without calculation when the building is less than 18.28 m in height and meets all requirements given in Section 6.4 of the standard. method 2 can be used for buildings and structures of any height that are regular in shape, provided the buildings are not sensitive to across-wind loading, vortex shedding, or instability due to galloping or flutter; or do not have a site for which channeling effects warrant special consideration. method 3 is a wind-tunnel test procedure that can be used in lieu of methods 1 and 2 for any building or structure. method 3 is recommended for buildings that possess any of the following characteristics :

  • Have nonuniform shapes.
  • Are flexible with natural frequencies less than 1 Hz.
  • Are subject to significant buffeting by the wake of upwind buildings or other


  • Are subject to accelerated flow of wind by channeling or local topographic features.

Wind tunnel studies is the good way to analysis wind load on building compare to analytical study. Wind tunnel studies are recommended for situations when wake buffeting may exist due to significant upwind obstructions such as hills or significant aerodynamic term that describes a turbulent fluid region on the downstream side of a body, where strong eddies are generated which may impose critical fluctuating wind loads on structures downstream especially if the frequency content of turbulence excites the resonant frequency of the downstream structure. This wake buffeting can be very important for slender towers and other dynamically sensitive structures where overall structural wind loads can be increased dramatically. Wind tunnel studies are also recommended for structure whose site location makes them subject to channeling effect caused by topographical or neighboring tall buiding such as in the city center or central business. In that situation the wind velocity from certain wind directions will be locally accelerated as the flow is squeezed between the upwind obstructions causing increased wind loading on the nearby structures.


Long Span Bridge Sectional Aeroelastic Wind Tunnel Testing

Wind tunnel test of a sectional model has becoming popular in long span bridge design processes. It is convenient to analyze the bridge deck aeroelastic behavior. The model is simple in structure, hence it can reduce the wind tunnel test cost accordingly, compare to full model. However, the model geometrical shape as well as structural dynamic properties must represent the actual bridge to be built, which is also called the prototype bridge.

Sectional model test has been widely understood as a good tool to predict the critical flutter instability as well as critical vortex induced vibration, particularly at initial deck structure development.  On the other hand, full model test is mostly performed when the engineer want to known the detail behavior of the whole bridge structure, particularly on the interaction of structural members.

In actual structure, the deck of a flexible bridge experiences several mode of oscillation. That is the excitation of its natural frequencies by external forces: life, seismic or wind load. The bridge aeroelasticity relates to wind load excitation.

Structural members can be excited by the wind. For instance, deck heaving, pylon torsion, swaying of suspension wire or other elements. The occurrence characteristics are depended on cross section of the structure, position of mounting, or wind flow characteristics. The phenomena can be catastrophic, such as flutter, or non-catastrophic such as: buffeting, galloping and vortex induced vibration (VIV). However, the non-catastrophic phenomenon contributes much to fatigue processes and discomfort of structure.

Vortex is a typical rotational flow (eddy flow) in the wake of an obstacle. The flow fluctuates in such a pattern which is known as Von Karman Vortex trails. If the frequency of vortex fluctuation coincides with one of the flexible structure natural frequency (fN), a resonance phenomenon occurs. The phenomenon is called Vortex Induced Vibration (VIV).  In case of a sectional model the oscillation could be on heaving, rolling or torsion motion.

A mathematical relation between vortex shedding fluctuation frequency (fvortex) , structure reference length (lref) and wind speed (U) is known as Strouhal formula. The formula produces a constant number for a specific structure, which is called Strouhal Number (St). The fvortex is also sometime called as Strouhal frequency (fs).

The St number is a non-dimensional number and one of the similarity parameter. It is often used to transform the wind tunnel speed of bridge model to the actual wind speed of prototype bridge. During a resonance fs = fN,  hence the St can be calculated. For flexible bridges, the value is in the range of 0.12 to 0.18. By measuring the frequency response of the model structure, the wind tunnel test could produce a relation between fs and U. In addition, the maximum oscillation during a resonance also represents the maximum displacement (Dmax) of the structure [Simiu,1996].

