Wind Loads on High-Rise Buildings by Using Five Major International Wind Codes and Standards

A high-rise building of height – 183 m was employed to evaluate similarities and differences of wind load calculations done by using five major wind codes and standards. Evaluation was done in both ultimate and serviceability limit conditions. Member forces in columns, and beams, compressive stress in shear walls and support reactions obtained from finite element modelling was used to assess building responses in ultimate limit condition. Along and across wind, accelerations and drift indices were engaged to estimate serviceability limit state performances. Available 3 second gust wind speeds are converted into mean hourly and 10 minute average wind speeds to calculate wind loads on building. Wind speeds with 5 years return period was used in building acceleration calculation. The simultaneous use of higher terrain-height multiplier and importance factor may be lead to over design, even in cyclone prone areas. The use of post disaster wind speed does not exceed the drift limit but exceeds threshold acceleration value in across insert wind acceleration.


Introduction
High-rise buildings are more susceptible to wind loads due to their excessive heights, use of lightweight materials for super structure and inner partition walls, low structural damping, etc. Thus, design of high-rise buildings against wind loads is always a challenging task for designers. However, due to scarcity of land and high land cost, and for the glory of developer, building skyscrapers in urban areas is indispensable. In recent time, there has been a national trend to build tall, slender tower type high rise buildings in Sri Lanka especially in Colombo city limit [1]. A properly designed high-rise building should satisfy both ultimate and serviceability limit state conditions against vertical and lateral loads. To ensure a properly designed building, wind loading standards are serving as a primary design tool at early design stage in the absence of more sophisticated testing methods such as Wind Tunnel tests, Computational Fluid Dynamic (CFD) simulations, A proper wind loading standard would be able to calculate wind load fairly accurately and be able to check serviceability criteria by considering both static and dynamic behaviour of the building. However, early wind loading standards are not covering either dynamic analysis or serviceability limit criteria such as maximum allowable wind acceleration. In Sri Lankan context, neither the most common wind loading standard CP3 Chapter V-Part 2: 1972 [2] nor the only available mandatory document on wind loading Design Manual--Design buildings for high winds, Sri Lanka‖ [3] covers the above two aspects. Therefore, Sri Lankan engineers employ different international wind loading standards to calculate wind loads on high-rise buildings. However, direct adaptation of these international standards leads to some problems such as adaptability of country specific factors in Sri Lankan conditions, compatibility of wind loading standards with other design standards, achieved level of risk level remaining unknown, effects of differences and similarities of wind load calculations of different standards, satisfaction of serviceability conditions of a building, etc. In this study, a 183 m tall office building was selected as a case study to compare wind load calculation done by using five major international wind loading standards. Those standards were selected on a rational basis by considering their popularity and familiarity in Sri Lanka. The differences and similarities of wind load calculations are addressed by using forces on structural members such as columns, beams, and shear walls obtained via finite element modelling (FEM). In this paper section 2 explains the reasons for selected codes and standards; section 3 illustrates the case study. Section 4 and 5 show the results for ultimate and serviceability limit states. Finally section 6 makes a conclusion about advantages and disadvantages of selected wind loading standards and proposes a suitable wind loading standard to assess wind loads on buildings in Sri Lanka.