Another interesting phenomenon to note is lock-in. The vortex will not induce the model in a single wind speed. It will induce the model in a range of wind speeds. Lock-in is the phenomenon of VIV where the resonance may occurs longer in a range of wind speeds.

Flutter is the loss of system structure capability to dissipate energy which is received from the wind. The total damping is zero when a flutter occurs. The total damping consists of structural damping and aerodynamic damping. Structural damping can be obtained easily by a modal testing or free vibration analysis. However, the aerodynamic damping is something difficult to determine because it is function of geometrical shape and wind properties, particularly wind speed. Therefore, a wind tunnel test is an important means to obtain the critical wind speed of flutter.

One of the practical method to estimate the flutter speed of a structure is known as Zimmermann method. The total damping and oscillation frequency of the model are measured for each wind speed to produce an indicator parameter which is called  flutter margin (Fm). The flutter occurs when Fm equal to zero.





Aero-Gas dynamics and Vibration Laboratory-Report


Structure and Machine Natural Frequency

Every machine and structure has  natural frequencies . if machines and structures are designed correctly, these natural frequencies will not affect the operation or reliability of the machines. In reality, however, a wide variety of fault condition are either caused by, or are affected by natural frequencies.

 It really is important to  understand what they are, how to detect them and how to correct them.

Plant resonances

The pipes,foundation, and rotating machinery in every plant have natural frequencies. If designed well, the natural frequencies are not excited (much). However, if a machine happened to be mounted on a structure that had a natural frequency equal to speed of the motor, the vibration level would grow considerably-there would be a resonance.

 Resonances amplify vibration. the measured vibration level must be 3 to 50 times higher then they would be normally. So, instead of vibrating at 0,,5 mm/sec, for example, the machine could vibrate at up to 25 mm/sec. the potential for structural failure or a catastrophic failure of the machine is high.

By definition, the natural frequency is the frequency of free vibration of a system. The frequency at which an undamped system with a single degree of freedom will oscillate upon  momentary displacement from its position.

 In simple term, if energy could be injected at all frequency of the structure it will vibrate at its natural frequencies. When a natural frequency is excited, the structure resonates, and the vibration amplitudes are amplified. So the stress can be increase up to 100 times (as compered to the stress at a frequency higher or lower than natural frequency.

 Critical speed

The term critical speed is typically used regarding very large rotors such as alarge steam turbines. These are flexible rotor. As the rotor approaches its natural frequency it will begin to flex. When the machine RPM coincides with the first mode of vibration, the speed is called the “critical speed’.

Why are resonances is important ?

When natural frequency is excited, the structure resonates. The amplitude of vibration will increase significantly, thus the stress of the machine increase significantly. The increased stress reduces the life of the machine component and structures. Weld crack, metal fatigues, bearing fail, and worse.


Without doing any special test, there are two basic ways to tell if you have resonance problem : unusual failures,and tell-tale signs in the spectrum.

  • Unusual failure :

When there machinery failures or structure failures that seem to be as a result of fatiguing and there is not any other explanation, then consider resonance as a possible cause. Structures should last a very long time, and fatigue failures should only occur after many year service.

Typically failures as a result of resonance include :

  1. Broken welds
  2. Cracked and leaking pipes
  3. Premature machine failure
  4. Broken or cracked shaft
  5. Foundation cracks
  • Four tell-tale signs in the spectrum :
  1. Unusually high peaks in the spectrum – peaks are amplified
  2. High vibration levels in one direction / axis but not in another
  3. Areas in the spectrum where the noise floor and any peaks in the vicinity seem to have been raised. 
  4. Peak that change amplitude when machine speed changes. Resonance is only excited under certain conditions.

Vibration Damages Towers (Case : Ferrybridge Cooling Towers Collapse)

On the 1st November 1965, during high winds, three out of a group of eight cooling towers at Ferrybridge ‘C’ Power Station collapsed, with the remaining towers sustaining severe structural damage.  The towers, each 375 feet high, had been constructed closer together than was usual and had greater shell diameters and shell surface area then any previous towers. The design and construction contract for the towers had been given to Film Cooling Towers (concrete) Ltd. in 1962.

High winds were considered to be the trigger for the collapse, but an inquiry found  the exact cause to be an amalgamation of several other factors in their design:

  • British Standard wind speeds had not been used in the design resulting in the design wind pressures at the top of the tower being 19% lower than it should have been.
  • Basic wind speed was interpreted and used as the average over a one minute period, whereas, in reality, the structures are susceptible to much shorter gusts.
  • The wind loading had been based on experiments using a single isolated tower.  The grouping of the towers created turbulence on the leeward ones – the ones that did actually collapse.
  • Safety margins had not really covered any uncertainties in the wind loadings.

There had been, it was decided, a serious underestimation of the wind loading in the initial design.

Fortunately, no one was killed or injured in the accident.

Ferrybridge Cooling Tower Collapse Ferrybridge Cooling Tower Collapse
A cooling tower comes crashing to the ground during high winds at Ferrybridge ‘C’ Power Station in 1965. The aftermath of the incident. Three
of the eight cooling towers were
completely destroyed.
The vibrations of slender structures are caused by cross-wind loads known as the phenomenon vortex shedding. Specific critical wind speeds around an object can create under and over pressures that makes it move (vibrate) in cross-wind direction. These forces can make bridges swing  , and chimneys or similar slender and light structures oscillate. Oscillation occur when the damping is small and the natural resonance frequency is close to the vortex shedding frequency.

Finite Element Method

Finite element method (FEM) is based on the idea of building a complicated structural object with simple element , or dividing a complicated structural object into small and manageable pieces (like a puzzle). The application of finite element can be found everywhere in everyday life, as well as in engineering.  if you remember when you was child you playing lego  and build some building, you playing puzzle this is the example of finite element method. but now the concept of  finite element method can be application for structural analysis.

Application of Finite Element in Engineering

  1. Mechanical/Aeronautics/Astronautics/Civil Engineering
  2. Structural static analysis (linear or non linear problem)
  3. Structural dynamic analysis  (linear or non linear problem)
  4. Thermal Analysis
  5. Biomechanics
  6. etc.

Available Commercial FEM Software :

  1. Msc Nastran (
  2. Msc Adams
  3. Ansys (
  4. abaqus (
  5. Ls-Dyna (
  6. Algor
  7. Cosmos
  8. etc.


High Rise Building Vibration

Some of the causes of vibration that occurs in tall buildings. problems that most of us know is a vibration in buildings due to earthquake. but there are other causes that led to the building vibrate faster, this is caused by wind loads acting on the surface of the building.harmonic load due to wind can cause buildings to vibrate at a level of discomfort for the occupants of the building. many factors that affect the wind loading on tall buildings,including the shape of the building, the area where the building is in, and climate around the building. while the factors that affect a large vibration that occurs in the building except for the wind load is the dynamic characteristics of the building structure. therefore it is very important for predicting the dynamic characteristics of tall buildings. one way to predict the dynamic characteristics of tall buildings is to use the finite element method. The following is an example of a tall building modeling to determine the dynamic characteristics with finite element method :

Vibration Measurement setup for Modal Testing

Vibration Measurement Setup

  1. Soft suspense
  2. Soft suspense
  3. Charge & Conditioning amplifier for shaker
  4. Charge & Conditioning amplifier for accelerometer
  5. Shaker
  6. MSA/DSA
  7. Computer
  8. Accelerometer
  9. Power Amplifier
  10.  SG : Signal generator
  11. LC : load cell

This setup is to measure Point FRF (Frequency Response Function) .  The excitation force provided by shaker and measured by a load cell that exist in the shaker and the vibration response will be read by the accelerometer (accelerometer sensor is used as to result in inertance and an accelerometer placed under and in line with the shaker because they want to know the point FRF). The signal from the accelerometer and load cell passed on to  the  conditioning  charge  amplifier to be strengthening. Once the signal has been amplified into MSA / DSA processed simultaneously for the next process is synchronization with existing software on the computer to display the results of FRF (frequency response  function) measurements. . Soft suspension is used to view the rigid body motion of the structure.