Selected codes and standards for the study
The selection of wind loading standards is mainly based on its ability to address dynamic response of a high-rise building. This is because of slenderness, low damping ratio and flow mechanisms such as vortex shedding which tend to cause high-rise building to excite dynamically. However, the most common wind loading standard use in Sri Lanka, CP3 Chapter V-Part2:1972 [2] could not be used to calculate wind loads on dynamically excited buildings as it employs a quasi-static approach. Therefore, many other international standards such as BS 6399 standard has a detailed method for dynamic analysis, which can be used for a building with height or length-tobreadth ratio greater than 5 and a first-mode of vibration of less than 1 Hz. By using the gust factor method, this standard encounters most of the dynamic nature of wind such as resonance factor, background factor, damping ratio, peak factor for both approaching wind and the building, size of the building and spectrum of the approaching wind. This standard employs a 3-second gust wind speed as the basic wind speed which allows adopting this code directly in Sri Lankan context. Instead of using gust factor, AS/NZS 1170.2:2002 [6] uses aerodynamic shape factor and dynamic response factor to analysis dynamic behaviour of high-rise buildings. The Aerodynamic shape factor adjusts factors such as correlation of pressure on opposite sides of a building, load shearing effects between adjacent areas, local pressure effect, etc. The dynamic factor considers encountering different characteristics of dynamic behaviour of the building and dynamic nature of the approaching wind. The basic wind speed used in latest Australian standard is mean hourly wind speed. The Euro standard, EN 1991-1-4:2005 [7] is the first real multinational wind loading standard. In this study it is used with the National annex prepared for United Kingdom. The dynamic nature of the wind loading is captured via structural factor, which takes in to account the effect on wind actions from the nonsimultaneous occurrence of peak wind pressures on surfaces together with the effect of the vibrations of the structure due to turbulence. Comparison of wind loading standards is a common objective accomplished by researchers before adopting them directly in any country other than its origin. Bashor and Kareem [9] compared six major international wind loading standards for both ultimate and serviceability limit states to identify parameters for further theoretical studies. A series of papers was published in Asia Pacific Conference of Wind Engineering (APCWE) to harmonize different international wind loading standards used in Asia-Pacific countries ( [10], [11], [12]). These studies employed base shear, bending moments and acceleration at the top of the building for the comparison purpose. Results of the comparison revealed that wind loads calculation using different codes and standards have significant discrepancies though they are evaluated for the same wind climate conditions. Aforementioned

3.
Case studies A 183m high Commonwealth Advisory Aeronautical Research Council (CAARC) standard building model [13] was selected as the case study for wind load calculation. The plan dimension of this building is 46 m x 30 m which gives height-to-breadth ratio 6.1 (>5) anticipating dynamic excitation due to wind loads. The same building model was used by previous researchers [10,11,12] to compare wind loads derived by using wind loading standards. For the load calculation purpose, this building is assumed as an office in an urban area of Sri Lanka. It is also designed as a 51 storey building with floor-to-floor height of 3.6 m. To represent the realistic structural behaviour, structural elements and internal spaces are designed in details. The basic structural form is wall-frame structure. The columns and beams form the frame and shear walls located around the lift and service core.
Within the service core a hard zoning lift system, washrooms and ducts are located. The dimensions of structural members designed according to vertical loads are given in Table 1.
A general purpose finite element computer program SAP 2000 [14] was used to model the 183 m building in order to obtain member forces for comparison. SAP2000 is object-based modelling software which automatically converts the object-based model into an element-based model that is used for analysis. This element-based model is called the analysis model and consists of traditional finite element objects such as frame elements, shell elements and joints (nodes). A frame element with 6 degree of freedom is modelled as a straight line, connecting two points which has its own local axis to define section properties, loads and displacements.

Basic wind speeds with different averaging time
The provisions of wind load calculation given in different wind loading standards are based on basic wind speeds with different averaging times. These averaging times are range from 3 second to 1 hour. However, the only available wind speed data in Sri Lanka are based on 3 second gust wind speed. Therefore, it is necessary to convert 3 second gust wind speeds into different averaging time wind speeds by using proper conversion factors. This conversion should be done cautiously as erroneous conversion might lead to have unrealistic wind loads. For this study, conversion was done as follows. First, 3 second gust wind speed is converted in to mean hourly wind speed as proposed by Cook [8] (Eq 1). This method is based on the terrain and building height factor given in BS

Wind induced forces
Wind loading standards only facilitate calculation of wind pressures on building faces at different heights. Multiplying calculated pressures with corresponding contributory areas will give the wind load at that height. However, this is not the actual load experienced by structural members as they have load sharing mechanisms. On the other hand structural members are designed by considering the actual loads acting on them rather than considering loads on the structure. The actual structural load acting on any member can be obtained by using the 3-D finite element model by applying external loads. Post disaster wind speeds in all three zones as shown in Table 1 were used for wind calculation. The use of higher post-disaster wind speeds compared to low wind speeds for normal structures is recommended based on views of previous researchers ( [17], [18]) considering; uncertainty of derivation of basic wind speeds, lower wind speeds compared to other countries with similar topographical and wind conditions, and as a conservative approach to achieve higher margin of safety, etc. Wind loads are calculated by using dynamic methods as given in standards to encounter anticipated dynamic response of the high-rise buildings. In addition to that, for wind load calculation by using AS 1170. 2 For the purpose of comparison, the obtained results are shown as normalised forces with respect to CP 3 Chapter V-Part 2:1972 [2], which is the most common practice in Sri Lanka. Wind induced forces such in columns, beams, supports and shells on 183 m high building in all three zones are shown in Figure 2 to 10.

Member forces
The maximum loads in columns and beams are observed for the load combination 1.2G+1.2Q+1.2W. Thus it can be considered as the governing load combination for structural design. According to Figures 2(a) and (b), there is a significant difference between loads derived for Australian standards and rest of standards especially in zone 1. The loads derived from Australian wind standards are much larger than loads derived from other standards in zone 1. The maximum differences in bending moments in column and beams are 1. . This concludes that the use of higher terrain-height multiplier and importance factor together may lead to overestimate of wind loads even in cyclone prone areas. Therefore, designer can decide to employ either importance factor or terrain-height multiplier for a design unless necessary to design highly cyclone resilient structure such as cyclone shelter. The effect of different wind loads calculations are clearly visible of reults obtained for load combination 1.0G + 1.4W, where effect of dead load is minimum. The larger wind loads resulted from Australian standards in zone 1 are again reflected from Figures3 (a) and (b). Column and beam loads obtained from zone 2 and zone 3 are approximtely same for all wind loading standards suggests that without using especial factors such as higher terrainheight multiplier,wind load calculations would be similar for all standards. However, the aforementioned fact should be closley inspected as member forces could be varied greatly not only in the use of especial factors but also the manner wind loads are applied on a building. For an example, the same load combination 1.0G + 1.4W displayed larger variation in zone 2 when wind flow perpendicular to 30m building side (Figure 7). There are rather higher coulum and beam loads in zone 2 and 3 for both Euro and British standars (Figure 7). This might be resulted from wind load application method engaged by those two standards. The division-by-parts rule leads to applying higher wind loads over a height range compared to linear variation of wind load with height as assumed in CP 3 Chapter V: Part 2:1972 [2]. Bending moments in coulum and beams reach a maximum for load case 1.4G + 1.4W due to the combine effect of higher dead load and wind loads. When wind flow perpendicular to 46 m side, the bending moments obtained for Australian standards can be as high as 1.85 and 1.4 for columns and beams respectively (Figures 4(a) and (b)). When wind flow perpendicular to 30m side, those corresponding values are 1.75 for coulmns and 1.4 for beams for both Australian standards (Figures 8(a) and  (b)). Wind load only case represents the individual differences in wind load calculations and its application on buildings. From Figures  5(a) and (b), it reflects that effect of used of both important factor and higher terrain height multiplier in AS 1170. 2:1989[5]. The combined effect of these two factors lead to have wind loads two times greater than wind loads derived from CP 3 Chapter V: Part 2:1972 [2]. The importance factor alone may increase the wind laod about 50% in columns. More surprisingly, EN 1991-4:2005 [7] yields higher wind loads compared to both BS 6399. 2:1997 [4] and CP 3 Chapter V: Part 2:1972 [2] due to the use of difference in wind load application and use of a new set of factor different from previous British standards. Particularly, the effect of the use of divison-by-parts rule is displayed in results of Figures 9 (a) and (b

Base reactions
Base reactions are principal measurements of calculated wind loads and its overall effects on the building. Base shear represents the total wind loads on the building and base moment indicates overturning moment resulted from those wind loads. The latter factor is also an indirect indication of wind load distribution along the building height. Figures 10 (a) and (b) show bending moment and shear force in three zones for wind directions perpendicular to 46m side and 30m side respectively. The larger base moment and base shear yielded from AS 1170. 2:1989[5] due to use of importance factor. In the zone 2 and zone 3, Euro codes yielded higher base moment as well as base shears values due to both higher wind load derivation and use of division-by-part rule for wind load disrtribution.  [2] values when wind flow perpendicular to 46 m long side. However, when wind flow perpendicular to 30 m side base moment and shear can be as high as1.6 and 1.8 in zone 1 and zones 2, 3 respectively. This is another evidence for difference arising from use of division-by-parts rule in newer British code compared to the previous British code of practice (Table 2).