Overview of Computational Fluid Dynamics Approaches

•Direct Numerical Simulation (DNS)
–Theoretically, all turbulent (and laminar / transition) flows can be simulated by numerically solving the          full Navier-Stokes equations
–Resolves the whole spectrum of scales. No modeling is required
–But the cost is too prohibitive!  Not practical for industrial flows
•Large Eddy Simulation (LES) type models
–Solves the spatially averaged N-S equations
–Large eddies are directly resolved, but eddies smaller than the mesh are modeled
–Less expensive than DNS, but the amount of computational resources and efforts are still too large for most          practical applications
•Reynolds-Averaged Navier-Stokes (RANS) models
–Solve time-averaged Navier-Stokes equations
–All turbulent length scales are modeled in RANS
    Various different models are available
–This is the most widely used approach for calculating industrial flows
-ANSYS Training Manual-

Why vibration is important?

In many ways people can expect to obtain an ideal machine viewed from the angle of vibration , which is a machine that produces no vibration at all. Such an ideal machine will greatly save energy because of all the energy given to the whole machine will be used to perform the work alone, whether pumping a fluid, compressing the air, crushing paper etc. Without generating by products in the form of vibration.
Apparently it is very unlikely because in terms of machining is not possible to get a very homogeneous material, fabrication machines without residual imbalance (residual unbalance) and rotating machinery moving and moving back and forth that does not cause friction with the other part.
What is then seen as a result of the onset of engine vibrations, is nothing but the result of abnormal circumstances such as  bolts slackness, engine parts, wear faster, become misaligned shafts, rotor unbalance, etc. into. Conditions mentioned above will raise the dissipation energy  because of vibration, causing resonance, and dynamic loads on the bearings. Cause and effect that occurs so will cause the machine immediately leads to damage (break down) which causes the engine to be shut down or automatically shut itself down because of the protection on the electrical system or its instrumentation.
A design and manufacturing excellent machine is trying to minimize imprecision such that get machines that level of vibration is very small (fine). A consequence is the price of the machine will be more expensive than machine level imprecision ”mediocre”. Even such machines .for sometimes can not be exported out of state as a result an advantage because it has a strategic impact; politics and national security.

Vibration Application (Modal Testing)

Modal testing can be applied for several purposes, including:
• Diagnostic tools
Modal testing can be used as a tool for diagnosing a failure of the structure, by knowing the characteristics of dynamic  structure by using the modal testing, the modal testing can be known personally frequency of the structure, obtained when a big frequency change in private with the initial condition of the structure is modeled using the method Finite element to obtain natural frequencies measured previously by way of modeling the damage that may occur in the structure. In this way we can diagnose structural damage that occurs.
Examples of damage diagnosis: Flutter on the aircraft, the chatter in machining processes, etc..

• Verification tools
One thing that is very important in designing the structure is to know the dynamic characteristics of the structure, to know we can use modal testing experimentally. With the experimental testing  can be known natural frequency, damping structure, and mode shape of vibration, this parameter is used as verification data generated by modeling the structure using the finite element method. Modeling with finite element can be modified such that the result resembles the results of modal testing experimental.Having verified the finite element model with data experimental then if there are minor modifications to the design of the structure of need not be experimental modal testing to determine the dynamic characteristics of the structure but simply by modifying the finite element model has been verified, this will save you the cost of product development. This can be done for model updating. moreover modal testing  can be used to verify that the product is conformed to the standards that apply or not.

• Optimization tools
In the design phase of the optimization process is needed in order to get a product that excels in all areas. modal testing can be used as a tool in the design optimization process. With a modal testing we can find out whether the damping, vibration, fatigue life of a structure meets the design criteria, if not it is necessary to redesign and perform finite element analysis to get a modal of parameters that meet the criteria or modifications performed on a prototype to get a better product.
One example is to perform sensitivity analysis that is if we make minor modifications to the structure then we can know personally frequency change is dominated by the rigidity or the mass of the structure.