Maximum stress in shear walls
Maximum compressive stress in shear walls can be observed for wind load derived by using AS 1170. 2:1989[5]in zone 1 due to use of higher terrain height multiplier. The normalized maximum stress is about 1.7 and 1.45 for wind flow perpendicular to 46m side and 30m side of the building respectively. However, when wind flow perpendicular to 30m side, Euro standards caused larger stress in shear walls in zones 2 and 3 because of higher wind loads acting on greater area of building sides due to use of divisio-by-part rule. Division-by-part rule also leads to generate higher wind loads, especailly wind flow perpendicular to 30m side for BS 6399.2:1997[4] compared to CP3 Chapter V -Part 2: 1972 [2]. .

Drift limit
The drift limit is used to evaluate the degree of wind sensitivity of the building. This is an important measurement to prevent damages on non-structural building components including partition walls.The maximum deflection at top of the building in serviceability limit condition is obtained by applying wind loads to the finite element 3-D model.  Table 3.
The generally accepted average drift index limit for high rise buildings is 1/500 [20]. By refering Table 3, only wind loads derived from AS 1170.2:1989 in zone 1 exceeds the generally accepted drift limit. This is because of larger wind loads resulted from use of both importance factor and higher terrain-height factor. However, all other cases satisfy the drift index requirement. In zone 3, all models well below the maximum drift limit, approximately by half of its threshold value. Thus, it can be concluded that even wind load calculation done by using post disaster wind speed will not cause conflict with drift index limit in tall buildings.  Table 3. Δzheight of the section of the structure upon which the wind pressure acts b-breadth of a structure, normal to the wind stream gR-peak factor for resonant response (10 min period) given by: =   c e n 600 log 2 nc = first mode natural frequency of vibration of a structure in the crosswind direction, in Hertz gv-peak factor for the upwind velocity fluctuations, which may be taken as 3.7 Ih-turbulence intensity Km-mode shape correction factor for crosswind acceleration Cfs-crosswind force spectrum coefficient generalized for a linear mode shape ζ -ratio of structural damping to critical damping of a structure Et-spectrum of turbulence in the approaching wind stream S -size reduction factor The acceleration values calculated are shown in Table 4. Euro code yields higher along wind acceleration values than Australian standards. However these along wind acceleration values are much less than across wind acceleration values due to slenderness of 183 m height building. In most of the cases, these across-

Conclusion
Major findings of this study can be concluded ed as 1.
Structural loads are varying according to the wind loads derived from the standards. Both Australian standards give higher forces in zone 1 due to use of importance factor (Mi) and/or use of higher terrain multiplier for cyclonic areas.

2.
Base shear and base moments are two major measurements on determining the effects of wind loads, which directly combine with the wind load values and their distribution along the height of the building.
Higher base moment and base shear are obtained from British and Euro codes because they use division by parts rule for contributing wind loads along the height of the building.

3.
Division -by -parts rule trends to apply higher loads even at the lower heights of the buildings and subsequently results in higher base moments for the building. Therefore, it is better to choose small strips at intermediate levels rather than choosing larger strips.

4.
Only wind loads calculated according to the AS 1170.2:1989 for 183 m high building in zone 1 exceed the drift limit. This could be a result of higher wind loads derived with the use of both importance factor (Mi) and the higher terrain multiplier for cyclonic areas.

5.
Along wind accelerations values calculated from Euro code are higher than the those values calculated from Australian codes except in zone 1. The higher serviceability wind speeds values ued for Astralian codes may be the reason for above difference.

6.
Across wind acceleration values are considerably higher than the along wind acceleration values. Thus greater concern should give to control across wind acceleration when designing high-rise buildings in Sri Lanka.